Effects of environmental heterogeneity on species diversity: A new process-based, multi-species, landscape simulation model (SHALOM)

Abstract

I designed and implemented a process-based, multi-species, spatially-explicit, object-oriented landscape simulation model that analyzes how high-level ecological complexity affects species diversity patterns (SHALOM). SHALOM has physical (landscape, habitat, cell, patch) and biological classes (population, species, community). At the local scale, populations grow continuously, affected by a community-level saturation effect, a species-habitat match, and demographic stochasticity. The global-scale processes are dispersal and catastrophic stochasticity. The model uses allometric relationships and energy as a common currency to bridge differences between different body-sized species located in habitats of different productivities. The model represents a new synthetic approach to study combined ecosystem, community and population processes. I solved the model's local-scale population growth equation analytically. For a population to have a positive carrying capacity, its death-rate-to-birth-rate ratio (d/b) should be greater than its match to the habitat it occupies. Body-size dependent birth and death rates show that d/b decreases with body size for eutherian mammals. Altogether, habitat specificity negatively scales with body size. I discuss this prediction in light of two macroecological patterns--geographic range vs. body size and species abundance vs. geographic range. I simulated a simple 4-patch landscape, each patch having a unique habitat. I simulated 26 species that differed only in body size. I used allometric values of eutherian mammals. The results show that interspecific competition reduces species diversity in each habitat and in the landscape. Stochasticity depresses mean population sizes, opening opportunities for species to avoid competitive exclusion. With stochasticity, habitats have different communities determined by which large species becomes locally extinct at random. Demographic and catastrophic stochasticities differ in their effects on species diversity. Dispersing individuals move between habitats and reestablish the local populations of their species. Dispersal neutralizes the randomness of the assemblages produced by stochasticity. I simulated a 16-patch landscape. Some habitats were unsuitable for several species. The results show that body size and species abundance have a log-normal relationship, and that geographic range increases non-linearly with log body size. These patterns are highly consistent with observed data.