Environmental heterogeneity in space and time: Effects on plant species diversity in an unproductive ecosystem

Areas that are spatially or temporally heterogeneous should, in theory, support higher species diversity than more uniform environments due to the provision of more niches and prevention of competitive exclusion. Using herbaceous plant communities in an unproductive limestone pavement habitat, I tested the hypothesis that diversity would be positively related to spatial and temporal heterogeneity in environmental factors at fine spatial scales and at seedling recruitment stages. Spatial heterogeneity in microsite composition was positively related to richness in both seedling and adult communities in the field, and many species showed microsite preferences in a seed addition experiment. Species richness was unimodally related to mean soil depth and relative elevation, and peak species richness was associated with maximal spatial heterogeneity in soil depth and microsite composition. This study thus suggests that unimodal richness gradients can be produced by spatial heterogeneity acting to constrain seedling recruitment.; Using a series of greenhouse experiments that mimic field water regimes, I determined the relationships between temporal variability in soil moisture and plant diversity. Higher temporal heterogeneity promoted the germination of more species, but increased post-germination mortality. The effects of heterogeneity thus depended on the life history stage considered. The number of species establishing after 4 months was negatively related to temporal variability in water supply in shallow soils but the relationship was unimodal in deeper soils. Unimodal relationships between elevation and species richness in the field are thus partly due to the effects of a spatial gradient in temporal heterogeneity in soil moisture exerting effects at both germination and establishment life stages. There was some evidence that competitive exclusion at high seedling densities led to declines in species richness at the extremes of the gradient in temporal variability in soil moisture. My thesis is that environmental heterogeneity acts to filter species from this community at early plant life stages, with maximal spatial heterogeneity and intermediate temporal heterogeneity allowing the greatest number of species to recruit. The importance of spatial and temporal heterogeneity in producing unimodal diversity gradients deserves wider consideration.