The perils of plasticity: dual resource pulses increase facilitation but destabilize populations of small-bodied cavity-nesters

Resource pulses within structured communities can lead to changes in the ecological roles of community members, particularly for species that exhibit plasticity in resource use. The red-breasted nuthatch Sitta canadensis is a facultative excavating cavity-nester that forages on seeds and insects, thus exhibits plasticity in both nesting habits and diet. In a long-term study of cavity-nesting vertebrates, we used point counts, and nest and vegetation surveys to examine the effects of two resource pulses of mountain pine bark beetle prey Dendroctonus ponderosae and tree cavities via excavator populations, on population densities and cavity reuse of red-breasted nuthatches, from 1997-2006. We observed a doubling in mean nut-hatch densities from 0.12 to 0.24 individuals ha-1 then a collapse later in the decade to 50% below endemic levels (0.06 individuals ha-1). These regional fluctuations were positively correlated with densities of trees recently infected by bark beetles. Because nuthatches range over large areas in winter, this regional correlation suggests that populations responded to the increase in winter food supply. At the site scale, general linear mixed effects models showed that nuthatch populations increased following years of high densities of downy woodpeckers Picoides pubescens, suggesting that downy woodpeckers were important facilitators, via cavity excavation. Type of nesting cavity used by nuthatches varied during the pulse, such that the use of existing cavities (in lieu of excavation) increased on sites that had harboured downy woodpecker nests in the previous year. We conclude that increased densities of cavity excavators allowed facultative excavators to increase their reuse of cavities, which may have contributed to the dramatic increases in nuthatch populations. However, nuthatch populations collapsed after the boom suggesting that this dual resource pulse may have destabilized populations by enabling densities to reach unsustainable levels. Thus, plasticity in resource use can have serious costs as well as benefits.