Meelis Pärtel, University of Tartu
Heather Reynolds, University of Indiana, USA
Have you noticed that environments that exhibit various conditions support higher species diversity? For example, one can probably find more species in an area that has patches of dry grasslands, wet meadows and ponds than in a uniform grassland area. This positive relationship between environmental heterogeneity and species diversity is a common knowledge in ecology, and forms a basis for several ecological theories and nature conservation decisions. But what if environmental conditions are heterogeneously distributed also within a community; for example, some patches are more fertile than others? Does this small-scale environmental heterogeneity also support more species? Several studies in plant communities have shown that environmental heterogeneity occurring at small spatial scales has a negative effect on plant co-existence. In this thesis, I shed further light on the negative heterogeneity-diversity relationship and describe mechanisms behind this pattern. We used data from previous studies, a computer simulation and a greenhouse experiment and found that small-scale heterogeneity can negatively impact on species diversity in two ways, depending on the relative sizes of plant individuals and habitat patches. If the patches are larger than individuals, heterogeneity can restrict dispersal between suitable patches that become more isolated. This decreases population sizes and lowers species diversity. If the patches are smaller than plant individuals, large and fast-growing species can quickly forage through different quality patches and take advantage of heterogeneous conditions, outcompeting small and slow-growing species. Small-scale heterogeneity is common in nature and can have significant impacts on community structure. Understanding the mechanisms behind a negative heterogeneity-diversity relationship can help to predict future changes in plant communities and aid conservation decisions.