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Community Variation (community + variation)
Selected AbstractsEffects of raised water levels on wet grassland plant communitiesAPPLIED VEGETATION SCIENCE, Issue 3 2009Sarah E. Toogood Abstract Questions: What are the effects of raised water levels on wet grassland plant communities and dynamics? To what extent do time since raised water levels, vegetation management and water regime influence community composition? Location: Pevensey Levels, southeast England, UK. Methods: Plant communities and hydrology were monitored during 2001-03 within 23 wet grassland meadows and pastures where water levels had been raised for nature conservation at different times over 21 years. Community variations were examined using species abundance and ecological traits. Results: Water regime, measured as duration of flooding, groundwater level and soil moisture was significantly related to plant community variation. Communities were divided into grasslands where inundation was shallow (,8 cm) and relatively short (,3 months) and sites where deeper flooding was prolonged (,5 months), supporting a variety of wetland vegetation. With increasing wetness, sites were characterised by more bare ground and wetland plants such as sedges, helophytes and hydrophytes, and species with a stress-tolerating competitive strategy. All sites showed considerable annual dynamics, especially those with substantially raised water levels. There were no significant relationships between time since water levels were raised and plant community composition. Grassland management exerted a limited influence upon vegetation compared to water regime. Conclusions: Grassland plant communities are responsive to raised water levels and have potential for a rapid transition to wetland vegetation, irrespective of grazing or cutting management. Creation or restoration of wet grasslands by (re)wetting is feasible but challenging due to the high dynamism of wetland plant communities and the need for substantially raised water levels and prolonged flooding to produce significant community changes. [source] Broad-scale environmental response and niche conservatism in lacustrine diatom communitiesGLOBAL ECOLOGY, Issue 5 2010Joseph R. Bennett ABSTRACT Aim, (1) To resolve theoretical debates regarding the role of environment versus dispersal limitation, the conservatism of niches across distances and the prevalence of environmental specialists in diatom communities. (2) To provide guidance on the use of diatom communities and other microbial analogues to analyse ecological response to environmental change. Location, Eight hundred and ninety-one lakes in five regional datasets from north-western Europe and four regional datasets from north-eastern North America. Methods, Lacustrine diatom communities were analysed at three scales: inter-continental, intra-continental and regional. Nested partial redundancy analyses (RDAs) were used to determine spatial versus environmental components of community variation. Weighted-averaging (WA) regression and calibration, as well as logistic and quadratic regressions, were used to detect niche conservatism and the prevalence of environmental specialists. Results, Community patterns indicate that dispersal limitation acts predominantly at the inter-continental scale, while at the regional (less than c. 1,000,000 km2) scale, a single environmental variable (pH) explains more than five times the community variation as spatial (dispersal-related) variables. In addition, pH niche components appear to be conserved at the inter-continental scale, and environmental specialization does not impose relative rarity, as specialists apparently readily disperse to suitable environments. Main conclusions, Analysis at multiple scales is clearly important in determining the influences of community variation. For diatom communities, dispersal limitation acts most strongly at the broadest scales, giving way to environment at the scales considered by most analyses. The availability of a wide variety of propagules with consistent niches across regions indicates that diatom communities reflect the succession of taxa according to local environmental conditions, rather than disequilibrium with the environment or adaptation of local populations. While multi-scale analyses must be undertaken for other groups to resolve debates over community drivers and determine appropriate scales for prediction, for diatoms (and probably other microbial communities), responses to environmental change can be inferred using analogue datasets from large geographic areas. [source] Do community-level models describe community variation effectively?JOURNAL OF BIOGEOGRAPHY, Issue 10 2010Andrés Baselga Abstract Aim, The aim of community-level modelling is to improve the performance of species distributional models by taking patterns of co-occurrence among species into account. Here, we test this expectation by examining how well three community-level modelling strategies (,assemble first, predict later', ,predict first, assemble later', and ,assemble and predict together') spatially project the observed composition of species assemblages. Location, Europe. Methods, Variation in the composition of European tree assemblages and its spatial and environmental correlates were examined with cluster analysis and constrained analysis of principal coordinates. Results were used to benchmark spatial projections from three community-based strategies: (1) assemble first, predict later (cluster analysis first, then generalized linear models, GLMs); (2) predict first, assemble later (GLMs first, then cluster analysis); and (3) assemble and predict together (constrained quadratic ordination). Results, None of the community-level modelling strategies was able to accurately model the observed distribution of tree assemblages in Europe. Uncertainty was particularly high in southern Europe, where modelled assemblages were markedly different from observed ones. Assembling first and predicting later led to distribution models with the simultaneous occurrence of several types of assemblages in southern Europe that do not co-occur, and the remaining strategies yielded models with the presence of non-analogue assemblages that presently do not exist and that are much more strongly correlated with environmental gradients than with the real assemblages. Main conclusions, Community-level models were unable to characterize the distribution of European tree assemblages effectively. Models accounting for co-occurrence patterns along environmental gradients did not outperform methods that assume individual responses of species to climate. Unrealistic assemblages were generated because of the models' inability to capture fundamental processes causing patterns of covariation among species. The usefulness of these forms of community-based models thus remains uncertain and further research is required to demonstrate their utility. [source] Do plant communities exist?JOURNAL OF VEGETATION SCIENCE, Issue 5 2000Evidence from scaling-up local species-area relations to the regional level Abstract. One long tradition in ecology is that discrete communities exist, at least in the sense that there are areas of relatively uniform vegetation, with more rapid change in species composition between them. The alternative extreme view is the Self-similarity concept , that similar community variation occurs at all spatial scales. We test between these two by calculating species-area curves within areas of vegetation that are as uniform as can be found, and then extrapolating the within-community variation to much larger areas, that will contain many ,communities'. Using the Arrhenius species-area model, the extrapolations are remarkably close to the observed number of species at the regional/country level. We conclude that the type of heterogeneity that occurs within ,homogeneous' communities is sufficient to explain species richness at much larger scales. Therefore, whilst we can speak of ,communities' for convenience, the variation that certainly exists at the ,community' level can be seen as only a larger-scale manifestation of micro-habitat variation. [source] Effects of raised water levels on wet grassland plant communitiesAPPLIED VEGETATION SCIENCE, Issue 3 2009Sarah E. Toogood Abstract Questions: What are the effects of raised water levels on wet grassland plant communities and dynamics? To what extent do time since raised water levels, vegetation management and water regime influence community composition? Location: Pevensey Levels, southeast England, UK. Methods: Plant communities and hydrology were monitored during 2001-03 within 23 wet grassland meadows and pastures where water levels had been raised for nature conservation at different times over 21 years. Community variations were examined using species abundance and ecological traits. Results: Water regime, measured as duration of flooding, groundwater level and soil moisture was significantly related to plant community variation. Communities were divided into grasslands where inundation was shallow (,8 cm) and relatively short (,3 months) and sites where deeper flooding was prolonged (,5 months), supporting a variety of wetland vegetation. With increasing wetness, sites were characterised by more bare ground and wetland plants such as sedges, helophytes and hydrophytes, and species with a stress-tolerating competitive strategy. All sites showed considerable annual dynamics, especially those with substantially raised water levels. There were no significant relationships between time since water levels were raised and plant community composition. Grassland management exerted a limited influence upon vegetation compared to water regime. Conclusions: Grassland plant communities are responsive to raised water levels and have potential for a rapid transition to wetland vegetation, irrespective of grazing or cutting management. Creation or restoration of wet grasslands by (re)wetting is feasible but challenging due to the high dynamism of wetland plant communities and the need for substantially raised water levels and prolonged flooding to produce significant community changes. [source] Effects of drought on avian community structureGLOBAL CHANGE BIOLOGY, Issue 8 2010THOMAS P. ALBRIGHT Abstract Droughts are expected to become more frequent under global climate change. Avifauna depend on precipitation for hydration, cover, and food. While there are indications that avian communities respond negatively to drought, little is known about the response of birds with differing functional and behavioural traits, what time periods and indicators of drought are most relevant, or how response varies geographically at broad spatial scales. Our goals were thus to determine (1) how avian abundance and species richness are related to drought, (2) whether community variations are more related to vegetation vigour or precipitation deviations and at what time periods relationships were strongest, (3) how response varies among avian guilds, and (4) how response varies among ecoregions with different precipitation regimes. Using mixed effect models and 1989,2005 North American Breeding Bird Survey data over the central United States, we examined the response to 10 precipitation- and greenness-based metrics by abundance and species richness of the avian community overall, and of four behavioural guilds. Drought was associated with the most negative impacts on avifauna in the semiarid Great Plains, while positive responses were observed in montane areas. Our models predict that in the plains, Neotropical migrants respond the most negatively to extreme drought, decreasing by 13.2% and 6.0% in abundance and richness, while permanent resident abundance and richness increase by 11.5% and 3.6%, respectively in montane areas. In most cases, response of abundance was greater than richness and models based on precipitation metrics spanning 32-week time periods were more supported than those covering shorter time periods and those based on greenness. While drought is but one of myriad environmental variations birds encounter, our results indicate that drought is capable of imposing sizable shifts in abundance, richness, and composition on avian communities, an important implication of a more climatically variable future. [source] The influence of multi-scale environmental variables on the distribution of terricolous lichens in a fog desertJOURNAL OF VEGETATION SCIENCE, Issue 6 2006Jennifer S. Lalley Abstract Question: How do environmental variables in a hyper-arid fog desert influence the distribution patterns of terricolous lichens on both macro- and micro-scales? Location: Namib Desert, Namibia. Methods: Sites with varying lichen species cover were sampled for environmental variables on a macro-scale (elevation, slope degree, aspect, proximity to river channels, and fog deposition) and on a micro-scale (soil structure and chemistry). Macro-scale and micro-scale variables were analysed separately for associations with lichen species cover using constrained ordination (DCCA) and unconstrained ordination (DCA). Explanatory variables that dominated the first two axes of the constrained ordinations were tested against a lichen cover gradient. Results: Elevation and proximity to river channels were the most significant drivers of lichen species cover in the macro-scale DCCA, but results of the DCA suggest that a considerable percentage of variation in lichen species cover is unexplained by these variables. On a micro-scale, sediment particle size explained a majority of lichen community variations, followed by soil pH. When both macro and micro-scale variables were tested along a lichen cover gradient, soil pH was the only variable to show a significant relationship to lichen cover. Conclusion: The findings suggest that landscape variables contribute to variations in lichen species cover, but that stronger links occur between lichen growth and small-scale variations in soil characteristics, supporting the need for multi-scale approaches in the management of threatened biological soil crust communities and related ecosystem functions. [source] | |||||||||||||