Individual differences in habitat selection mediate landscape level predictions of a functional response

Abstract

Predicting future space use by animals requires models that consider both habitat availability and individual differences in habitat selection. The functional response in habitat selection posits animals adjust their habitat selection to availability, but population-level responses to availability may differ from individual responses. Generalized functional response (GFR) models account for functional responses by including fixed effect interactions between habitat availability and selection. Population-level resource selection functions instead account for individual selection responses to availability with random effects. We compared predictive performance of both approaches using a functional response in elk (Cervus canadensis) selection for mixed forest in response to road proximity, and avoidance of roads in response to mixed forest availability. We also investigated how performance changed when individuals responded differently to availability from the rest of the population. Individual variation in road avoidance decreased performance of both models (random effects: β = 0.69, 95% CI 0.47, 0.91; GFR: β = 0.38, 95% CI 0.05, 0.71). Changes in individual road and forest availability affected performance of neither model, suggesting individual responses to availability different from the functional response mediated performance. We also found that overall, both models performed similarly for predicting mixed forest selection (F_{1, 58} = 0.14, p = 0.71) and road avoidance (F_{1, 58} = 0.28, p = 0.60). GFR estimates were slightly better, but its larger number of covariates produced greater variance than the random effects model. Given this bias-variance trade-off, we conclude that neither model performs better for future space use predictions.

Box 1 The link between individual differences and the functional response

The functional response in habitat selection posits that animals adjust their habitat selection as availability changes (Mysterud and Ims 1998). However, agreement between the population-level functional response and individual habitat selection depends on how much individual variation affects the ability of the functional response to predict the selection of individuals when availability changes.

In Fig. 7, each coloured line represents a single individual. Individuals exhibit a behavioural reaction norm for a habitat, the direction and magnitude of which varies along an environmental gradient as an individual’s home range changes between two time periods. The functional response for the habitat is positive, i.e., the slope of the increase in selection for the habitat increases along the environmental gradient. Such a response could occur if the environmental gradient measures risk of predation, and the habitat provides cover from that risk. When all individuals respond similarly to the environmental gradient as their positions vary along the environmental gradient, their individual behavioural reaction norms (BRNs) are correlated with the functional response pattern (panel a). In such cases, the functional response is a good predictor of individual selection.

However, if some individuals exhibit a response to the environmental gradient that differs from the majority of the population, the functional response is less reliable for predicting individual selection (panel b). Deviation from the functional response could occur if individuals exhibit consistent differences in their habitat selection regardless of the environmental gradient. In such cases, the mean selection by all individuals in the population (dashed line) is likely to be a better predictor than the functional response because it minimizes the variation between all individuals and their predicted selection.