Ecologists often assume that fragmentation correlates negatively with species diversity because small populations and patch isolation have a predictable negative impact on population survival probability. However, recent empirical research indicates that landscapes (meta-habitats) with small patches may support more species, but it leaves conditions and mechanisms largely unexplored. We also know that when patches interact through dispersal, a region may host many metapopulations of different species. The size of each population might be enough to offset the fragmentation, to persist, and thus maintain species regionally. We hypothesized that dispersal might enhance or depress richness depending on its effectiveness and that patch size affects the dispersal effectiveness. Our approach: To explore the effect of patch size, we created small, medium, and large patch landscapes and monitored the connectivity among patches (percentage of shared boundaries among equally suitable patches). The dependent variables included species richness and population density. We used an agent-based, spatially explicit metacommunity model where species gain energy on ‘suitable’ patches, with dispersal and reproduction costing energy and species interactions imparting costs and rewards on the gradient from negative to positive interactions.
Results/Conclusions
We found that landscapes with many small patches maintained twice as many species as landscapes with patches about twice as large in area. The patch connectivity had a much more significant positive effect in small patch landscapes. This effect was again much more significant than the same connectivity level in landscapes with the largest patches. However, low connectivity significantly negatively impacted richness, particularly in small patch landscapes. Surprisingly, combined species density was higher in the smallest patch landscapes, even if it was lower per patch than in the largest ones. In short, small patch landscapes maintained high biodiversity when connected and low diversity when isolated. Meta-analyses of empirical patterns likely include both situations and, depending on their mix, may reveal either type of effect.
A plausible interpretation is that a meta-habitat comprising many small and different patches (high heterogeneity) is conducive to the coexistence of many different species. Patch connectivity substantially increases richness by rescuing small populations from local extinction. Connectivity among large patches also improves richness, but the effect is negligible. This interactive effect between patch size and connectivity may create a stage for high richness and dispersal by stabilizing species persistence.
