Examining the relative influence of animal movement patterns and mortality models on the distribution of animal transported subsidies

Abstract

Active subsidies are animal-transported consumption (e.g., predators, pathogens, parasites) and resources (e.g., prey, detritus, nutrients) moved between ecosystems. Animal movement behavior has the potential to mediate the extent and intensity of active subsidies and corresponding ecosystem responses. Animal movement behaviors could interact with mortality risk to affect active subsidy distributions and ecosystem impacts but have rarely been examined in existing ecosystem models of spatial subsidies. Movement ecologists typically simulate animal dispersal and foraging movements using stochastic random walk models such as correlated random walk (CRW) and Lévy walk (LW) models. Movement models generally implement mortality as an instantaneous mortality rate (i.e., probability of death per step). Variation in CRW and LW movement patterns, in combination with mortality rate level and model, can likely change emergent subsidy distributions and impact. Using a spatially explicit individual-based model (IBM), we quantify how variation in the straightness (i.e., CRW) and step length (i.e., LW) of animal movement patterns with variation in mortality rate and model (i.e., distance-dependent versus time-dependent function) alter the intensity and extent of consumer and nutrient subsidy distributions. Movement pattern was a dominant determinant of subsidy displacement with more pronounced effects on consumer (i.e., living) subsidy (e.g., predators) patterns. Mortality rate and mortality model (i.e., distance-dependent versus time-dependent on LW) strongly predicted subsidy density with stronger effects on nutrient (i.e., dead) subsidy (e.g., feces) patterns. Consumer subsidies were deposited farther and at lower densities than nutrient subsidies. Given lower mortality rate, movements with more variable step lengths displaced both nutrient and consumer subsidies farther and at lower densities than straighter movements. Movements with more variable step lengths also resulted in greater nutrient subsidy numbers at lower densities with higher mortality rate in distance-dependent compared to time-dependent mortality models. We conclude that ecosystem modeling frameworks that incorporate interaction between animal movement behavior and mortality conditions will enhance insights into active subsidy distributions.