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.

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Relative importance of competition and plant–soil feedback, their synergy, context dependency and implications for coexistence
Lekberg, Ylva ; Bever, James D. ; Bunn, Rebecca A. ; Callaway, Ragan M. ; Hart, Miranda M. ; Kivlin, Stephanie N. ; Klironomos, John ; Larkin, Beau G. ; Maron, John L. ; Reinhart, Kurt O. ; Remke, Michael ; Putten, Wim H. van der - \ 2018
Ecology Letters 21 (2018)8. - ISSN 1461-023X - p. 1268 - 1281.
Additive interaction - coexistence - competition - facilitation - meta-analysis - mutualist - pathogen - plant–soil feedback - resource gradient - soil biota

Plants interact simultaneously with each other and with soil biota, yet the relative importance of competition vs. plant–soil feedback (PSF) on plant performance is poorly understood. Using a meta-analysis of 38 published studies and 150 plant species, we show that effects of interspecific competition (either growing plants with a competitor or singly, or comparing inter- vs. intraspecific competition) and PSF (comparing home vs. away soil, live vs. sterile soil, or control vs. fungicide-treated soil) depended on treatments but were predominantly negative, broadly comparable in magnitude, and additive or synergistic. Stronger competitors experienced more negative PSF than weaker competitors when controlling for density (inter- to intraspecific competition), suggesting that PSF could prevent competitive dominance and promote coexistence. When competition was measured against plants growing singly, the strength of competition overwhelmed PSF, indicating that the relative importance of PSF may depend not only on neighbour identity but also density. We evaluate how competition and PSFs might interact across resource gradients; PSF will likely strengthen competitive interactions in high resource environments and enhance facilitative interactions in low-resource environments. Finally, we provide a framework for filling key knowledge gaps and advancing our understanding of how these biotic interactions influence community structure.

Shifts of community composition and population density substantially affect ecosystem function despite invariant richness
Spaak, Jurg W. ; Baert, Jan M. ; Baird, Donald J. ; Eisenhauer, Nico ; Maltby, Lorraine ; Pomati, Francesco ; Radchuk, Viktoriia ; Rohr, Jason R. ; Brink, Paul J. van den; Laender, Frederik De - \ 2017
Ecology Letters 20 (2017)10. - ISSN 1461-023X - p. 1315 - 1324.
Algae - biodiversity - coexistence - community ecology - modelling - primary production

There has been considerable focus on the impacts of environmental change on ecosystem function arising from changes in species richness. However, environmental change may affect ecosystem function without affecting richness, most notably by affecting population densities and community composition. Using a theoretical model, we find that, despite invariant richness, (1) small environmental effects may already lead to a collapse of function; (2) competitive strength may be a less important determinant of ecosystem function change than the selectivity of the environmental change driver and (3) effects on ecosystem function increase when effects on composition are larger. We also present a complementary statistical analysis of 13 data sets of phytoplankton and periphyton communities exposed to chemical stressors and show that effects on primary production under invariant richness ranged from −75% to +10%. We conclude that environmental protection goals relying on measures of richness could underestimate ecological impacts of environmental change.

Rapid diversity loss of competing animal sppecies in well-connected landscapes
Schippers, P. ; Hemerik, L. ; Baveco, J.M. ; Verboom, J. - \ 2015
PLoS ONE 10 (2015)8. - ISSN 1932-6203 - 17 p.
woodpecker dendrocopos-medius - squirrel sciurus-carolinensis - great spotted woodpeckers - neutral-theory - habitat fragmentation - riverine forest - climate-change - coexistence - abundance - metapopulations
Population viability of a single species, when evaluated with metapopulation based landscape evaluation tools, always increases when the connectivity of the landscape increases. However, when interactions between species are taken into account, results can differ. We explore this issue using a stochastic spatially explicit meta-community model with 21 competing species in five different competitive settings: (1) weak, coexisting competition, (2) neutral competition, (3) strong, excluding competition, (4) hierarchical competition and (5) random species competition. The species compete in randomly generated landscapes with various fragmentation levels. With this model we study species loss over time. Simulation results show that overall diversity, the species richness in the entire landscape, decreases slowly in fragmented landscapes whereas in well-connected landscapes rapid species losses occur. These results are robust with respect to changing competitive settings, species parameters and spatial configurations. They indicate that optimal landscape configuration for species conservation differs between metapopulation approaches, modelling species separately and meta-community approaches allowing species interactions. The mechanism behind this is that species in well-connected landscapes rapidly outcompete each other. Species that become abundant, by chance or by their completive strength, send out large amounts of dispersers that colonize and take over other patches that are occupied by species that are less abundant. This mechanism causes rapid species loss. In fragmented landscapes the colonization rate is lower, and it is difficult for a new species to establish in an already occupied patch. So, here dominant species cannot easily take over patches occupied by other species and higher diversity is maintained for a longer time. These results suggest that fragmented landscapes have benefits for species conservation previously unrecognized by the landscape ecology and policy community. When species interactions are important, landscapes with a low fragmentation level can be better for species conservation than well-connected landscapes. Moreover, our results indicate that metapopulation based landscape evaluation tools may overestimate the value of connectivity and should be replaced by more realistic meta-community based tools.
Spatial heterogeneity of plant–soil feedback affects root interactions and interspecific competition
Hendriks, M. ; Ravenek, J. ; Smit-Tiekstra, A.E. ; Paauw, J.W.M. van der; Caluwe, H. de; Putten, W.H. van der; Kroon, H. de; Mommer, L. - \ 2015
New Phytologist 207 (2015)3. - ISSN 0028-646X - p. 830 - 840.
nutrient heterogeneity - species-diversity - population-dynamics - relative abundance - deciduous woodland - temporal variation - borne pathogens - grassland - community - coexistence
Plant-soil feedback is receiving increasing interest as a factor influencing plant competition and species coexistence in grasslands. However, we do not know how spatial distribution of plant-soil feedback affects plant below-ground interactions. We investigated the way in which spatial heterogeneity of soil biota affects competitive interactions in grassland plant species. We performed a pairwise competition experiment combined with heterogeneous distribution of soil biota using four grassland plant species and their soil biota. Patches were applied as quadrants of 'own' and 'foreign' soils from all plant species in all pairwise combinations. To evaluate interspecific root responses, species-specific root biomass was quantified using real-time PCR. All plant species suffered negative soil feedback, but strength was species-specific, reflected by a decrease in root growth in own compared with foreign soil. Reduction in root growth in own patches by the superior plant competitor provided opportunities for inferior competitors to increase root biomass in these patches. These patterns did not cascade into above-ground effects during our experiment. We show that root distributions can be determined by spatial heterogeneity of soil biota, affecting plant below-ground competitive interactions. Thus, spatial heterogeneity of soil biota may contribute to plant species coexistence in species-rich grasslands.
Alternative stable states and alternative endstates of community assembly through intra- and interspecific positive and negative interactions
Gerla, D.J. ; Mooij, W.M. - \ 2014
Theoretical Population Biology 96 (2014). - ISSN 0040-5809 - p. 8 - 18.
ecological communities - allee - facilitation - dynamics - model - restoration - populations - transitions - coexistence - competition
Positive and negative interactions within and between species may occur simultaneously, with the net effect depending on population densities. For instance, at low densities plants may ameliorate stress, while competition for resources dominates at higher densities. Here, we propose a simple two-species model in which con- and heterospecifics have a positive effect on per capita growth rate at low densities, while negative interactions dominate at high densities. The model thus includes both Allee effects (intraspecific positive effects) and mutualism (interspecific positive effects), as well as intra- and interspecific competition. Using graphical methods we derive conditions for alternative stable states and species coexistence. We show that mutual non-invasibility (i.e. the inability of each species to invade a population of the other) is more likely when species have a strong positive effect on the own species or a strong negative effect on the other species. Mutual non-invasibility implies alternative stable states, however, there may also be alternative stable states at which species coexist. In the case of species symmetry (i.e. when species are indistinguishable), such alternative coexistence states require that if the positive effect exerted at low densities at the own species is stronger than on the other species, the negative effect at higher densities is also stronger on the own species than on the other species, or, vice versa, if the interspecific positive effects at low densities are stronger than the intraspecific effects, the negative effects at higher densities are also stronger between species than within species. However, the reachability of alternative stable states is restricted by the frequency and density at which species are introduced during community assembly, so that alternative stable states do not always represent alternative endstates of community assembly.
Grazing-induced changes in plant–soil feedback alter plant biomass allocation
Veen, C.F. ; Vries, S. de; Bakker, E.S. ; Putten, W.H. van der; Olff, H. - \ 2014
Oikos 123 (2014)7. - ISSN 0030-1299 - p. 800 - 806.
tallgrass prairie - borne pathogens - invasive plant - grassland - herbivores - community - competition - coexistence - defoliation - diversity
Large vertebrate herbivores, as well as plant–soil feedback interactions are important drivers of plant performance, plant community composition and vegetation dynamics in terrestrial ecosystems. However, it is poorly understood whether and how large vertebrate herbivores and plant–soil feedback effects interact. Here, we study the response of grassland plant species to grazing-induced legacy effects in the soil and we explore whether these plant responses can help us to understand long-term vegetation dynamics in the field. In a greenhouse experiment we tested the response of four grassland plant species, Agrostis capillaris, Festuca rubra, Holcus lanatus and Rumex acetosa, to field-conditioned soils from grazed and ungrazed grassland. We relate these responses to long-term vegetation data from a grassland exclosure experiment in the field. In the greenhouse experiment, we found that total biomass production and biomass allocation to roots was higher in soils from grazed than from ungrazed plots. There were only few relationships between plant production in the greenhouse and the abundance of conspecifics in the field. Spatiotemporal patterns in plant community composition were more stable in grazed than ungrazed grassland plots, but were not related to plant–soil feedbacks effects and biomass allocation patterns. We conclude that grazing-induced soil legacy effects mainly influenced plant biomass allocation patterns, but could not explain altered vegetation dynamics in grazed grasslands. Consequently, the direct effects of grazing on plant community composition (e.g. through modifying light competition or differences in grazing tolerance) appear to overrule indirect effects through changes in plant–soil feedback.
Arbuscular mycorrhizal impacts on competitive interactions between Acacia etbaica and Boswellia papyrifera seedlings under drought stress
Birhane, E. ; Sterck, F.J. ; Bongers, F. ; Kuyper, T.W. - \ 2014
Journal of Plant Ecology 7 (2014)3. - ISSN 1752-9921 - p. 298 - 308.
interspecific competition - plant-growth - trade-offs - fungi - frankincense - infection - traits - associations - populations - coexistence
Aims Arbuscular mycorrhizal fungi can have a substantial effect on the water and nutrient uptake by plants and the competition between plants in harsh environments where resource availability comes in pulses. In this study we focus on interspecific competition between Acaia etbaica and Boswellia papyrifera that have distinctive resource acquisition strategies. We compared the extent of interspecific competition with that of intraspecific competition. Methods In a greenhouse study we examined the influence of Arbuscular Mycorrhiza (AM) and pulsed water availability on competitive interactions between seedlings of the rapidly growing species A. etbaica and the slowly growing species B. papyrifera. A factorial experimental design was used. The factors were AM, two water levels and five species combinations Important Findings Seedlings of both species benefitted from AM when grown alone, and the positive growth response to pulsed water availability in B. papyrifera seedlings was in contrast with the negative growth response for A. etbaica seedlings. AM also affected the competitive performance of both species. B. papyrifera was not affected by intraspecific competition, whereas A. etbaica was negatively affected compared to the seedlings grown alone. This effect was stronger in the presence of AM. In interspecific competition, A. etbaica outcompeted B. papyrifera. Mycorrhiza and pulsed water availability did not affect the outcome of interspecific competition, and the aggressivity index of A. etbaica remained unchanged. The extent to which AM influences plant competition in a drought-stressed environment may depend on belowground functional traits of the species. AM and pulsed water availability could modify the balance between intraspecific and interspecific competition. By affecting the balance between intraspecific and interspecific competition, both factors could impact the establishment and survival of seedlings.
Disturbance–diversity relationships for soil fauna are explained by faunal community biomass in a salt marsh
Thakur, M.P. ; Berg, M.P. ; Eisenhauer, N. ; Langevelde, F. van - \ 2014
Soil Biology and Biochemistry 78 (2014). - ISSN 0038-0717 - p. 30 - 37.
species-diversity - intermediate disturbance - richness - productivity - coexistence - succession - patterns - competition - collembola - dynamics
Disturbance–diversity relationships have long been studied in ecology with a unimodal relationship as the key prediction. Although this relationship has been widely contested, it is rarely tested for soil invertebrate fauna, an important component of terrestrial biodiversity. We tested disturbance–diversity relationships for soil meso- and macrofauna in a salt marsh where periodic sea water inundation and cattle grazing occur as stressors. We hypothesized a unimodal inundation frequency–diversity relationship, whereas we expected grazing to overrule the effects of inundation frequency due to its large effects on the habitat of soil fauna. We found a negative relationship between inundation frequency and diversity at the ungrazed sites and no relationship at the grazed sites. Moreover, we found a negative relationship between community biomass and diversity for soil fauna that may have caused this negative disturbance–diversity relationship. Community biomass at the intermediate inundation frequency increased due to the dominance of Orchestia gammarellus (a macro-detritivore species), which could exploit low quality litters at the ungrazed sites. We highlight that the negative relationship between faunal community biomass and faunal diversity may influence disturbance–diversity relationships and illustrate that total biomass distribution of feeding guilds of soil fauna can improve our understanding of the soil fauna response to stressors in salt marshes.
Nile perch (Lates niloticus, L.) and cichlids (Haplochromis spp.) in Lake Victoria: could prey mortality promote invasion of tis predator?
Wolfshaar, K.E. van de; Hille Ris Lambers, R. ; Goudswaard, P.C. ; Rijnsdorp, A.D. ; Scheffer, M. - \ 2014
Theoretical Ecology 7 (2014)3. - ISSN 1874-1738 - p. 253 - 261.
east-africa - mwanza gulf - human impact - ecosystems - productivity - coexistence - competition - recovery - kyoga
The invasion of Nile perch into Lake Victoria is one of the iconic examples of the destructive effect of an introduced species on an ecosystem but no convincing explanation exists of why Nile perch only increased dramatically after a 25 year lag. Here, we consider this problem using a mathematical model that takes into account interactions between Nile perch and its cichlid prey. We examined competing hypotheses to explain Nile perch invasion and show that suppression of juvenile Nile perch by cichlids may cause the system to have two alternative stable states: one with only cichlids and one with coexistence of cichlids and Nile perch. Without cichlid predation on Nile perch, alternative stable states did not occur. Our analysis indicates that cichlid mortality, for example fishing mortality, may have induced the observed shift between the states.
Repeated parallel evolution reveals limiting similarity in subterranean diving beetles
Vergnon, R.O.H. ; Leijs, P. ; Nes, E.H. van; Scheffer, M. - \ 2013
American Naturalist 182 (2013)1. - ISSN 0003-0147 - p. 67 - 75.
species-diversity - competition - dytiscidae - patterns - convergence - coexistence - coleoptera - divergence - morphology - community
The theory of limiting similarity predicts that co-occurring species must be sufficiently different to coexist. Although this idea is a staple of community ecology, convincing empirical evidence has been scarce. Here we examine 34 subterranean beetle communities in arid inland Australia that share the same habitat type but have evolved in complete isolation over the past 5 million years. Although these communities come from a range of phylogenetic origins, we find that they have almost invariably evolved to share a similar size structure. The relative positions of coexisting species on the body size axis were significantly more regular across communities than would be expected by chance, with a size ratio, on average, of 1.6 between coexisting species. By contrast, species' absolute body sizes varied substantially from one community to the next. This suggests that self-organized spacing according to limiting-similarity theory, as opposed to evolution toward preexisting fixed niches, shaped the communities. Using a model starting from random sets of founder species, we demonstrate that the patterns are indeed consistent with evolutionary self-organization. For less isolated habitats, the same model predicts the coexistence of multiple species in each regularly spaced functional group. Limiting similarity, therefore, may also be compatible with the coexistence of many redundant species. © 2013 by The University of Chicago.
Consequences of plant–soil feedbacks in invasion
Suding, K.N. ; Harpole, W.S. ; Fukami, T. ; Kulmatiski, A. ; MacDougall, A.S. ; Stein, C. ; Putten, W.H. van der - \ 2013
Journal of Ecology 101 (2013)2. - ISSN 0022-0477 - p. 298 - 308.
microbial community structure - acacia-longifolia - biota - restoration - pathogens - dynamics - ecology - accumulation - invasiveness - coexistence
1. Plant species can influence soil biota, which in turn can influence the relative performance of plant species. These plant–soil feedbacks (PSFs) have been hypothesized to affect many community-level dynamics including species coexistence, dominance and invasion. 2. The importance of PSFs in exotic species invasion, although widely hypothesized, has been difficult to determine because invader establishment necessarily precedes invader-mediated PSFs. Here, we combine a spatial simulation model of invasion that incorporates PSFs with a meta-analysis that synthesizes published case studies describing feedbacks between pairs of native and exotic species. 3. While our spatial model confirmed the link between positive soil feedbacks (‘home’ advantage) for exotic species and exotic species spread, results were dependent on the initial abundance of the exotic species and the equivalence of dispersal and life history characteristics between exotic and native species. 4. The meta-analysis of 52 native–exotic pairwise feedback comparisons in 22 studies synthesized measures of native and exotic performance in soils conditioned by native and exotic species. The analysis indicated that the growth responses of native species were often greater in soil conditioned by native species than in soil conditioned exotic species (a ‘home’ advantage). The growth responses of exotic species were variable and not consistently related to species soil-conditioning effects. 5. Synthesis. Overlaying empirical estimates of pairwise PSFs with spatial simulations, we conclude that the empirically measured PSFs between native and exotic plant species are often not consistent with predictions of the spread of exotic species and mono-dominance. This is particularly the case when exotic species are initially rare and share similar dispersal and average fitness characteristics with native species. However, disturbance and other processes that increase the abundance of exotic species as well as the inclusion of species dispersal and life history differences can interact with PSF effects to explain the spread of invasive species
Dominos in the dairy: An analysis of transgenic maize in Dutch dairy farming
Groeneveld, R.A. ; Wesseler, J.H.H. ; Berentsen, P.B.M. - \ 2013
Ecological Economics 86 (2013). - ISSN 0921-8009 - p. 107 - 116.
genetically-modified crops - non-gm crops - environmental benefits - property-rights - coexistence - externalities - biotechnology - patterns - adoption - impact
EU member states require farmers growing transgenic maize to respect a minimum distance from fields with non-transgenic maize. Previous studies have theoretically argued that such minimum distance requirements may lead to a so-called ‘domino effect’ where farmers who want to grow transgenic maize are forced to grow the non-transgenic variety and in turn impose the same constraints on their neighbors. This article applies a spatially explicit farm model to a dairy region in the Southern Netherlands to assess how farmers growing non-transgenic maize limit other farmers' potential to grow transgenic herbicide-resistant maize. The results indicate that the minimum distance requirements can severely limit the benefits from herbicide resistant maize. Having different land use options in one farm, however, enables dairy farmers to grow transgenic maize despite having one or more neighbors growing non-transgenic maize. We also find that the share of the domino effect in the overall impact of minimum distance requirements decreases with the density of farmers not growing transgenic maize.
MM2-Thalamic Creutzfeldt-Jacob Disease: Neuropathological, Biochemical and Transmission Studies Identify a Distinctive Prion Strain
Moda, F. ; Suardi, S. ; Fede, G. Di; Indaco, A. ; Limido, L. ; Vimercati, C. ; Ruggerone, M. ; Campagnani, I. ; Langeveld, J.P.M. ; Terruzzi, A. ; Brambilla, A. ; Zerbi, P. ; Fociani, P. ; Bishop, T. ; Will, G.W. ; Manson, J.C. ; Giaccone, G. ; Tagliavini, F. - \ 2012
Brain Pathology 22 (2012)5. - ISSN 1015-6305 - p. 662 - 669.
sporadic fatal insomnia - prpsc types - protein - variant - brain - classification - cooccurrence - degeneration - coexistence - scrapie
In CreutzfeldtJakob disease (CJD), molecular typing based on the size of the protease resistant core of the disease-associated prion protein (PrPSc) and the M/V polymorphism at codon 129 of the PRNP gene correlates with the clinico-pathologic subtypes. Approximately 95% of the sporadic 129MM CJD patients are characterized by cerebral deposition of type 1 PrPSc and correspond to the classic clinical CJD phenotype. The rare 129MM CJD patients with type 2 PrPSc are further subdivided in a cortical and a thalamic form also indicated as sporadic fatal insomnia. We observed two young patients with MM2-thalamic CJD. Main neuropathological features were diffuse, synaptic PrP immunoreactivity in the cerebral cortex and severe neuronal loss and gliosis in the thalamus and olivary nucleus. Western blot analysis showed the presence of type 2A PrPSc. Challenge of transgenic mice expressing 129MM human PrP showed that MM2-thalamic sporadic CJD (sCJD) was able to transmit the disease, at variance with MM2-cortical sCJD. The affected mice showed deposition of type 2A PrPSc, a scenario that is unprecedented in this mouse line. These data indicate that MM2-thalamic sCJD is caused by a prion strain distinct from the other sCJD subtypes including the MM2-cortical form.
Emergent neutrality leads to multimodal species abundance distributions
Vergnon, R. ; Nes, E.H. van; Scheffer, M. - \ 2012
Nature Communications 3 (2012). - ISSN 2041-1723 - 6 p.
body-size - community - biodiversity - coexistence - diversity - ecology - model - similarity - paradox - niche
Recent analyses of data sampled in communities ranging from corals and fossil brachiopods to birds and phytoplankton suggest that their species abundance distributions have multiple modes, a pattern predicted by none of the existing theories. Here we show that the multimodal pattern is consistent with predictions from the theory of emergent neutrality. This adds to the observations, suggesting that natural communities may be shaped by the evolutionary emergence of groups of similar species that coexist in niches. Such self-organized similarity unifies niche and neutral theories of biodiversity.
Trait assembly of woody plants in communities across sub-alpine gradients: Identifying the role of limiting similarity
Yan, B. ; Zhang, J. ; Liu, Y. ; Li, Z. ; Huang, X. ; Yang, W. ; Prinzing, A. - \ 2012
Journal of Vegetation Science 23 (2012)4. - ISSN 1100-9233 - p. 698 - 708.
functional traits - amazonian forest - neutral theory - niche - coexistence - height - growth - consequences - convergence - equivalence
Questions - Plant species can be assembled into communities through habitat filtering or species competition, but their relative roles are still debated. We do not know whether there is limited similarity between co-existing species when accounting for the parallel effect of abiotic habitat filtering and biotic competition. By accounting for such effects, we test the predictions of three theories (classic niche theory, the Hubbell neutrality theory and the Scheffer and van Nes theory) of community assembly. Location - Two vegetation transitions (a grazing gradient and a timber line ecotone) in a sub-alpine area of western Sichuan Province, China (31° 51'N, 102° 41'E). Methods - We used a null model to investigate the above plant community assembly theories on two sub-alpine gradients of woody vegetation. In the null model, species traits were constrained between the maximum and minimum trait values of observed communities to test the principle of limiting similarity between co-existing species by testing for even spacing of traits. We analysed traits characterizing growth strategies of stems, leaves and twigs, measured at the level of individuals in situ. Results - After accounting for variations in trait range, it became evident that six out of eight traits showed significantly uneven spacing within some plots, notably towards the forest end of the gradient, i.e. under increased competition pressure among woody plants. The Wilcoxon rank test showed that seven out of eight traits were significantly unevenly spaced within plots. The two transitions studied showed surprisingly similar patterns, despite their dissimilar precise drivers.
Sustained dynamic transience in a Lotka–Volterra competition model system for grassland species
Geijzendorffer, I.R. ; Werf, W. van der; Bianchi, F.J.J.A. ; Schulte, R.P.O. - \ 2011
Ecological Modelling 222 (2011)15. - ISSN 0304-3800 - p. 2817 - 2824.
north-carolina grassland - trifolium-repens - neighbor relationships - plant-communities - permanent pasture - white clover - growth - coexistence - diversity - patterns
Theoretical approaches, such as the Lotka–Volterra framework, enable predictions about long term species coexistence based on stability criteria, but generally assume temporal constancy of system equations and parameters. In real world systems, temporal variability may interfere with the attainment of stable states. Managed grassland ecosystems in Northwestern Europe experience structural periodic fluctuations in environmental conditions: the seasons. In addition, periodic disturbances such as cutting are very common. Here we show, using a Lotka–Volterra system applied to grassland species with empirically derived parameters, that seasonal variability can result in a time dependent equilibrium and redirection of displacement processes. Parameter estimates differed between species and – in most cases – between the seasons. As a result, five of the fifteen tested species combinations had different outcomes of species interactions between seasons. This indicates that systems remain in dynamic transience over the year as the equilibrium changes and the species composition of the system follows the equilibrium without ever attaining it. The non-attainment of the steady state enables coexistence of species even if there is competitive exclusion in one of the seasons. For three of the fifteen species combinations, cutting frequency affected the long-term coexistence patterns. Cutting resets the biomass of competing species and favours during regrowth those species that have a high growth rate, which can alter species coexistence in comparison to a Lotka–Volterra model without cutting. The Lotka–Volterra framework with seasonally changing empirical parameters predicts coexistence as a possible outcome of systems that in component seasons are characterised by exclusion, and vice versa
A continental analysis of correlations between tree patterns in African savannas and human and environmental variables
Groen, T.A. ; Langevelde, F. van; Vijver, C.A.D.M. van de; Raad, A.L. de; Leeuw, J. de; Prins, H.H.T. - \ 2011
Journal of Arid Environments 75 (2011)8. - ISSN 0140-1963 - p. 724 - 733.
southern-africa - semiarid savannas - density patterns - fire frequency - woody cover - coexistence - vegetation - stability - ecosystems - woodlands
This study analyses possible relationships between natural processes taking place in savannas and the tree patterns found in savannas. This can lead to new hypotheses about which processes are driving savanna physiognomy. To do so tree patterns were quantified for African savannas from historical aerial photographs applying frequently used landscape metrics. Also, additional data for these areas were collected to quantify the processes taking place at these locations. Correlations between tree pattern indices and explaining factors were analysed. We found a negative trend between tree cover and density of sheep and goats, but no relationship between tree cover and density of cattle, suggesting that small livestock have an effect on tree cover, but that larger livestock (or obligate grazers) do not. Also, a positive correlation between human population density and tree cover was found. Possible explanations for the found relations are discussed. Subsequent ways to analyse the latter correlation are discussed, and the potential of the presented historical database of aerial photographs is highlighted
Ontogenetic Diet Shifts Result in Niche Partitioning between Two Consumer Species Irrespective of Competitive Abilities
Schellekens, T. ; Roos, A.M. ; Persson, L. - \ 2010
American Naturalist 176 (2010)5. - ISSN 0003-0147 - p. 625 - 637.
complex life-cycles - crustacean zooplankton - apparent competition - size range - body-size - coexistence - communities - resource - environments - populations
Tilman's theory predicts the outcome of competition between two consumers sharing two resources on the basis of the shape of zero net-growth isoclines (ZNGIs). In his theory, intra-specific differences in resource use are not accounted for. Here we extend this theory to include situations where organisms undergo ontogenetic diet shifts, as these characterize the life histories of many species. In a situation that without diet shifts would lead to neutral coexistence of consumer species, we investigate whether ontogenetic diet shifts lead to niche partitioning. We analyze a model describing competition for two resources between two competitors with distinctive diets over ontogeny, using copepods (showing ontogenetic diet shifts) and daphnids (not showing ontogenetic diet shifts) as appropriate representatives. We show that an ontogenetic diet shift affects the shape of the ZNGI, changing it from reflecting perfectly substitutable resources to reflecting essential resources. Furthermore, we show that resource supply determines population stage structure and stage-dependent resource consumption in copepods and influences the competitive outcome with daphnids. In particular, we show that in itself, an ontogenetic diet shift can provide a competitive advantage if the supply of the adult resource is lower than the supply of the juvenile resource but that it always causes a disadvantage if the supply of the adult resource exceeds that of the juvenile resource.
Food Quality in Producer-Grazer Models: A Generalized Analysis
Stiefs, D. ; Voorn, G.A.K. van; Kooi, B.W. ; Feudel, U. ; Gross, T. - \ 2010
American Naturalist 176 (2010)3. - ISSN 0003-0147 - p. 367 - 380.
predator-prey interactions - population-dynamics - functional-response - stability - systems - stoichiometry - enrichment - connectance - coexistence - competition
Stoichiometric constraints play a role in the dynamics of natural populations but are not explicitly considered in most mathematical models. Recent theoretical works suggest that these constraints can have a significant impact and should not be neglected. However, it is not yet resolved how stoichiometry should be integrated in population dynamical models, as different modeling approaches are found to yield qualitatively different results. Here we investigate a unifying framework that reveals the differences and commonalities between previously proposed models for producer-grazer systems. Our analysis reveals that stoichiometric constraints affect the dynamics mainly by increasing the intraspecific competition between producers and by introducing a variable biomass conversion efficiency. The intraspecific competition has a strongly stabilizing effect on the system, whereas the variable conversion efficiency resulting from a variable food quality is the main determinant for the nature of the instability once destabilization occurs. Only if the food quality is high can an oscillatory instability, as in the classical paradox of enrichment, occur. While the generalized model reveals that the generic insights remain valid in a large class of models, we show that other details such as the specific sequence of bifurcations encountered in enrichment scenarios can depend sensitively on assumptions made in modeling stoichiometric constraints.
Honey, I cooled the cods: Modelling the effect of temperature on the structure of Boreal/Arctic fish ecosystems
Pope, J.G. ; Falk-Pedersen, J. ; Jennings, S. ; Rice, J.C. ; Gislason, H. ; Daan, N. - \ 2009
Deep-Sea Research. Part II, tropical studies in oceanography 56 (2009)21-22. - ISSN 0967-0645 - p. 2097 - 2107.
north-sea cod - gadus-morhua - species-diversity - transient dynamics - predator-prey - marine fish - food webs - coexistence - size - biodiversity
Historically colder regions of the North Atlantic had fisheries dominated by only a few fish species; principally cod and capelin. Possible population dynamic mechanisms that lead to such dominance are investigated by considering how a charmingly simple published multispecies model of the North Sea would react if the system operated at a lower temperature. The existing model equations were modified to describe temperature effects on growth, fecundity and recruitment and the model was rerun based on typical temperatures for the North Sea and a colder system. The results suggest that total fish biomass in the colder system increases but the community is more vulnerable to a given rate of fishing mortality. In the colder system, within species density dependence is reduced but relative predation rates are higher. Consequently, intermediate-sized species are vulnerable to relatively high levels of predation throughout their life history and tend to be excluded, leading to a system dominated by small and large species. The model helps to explain how temperature may govern coexistence and competitive exclusion in fish communities and accounts for the observed dominance of small and large species in Boreal/Arctic ecosystems.
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