Abstract
Spatial variation in resources is a fundamental driver of habitat quality but the realized value of resources at any point in space may depend on the effects of conspecifics and stochastic factors, such as weather, which vary through time. We evaluated the relative and combined effects of habitat resources, weather, and conspecifics on habitat quality for ferruginous pygmy-owls (Glaucidium brasilianum) in the Sonoran Desert of northwest Mexico by monitoring reproductive output and conspecific abundance over 10 years in and around 107 territory patches. Variation in reproductive output was much greater across space than time, and although habitat resources explained a much greater proportion of that variation (0.70) than weather (0.17) or conspecifics (0.13), evidence for interactions among each of these components of the environment was strong. Relative to habitat that was persistently low in quality, high-quality habitat buffered the negative effects of conspecifics and amplified the benefits of favorable weather, but did not buffer the disadvantages of harsh weather. Moreover, the positive effects of favorable weather at low conspecific densities were offset by intraspecific competition at high densities. Although realized habitat quality declined with increasing conspecific density suggesting interference mechanisms associated with an Ideal Free Distribution, broad spatial heterogeneity in habitat quality persisted. Factors linked to food resources had positive effects on reproductive output but only where nest cavities were sufficiently abundant to mitigate the negative effects of heterospecific enemies. Annual precipitation and brooding-season temperature had strong multiplicative effects on reproductive output, which declined at increasing rates as drought and temperature increased, reflecting conditions predicted to become more frequent with climate change. Because the collective environment influences habitat quality in complex ways, integrated approaches that consider habitat resources, stochastic factors, and conspecifics are necessary to accurately assess habitat quality.
Highlights
Understanding how the environment affects the fitness realized by individuals is a fundamental aspect of ecology
We assessed habitat quality for ferruginous pygmy-owls at the scale of individual territory patches by estimating magnitudes of spatial and temporal variation in reproductive output (R), and assessing the relative contribution and specific effects of factors associated with three general components of the environment
Factors associated with each component had important effects, habitat resources were much more influential than conspecifics or temporal factors such as weather
Summary
Understanding how the environment affects the fitness realized by individuals is a fundamental aspect of ecology. Environmental factors that vary in space and time influence habitat quality by affecting the fitness realized by occupants in a given habitat. Fitness is often defined as an individual’s contribution to population growth [1,2]. Habitat quality or habitat fitness potential (sensu [3]) can be defined as the contribution of individuals in a specific habitat to population growth over periods that exceed the generation time of the focal species [4,5,6]. Habitat quality should affect settlement choices by individuals because those choices have important demographic consequences and are under natural selection [7]. Understanding factors that influence habitat quality can elucidate important selective pressures and guide management
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