Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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Potatoes, pathogens and pests : effects of genetic modifi cation for plant resistance on non-target arthropods
Lazebnik, Jenny - \ 2017
Wageningen University. Promotor(en): Joop van Loon; Marcel Dicke. - Wageningen : Wageningen University - ISBN 9789463431620 - 151
solanum tuberosum - potatoes - oomycetes - phytophthora infestans - genetic engineering - transgenic plants - disease resistance - risk assessment - nontarget organisms - arthropods - insect pests - herbivores - trophic levels - ecological risk assessment - greenhouse experiments - field experimentation - aardappelen - oömyceten - genetische modificatie - transgene planten - ziekteresistentie - risicoschatting - niet-doelorganismen - geleedpotigen - insectenplagen - herbivoren - trofische graden - ecologische risicoschatting - kasproeven - experimenteel veldonderzoek

Currently, fungicides are necessary to protect potato crops against late blight, Phytophthora infestans, one of the world’s most damaging crop pathogens. The introgression of plant resistance genes from wild potato species targeted specifically to the late blight pathogen into susceptible potato varieties may alleviate the environmental impact of chemical control. Genetically modified plants are subject to an environmental risk assessment, and this includes testing for risks to the non-target arthropod community associated with the crop. The thesis begins with a review about the main plant defense responses and their role in influencing sequential interactions between herbivores and plant pathogens. The experimental chapters each focus on different aspects of the interaction between potato plants (both resistant and susceptible), the target pathogen (P. infestans) and several non-target insects. With each chapter, the scope widens: from the molecular gene expression in potato leaves in response to sequential attacks, to field scale biodiversity analyses. At the molecular level, one of the main findings was that the genomic position of the Rpi-vnt1 insertion conferring resistance to P. infestans influenced potato gene expression measured in leaves, when interacting with the non-target insect pests Myzus persicae (Green peach aphid) and Leptinotarsa decemlineata (Colorado potato beetle). Insect performance differed between the resistant GM and susceptible non-GM comparator. In the following chapter, the differences in insect performance were tested across a range of conventionally bred cultivars varying in resistance to P. infestans. Differences in M. persicae performance between several cultivars greatly outweighed the differences previously detected between the GM and non-GM comparator. These results are crucial in shaping the way risk is assessed in the context of GM crops, and these results are supported in our experiments assessing effects on biodiversity with pitfall traps in the field. The third trophic level was also addressed by comparing the performance of the parasitoid Aphidius colemani reared on GM and non-GM fed aphids, both with an without exposure to P. infestans. Differences in parasitoid performance were only found on the susceptible cultivar when inoculated with P. infestans. In the last experimental chapter the risk assessment is taken to the field comparing pitfall trap catches over two years and in two countries. Different methods for statistical analysis of biodiversity data were compared to arrive at recommendations for such analysis in the framework of environmental risk assessments. Drawing on these lessons, the discussion ends with ideas for the development of protocols for environmental risk assessments in the light of expected scientific progress in agricultural biotechnology.

Large herbivores as a driving force of woodland-grassland cycles : the mutual interactions between the population dynamics of large herbivores and vegetation development in a eutrophic wetland
Cornelissen, Perry - \ 2017
Wageningen University. Promotor(en): Frank Berendse; Karle Sykora, co-promotor(en): Jan Bokdam. - Wageningen : Wageningen University - ISBN 9789463430159 - 151
grasslands - woodlands - herbivores - population dynamics - vegetation - wetlands - graslanden - bosgebieden - herbivoren - populatiedynamica - vegetatie

This thesis examines the mutual interactions between the population dynamics of large herbivores and wood-pasture cycles in eutrophic wetlands. Therefore, habitat use and population dynamics of large herbivores, the effects of large herbivores on vegetation development, and the mutual interactions between vegetation development and herbivore population dynamics were studied in the eutrophic wetland the Oostvaardersplassen. At the Oostvaardersplassen cattle, horses and red deer were introduced in a fenced area with no predators, and population numbers are bottom-up controlled by food supply. The study showed that high densities of cattle, horses and red deer were able to break down woody vegetation and create grasslands. As the populations of large herbivores increased, the amount of the preferred grass available per animal decreased. This forced the large herbivores to use other food plants in other vegetation types, such as scrub, and transforming these into grasslands. In this way, the large herbivores facilitated high numbers of geese. As geese can clip the grass very short (<2 cm), they forced the large herbivores even more to forage in alternative vegetation types. Cattle, the largest herbivore in the system, were the first to experience the negative consequences of this strong competition, and their numbers declined. This raises the question whether an assemblage of bottom-up regulated populations of cattle, horses and red deer, or other large herbivores, can sustainably coexist under these circumstances. The results of our modelling study and experiences in the field suggest that resource partitioning may be a more reliable mechanism for long term coexistence than temporal variability due to climatic extremes or disease outbreaks. The best way to provide opportunities for resource partitioning in the Oostvaardersplassen is to enlarge the area and connect it to other reserves in order to increase the heterogeneity of the grazed system. Although the results of our model suggest that weather variability and presence of geese gave minor opportunities for the coexistence of large herbivores, both factors were necessary for creating windows of opportunity for the establishment of thorny shrubs. Weather variability creates strong reductions of the large herbivore populations while geese influence the maximum and minimum numbers, which are lower when geese are present. The effects of geese on the minimum numbers are small, but apparently sufficient to make the wood-pasture cycle operate. This raises another question whether a large predator, such as the wolf, could have similar effects on these ecosystems as the geese in the model. The impact of geese combined with a possible positive effect of wolves on wood-pasture cycles could perhaps increase the frequency of the windows of opportunity and increase the survival of established thorny shrubs. Until now, we have seen that a few conditions for the wood-pasture cycle are met by the herbivores. However, a few important requirements are not satisfied: (a) a temporary reduction of large herbivore numbers allowing the establishment of light demanding thorny shrubs and the development of thorny scrubland within the created grasslands; (b) the establishment of palatable trees within these thorny scrubs; (c) the formation of closed canopies which shade out the shrubs and lead to unprotected groups of trees and groves. This means that we still cannot conclude if the large herbivores are a driving force for the whole cycle in a highly productive environment. As long as we have not experienced a complete wood-pasture cycle in the Oostvaardersplassen or any other area, it remains to be seen what will happen in the future. Whatever the outcome will be, the results of our study suggest that some adjustments would benefit the Oostvaardersplassen-system such as increasing heterogeneity through connecting the area with other large nature reserves. This will not only increase opportunities for resource and space partitioning and thus increase opportunities for the coexistence of the large herbivores, but also for wood-pasture cycles and increased biodiversity.

Arriving at the right time : a temporal perspective on above-belowground herbivore interactions
Wang, Minggang - \ 2016
Wageningen University. Promotor(en): Wim van der Putten, co-promotor(en): T.M. Bezemer; A. Biere. - Wageningen : Wageningen University - ISBN 9789462578142 - 174 p.
herbivores - aboveground belowground interactions - herbivory - defence mechanisms - roots - leaves - mycorrhizas - population dynamics - soil biology - herbivoren - boven- en ondergrondse interacties - herbivorie - verdedigingsmechanismen - wortels - bladeren - mycorrhizae - populatiedynamica - bodembiologie
Tales on insect-flowering plant interactions : the ecological significance of plant responses to herbivores and pollinators
Lucas Gomes Marques Barbosa, D. - \ 2015
Wageningen University. Promotor(en): Marcel Dicke, co-promotor(en): Joop van Loon. - Wageningen : Wageningen University - ISBN 9789462572119 - 207
bloeiende planten - insecten - insect-plant relaties - plant-herbivoor relaties - herbivoren - bestuivers (dieren) - trofische graden - parasitoïden - herbivoor-geinduceerde plantengeuren - flowering plants - insects - insect plant relations - plant-herbivore interactions - herbivores - pollinators - trophic levels - parasitoids - herbivore induced plant volatiles - cum laude
cum laude graduation
African wildlife and people : finding solutions where equilibrium models fail
Poshiwa, X. - \ 2013
Wageningen University. Promotor(en): Herbert Prins; Ekko van Ierland, co-promotor(en): Ignas Heitkonig; Rolf Groeneveld. - Wageningen : Wageningen UR - ISBN 9789461737618 - 173
wild - extensieve weiden - evenwicht - droogte - mensen - herbivoren - vee - verstoring - ecologische verstoring - zimbabwe - afrika - wildlife - rangelands - equilibrium - drought - people - herbivores - livestock - disturbance - ecological disturbance - africa

Grazing systems, covering about half of the terrestrial surface, tend to be either equilibrial or non-equilibrial in nature, largely depending on the environmental stochasticity.The equilibrium model perspective stresses the importance of biotic feedbacks between herbivores and their resource, while the non-equilibrium model perspective stresses stochastic abiotic factors as the primary drivers of vegetation and herbivore dynamics.In semi-arid and arid tropical systems, environmental stochasticity is rather high, making the systems essentially non-equilibrial in nature, suggesting that feedback between livestock and vegetation is absent or at least severely attenuated for much of the time. In southern Africa, range and livestock management however, has been built around the concept of range condition class and the practices of determining carrying capacities and manipulating livestock numbers and grazing seasons to influence range condition. This management approach is derived from the equilibrium or climax concept of Clementsian succession. The erratic and variable rainfall in many pastoral areas of Africa poses a fundamental challenge to this conventional notion of carrying capacity in range management. This realization has caused a shift towards models that embrace non-equilibrium dynamics in ecosystems. The main concern is that application of the range model may contribute to mismanagement and degradation of some rangeland ecosystems. However, only a few studies in rangelands have empirically tested the non-equilibrium hypothesis leading to the debate on rangeland dynamics remaining unresolved.

Across the savannas of Africa, grasslands are being changed into cultivation due to increasing human population, at the expense of decreasing wildlife populations. African savannas however, still contain pockets of wilderness surviving as protected areas, but even there, species richness of large mammals is decreasing. The inevitable result is the loss of most of the wild plants and animals that occupy these natural habitats, at the same time threatening the well-being of the inhabitants of these savannas. Hence, to facilitate the management of arid and semi-arid savannas for both biological conservation and sustainable use (improving human welfare) an improved understanding of the complex dynamics of these savannas is critical. Furthermore, it is widely recognized that a high level of uncertainty typifies the lives of rural farmers in developing countries.Non-equilibrium dynamics bring additional uncertainty and risk to the system.However, attempts to understand efficient and sustainable ways to improve biodiversity and human welfare in systems showing non-equilibrium dynamics have been rare.The behaviour of non-equilibrium systems is characterised as more dynamic and less predictable than equilibrium systems. Therefore, non-equilibrium dynamics in dryland ecosystems present a different kind of management problem for both livestock and wildlife systems since their management has been dictated by the equilibrium assumption. Additionally, loss of biodiversity is regarded today as one of the great unsolved environmental problems.Faced with this biodiversity crisis, the challenge is to find ways to respond in a flexible way to deal with uncertainty and surprises brought about by non-equilibrium dynamics.

In this thesis I use a bioeconomic approach in analyzing the implications of non-equilibrium dynamics for the efficient and sustainable management of wildlife and livestock in dryland grazing systems. The study area for this thesis is southeastern lowveld of Zimbabwe.

In chapter 2, I investigate the role of abiotic and biotic factors in determining plant species composition. While early studies emphasized the importance of edaphic and environmental controls on plant species distribution and spatial variation in vegetation composition, recent studies have documented the importance of both natural and anthropogenic disturbances in this respect. At a regional scale vegetation structure (i.e., grass/tree ratio) and species composition in savannas is largely determined by precipitation, whereas at the nested landscape-scale vegetation structure and composition is more prominently determined by geologic substrate, topography, fire and herbivory. Chapter 2, shows that at the landscape scale, abiotic variables such as rainfall and soil fertility override the effect of humans and livestock on the herbaceous and the woody plant composition.

Then, in Chapter 3, I ask the question whether there is something like non-equilibrium and what are the impacts of such dynamics on cattle herd dynamics? I studied the relevance of non-equilibrium theory to my study area by testing whether annual changes in cattle numbers showed the presence of crashes and if so, what were the factors best explaining those crashes and what age and sex classes of cattle were most vulnerable to such crashes? Chapter 3 showed that crashes in annual cattle numbers were evident and were best explained by rainfall and NDVI and their lags. Immigration i.e., movement in of animals was also an important factor in years when rainfall was below the threshold and so it was a possible source of cattle recovery after a crash together with high calving rates. In years when rainfall was above the rainfall threshold, NDVI explained more variation in annual changes of livestock. The impacts of crashes were greater on calves than other cattle age categories thus explaining why there are legacy effects (lags) in cattle numbers that can only partly be offset by cattle purchases from elsewhere because of poverty or lack of surplus stock elsewhere. These findings make the southeastern lowveld system to be dominated by non-equilibrium dynamics.

The welfare of local people is the issue that I focused on in my economic section of this thesis (Chapters 4 and 5). I addressed the question of how risks of fluctuations in household income can be managed in order to improve human welfare. The expectation was that in systems exhibiting non-equilibrium dynamics people can improve their welfare by exploiting a combination of wildlife and agricultural activities (livestock and cropping) in their attempts to reduce fluctuations in their annual welfare. This would be possible if the risks in wildlife and agro-pastoral systems were sufficiently different. Exploiting different sources of income requires efficient allocation of resources. The most prominent resource is land and land varies spatially in quality and ecological resources require spatial connectivity. Therefore the spatial dimension is important in this allocation.

In Chapter 4 I asked the question: To what extent can wildlife income buffer rural households’ incomes against fluctuations in rainfall? I studied the extent to which wildlife derived income can buffer local households’ income against fluctuations due to rainfall. The addition of wildlife as an asset for rural farmers’ portfolio of assets showed that wildlife can be used as a hedge asset to offset risk from agricultural production without compromising on return. However, the power of diversification using wildlife is limited because revenues from agriculture and wildlife assets were positively correlated. However, the correlation was very weak (only 0.4 and the explained variance thus only be 16 %) which gives ample scope for buffering. Therefore, revenues from wildlife have potential to reduce household income fluctuations due to drought, but only to a limited extent.

In Chapter 5 the question was: From a theoretical perspective, can wildlife income have an insurance value to local people? I used a modelling approach to study the extent to which wildlife income offers an insurance value to local people against fluctuating annual rainfall. Findings did not support the common assertion that wildlife can offer insurance to local people against income fluctuations due to rainfall fluctuations. The failure by wildlife income to offer insurance value to local people could be explained by high costs of harvesting the wildlife resource and high densities of both human and livestock populations in southeastern lowveld.As corollary I draw the conclusion that wildlife cannot pay its way in these rangelands as long as there are high densities of people as shown in Chapter 5. Definitely wildlife income becomes insufficient if long-term sustainability of wildlife resources is considered.

Chapter 6, finally synthesizes the conclusions that can be drawn from the preceding chapters and puts the issues addressed in a broader context. In summary, this thesis shows evidence of non-equilibrium dynamics in semi-arid grazing systems. Furthermore, the small contribution of wildlife income to local people’s welfare goes to show the widely shared view that financial rewards generated through integrated conservation and development programmes such as CAMPFIRE have generally been seen as insufficient. This led me to suggest that if we have a moral or ethical obligation to protect wildlife species, then an important way for people to meet their aspirations economically was suggested by Malthus.

Multitrophic interactions on a range-expanding plant species
Fortuna, T.F.M. - \ 2013
Wageningen University. Promotor(en): Louise Vet; J.A. Harvey. - Wageningen : Wageningen UR - ISBN 9789461737656 - 229
planten - invasieve soorten - geïntroduceerde soorten - herbivoren - ongewervelde dieren - natuurlijke vijanden - predatoren - parasitoïden - multitrofe interacties - bunias orientalis - verdedigingsmechanismen - plants - invasive species - introduced species - herbivores - invertebrates - natural enemies - predators - parasitoids - multitrophic interactions - defence mechanisms

Studies on the ecological impacts of exotic invasive plants have mainly focused on inter-continental invasions. However due to global environmental changes, a rapid increase in intra-continental range-expanding plants has been observed. In this context, multitrophic interactions between exotic plants, native herbivores and their natural enemies have been largely ignored. This thesis aimed at examining how an exotic range-expanding plant interacts with aboveground insect herbivores and their natural enemies and how it can contribute to the successful establishment of the exotic plant. In addition, it examines how resistance traits of different populations of the range-expander affect the behaviour and performance of herbivores and their natural enemies in the new habitat. Bunias orientalis (Capparales: Brassicaceae) is perennial plant from extreme south-eastern Europe and Asia that has recently expanded its range and become invasive in northern and central parts of Europe. In the Netherlands, it is considered naturalized but non-invasive.

Firstly, using a community approach, I found that Bu. orientalis suffered less herbivore damage and harboured smaller invertebrate communities than sympatric native Brassicaceae in the Netherlands. The exotic plant has been found of low quality for the larval growth of the specialist herbivore (Pieris brassicae). Furthermore, two of its gregarious parasitoids were differentially affected by the quality of the exotic plant. The pupal parasitoid (Pteromalus puparum) survived better than the larval parasitoid (Cotesia glomerata), and the latter parasitized less hosts on the exotic than on native plants. Therefore, the herbivore can be selected to adapt to the new plant by conferring an enemy free space to the herbivore. In this case, a plant shift by the specialist herbivore might occur and thus preventing the further spread of the exotic plant. Conversely, in the field I found greater carnivore pressure on Bu. orientalis compared to other native Brassicaceae, particularly in the peak of arthropod abundance. Hence, top-down forces exerted by herbivore natural enemies may act in concert with bottom-up control of plant resistance traits to counteract herbivore plant shift and promote the successful range expansion of the exotic plant.

Secondly, using a biogeographical approach, I found a considerable intraspecific variation in defence traits (trichomes, glucosinolates, metabolic fingerprints) of Bu. orientalis populations from the native and the exotic range. Plants collected in the native range were better defended than their exotic conspecifics. This variation matched with the performance of a generalist herbivore (Mamestra brassicae) and its parasitoid (Microplitis mediator), which developed poorly in plants from the native range. The results suggest that the defensive mechanisms of Bu. orientalis might have been counter-selected during the range expansion of the exotic plant. Further studies, however, need to examine if enemy release in the new range is followed by an increase in performance of the exotic plant. Finally, a comparative study of multitrophic interactions, both above- and belowground, in the plant native range and along the transect of its range expansion can help to clarify the mechanisms underlying the invasive success of Bu. orientalis.

Uit de wetenschap : hoe tel je wilde hoefdieren?
Groot Bruinderink, G.W.T.A. ; Dekker, J. ; Cornelissen, P. - \ 2013
Vakblad Natuur Bos Landschap 10 (2013)3. - ISSN 1572-7610 - p. 14 - 16.
wildbeheer - grote grazers - herbivoren - natuurgebieden - monitoring - populatiedichtheid - zuidelijk flevoland - wildlife management - large herbivores - herbivores - natural areas - population density
Tellen van wilde hoefdieren wordt steeds belangrijker, onder andere vanwege eventuele beperking van de wildstand. Maar hoe moeten herbivoren geteld worden om een goed beeld te krijgen van de aantallen? Begrippen als precisie (herhaling van tellingen), nauwkeurigheid (geschatte waarneming t.o.v. werkelijkheid) en betrouwbaarheid (systematische fouten)
The bulldozer herbivore: how animals benefit from elephant modifying an African savanna
Kohi, E. - \ 2013
Wageningen University. Promotor(en): Herbert Prins, co-promotor(en): Fred de Boer. - S.l. : s.n. - ISBN 9789461735355 - 170
herbivoren - loxodonta africana - afgrazen - voedingsgewoonten - habitats - diergedrag - dierecologie - savannen - afrika - herbivores - browsing - feeding habits - animal behaviour - animal ecology - savannas - africa

Herbivore-vegetation interactions are important structuring forces in savanna that modify the availability and quality of forage resources. Elephant for example, are known for their ability to change the vegetation structure through toppling trees, uprooting, snapping, debarking and breaking branches. Controlling the number of elephant is a common response of wildlife managers who think that the increase of elephant will further destroy the habitat and hence cause loss of biodiversity. However, our knowledge on how elephant feeding habits affect other large herbivore species in habit use is limited. Therefore, the question in this thesis is: What is the impact of elephant feeding habits on species diversity of large herbivores in African savanna? To answer this question, it is important to understand the responses of trees when impacted by elephant. What proportion of the browsed biomass is made available after a tree is pushed over or snapped by elephant? How is the forage quality affected? Is the seasonal and intensity of browsing affecting forage availability? Field experiments and field surveys were used to investigate the tree’s response and herbivore species responses to elephant impact. These experiments were (1) simulation of timing and intensity of browsing (hand defoliation) and (2) manipulation of vegetation i.e. simulated pushed over trees, uprooted trees (tree removal) and snapped trees (tree cut at the stem). The field survey involved measuring impacted trees by elephant. Leaf biomass and quality of pushed over, snapped and uprooted trees were measured. The defoliation experiment was conducted in the roan antelope enclosure in Kruger National Park South Africa, and the vegetation manipulation experiment was conducted in the Umbabati Private Nature Reserve, South Africa.

The results indicate that elephant foraging habits change the distribution of forage, increasing the forage availability at lower feeding heights, which means that accessibility of forage to medium and small herbivores increases. Elephant browsing also improved forage quality and availability in the dry season, which is very important to browsing animals. A high intensity of browsing by elephant in the wet season increased the dry season forage, because the amount of new regrowth (leave compensation) is proportional to the amount of leaves that was removed. Elephant therefore initiate inter and intra-species facilitation processes. Inter-species facilitation occurs when other herbivores species utilize the regrowth stimulated by elephant, whereas intra-species facilitation occurs when the browse resource is exploited by other elephants. The herbivore responses to elephant browsing clearly indicated that facilitation effects occur, especially for certain guilds when selecting their habitat. For example, small predation-sensitive herbivore species (steenbok, impala and common duiker) preferred completely opened up areas, whereas large herbivores were less affected in their habitat preference by elephant impact. Greater kudu selected pushed over and control plots and rarely visited opened up areas. These differential response of herbivores species to elephant impact resulted in a high species richness of large mammals in elephant impacted areas. In conclusion, elephant feeding habits play a major role in structuring the herbivore assemblage/community through modifying the vegetation. Resource heterogeneity increased under the influence of elephant feeding, in particular through increasing the accessibility of leaf biomass at lower feeding heights, increasing green leaf availability in the dry season, and improving the nutrient content in re-growth. Moreover, habitat selection, especially that of small herbivores was positively influenced by elephant impact. With these findings, this study contributes to a better understanding of the role of elephant feeding habits and its cascading effects to other herbivore species.

Temporal dynamics of induced responses in Brassica juncea
Mathur, V. - \ 2012
Wageningen University. Promotor(en): Louise Vet; N.M. van Dam, co-promotor(en): A.S. Reddy. - S.l. : s.n. - ISBN 9789461733085 - 219
brassica juncea - insectenplagen - insect-plant relaties - gastheer parasiet relaties - herbivoren - brassicaceae - entomologie - ecologie - herbivoor-geinduceerde plantengeuren - insect pests - insect plant relations - host parasite relationships - herbivores - entomology - ecology - herbivore induced plant volatiles

Plants induce a suite of direct and indirect responses after herbivore attack. For utilizing these plant attributes in agriculture, it is important to compare the relative importance of these two defence types in the same plant species. Precise timing of these responses is vital for their effect on the herbivores feeding on the plant.
In my thesis, I present an analysis of the temporal dynamics of various direct and indirect responses in Indian/ Brown mustard, Brassica juncea due to herbivory by a generalist Spodoptera spp, a specialist Plutella xylostella or both of these herbivores together. Morphological (leaf size and trichome densities) and chemical (volatiles, glucosinolates, amino acids, sugars) responses were analysed over temporal range of 24 hours to 20 days. The effect of direct systemic responses was assessed using a specialist (Plutella xylostella) and a generalist (Spodoptera litura) herbivore. In addition, to investigate indirect responses, orientation experiments were conducted on the generalist (Cotesia marginiventris) and specialist (C. plutellae) parasitoids. Results were validated with analysis of genes involved in signalling and biosynthetic pathways that govern the expression of these responses.
For the first time, the presence of extrafloral nectar (EFN) in Brassicaceae is being reported. Their sugar and amino acid contents were measured in five different varieties of B. juncea. The indirect function of these EFN droplets was assessed by testing their inducibility and attraction and survival of parasitoids on them.
These studies thus show that although morphological responses are slower than chemical responses, they also contribute to induced plant resistance in a relatively short time span. Moreover, before considering induced responses as resistance factors, their effect should be assessed at different points in time with both generalist and specialist herbivores.

The ecology of plant secondary metabolites : from genes to global processes
Iason, G.R. ; Dicke, M. ; Hartley, S.E. - \ 2012
Cambridge [etc.] : Cambridge University Press (Ecological reviews ) - ISBN 9780521157124 - 335
secundaire metabolieten - planten - plantensamenstelling - verdedigingsmechanismen - ziekteresistentie - gastheer parasiet relaties - ecologie - genen - biosynthese - herbivoren - secondary metabolites - plants - plant composition - defence mechanisms - disease resistance - host parasite relationships - ecology - genes - biosynthesis - herbivores
Plant secondary metabolites (PSM) such as terpenes and phenolic compounds are known to have numerous ecological roles, notably in defence against herbivores, pathogens and abiotic stresses and in interactions with competitors and mutualists. This book reviews recent developments in the field to provide a synthesis of the function, ecology and evolution of PSM, revealing our increased awareness of their integrative role in connecting natural systems. It emphasises the multiple roles of secondary metabolites in mediating the interactions between organisms and their environment at a range of scales of ecological organisation, demonstrating how genes encoding for PSM biosynthetic enzymes can have effects from the cellular scale within individual plants all the way to global environmental processes. A range of recent methodological advances, including molecular, transgenic and metabolomic techniques, are illustrated and promising directions for future studies are identified, making this a valuable reference for researchers and graduate students in the field.
Climate change induced range-expanding plants : aboveground and belowground interactions
Morriën, W.E. - \ 2011
Wageningen University. Promotor(en): Wim van der Putten. - [S.l.] : S.n. - ISBN 9789085859376 - 144
planten - geïntroduceerde soorten - klimaatverandering - bodemfauna - vrijlevende nematoden - bodembacteriën - rizosfeer - plaagresistentie - ziekteresistentie - herbivoren - interacties - plants - introduced species - climatic change - soil fauna - free living nematodes - soil bacteria - rhizosphere - pest resistance - disease resistance - herbivores - interactions

Burning of fossil fuels has raised the level of atmospheric carbon dioxide, which contributes to global climate warming. As a result the mean earth surface temperature has increased faster in the past decades than in the previous thousands of years before. This rapid climate warming together with habitat fragmentation and other land use changes puts a major pressure on many plants and animals. They should either adapt to the warmer climate conditions or disperse in order to keep up with their optimal climatic conditions. Range expansion brings new interactions within the ecosystem in the new range. This can lead to potential benefits, for example range shifting species that do not encounter natural enemies in the new range might become invasive. Although invasive species are a well-studied phenomenon, there is relatively little known about the general mechanisms of biological invasions under climate change. In this thesis I focus on plant species that expand range due to current climate warming. I examined how these range-expanding plants interact with aboveground herbivorous insects and - mostly - how they establish belowground interactions with components of the soil food web. I examined how these interactions in the new range may play a role in the successful establishment of climate change induced range-expanding plants in plant communities of the new range. The focus of my study was on riverine (riparian) areas along the great rivers in the Netherlands, which are well connected with southern Europe by the Rhine and Rhine-Danube canal.

In the first experiment we examined exotic plant exposure to aboveground and belowground enemies. We used plants that originated from Eurasia (intra-continental range expanders) and plants that originated from other continents (inter-continental range expanders). We compared these exotic plants with phylogenetically related natives. We grew the plants with and without non-coevolved polyphagous (generalist) herbivores, a locust Schistocerca gregaria and an aphid Myzus persicae. We also exposed all plants to a general soil community from the invaded range and compared their plant-soil feedback responses. Then I tested how individual plants responded to aboveground and belowground plant enemies and I compared this to their combined effects. I also tested whether the strength of aboveground control by generalist shoot-feeding insects was indicative of the strength of belowground control by plant-soil feedback.

In the next study I examined how the soil nematode community from the new range responds to exotic plant species compared to related native plants species. As a follow up, I determined the rhizosphere community composition of bacteria, fungi, arbuscular mycorrhizal fungi (AMF) and fusaria. All groups of microbes were analyzed qualitatively and the non-mycorrhizal fungal biomass and fusaria were also analyzed quantitatively. I tested the hypothesis that range-expanding plant species have a different rhizosphere microbial community composition than natives.

Finally, I compared the early establishment of range-expanding exotics and phylogenetically related plant species that are native in the invaded habitats. In a greenhouse I grew five range-expanding plant species and five related natives in sterilized and non-sterile inoculated soils from the new range, both alone and with a background community of plant species present in the invaded habitat. In the field, I grew the same plants species in artificially created sparse and dense plant communities. I tested whether range-expanding exotic plant species establish better under competition with native vegetation than phylogenetically related natives, because exotics may benefit from less negative interactions with the soil community compared to natives.

Nutrients in an African Savanna: the consequences of supply heterogeneity for plants and animals
Waal, C. van der - \ 2010
Wageningen University. Promotor(en): Herbert Prins; H. de Kroon, co-promotor(en): Ignas Heitkonig. - [S.l. : S.n. - ISBN 9789085856740 - 151
voedingsstoffen - bodemvruchtbaarheid - heterogeniteit - savannen - bomen - grassen - biologische mededinging - herbivoren - begrazing - afgrazen - foerageren - vegetatie - semi-aride klimaatzones - schaalverandering - voedingsstoffenbeschikbaarheid - zuid-afrika - nutrients - soil fertility - heterogeneity - savannas - trees - grasses - biological competition - herbivores - grazing - browsing - foraging - vegetation - semiarid zones - scaling - nutrient availability - south africa
In savannas, trees and grasses co-exist and share resources such as water and
nutrients. The ratio between the tree and grass components (i.e., vegetation structure)
importantly controls productivity, animal assemblages and earth-atmosphere
feedbacks. As the structure of savanna vegetation is inherently unstable and easily
disturbed, finding out how the vegetation structure is controlled is of great importance
for rangeland management and conservation. Currently four factors are believed to
control the vegetation structure in savannas; namely, water, nutrients, herbivory and
fire. While the water and fire factors have been intensely studied, the roles of nutrients
and herbivores as factors are less well known. Improving our understanding of the
role of nutrients in savannas is relevant, because it is increasingly realised that global
change also alter the availability of nutrients, e.g., increased nitrogen deposition. How
savanna systems respond to nutrient perturbations is uncertain. Changes in soil
nutrient availability are also likely to feed back to changes in forage quality, which
may influence large herbivore use and vegetation impact patterns, thus indirectly
influencing vegetation structure. Moreover, it is increasingly realised that not only
changes in the availability of nutrients influence plants and subsequently herbivores,
but also how nutrients become spatially available. In fact, new ideas suggest that
differences in the scale of spatial resource heterogeneity may control how resources
are partitioned between co-existing species differing in size, e.g., large trees coexisting
with small grasses in savanna systems. To test how changes in nutrient
availability and spatial configurations influence savanna systems, several field
experiments were conducted in a semi-arid savanna in South Africa.
In the study area we found that nutrient (N, P and K) availability negatively
affected tree (Colophospermum mopane) seedling establishment in fertilization
experiments. Also, trees failed to re-colonize nutrient-rich kraal sites that were
abandoned almost half a century ago. In dry savannas, it is currently believed that the
success of tree seedling establishment exercises large control over the relative
dominance of trees, thus an increase in nutrient availability may feed back to a
structurally more open vegetation state. Different explanations may account for
constrained tree seedling establishment under fertile soil conditions. We tested the
hypothesis that the intensification of grass competition reduces tree seedling
recruitment in fertile environments. In controlled competition experiments it was
shown that negative nutrient effects on tree seedlings only occur when seedlings were
competing with grasses in mixtures. Furthermore, we found that adding both water
and nutrients to tree seedling-grass plant mixtures cancelled the negative effects of
added nutrients on tree seedlings. Thus the suppressing effect of increased nutrient
availability on seedlings appears to operate indirectly through the pre-empting of soil
water resources by vigorous herbaceous growth under fertile conditions. Since woody
seedlings are vulnerable to water stress, increased nutrient-induced water stress
translates into higher mortality rates and suppressed growth of tree seedlings in fertile
areas. In support, transplanted tree seedling mortality increased during a mid-season
drought as local fertilizer concentration increased. In conclusion, intensified
herbaceous competition under fertile soil conditions appears to be a viable
mechanism explaining poor tree seedling recruitment under fertile soil conditions.
Savannas and nutrients
While establishing tree seedlings suffer under fertile conditions, our data
suggest that established (mature) tree growth benefitted from an increase in nutrient
availability, especially following an increase in N availability. With regards to
increased atmospheric N deposition, we predict that tree cover may initially expand
following nutrient enrichment in dry savannas, although tree cover responses may be
insensitive to current levels of N deposition. However, in the long-term tree cover is
expected to decline, because of constrained tree recruitment that appears to be more
sensitive to small increases in N availability.
The relative availability of nutrients such as N and P may also influence how
resources are partitioned between co-existing trees and grasses. In a field experiment
we found evidence that trees were relatively more limited by N than P availability. An
East African study has shown that grasses underneath trees are more P than N limited
and we found evidence that the competiveness of sub-canopy grasses in our study
increased when only P was supplied. The relative availability of N vs. P may therefore
offer an additional axis governing resource partitioning between trees (non N-fixing)
and grasses in savanna systems. This supports the idea that organisms with a high
growth potential, apparently grasses in savannas, have relatively high P requirements
to sustain rapid protein synthesis, while slower growing organisms such as trees are
more limited by the availability of N.
The availability of soil nutrients strongly mediated where large herbivores
concentrated their impact in the landscape. Both grazers and browsers responded
positively to fertilization, apparently via the control that soil nutrient availability has
on forage quality. Tree and grass leaf N and P concentrations increased and
condensed tannin concentrations in trees decreased following fertilization. Under high
local soil nutrient concentrations the vegetation biomass was in some instances
reduced below control biomass by herbivores, indicating that top-down herbivore
effects potentially override bottom-up nutrient effects under fertile conditions.
In ecology, it is increasingly realized that it is not only the availability of
nutrients, but also how nutrients become spatially available that matters. Data from a
large field experiment where a gradient in the scale of nutrient patchiness (i.e., patch
grains sizes 2 x 2 m, 10 x 10 m or 50 x 50 m) was created, suggested that the scale of
nutrient patchiness controlled the partitioning of resources between co-existing trees
and grasses. For the same local fertilizer concentration, tree leaf quality was
unresponsive to fertilization in small patches, but responded in the larger patches.
Grass leaf quality increased with local fertilizer concentration regardless of patch size.
The differential responses of trees and grasses to scale differences subsequently
modulated the responses of the browser and grazer guilds. For the same high local
fertilizer concentration, grazers responded to both fine and coarse scales of nutrient
patchiness, while browsers responded only to the coarse scale of nutrient patchiness.
In turn, the selective grazing in the fine scale of nutrient patchiness treatment,
apparently stimulated tree growth. In the coarser scale of patchiness treatment both
browsers and grazer impact intensified locally. Thus the scale of nutrient patchiness
controlled nutrient partitioning between trees and grasses, which was apparently
closely tracked by the large herbivore assemblage, resulting in differential local
impacts on the tree and grass layers. Apart from local effects, the scale of nutrient
patchiness may also regulate the use and quality of forage resources at larger (e.g.,
landscape) scales. In the large field fertilizer experiment, calculations suggest that the
total herbaceous off-take by grazers peaked where the same fertilizer amount (15 kg N
plot-1) was spread over the whole plot surface area rather than concentrated in 10 x 10
m patches. Thus, how nutrients are distributed in an area controls secondary
productivity and where herbivores concentrate their impact.
The scale related patterns observed in the large fertilizer experiment may have
been reinforced by plastic responses (e.g., fine root proliferation in nutrient-rich soil)
to heterogeneous nutrient supplies of trees in the coarse scale treatments and grasses
in the fine scale treatments, respectively. To test this, the same nutrient amount was
supplied at two different scales of patchiness to focal trees with their associated
grasses. Two years after fertilization, large-scale fertilized trees showed increased
shoot growth and increased leaf N concentrations compared to small-scale fertilized
trees receiving the same N amount. Conversely, trees in a small-scale configuration
fertilized with P showed negative responses compared to large-scale counterparts.
These results suggest that differences in the local scale of nutrient patchiness also
influence how nutrients are partitioned between co-existing trees and grasses.
Herbivores not only respond to nutrient heterogeneity, but may also create
spatial heterogeneity in nutrient availability, which, in turn, may influence the
vegetation structure of savannas. We studied the current soil nutrient status, tree and
grass biomass patterns and large herbivore use of nine former livestock holding pen
areas (kraals) in a semi-arid, nutrient poor savanna. These were contrasted with
nearby control sites located in the surrounding landscape. The kraals, formerly
enriched by livestock dung and urine, were abandoned around 1970 and since then
wildlife replaced livestock in these parts. We found that around 40 years later, kraal
soil had elevated concentrations of inorganic N, extractable P, K, Ca and Mg
compared to control sites, which resulted in high quality forage in kraal sites. Trees
also failed to invade these sites, thus kraals remained as structurally open patches in
the otherwise dense savanna. Evidently, wild large herbivores maintain the high
nutrient status of kraal sites, probably by importing nutrients into these sites and by
accelerating local nutrient cycling. In turn, the increased local nutrient availability
prevents tree seedlings from establishing under these fertile conditions.
Finally, this study provided evidence that changes in the availability of
nutrients influenced the success of woody seedling establishment, which may feed
back to changes in the relative proportions of trees and grasses in dry savannas.
Furthermore, this study supports the new idea that the scale of resource heterogeneity
influences how resources are partitioned between co-existing trees and grasses, which,
in turn, modulated browser vs. grazer use and impact patterns on the vegetation.
In conclusion, this study provides new information on nutrient-plant-herbivore
interactions in a dry savanna with potentially important implications for the
management of dry savannas in general.
Beyond the here and now : herbivore ecology in a spatial-temporal context
Knegt, H.J. de - \ 2010
Wageningen University. Promotor(en): Herbert Prins; Andrew Skidmore, co-promotor(en): Frank van Langevelde. - [S.l. : S.n. - ISBN 9789085856283 - 140
herbivoren - loxodonta africana - ecologie - vegetatie - patronen - habitatselectie - habitats - dierecologie - milieu - afrika - herbivores - ecology - vegetation - patterns - habitat selection - animal ecology - environment - africa
Ecological phenomena at a certain location often not only depend on the current characteristics of that location itself, but also on characteristics of the landscape surrounding the site or influences from the past. In other words, in order to be able to understand ecological processes here and now, we often need information that is beyond here and now. This thesis investigates the role of such spatial-temporal context on the relationships between species and their environment. The focus is on large mammalian herbivores, an in particular the factors determining the movement and distribution of African elephants (Loxodonta africana) in Kruger National Park (KNP), South Africa.

This thesis covers four main themes. First, it is shown that spatial-temporal variation in the biotic and abiotic environment creates predictable seasonal and daily cyclic patterns in the distribution of elephants in KNP. Then, herbivores are shown not only to respond to environmental heterogeneity: they also can create distinct spatial patterning in savanna vegetation when foraging initiates a positive plant-herbivore feedback and when the foraging process is spatially explicit. This leads to specific sites being revisited often, and therefore the patterns of site revisitation by elephants in KNP are studied, mainly in relation to surface water availability and vegetation characteristics. Lastly, this thesis focuses on the scale-dependent response of organisms to environmental heterogeneity, showing the consequences of a mismatch in the scale of analysis on statistical inference, and then focusing on finding the appropriate spatial scales to analyse and predict the spatial distribution of elephants in KNP. Together, the chapters presented in this thesis highlight the importance of explicitly considering the scale and context dependency of species-environment relationships, and demonstrate methods to take issues of spatial-temporal scale and context into account. These methods have the potential to increase our understanding of ecological phenomena and therefore may lead to better management of natural resources.

Plant-mediated multitrophic interactions between aboveground and belowground insects
Soler Gamborena, R. - \ 2009
Entomologische Berichten 69 (2009)6. - ISSN 0013-8827 - p. 202 - 210.
planten - insecten - bodeminsecten - herbivoren - trofische graden - brassicaceae - insect-plant relaties - multitrofe interacties - plant-herbivoor relaties - plants - insects - soil insects - herbivores - trophic levels - insect plant relations - multitrophic interactions - plant-herbivore interactions
Het is bekend dat planten als verticale communicatiekanalen kunnen fungeren tussen onder-en bovengrondse plantetende insecten. Worteleters veroorzaken veranderingen in biomassa en de chemische samenstelling van de bovengrondse delen van een plant. Deze veranderingen kunnen de overleving, de groei en de ontwikkeling van bladeters beïnvloeden. In het promotieproject werd onderzocht of en hoe de wisselwerking tussen ruimtelijk gescheiden insecten beperkt is tot planteneters, dan wel uitgebreid kan worden naar hogere trofische niveaus zoals bijvoorbeeld parasitaire wespen, de natuurlijke vijanden van de plantenetende insecten. Tijdens de 20e Nederlandse Entomologendag (19 december 2008) is de eerste NEV dissertatieprijs uitgereikt aan dit proefschrift
Satisfying giant appetites : mechanisms of small scale foraging by large African herbivores
Pretorius, Y. - \ 2009
Wageningen University. Promotor(en): Herbert Prins; R. Slotow, co-promotor(en): Fred de Boer. - [S.l. : S.n. - ISBN 9789085854784 - 141
herbivoren - afrika - ruwvoer (forage) - foerageren - zoekgedrag - morfologie - monddelen - voedingsstoffenopname (mens en dier) - lichaamssamenstelling - lichaamsgewicht - lineair programmeren - allometrie - grote grazers - herbivores - africa - forage - foraging - searching behaviour - morphology - mouthparts - nutrient intake - body composition - body weight - linear programming - allometry - large herbivores
Variation in body mass allows for resource partitioning and co-existence of different species. Body mass is also seen as the main factor governing nutrient requirements in herbivores as metabolic rate and requirements have often been found to scale to ¾ power of body mass. Although the consequences of body mass on foraging behaviour of herbivores has been extensively studied, the mechanism behind how body mass differences determines the small scale foraging patterns of especially larger herbivores, has up to now been unclear. In this study, I looked at how body mass and small scale vegetation characteristics shaped the mouth morphology of herbivores and how body mass of a herbivore affects the scale at which intake is maximized. The results indicate that the dilution of plant mass and more specifically leaf mass in space requires that mega-herbivores such as elephant have enlarged soft mouth parts to compensate for this dilution. Finally, I demonstrate, using linear programming techniques with multiple nutrients as constraints, how a mega-herbivore’s daily diet choice is determined by forage abundance whereas a small herbivore is more constrained by fibre.
Geography of mammalian herbivores in the Indian Trans-Himalaya: patterns and processes
Namgail, T. - \ 2009
Wageningen University. Promotor(en): Herbert Prins, co-promotor(en): Sip van Wieren. - [S.l.] : S.n. - ISBN 9789085855248 - 122
herbivoren - zoogdieren - zoögeografie - geografische verdeling - india - biogeografie - ecotonen - soortendiversiteit - soortenrijkdom - grote grazers - himalaya - herbivores - mammals - zoogeography - geographical distribution - biogeography - ecotones - species diversity - species richness - large herbivores
Animals need adequate resources so that their populations not only survive but thrive. So they seek places that can best provide them. Yet, they face several challenges, while obtaining these resources, e.g., predators, competitors and physical obstacles: mountains and rivers. Some animals are better-equipped to overcome these challenges, and are widely distributed, while others are not. These differences generate uneven pattern of distribution of life on earth. Tsewang Namgail’s study on the mammalian herbivores in the arid regions of the Himalayan mountains shows that interspecific competition is a major factor determining distribution and diversity patterns of these animals. Topography is also an important factor determining their coexistence, and thus it plays a crucial role in the formation and maintenance of herbivore assemblies in these drier, alpine regions. The study highlights that herbivores change their diet spectrum in response to the number of other herbivore species in an assemblage, and therefore emphasizes the inclusion of interspecific interactions in species distribution models.
Herbivore-induced indirect defense of Arabidopsis : ecogenomic approach to the role of infochemicals in parasitoid attraction
Snoeren, T.A.L. - \ 2009
Wageningen University. Promotor(en): Marcel Dicke. - [S.l. : S.n. - ISBN 9789085853596 - 169
arabidopsis - pieris rapae - verdedigingsmechanismen - herbivoren - jasmonzuur - herbivoor-geinduceerde plantengeuren - insect-plant relaties - transcriptomics - defence mechanisms - herbivores - jasmonic acid - herbivore induced plant volatiles - insect plant relations
Plants defend themselves against herbivorous insects with the induced production of volatiles that attract the enemies of the herbivores. In this thesis I used an ecogenomic approach to study the role of signal-transduction pathways that regulate the induction of herbivore-induced plant volatiles (HIPV) and the effects of HIPVs on parasitoid host-finding behavior. To this end, I have combined transcriptomics, metabolite analyses and insect behavioral analyses.

Nine Arabidopsis thaliana accessions were screened for the emission of HIPVs. The accessions varied in the emission rate of Pieris rapae–induced volatiles after folivory or treatment with the herbivory-mimicking plant hormone jasmonic acid (JA). The relevance of this observed variation in the emission of JA-induced volatiles for host location was tested with Diadegma semiclausum parasitoids. Furthermore, the accessions also varied in transcript levels of genes that are (putatively) involved in the production of some of the recorded HIPV-compounds.
The oxylipin JA is the key plant hormone involved in the induction of the HIPV-blend emitted in response to caterpillar folivory. Mutant plants affected in the oxylipin signal-transduction pathway were studied to assess the effects of JA and its oxylipin intermediates 12-oxo-phytodienoate (OPDA) and dinor-OPDA (dnOPDA) on HIPV emission and attraction of the parasitoid D. semiclausum. In contrast to the effect of JA on the induced production of HIPVs, dnOPDA and OPDA were found to have no and little effect, respectively. The HIPV-compound methyl salicylate was shown to be JA-regulated and its abundance in the headspace varied among accessions. The contribution of methyl salicylate to parasitoid attraction was investigated. Bioassays with P. rapae-infested transgenic plants, lacking MeSA production, showed that MeSA negatively influenced D. semiclausum host-finding behavior.
Mutant plants were also studied to assess whether JA and its intermediates affected the induction of genes potentially involved in defense. The different oxylipins were shown to have distinct roles in induced defense signaling. Jasmonic acid had the strongest effect on transcript levels of defense-related genes from the oxylipin- and shikimate signal-transduction pathway. Minor roles were observed for OPDA and dnOPDA in the induction of one of these genes.

Utilizing an ecogenomic approach has provided new insight into the mechanisms underlying insect-plant interactions and holds promising opportunities.

Economics of controlling invasive species: the case of Californian thistle in New Zealand
Chalak, S.M. - \ 2009
Wageningen University. Promotor(en): Ekko van Ierland, co-promotor(en): Arjan Ruijs. - [S.l. : S.n. - ISBN 9789085853213 - 140
milieu - economie - invasie - onkruiden - biologische bestrijding - uitsterven - risico - herbivoren - dynamisch programmeren - verspreiding - concurrentie tussen planten - stuifmeelconcurrentie - nieuw-zeeland - cirsium arvense - milieueconomie - verspreiding van planten - environment - economics - invasion - weeds - biological control - extinction - risk - herbivores - dynamic programming - dispersal - plant competition - pollen competition - new zealand - environmental economics - plant dispersal
Invasive species, Economics, Californian thistle, New Zealand, Stochastic, Dynamic programming, Biological control, Extinction risk, Herbivory, Dispersal, Competition
Invasive species are one of the most significant threats to biodiversity and agricultural production systems leading to huge worldwide economic damages. This thesis has two main aims. The first aim is to analyse the control of an invasive plant in an agricultural system, using the case study of the Californian thistle in New Zealand. The second aim is to study the negative externalities that controlling invasion in agriculture can pose to ecosystems.
To achieve the first aim, both deterministic and stochastic dynamic programming models are set up to find cost effective methods to tackle the problem of Californian thistle. I make a contribution to the literature by performing a dynamic and stochastic programming analysis in which two different categories of control strategies are considered, each with different dynamics. Models are set up with a discrete decision variable consisting of 62 feasible combinations of integrated control strategies. For the second aim I introduce a novel modelling approach in which two compartments are distinguished: a managed compartment where locally a herbivore is introduced to control a weed, and a natural compartment where the same herbivore species can attack a wild plant species. The main processes are herbivory, competition, dispersal and control.
I conclude that bioeconomic modelling is an important tool in analysing optimal management strategies for the control of invasive species and that annual and once and for all choices need to be integrated in the analysis. A stochastic approach is appropriate but does not necessarily lead to different results, depending on the parameter values and the setup of the model. Finally, the method illustrates that an integrated analysis of the economic system and the ecological system is required to assess the risk of extinction of natural plant species. This risk depends on species interactions which in this thesis are competition, dispersal and herbivory. I conclude that a control measure can protect the desirable wild plant species and increase benefits obtained from the ecosystem.
For the policy implications, I conclude that there are several strategies to control invasive species, which can be integrated combinations of control options. The optimal strategy depends on the costs and benefits of the control options. In the case study for the Californian thistle I found that the optimal strategy is a combination of methods. For the interaction between agricultural and natural system I conclude that introducing a biological agent to the agricultural system can cause extinction of a desirable plant in the natural system. The main processes are competition, herbivory and dispersal. These processes are important and need to be analysed in detail before introducing the biological agent. I conclude that the optimal strategy to control the introduced biological agent also depends on interaction of species through competition, dispersal and herbivory.

Infochemical use in Brassica-insect interactions : a phenotypic manipulation approach to induced plant defences
Bruinsma, M. - \ 2008
Wageningen University. Promotor(en): Marcel Dicke, co-promotor(en): Joop van Loon. - [S.l.] : S.n. - ISBN 9789085049265 - 169
brassicaceae - verdediging - herbivoren - jasmonzuur - pieris rapae - pieris brassicae - fenotypen - plant-herbivoor relaties - defence - herbivores - jasmonic acid - phenotypes - plant-herbivore interactions
Plants have developed a range of strategies to defend themselves against herbivore attack. Defences can be constitutive, i.e. always present independent of attack, or induced, i.e. only elicited when the plant is under attack. In this thesis, I focused on induced chemical defence responses of plants and the response of associated insects to these phenotypic changes in plants. Herbivore attack is known to induce chemical defences in Brassicaceous plants. Using several elicitors and inhibitors of different steps of the signalling pathways underlying herbivore-induced plant responses, I studied how induced infochemicals affect interactions with associated insects.

Jasmonic acid (JA) is a key plant hormone in the octadecanoid signalling pathway known to be involved in herbivore-induced plant defences. Application of JA can induce plant responses that are similar, although not identical, to herbivore feeding. Two specialist herbivores of Brassicaceous plants, the butterflies Pieris rapae and P. brassicae, preferred to oviposit on non-induced plants over JA-induced plants. Development of P. rapae caterpillars was shown to be reduced, suggesting that oviposition avoidance on JA-induced plants is adaptive. The levels of glucosinolates, secondary metabolites of Brassicaceous plants that are used by Pieris butterflies as oviposition stimulants, could not explain the observed oviposition preference of the butterflies.

JA-induced changes in the plants also affected members of the third trophic level. Volatile emission of JA-induced plants attracted parasitoid wasps to the plants. Parasitoid attraction to JA-induced plants was shown to depend on dose and induction time. However, using JA to induce phenotypic changes had effects different from those induced by herbivores, both chemically and ecologically. Volatile emission of JA-induced and herbivore-induced plants differed; whereas JA-induced plants emitted larger amounts of volatiles, the parasitoids preferred herbivore-induced plants over JA-treated ones.

Early events in plant defence responses, involved in attacker recognition, are damage-induced modulations of ion channel activities resulting in ion imbalances. The fungal elicitor alamethicin, an ion channel-forming peptide mixture, was used to mimic early steps in defence responses. Alamethicin treatment increased attractiveness of plants to parasitoid wasps. Although volatile emission of alamethicin-treated plants was much lower, they were equally attractive as JA-treated plants. This indicates that quality rather than quantity of induced plant volatile blends is important to parasitoids.

Besides chemical elicitation of herbivore-induced responses, which is a widely applied approach, plant defence responses can also be chemically inhibited. This provides the opportunity to inhibit the rate of specific enzymatic steps in a signal-transduction pathway. Furthermore, visual cues associated with feeding damage can be present (and similar) in control- and inhibitor-treated plants. Phenidone is a compound that inhibits lipoxygenase, an enzyme catalyzing an early step in the octadecanoid pathway. Parasitoid attraction was reduced when the plants were treated with phenidone before infestation.

Also herbivore oviposition preference was shown to be affected by inhibition of this signalling pathway. Herbivores can differ in their oviposition preferences. I studied two specialist herbivores with different oviposition preferences: Pieris brassicae avoids oviposition on herbivore-induced plants, whereas Plutella xylostella prefers to oviposit on Pieris-infested plants. I showed that these preferences have a chemical basis and are dependent on octadecanoid signalling, since treatment with the lipoxygenase inhibitor phenidone eliminated herbivore-induced oviposition avoidance or preference.

Thus far, most of the studies on induced plant defences have been done with vegetative plants. However, since reproduction and defence are both processes that require energy and nutrients, this could result in a trade-off. Herbivore feeding on leaves, flowers or roots is known to affect pollinator visitation, but the mechanisms mediating this change have not been addressed. Effects of induction with JA on nectar secretion and pollinator visitation to flowers were investigated. JA-induced plants secreted less nectar, but the sugar concentrations did not change. Also visitation of honeybees and syrphid flies did not change upon JA induction.

These results show the complexity of induced plant defence responses and the variety of behavioural responses of insects on different trophic levels. Combining the phenotypic manipulation approach to induced plant defences, as used in this thesis, with molecular genetic techniques and building on recent developments in plant biochemistry provides a promising way forward towards enhanced understanding of the intricate interactions between plants and insects.
Multitrofe interacties tussen onder- en bovengrondse insecten via de gezamenlijke waardplant
Soler Gamborena, R. - \ 2008
Gewasbescherming 39 (2008)2. - ISSN 0166-6495 - p. 64 - 66.
planten - insecten - bodeminsecten - herbivoren - trofische graden - waardplanten - insect-plant relaties - multitrofe interacties - plant-herbivoor relaties - plants - insects - soil insects - herbivores - trophic levels - host plants - insect plant relations - multitrophic interactions - plant-herbivore interactions
De resultaten van dit proefschrift laten zien dat het voor de bestrijding van bovengrondse plaaginsecten van groot belang is te weten welke ondergrondse insecten aanwezig zijn en wat er rond de wortel van de plant gebeurt, en vice versa. Zulke bovengrondse-ondergrondse interacties tussen insecten via veranderingen in de in de waardplant kunnen namelijk de efficiëntie van bestrijdingsprogramma's significant beïnvloeden
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