Habitat Heterogeneity and Algal-Grazer Interactions in Streams: Explorations with a Spatially Explicit Model

Abstract

We present results from a spatially explicit model that makes predictions about how physical habitat heterogeneity mediates algal-grazer interactions. We simulated the responses of mean algal biomass and variability in the distribution of algae to 2 levels of physical habitat heterogeneity (1 vs. 15 boulders) under 2 velocity regimes (slow vs. fast riffles). Algal growth occurred locally in a spatially explicit grid where local current was specified for each of the 60 × 30 grid cells. Grazer movement and foraging were simulated using an individual-based model, in which the direction and rate of movement of individual grazers were determined at each time step in relation to local current and algal standing crop. Model parameterization was based on field observations, laboratory experiments, and literature review. In each of the 4 simulations, algae were first allowed to grow in the absence of grazing to near carrying capacity. Neither habitat heterogeneity nor riffle velocity regime had a significant effect on the mean biomass or spatial variability of algae. Next, algae were exposed to 3 simulated grazer densities and the subsequent effects on algal biomass and patchiness were determined after prolonged contact. We performed replicate simulation runs to allow statistical inferences to be drawn about differential responses of algae to the various "treatments." As expected, mean algal biomass declined in proportion to grazer density. Higher habitat heterogeneity resulted in reduced algal biomass for low and intermediate grazer densities, but only in the slow riffle. Otherwise, the effects of specified grazer densities did not vary between slow and fast riffles. Variability in algal biomass distribution was measured in 2 ways. First, a spatially explicit index of relative patchiness showed a gradual increase under low grazer density but a transient peak followed by decline under moderate and intense grazing pressure, a pattern observed for both slow and fast current regimes. In the slow riffle alone, heterogeneity and grazer density interacted to influence algal patchiness. A second, non-spatially explicit index of variability in algal biomass, the coefficient of variation, increased as grazer density increased regardless of current regime. Higher variability in the distribution of algal biomass tended to be associated with greater habitat heterogeneity, especially in the slow riffle. The simulations suggested that spatial heterogeneity and ambient current velocity regime mediate algal-grazer interactions. The extent to which grazers crop biomass and create spatial variability in the distribution of algal biomass may depend not only on grazer density but also on the interaction between grazer density, physical habitat heterogeneity, and stream velocity. Given this complexity, empirical approaches to understanding algal-grazer interactions in spatially and temporally heterogeneous stream systems are likely to be greatly augmented by modelling, which allows different environmental conditions and grazer foraging behaviors to be simulated.