Is density-dependent resource harvest a reliable habitat indicator for conservation and management?

Adaptive foraging decisions emerge from environmental conditions that alter fitness. Under most circumstances fitness varies with population density. So it should be possible to use density-dependent foraging not only as a surrogate for fitness, but also as an assay for habitat quality. We tested this prediction by measuring the quitting-harvest rates of red-backed voles foraging in artificial resource patches located in three different habitats where we manipulated population density. Quitting-harvest rates were less in safe foraging trays located under cover than in risky trays in the open. Voles avoided risky trays in old field and alder field habitats, and foraged all trays in a dense alder forest habitat to similar low harvest rates. Quitting-harvest rates declined with density in experiments conducted in 2005 but not in similar experiments during 2006. We interpret these complex results as evidence that rapid food renewal during the 2006 experiments increased the voles' foraging costs, and their quitting-harvest rates, in artificial food patches. Continued foraging in the trays would compromise the voles' abilities to forage elsewhere in the habitat. This conclusion is bolstered by significant differences in rainfall between years that precipitated vegetation recovery in 2006 shortly before we initiated our experiments. Our results demonstrate that we may not be able to use behavior alone to estimate population density in variable environments. So we outline more effective research designs that should allow other scientists to not only detect density-dependent foraging, but also spatial and temporal variation in habitat quality.