Data from: Density-dependent, central-place foraging in a grazing herbivore: competition and trade-offs in time allocation near water

Optimal foraging theory addresses one of the core challenges of ecology:
predicting the distribution and abundance of species. Tests of hypotheses
of optimal foraging, however, often focus on a single conceptual model
rather than drawing upon the collective body of theory, precluding
generalization. Here we demonstrate links between two established
theoretical frameworks predicting animal movements and resource use:
central-place foraging and density-dependent habitat selection. Our goal
is to better understand how the nature of critical, centrally placed
resources like water (or minerals, breathing holes, breeding sites, etc.)
might govern selection for food (energy) resources obtained elsewhere – a
common situation for animals living in natural conditions. We empirically
test our predictions using movement data from a large herbivore
distributed along a gradient of water availability (feral horses, Sable
Island, Canada, 2008–2013). Horses occupying western Sable Island obtain
freshwater at ponds while in the east horses must drink at self-excavated
wells (holes). We studied the implications of differential access to water
(time needed for a horse to obtain water) on selection for vegetation
associations. Consistent with predictions of density-dependent habitat
selection, horses were reduced to using poorer-quality habitat (heathland)
more than expected close to water (where densities were relatively high),
but were free to select for higher-quality grasslands farther from water.
Importantly, central-place foraging was clearly influenced by the type of
water-source used (ponds vs. holes, the latter with greater time
constraints on access). Horses with more freedom to travel (those using
ponds) selected for grasslands at greater distances and continued to
select grasslands at higher densities, whereas horses using water holes
showed very strong density-dependence in how habitat could be selected.
Knowledge of more than one theoretical framework may be required to
explain observed variation in foraging behavior of animals where multiple
constraints simultaneously influence resource selection.