Abstract
Density‐dependent competition for food reduces vital rates, with juvenile survival often the first to decline. A clear prediction of food‐based, density‐dependent competition for large herbivores is decreasing juvenile survival with increasing density. However, competition for enemy‐free space could also be a significant mechanism for density dependence in territorial species. How juvenile survival is predicted to change across density depends critically on the nature of predator–prey dynamics and spatial overlap among predator and prey, especially in multiple‐predator systems. Here, we used a management experiment that reduced densities of a generalist predator, coyotes, and specialist predator, mountain lions, over a 5‐year period to test for spatial density dependence mediated by predation on juvenile mule deer in Idaho, USA. We tested the spatial density‐dependence hypothesis by tracking the fate of 251 juvenile mule deer, estimating cause‐specific mortality, and testing responses to changes in deer density and predator abundance. Overall juvenile mortality did not increase with deer density, but generalist coyote‐caused mortality did, but not when coyote density was reduced experimentally. Mountain lion‐caused mortality did not change with deer density in the reference area in contradiction of the food‐based competition hypothesis, but declined in the treatment area, opposite to the pattern of coyotes. These observations clearly reject the food‐based density‐dependence hypothesis for juvenile mule deer. Instead, our results provide support for the spatial density‐dependence hypothesis that competition for enemy‐free space increases predation by generalist predators on juvenile large herbivores.
Highlights
Competition for resources is considered the primary mechanism driving density dependence (Bonenfant et al, 2009; Sinclair, 1989), a process especially important to the population dynamics of large mammals (Fowler, 1987)
As predicted by predator–prey modeling (Messier, 1995; Pech et al, 1995; Sinclair et al, 1998) and two-predator–one-prey models (Northfield et al 2017), we predicted that when mountain lions were reduced, we should observe a density-dependent decline in mountain lion predation, but not in the reference area because deer territoriality limits the increase of overall deer density within high-elevation habitats following source–sink dynamics predicted by ideal despotic distribution (Figure 2, Table 1; Mosser et al, 2009; Northfield et al 2017; Stoner et al, 2018)
Safe space in high quality, higher elevation territories diminished for maternal mule deer, increasing exposure of their juveniles to predation risk by coyotes as mule deer density increased in lower elevation, higher coyote risk areas (Figure 2)
Summary
Competition for resources is considered the primary mechanism driving density dependence (Bonenfant et al, 2009; Sinclair, 1989), a process especially important to the population dynamics of large mammals (Fowler, 1987). As predicted by predator–prey modeling (Messier, 1995; Pech et al, 1995; Sinclair et al, 1998) and two-predator–one-prey models (Northfield et al 2017), we predicted that when mountain lions were reduced, we should observe a density-dependent decline in mountain lion predation, but not in the reference area because deer territoriality limits the increase of overall deer density within high-elevation habitats following source–sink dynamics predicted by ideal despotic distribution (Figure 2, Table 1; Mosser et al, 2009; Northfield et al 2017; Stoner et al, 2018)
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