Effects of animal movement strategies and costs on the distribution of active subsidies across simple landscapes

Abstract

Cross-ecosystem transfer of resources (spatial subsidies) can greatly impact recipient ecosystems. Many subsidies are actively moved by animals, which regularly transfer nutrients within and among ecosystems. Researchers have yet to integrate knowledge of animal movement and spatial subsidies to enhance predictions of subsidy spatial distribution and ecosystem effects. To examine this, we implemented a spatially explicit simulation model of animals that switch habitats due to behavioral or ontogenetic shifts. We explored how movement strategy (correlated random walk) and patterns of mortality risk affected the spatial distribution of living (consumer subsidies) and dead individuals (nutrient/energy subsidies). We ran models varying the correlation in a correlated random walk and explored four patterns of mortality risk: uniform mortality, higher mortality in the edge at the habitat boundary, a decreasing gradient and an increasing gradient of mortality risk as individuals move away from the boundary. For each scenario, we calculated the maximum extent, the distance of peak density and the peak density (a measure of maximum impact of the subsidy) of living and dead individuals. As expected, subsidy impact declined as deposition distance increased. Straighter movements resulted in deposition farther beyond the local habitat boundary with lower impact than more sinuous movements. Similarly, consumer subsidies were deposited farther from the boundary with lower impact than nutrient/energy subsidies. Patterns of mortality risk also affected the impact and deposition distance but to a lesser degree. Uniform mortality and increasing gradients of mortality risk deposited subsidies farther from the habitat boundary than did edge mortality and decreasing gradients. Edge mortality scenarios also resulted in higher densities of subsidies than other patterns of mortality risk. Our simulations represent a very simple, first attempt at using movement ecology to predict the spatial distribution and impacts of subsidies. More complex models and empirical tests are necessary to further assess movement ecology's utility for predicting subsidies.