The contributions of resource availability and social forces to foraging distributions: a spatial lag modelling approach

The spatial distribution of foraging animals at a given time simultaneously depends on (1) exogenous environmental variables such as resource availability and abiotic habitat characteristics, and (2) the endogenous variable social aggregation made up of the opposing mechanisms of conspecific attraction and repulsion. We developed an exogenous environment–social aggregation model to analyse the spatial distributions of six abundant shorebird species in the Dutch Wadden Sea at resolutions of 150×150, 200×200 and 250×250m. We used these resolutions to check the robustness of the estimates to the modifiable areal unit problem. We estimated the model parameters by spatial autoregression. This approach enables, among others, estimation of the direct and indirect effects of an exogenous environmental variable on animal density. The former is given by the regression coefficient and the latter, which is due to the amplification of the direct effect by social aggregation, by the spatial multiplier. At all resolution levels and for all species, the explanatory power of social aggregation, measured by Nagelkerke R2, was larger than the combined contribution of the exogenous environmental variables food availability, silt content and mudflat elevation. Social aggregation was stronger for dunlin, Calidris alpina, red knot, Calidris canutus, and curlew, Numenius arquata, than for oystercatcher, Haematopus ostralegus, grey plover, Pluvialis squatarola, and bar-tailed godwit, Limosa lapponica. The total impacts (that is, direct effect plus all indirect impacts) of the exogenous environmental predictors tended to exceed substantially the direct effects (which tend to be the only ones examined in studies on foraging distributions). ► Lack of methods hampers research on the spatial distribution of social foragers. ► Spatial autoregression enables estimation of social and environmental factors. ► In shorebirds, the explanatory power of attraction outweighs environmental factors. ► Total environmental impacts, i.e. impacts amplified by social forces, are obtained.