VARIABLE RESOURCE AVAILABILITY WHEN RESOURCE REPLENISHMENT IS CONSTANT: THE COUPLING OF PREDATORS AND PREY

Abstract.—Although food resources are thought to limit many populations, the extent to which the population dynamics of predators and prey are coupled is rarely known. We examined a sedentary population of Red Crossbills (Loxia curvirostra L. complex) that relies on seeds in cones that accumulate in the canopy of Rocky Mountain Lodgepole Pine (Pinus contorta latifolia Engelm.). Nearly constant annual seed production and gradual weathering over many years of initially impenetrable cones in the tree canopy results in a continuous and perhaps roughly constant replenishment of accessible seeds. However, seed availability varies seasonally. We estimated the seasonal variation in the energy demands of the study population. Our results demonstrate that seed predation by these sedentary Red Crossbills potentially drives the seasonal variation in seed availability and likely causes the Red Crossbill population to be regulated. The results are also consistent with a nearly constant replenishment of accessible seeds. In its apparent population stability this sedentary crossbill differs greatly from many other crossbills, which often vary in abundance by several orders of magnitude fro