Environmental geometry and heterogeneity-diversity relationships in spatially explicit simulated communities

Question: How are heterogeneity-diversity relationships (HDRs) influenced by spatial structure in environmental variables, sampling grain and the extent of niche differentiation? Methods: We developed a spatially explicit simulation model incorporating variable dispersal distances and competition strength on fractal landscapes. By varying the grain used to sample these models, we examined scaling patterns in HDR metrics at fine scales (sampling grain from 100 to 10 000 individuals, sampling extent ca. 260 000 individuals). Results: Environmental geometry exerts an important influence on the ecological processes responsible for HDRs. Unique geometric characteristics of individual landscapes can greatly influence emergent community properties; field studies frequently use inadequate sample sizes to account for this phenomenon. Two opposing processes influence spatial scaling of HDRs: variance partitioning, which favours smaller-grained samples, and mass effects, which favour larger-grained samples. In assessing HDRs, diversity is more sensitive than species richness, and should be the preferred measure in field studies. The environmental geometry and age of a community interact: compared to high fractal dimension landscapes, low fractal dimension landscapes are slower to develop HDRs, but in the long term their HDRs will be higher. Conclusions: Our study demonstrates that, despite the superficial simplicity of the concept, HDRs vary in complex and non-intuitive ways, and warrant further theoretical and empirical study. More generally, environmental geometry is likely to exert a strong influence on many emergent community processes, but we do not yet have a firm understanding of this relationship. © 2012 International Association for Vegetation Science.