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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A regional benthic fauna assessment method for the Southern North Sea using Margalef diversity and reference value modelling
Loon, Willem M.G.M. van; Walvoort, Dennis J.J. ; Hoey, Gert van; Vina-Herbon, Christina ; Blandon, Abigayil ; Pesch, Roland ; Schmitt, Petra ; Scholle, Jörg ; Heyer, Karin ; Lavaleye, Marc ; Phillips, Graham ; Duineveld, Gerard C.A. ; Blomqvist, Mats - \ 2018
Ecological Indicators 89 (2018). - ISSN 1470-160X - p. 667 - 679.
AMBI - Benthic assessment method - Fishing pressure - Index - ITI - Margalef diversity - Marine benthos - MMI - Model - MSFD - Multi-metric index - Organic enrichment - OSPAR - PIE - Reference value estimation - Sedimentation - Shannon index - SN - SNA - Species richness
The aims of this study are to develop an optimized method for regional benthic fauna assessment of the Southern North Sea which (a) is sensitive and precise (quantified as the slope and the R2 value of the pressure-impact relationships, respectively) for the anthropogenic pressures bottom fishing and organic enrichment, (b) is suitable for estimating and modelling reference values, (c) is transparent, (d) can be efficiently applied using dedicated software; and to apply this method to benthic data from the Southern North Sea. Margalef diversity appeared to be the best performing benthic index regarding these criteria, even better than several Multi-Metric Indices (MMIs) containing e.g. AMBI (AZTI Marine Biotic Index) and ITI (Infaunal Trophic Index). Therefore, this relatively simple and very practical index, including a new reference value estimation and modelling method, and BENMMI software were selected as a common OSPAR (Oslo Paris convention) method for the benthic fauna assessment of the Southern North Sea. This method was applied to benthic fauna data from the Southern North Sea collected during the period 2010–2015. The results in general show lower normalized Margalef values in coastal areas, and higher normalized Margalef values in deeper offshore areas. The following benthic indices were compared in this study: species richness, Margalef diversity, SNA index, Shannon index, PIE index, AMBI, ITI. For each assessment area, the least disturbed benthic dataset was selected as an adjacent 6 year period with, on average, the highest Margalef diversity values. For these datasets, the reference values were primarily set as the 99th percentile values of the respective indices. This procedure results in the highest stable reference values that are not outliers. In addition, a variable percentile method was developed, in which the percentile value is adjusted to the average bottom fishing pressure (according to data from the International Council for the Exploration of the Sea, ICES) in the period 2009–2013. The adjusted percentile values were set by expert judgement, at 75th (low fishing pressure), 95th (medium fishing pressure) and 99th (high fishing pressure) percentile. The estimated reference values for Margalef diversity correlate quite well with the median depth of the assessment areas using a sigmoid model (pseudo-R2 = 0.86). This relationship between depth and Margalef diversity was used to estimate reference values in case an assessment area had insufficient benthic data For testing the effects of bottom fishing pressure, normalized index values (NIV; index value divided by reference value) were used. The rationale for using NIVs is the assumption that, although a certain level of bottom fishing pressure will have a larger absolute effect on more biodiverse benthic communities in deeper waters than on more robust and less biodiverse coastal benthic communities, the relative effects (tested using NIVs) are comparable. A clear exponentially decreasing relationship (R2 = 0.26–0.27, p < 0.00001) was found between both bottom surface and subsurface fishing activity (penetration depth <2 cm and >2 cm, respectively) and normalized Margalef diversity values, with an asymptotic normalized Margalef value of 0.45 at a subsurface fishing activity >2.3 sweeps/year. This asymptotic value is predominantly found in coastal waters, and probably shows that the naturally more robust coastal benthic communities have been transformed into resilient benthic communities, which rapidly recover from increasing fishing pressure.
Soil-wood interactions : Influence of decaying coniferous and broadleaf logs on composition of soil fungal communities
Wal, Annemieke van der; klein Gunnewiek, Paulien ; Boer, Wietse de - \ 2017
Fungal Ecology 30 (2017). - ISSN 1754-5048 - p. 132 - 134.
Cord-forming fungi - Illumina MiSeq sequencing of ITS - LOGLIFE - Soil fungal communities - Species richness - Tree species - Wood decomposition - Wood leachates

Wood-inhabiting fungi may affect soil fungal communities directly underneath decaying wood via their exploratory hyphae. In addition, differences in wood leachates between decaying tree species may influence soil fungal communities. We determined the composition of fungi in 4-yr old decaying logs of Larix kaempferi and Quercus rubra as well as in soil directly underneath and next to logs. Fungal community composition in soil covered by logs was different from that in wood and uncovered soil and was clearly influenced by the tree species. Soil fungal species richness under logs was lower than in uncovered soil but higher than in decaying wood. The amount of exploratory hyphae of log-inhabiting fungi was only high close to decaying logs. In conclusion, there is a small but significant effect of decaying coniferous and broadleaf logs on soil fungal communities directly underneath logs, likely affected by differences in wood chemistry and fungal preference between tree species.

Root chemistry and soil fauna, but not soil abiotic conditions explain the effects of plant diversity on root decomposition
Chen, Hongmei ; Oram, Natalie J. ; Barry, Kathryn E. ; Mommer, Liesje ; Ruijven, Jasper van; Kroon, Hans de; Ebeling, Anne ; Eisenhauer, Nico ; Fischer, Christine ; Gleixner, Gerd ; Gessler, Arthur ; González Macé, Odette ; Hacker, Nina ; Hildebrandt, Anke ; Lange, Markus ; Scherer-lorenzen, Michael ; Scheu, Stefan ; Oelmann, Yvonne ; Wagg, Cameron ; Wilcke, Wolfgang ; Wirth, Christian ; Weigelt, Alexandra - \ 2017
Oecologia 185 (2017)3. - ISSN 0029-8549 - p. 499 - 511.
Functional groups - Jena Experiment - Root litter - SEM - Species richness
Plant diversity influences many ecosystem functions including root decomposition. However, due to the presence of multiple pathways via which plant diversity may affect root decomposition, our mechanistic understanding of their relationships is limited. In a grassland biodiversity experiment, we simultaneously assessed the effects of three pathways—root litter quality, soil biota, and soil abiotic conditions—on the relationships between plant diversity (in terms of species richness and the presence/absence of grasses and legumes) and root decomposition using structural equation modeling. Our final structural equation model explained 70% of the variation in root mass loss. However, different measures of plant diversity included in our model operated via different pathways to alter root mass loss. Plant species richness had a negative effect on root mass loss. This was partially due to increased Oribatida abundance, but was weakened by enhanced root potassium (K) concentration in more diverse mixtures. Equally, grass presence negatively affected root mass loss. This effect of grasses was mostly mediated via increased root lignin concentration and supported via increased Oribatida abundance and decreased root K concentration. In contrast, legume presence showed a net positive effect on root mass loss via decreased root lignin concentration and increased root magnesium concentration, both of which led to enhanced root mass loss. Overall, the different measures of plant diversity had contrasting effects on root decomposition. Furthermore, we found that root chemistry and soil biota but not root morphology or soil abiotic conditions mediated these effects of plant diversity on root decomposition.
How bird clades diversify in response to climatic and geographic factors
Rodríguez-Castañeda, Genoveva ; Hof, Anouschka R. ; Jansson, Roland - \ 2017
Ecology Letters 20 (2017)9. - ISSN 1461-023X - p. 1129 - 1139.
Climate stability - Ecological niche modelling - Energy availability - Geographic area - Habitat diversity - Phylogenetic independent contrasts - Sister group comparisons - Species richness - Temperate - Tropical
While the environmental correlates of global patterns in standing species richness are well understood, it is poorly known which environmental factors promote diversification (speciation minus extinction) in clades. We tested several hypotheses for how geographic and climatic variables should affect diversification using a large dataset of bird sister genera endemic to the New World. We found support for the area, evolutionary speed, environmental predictability and climatic stability hypotheses, but productivity and topographic complexity were rejected as explanations. Genera that had accumulated more species tend to occupy wider niche space, manifested both as occurrence over wider areas and in more habitats. Genera with geographic ranges that have remained more stable in response to glacial-interglacial changes in climate were also more species rich. Since many relevant explanatory variables vary latitudinally, it is crucial to control for latitude when testing alternative mechanistic explanations for geographic variation in diversification among clades.
Naturalized alien flora of the world : Species diversity, taxonomic and phylogenetic patterns, geographic distribution and global hotspots of plant invasion
Pyšek, Petr ; Pergl, Jan ; Essl, Franz ; Lenzner, Bernd ; Dawson, Wayne ; Kreft, Holger ; Weigelt, Patrick ; Winter, Marten ; Kartesz, John ; Nishino, Misako ; Antonova, Liubov A. ; Barcelona, Julie F. ; Cabezas, Francisco J. ; Cárdenas, Dairon ; Cárdenas-Toro, Juliana ; Castaño, Nicolás ; Chacón, Eduardo ; Chatelain, Cyrille ; Dullinger, Stefan ; Ebel, Aleksandr L. ; Figueiredo, Estrela ; Fuentes, Nicol ; Genovesi, Piero ; Groom, Quentin J. ; Henderson, Lesley ; Inderjit, ; Kupriyanov, Andrey ; Masciadri, Silvana ; Maurel, Noëlie ; Meerman, Jan ; Morozova, Olga ; Moser, Dietmar ; Nickrent, Daniel L. ; Nowak, Pauline M. ; Pagad, Shyama ; Patzelt, Annette ; Pelser, Pieter B. ; Seebens, Hanno ; Shu, Wen Sheng ; Thomas, Jacob ; Velayos, Mauricio ; Weber, Ewald ; Wieringa, Jan J. ; Baptiste, María P. ; Kleunen, Mark Van - \ 2017
Preslia 89 (2017)3. - ISSN 0032-7786 - p. 203 - 274.
Alien species - Distribution - Global Naturalized Alien Flora (GloNAF) database - Invasive species - Islands - Life history - Mainland - Naturalized species - Phylogeny - Plant invasion - Regional floras - Species richness - Taxonomy - Zonobiome
Using the recently built Global Naturalized Alien Flora (GloNAF) database, containing data on the distribution of naturalized alien plants in 483 mainland and 361 island regions of the world, we describe patterns in diversity and geographic distribution of naturalized and invasive plant species, taxonomic, phylogenetic and life-history structure of the global naturalized flora as well as levels of naturalization and their determinants. The mainland regions with the highest numbers of naturalized aliens are some Australian states (with New South Wales being the richest on this continent) and several North American regions (of which California with 1753 naturalized plant species represents the world's richest region in terms of naturalized alien vascular plants). England, Japan, New Zealand and the Hawaiian archipelago harbour most naturalized plants among islands or island groups. These regions also form the main hotspots of the regional levels of naturalization, measured as the percentage of naturalized aliens in the total flora of the region. Such hotspots of relative naturalized species richness appear on both the western and eastern coasts of North America, in north-western Europe, South Africa, south-eastern Australia, New Zealand, and India. High levels of island invasions by naturalized plants are concentrated in the Pacific, but also occur on individual islands across all oceans. The numbers of naturalized species are closely correlated with those of native species, with a stronger correlation and steeper increase for islands than mainland regions, indicating a greater vulnerability of islands to invasion by species that become successfully naturalized. South Africa, India, California, Cuba, Florida, Queensland and Japan have the highest numbers of invasive species. Regions in temperate and tropical zonobiomes harbour in total 9036 and 6774 naturalized species, respectively, followed by 3280 species naturalized in the Mediterranean zonobiome, 3057 in the subtropical zonobiome and 321 in the Arctic. The New World is richer in naturalized alien plants, with 9905 species compared to 7923 recorded in the Old World. While isolation is the key factor driving the level of naturalization on islands, zonobiomes differing in climatic regimes, and socioeconomy represented by per capita GDP, are central for mainland regions. The 11 most widely distributed species each occur in regions covering about one third of the globe or more in terms of the number of regions where they are naturalized and at least 35% of the Earth's land surface in terms of those regions' areas, with the most widely distributed species Sonchus oleraceus occuring in 48% of the regions that cover 42% of the world area. Other widely distributed species are Ricinus communis, Oxalis corniculata, Portulaca oleracea, Eleusine indica, Chenopodium album, Capsella bursa-pastoris, Stellaria media, Bidens pilosa, Datura stramonium and Echinochloa crus-galli. Using the occurrence as invasive rather than only naturalized yields a different ranking, with Lantana camara (120 regions out of 349 for which data on invasive status are known), Calotropis procera (118), Eichhornia crassipes (113), Sonchus oleraceus (108) and Leucaena leucocephala (103) on top. As to the life-history spectra, islands harbour more naturalized woody species (34.4%) thanmainland regions (29.5%), and fewer annual herbs (18.7% compared to 22.3%). Ranking families by their absolute numbers of naturalized species reveals that Compositae (1343 species), Poaceae (1267) and Leguminosae (1189) contribute most to the global naturalized alien flora. Some families are disproportionally represented by naturalized aliens on islands (Arecaceae, Araceae, Acanthaceae, Amaryllidaceae, Asparagaceae, Convolvulaceae, Rubiaceae, Malvaceae), and much fewer so on mainland (e.g. Brassicaceae, Caryophyllaceae, Boraginaceae). Relating the numbers of naturalized species in a family to its total global richness shows that some of the large species-rich families are over-represented among naturalized aliens (e.g. Poaceae, Leguminosae, Rosaceae, Amaranthaceae, Pinaceae), some under-represented (e.g. Euphorbiaceae, Rubiaceae), whereas the one richest in naturalized species, Compositae, reaches a value expected from its global species richness. Significant phylogenetic signal indicates that families with an increased potential of their species to naturalize are not distributed randomly on the evolutionary tree. Solanum (112 species), Euphorbia (108) and Carex (106) are the genera richest in terms of naturalized species; over-represented on islands are Cotoneaster, Juncus, Eucalyptus, Salix, Hypericum, Geranium and Persicaria, while those relatively richer in naturalized species on the mainland are Atriplex, Opuntia, Oenothera, Artemisia, Vicia, Galium and Rosa. The data presented in this paper also point to where information is lacking and set priorities for future data collection. The GloNAF database has potential for designing concerted action to fill such data gaps, and provide a basis for allocating resources most efficiently towards better understanding and management of plant invasions worldwide.
Exploring the floristic diversity of tropical Africa
Sosef, Marc S.M. ; Dauby, Gilles ; Blach-Overgaard, Anne ; Burgt, Xander van der; Catarino, Luís ; Damen, Theo ; Deblauwe, Vincent ; Dessein, Steven ; Dransfield, John ; Droissart, Vincent ; Duarte, Maria Cristina ; Engledow, Henry ; Fadeur, Geoffrey ; Figueira, Rui ; Gereau, Roy E. ; Hardy, Olivier J. ; Harris, David J. ; Heij, Janneke de; Janssens, Steven ; Klomberg, Yannick ; Ley, Alexandra C. ; Mackinder, Barbara A. ; Meerts, Pierre ; Poel, Jeike L. van de; Sonké, Bonaventure ; Stévart, Tariq ; Stoffelen, Piet ; Svenning, Jens Christian ; Sepulchre, Pierre ; Zaiss, Rainer ; Wieringa, Jan J. ; Couvreur, Thomas L.P. - \ 2017
BMC Biology 15 (2017)1. - ISSN 1741-7007
Botanical exploration - Digitization - Floristic patterns - Herbarium specimens - Plant growth form - Species richness - Tropical forests
Background: Understanding the patterns of biodiversity distribution and what influences them is a fundamental pre-requisite for effective conservation and sustainable utilisation of biodiversity. Such knowledge is increasingly urgent as biodiversity responds to the ongoing effects of global climate change. Nowhere is this more acute than in species-rich tropical Africa, where so little is known about plant diversity and its distribution. In this paper, we use RAINBIO - one of the largest mega-databases of tropical African vascular plant species distributions ever compiled - to address questions about plant and growth form diversity across tropical Africa. Results: The filtered RAINBIO dataset contains 609,776 georeferenced records representing 22,577 species. Growth form data are recorded for 97% of all species. Records are well distributed, but heterogeneous across the continent. Overall, tropical Africa remains poorly sampled. When using sampling units (SU) of 0.5°, just 21 reach appropriate collection density and sampling completeness, and the average number of records per species per SU is only 1.84. Species richness (observed and estimated) and endemism figures per country are provided. Benin, Cameroon, Gabon, Ivory Coast and Liberia appear as the botanically best-explored countries, but none are optimally explored. Forests in the region contain 15,387 vascular plant species, of which 3013 are trees, representing 5-7% of the estimated world's tropical tree flora. The central African forests have the highest endemism rate across Africa, with approximately 30% of species being endemic. Conclusions: The botanical exploration of tropical Africa is far from complete, underlining the need for intensified inventories and digitization. We propose priority target areas for future sampling efforts, mainly focused on Tanzania, Atlantic Central Africa and West Africa. The observed number of tree species for African forests is smaller than those estimated from global tree data, suggesting that a significant number of species are yet to be discovered. Our data provide a solid basis for a more sustainable management and improved conservation of tropical Africa's unique flora, and is important for achieving Objective 1 of the Global Strategy for Plant Conservation 2011-2020. In turn, RAINBIO provides a solid basis for a more sustainable management and improved conservation of tropical Africa's unique flora.
How to efficiently obtain accurate estimates of flower visitation rates by pollinators
Fijen, Thijs P.M. ; Kleijn, David - \ 2017
Basic and Applied Ecology 19 (2017). - ISSN 1439-1791 - p. 11 - 18.
Crop systems - Minimum observation duration - Observation protocol - Pollination - Species richness - Time of day - Visitation rate - Weather
Regional declines in insect pollinators have raised concerns about crop pollination. Many pollinator studies use visitation rate (pollinators/time) as a proxy for the quality of crop pollination. Visitation rate estimates are based on observation durations that vary significantly between studies. How observation duration relates to the accuracy of the visitation rate estimate is, however, unknown. We studied this relationship using six day-long observations (06:00. h-19:00. h) in leek-seed production fields (totalling 78. h). We analysed beyond which point in time observing longer did not significantly improve the accuracy of the visitation rate estimate (minimum observation duration). We furthermore explored the relationship between the minimum observation duration and visitation rate, time of day and temperature. We found that the minimum observation duration (mean. ±. SD: 24. ±. 11.9. min) was significantly related to visitation rate, where the observation time required to obtain accurate estimates decreased with increasing visitation rate. Minimum observation duration varied greatly between days and between fields but not within days. Within days, the visitation rates differed significantly only between the hour-intervals 06:00. h-07:00. h (lowest visitation rate) and 09:00. h-11:00. h (highest rate). Minimum observation duration decreased up to around 22. °C beyond which it remained fairly stable. Surprisingly, even after three day-long observations on the same plant we found new pollinator species visiting the flowers, suggesting that species-richness estimates based on plant observations alone probably underestimate true species richness. Because especially between-day variation in visitation rate on single plants can be large, reliable estimates of the pollinator visitation rate during the plant's flowering time require observations on multiple days. Standardising the number of pollinators rather than the time to observe (standardised pollinator timing approach: time to n-pollinator visits) may provide more consistent accurate assessments of visitation rate, especially for studies that use gradients in visitation rates to examine the contribution of pollinators to crop pollination.
Species-rich grassland can persist under nitrogen-rich but phosphorus-limited conditions
Dobben, Han F. van; Wamelink, Wieger ; Slim, Pieter A. ; Kamiński, Jan ; Piórkowski, Hubert - \ 2017
Plant and Soil 411 (2017)1. - ISSN 0032-079X - p. 451 - 466.
Biomass production - Co-limitation - Drained peat - Grassland - Mowing - Nutrient limitation - Poland - Species richness

Aim: Deposition of nitrogen is assumed to cause loss of botanical diversity, probably through increased production and exclusion of less competitive species. However, if production is (co-)limited by phosphorus, acceleration of the phosphorus cycle may be responsible for the diversity loss and, where that is the case, nitrogen emission reduction may turn out to be an ineffective mitigation strategy. Here we study the feasibility of this mechanism through adding potassium and phosphorus to grassland where nitrogen limitation is absent. Methods: We made vegetation relevés in a long-term agricultural fertilisation experiment where potassium, phosphorus and nitrogen were being added to grassland on drained peat where nitrogen availability was high, even in unfertilised plots. We applied a multivariate analysis to investigate the effect of additions of K, K + P and K + P + N on the species composition. Results: Unfertilised plots had a very low biomass production and were rich in plant species despite their high nitrogen availability. Addition of potassium led to a strongly increased production but did not result in a reduction of species numbers. Phosphorus in addition to potassium increased production still further and decreased species numbers, most notably the number of endangered species. Conclusions: Even under nitrogen rich conditions species richness may be high in grasslands where phosphorous provides a limitation to plant growth. Phosphorus limitation and phosphorus enrichment are both common in grassland, at least in north-western Europe. Part of the general decrease in species numbers that is commonly ascribed to nitrogen enrichment may therefore be due to phosphorus enrichment. If phosphorus and nitrogen are co-limiting (which is often the case) the current nitrogen emission reduction policies may be effective, but not sufficient to restore grassland diversity to its pre-industrial level.

Context-dependent environmental quality standards of soil nitrate for terrestrial plant communities
Goethem, Thomas M.W.J. van; Schipper, Aafke M. ; Wamelink, Wieger ; Huijbregts, Mark A.J. - \ 2016
Journal of Environmental Management 81 (2016). - ISSN 0301-4797 - p. 681 - 686.
Ecosystem vulnerability - Forests - Grasslands - Interaction effects - Nitrate - PH - Quantile regression - Red List species - Species richness

Environmental quality standards (EQS) specify the maximum permissible concentration or level of a specific environmental stressor. Here, a procedure is proposed to derive EQS that are specific to a representative species pool and conditional on confounding environmental factors. To illustrate the procedure, a dataset was used with plant species richness observations of grasslands and forests and accompanying soil nitrate-N and pH measurements collected from 981 sampling sites in the Netherlands. Species richness was related to soil nitrate-N and pH with quantile regression allowing for interaction effects. The resulting regression models were used to derive EQS for nitrate conditional on pH, quantified as the nitrate-N concentrations at a specific pH level corresponding with a species richness equal to 95% of the species pool, for both grasslands and forest communities. The EQS varied between 1.8 mg/kg nitrate-N at pH 9-65 mg/kg nitrate-N at pH 4. EQS for forests and grasslands were similar, but EQS based on Red List species richness were considerably lower (more stringent) than those based on overall species richness, particularly at high pH levels. The results indicate that both natural background pH conditions and Red List species are important factors to consider in the derivation of EQS for soil nitrate-N for terrestrial ecosystems.

Host body size and the diversity of tick assemblages on Neotropical vertebrates
Esser, Helen J. ; Foley, Janet E. ; Bongers, Frans ; Herre, Edward Allen ; Miller, Matthew J. ; Prins, Herbert H.T. ; Jansen, Patrick A. - \ 2016
International Journal for Parasitology: Parasites and Wildlife 5 (2016)3. - ISSN 2213-2244 - p. 295 - 304.
20/80 Rule - Panama - Parasite fauna - Pareto principle - Proportional similarity - Species richness

Identifying the factors that influence the species diversity and distribution of ticks (Acari: Ixodida) across vertebrate host taxa is of fundamental ecological and medical importance. Host body size is considered one of the most important determinants of tick abundance, with larger hosts having higher tick burdens. The species diversity of tick assemblages should also be greater on larger-bodied host species, but empirical studies testing this hypothesis are lacking. Here, we evaluate this relationship using a comparative dataset of feeding associations from Panama between 45 tick species and 171 host species that range in body size by three orders of magnitude. We found that tick species diversity increased with host body size for adult ticks but not for immature ticks. We also found that closely related host species tended to have similar tick species diversity, but correcting for host phylogeny did not alter the relationships between host body size and tick species diversity. The distribution of tick species was highly aggregated, with approximately 20% of the host species harboring 80% of all tick species, following the Pareto principle or 20/80 Rule. Thus, the aggregated pattern commonly observed for tick burdens and disease transmission also holds for patterns of tick species richness. Our finding that the adult ticks in this system preferentially parasitize large-bodied host species suggests that the ongoing anthropogenic loss of large-bodied vertebrates is likely to result in host-tick coextinction events, even when immature stages feed opportunistically. As parasites play critical roles in ecological and evolutionary processes, such losses may profoundly affect ecosystem functioning and services.

Changes in grass plant populations and temporal soil seed bank dynamics in a semi-arid African savanna : Implications for restoration
Tessema, Zewdu K. ; Boer, Fred de; Prins, Herbert H.T. - \ 2016
Journal of Environmental Management 182 (2016). - ISSN 0301-4797 - p. 166 - 175.
Grass plant population - Grazing intensity - Growth stages - Life–forms - Seed density - Species richness

The re-colonization or recovery of grass species after disappearance due to heavy grazing depends on the presence of persistent soil seed banks that might be accumulated over time from the aboveground vegetation. Moreover, successful plant recruitment is a function of seed production, seed germination and seedling survival, which can be mechanistically understood through studying the life cycle processes of grass species populations under field conditions. Therefore, we studied the number of germinable seeds, species richness and life-forms in the soil seed banks under light and heavy grazing conditions, and the changes in grass species populations in a semi-arid savanna of Ethiopia. Accordingly, a total of 103 species (15 perennial and 29 annual grasses, 6 legumes, 52 forbs and 1 woody species) emerged from the soil samples collected. Lightly grazed sites had a higher seed density compared with heavily grazed sites. The seed density increased over the first three months of soil sampling and decreased thereafter. Perennial grasses dominated the light grazing sites, whereas annual species dominated the heavily grazed sites, indicating that perennial grasses were replaced by annual species in the soil seed bank through grazing. The mean mortality rate from the seedling stage to adult plants was 65%. The seed-to-seedling stage was found to be the most critical transitional stage for grass survival. High seedling mortality in the aboveground vegetation and depletion of seeds in the soil seed banks as a result of sustained heavy grazing can lead to local extinction and disappearance of perennial grasses in semi-arid Ethiopian savannas.

Phylogenetic diversity of Amazonian tree communities
Honorio Coronado, E.N. ; Dexter, K.G. ; Pennington, R.T. ; Chave, Jérôme ; Lewis, S.L. ; Alexiades, M.N. ; Alvarez, Esteban ; Alves de Oliveira, Atila ; Amaral, J.L. ; Araujo-Murakami, Alejandro ; Arets, E.J.M.M. - \ 2015
Diversity and Distributions 21 (2015)11. - ISSN 1366-9516 - p. 1295 - 1307.
Amazon basin - Eudicots - Magnoliids - Monocots - Phylogenetic diversity - Species richness

Aim: To examine variation in the phylogenetic diversity (PD) of tree communities across geographical and environmental gradients in Amazonia. Location: Two hundred and eighty-three c. 1 ha forest inventory plots from across Amazonia. Methods: We evaluated PD as the total phylogenetic branch length across species in each plot (PDss), the mean pairwise phylogenetic distance between species (MPD), the mean nearest taxon distance (MNTD) and their equivalents standardized for species richness (ses.PDss, ses.MPD, ses.MNTD). We compared PD of tree communities growing (1) on substrates of varying geological age; and (2) in environments with varying ecophysiological barriers to growth and survival. Results: PDss is strongly positively correlated with species richness (SR), whereas MNTD has a negative correlation. Communities on geologically young- and intermediate-aged substrates (western and central Amazonia respectively) have the highest SR, and therefore the highest PDss and the lowest MNTD. We find that the youngest and oldest substrates (the latter on the Brazilian and Guiana Shields) have the highest ses.PDss and ses.MNTD. MPD and ses.MPD are strongly correlated with how evenly taxa are distributed among the three principal angiosperm clades and are both highest in western Amazonia. Meanwhile, seasonally dry tropical forest (SDTF) and forests on white sands have low PD, as evaluated by any metric. Main conclusions: High ses.PDss and ses.MNTD reflect greater lineage diversity in communities. We suggest that high ses.PDss and ses.MNTD in western Amazonia results from its favourable, easy-to-colonize environment, whereas high values in the Brazilian and Guianan Shields may be due to accumulation of lineages over a longer period of time. White-sand forests and SDTF are dominated by close relatives from fewer lineages, perhaps reflecting ecophysiological barriers that are difficult to surmount evolutionarily. Because MPD and ses.MPD do not reflect lineage diversity per se, we suggest that PDss, ses.PDss and ses.MNTD may be the most useful diversity metrics for setting large-scale conservation priorities.

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