Population fecundity is tied to variation in acorn crop size (Koenig et al. This species lives in variable-sized social groups and specializes on acorns ( Quercus spp.) as a critical, yet highly variable, winter food source (Koenig et al. The cooperatively breeding acorn woodpecker ( Melanerpes formicivorus) has exhibited an increase in both geographical distribution and local population size throughout the Pacific Coast of continental North America and has a positive intraspecific abundance-occupancy relationship (Fig. The relationship between occupancy and abundance can further be complicated in cooperatively breeding taxa, as their local abundance may be determined by changes in mean social group size or the number of social groups. Unfortunately, a historical mismatch in the temporal and spatial scales at which population dynamics and landscape heterogeneity are quantified has often confounded the understanding of population-level processes (Wiens 1989 Lima and Zollner 1996 Wiegand et al. Conversely, when resource abundance varies temporally, the abundance and occupancy of species reliant on those resources is highest during or after the resource peak (Levey 1988, Beehler and Mulia 1995, Schnurr et al. When resources are consistently available, yet temporally variable, populations of species with high resource tracking remain at high overall abundance with variable occupancy across sites, while species with poor resource tracking have lower overall abundance and more consistent site occupancy (Rey 1995 Afonso and Eterovick 2007). In addition to spatial variation, temporal variation in resource availability may also determine abundance and occupancy (Hanski 1993, Gaston et al. Both local and landscape processes may therefore affect occupancy and demography, potentially in competing ways (Pearson 1993 Koper and Schmiegelow 2006). 2008) and thus decreasing local abundance due to emigration or decreased fecundity (Borregaard and Rahbek 2010). Similarly, landscape-level changes in habitat associated with fragmentation or succession can affect habitat quality, resulting in some patches becoming unsuitable (Vander Haegen et al. Pulses of resources can result in large increases in abundance when organisms disperse to resource-rich locations or increase their fecundity (Edworthy et al. Species occupancy and abundance are often determined by resource availability both locally and regionally (García and Ortiz-Pulido 2004 Prange et al. 1997 Borregaard and Rahbek 2010 Faulks et al. Various mechanisms have been proposed to explain intraspecific abundance-occupancy relationships, but none has received unequivocal support (Gaston et al. Moreover, the findings support the role of resources in driving changes in abundance and occupancy at a landscape scale, suggesting that colonization of marginal habitat drives the positive occupancy-abundance relationship in this cooperatively breeding species.Ī positive relationship between the abundance and site occupancy of a species is a fundamental pattern in ecology, but the population dynamics underlying this relationship are poorly understood (Andrewartha and Birch 1954 Gaston et al. Our results indicate that scale-dependent processes can result in paradoxical relationships in systems with spatial and temporal resource heterogeneity. Group size increased with neighborhood acorn crop two years earlier but groups were smaller in territories with more canopy cover. The increase in abundance was largely determined by the formation of new social groups, driven by a landscape-level expansion of canopy cover and its interaction with neighborhood-level acorn abundance, indicative of long-term increases in overall acorn productivity. Using model selection, we contrasted competing hypotheses on the effects of changing resource availability on occupancy and social group size across three spatial scales: territory, neighborhood, and landscape. We investigated the role of annual variation in food availability and long-term changes in habitat composition as predictors of population dynamics. In this 34-year study of acorn woodpeckers ( Melanerpes formicivorus), a cooperatively breeding species, we observed a paradoxical response to changes in habitat composition: despite a reduction in the availability of high-quality breeding habitat, the population increased considerably. Resource dynamics and habitat changes have been suggested as potential drivers, but long-term studies relating them to abundance and occupancy are rare. Spatial occupancy and local abundance of species often positively covary, but the mechanisms driving this widespread relationship are poorly understood.
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