Interactions between environmental variability and immigration rate control patterns of species diversity

Abstract

Theories focussing on local competition processes predict that species diversity is maximised with high-frequency environmental variability or alternatively with intermediate frequencies. The models coupling regional processes to local explanations of diversity patterns predict that the immigration rate from a regional species pool can strongly increase diversity. However, the interaction between local and regional processes in temporally fluctuating environment has received little attention. We explored in a simulation study how the patterns of species diversity are affected by the frequency spectrum of stochastic environmental variations and density independent immigration rate in a competition model. Environmental variations were assumed to affect the growth rate of competing species. The growth rate of a species had a unimodal response to a continuous environmental variable, such as salinity and temperature. The location of optimum and breadth of the growth response in the environmental niche axis is treated as a species-dependent trait. Consequently, environmental variations can affect competitive ranking of the species. Decreasing the frequency of environmental variations produced monotonically decreasing or unimodal “intermediate disturbance type” diversity patterns in the competition model. The level of diversity and type of the pattern was determined by immigration rate, and the number and traits of the species initially placed along the environmental niche axis. Immigration generally increased diversity, but the amount of increase in intermediate immigration rates was strongly modified by the frequency spectrum of the environmental variation. Decreasing the frequency of the environmental variation always produced a monotonically decreasing diversity pattern in a two-species system where the growth optima are positioned symmetrically around the mode of the environmental noise. However, increasing the species number and asymmetry of optima makes the diversity pattern sensitive to immigration rate: diversity can be very low and unaffected by the frequency of environmental variation in the absence of immigration, peak in low-frequency environment with small immigration rate (i.e. produce the IDH type pattern), or decrease monotonically with high immigration rate. Diversity is most sensitive to small changes in the initial species number and position of the species growth optima when environmental fluctuations have high frequency. That immigration can change the impact of the frequency of the environmental variation on diversity from monotonically decreasing to unimodal (IDH type) pattern emphasizes that the regional process can be important for explanations of the diversity patterns, especially when the immigration rates are uncorrelated to the local conditions. The strong interactions between community properties, immigration rate, and the spectrum of environmental variation suggest that small changes in community properties or in dispersal rates can have a large impact on species diversity.