Using dynamic foodscape models to assess bottom-up constraints on population performance of herbivores

Resource heterogeneity governs a multitude of ecological processes, but
the mechanisms by which heterogeneity influences population performance
are not fully resolved. Because optimizing behavior is challenging in
heterogeneous landscapes, individual variation in foraging and movement
strategies is common, and understanding the consequences of that variation
is one of the most pressing challenges in modern ecology. In theory, such
consequences should be modulated at least in part by nutrition, which
directly influences discretionary energy available for growth and
reproduction. We developed a series of linked dynamic models for
predicting (i) spatiotemporal variation in the foodscapes available to
seven distinct populations of elk (Cervus canadensis) in Idaho, USA, and
(ii) variation in pregnancy rates among those populations as a function of
foodscape use and availability. Foodscape models, which predicted
variation in suitable forage biomass (biomass of forage that met or
exceeded requirements of female elk at peak lactation), generally
performed well, with adjusted R2 values ranging from 0.34 to 0.51.
Patterns of foodscape use differed among populations and years, with some
populations showing selection for the foodscape and others exhibiting
indifference or even avoidance of high-quality forage resources. Pregnancy
rates ranged from 66–100%, and our top model relating pregnancy to metrics
of forage availability explained 41% of the variation among 20 elk
population-years. Our top model relating pregnancy to foodscape use by elk
explained 57% of the variation in pregnancy rates among 12
population-years. Pregnancy rates were influenced more strongly by
heterogeneity in foodscape use and availability than by differences in
mean or maximum suitable biomass among populations. Our results suggest
that population performance of elk was modulated both by the availability
of high-quality forage and by factors that constrained use of the
foodscape by elk. The dynamic modeling approach we developed for linking
nutritional resources to herbivore performance is generalizable to many
other species and systems, and can be used by wildlife managers to assess
whether herbivore populations might be limited by bottom-up factors.