Spatial, temporal, and density-dependent components of habitat quality for a desert owl

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.

Fig 1. Study area in northwest Mexico showing distribution of territory patches used by ferruginous pygmy-owls and the location of weather stations.

Territory patches were located in two major vegetation communities and weather stations were located near Sasabe, Sonoyta, Cucurpe, Magdelena, and Altar. Regional patch occupancy was estimated in 11 regions: San Miguel, upper Magdalena, Magdalena-Coyotillo, Busani, upper Alter, lower Altar, upper Sasabe, lower Sasabe, upper Plomo, lower Plomo, and Sonoyta. Territory patches were 50 ha in area and are not shown to scale; the study area was approximately 20,000 km².

Fig 2. Interactive effects of abundance of potential nest sites and other habitat factors on reproductive output of ferruginous pygmy-owls in northwest Mexico, 2001–2010.

Estimates of reproductive output are based on the top-ranked models that include each of the habitat factors represented as summarized in Table 3.

Fig 3. Effect of habitat factors on reproductive output of ferruginous pygmy-owls in northwest Mexico, 2001–2010.

Lower right figure shows the number of negative heterospecific interactions observed divided by the total number of territory patches in each group across a gradient of increasing abundance of potential nest sites. Filled circles in upper figures are patches in semi-desert grasslands whereas those in the lower figure are patches where we observed negative heterospecific interactions. Estimates of reproductive output are based on model 3 in Table 3. Inset in upper left figure shows means (± SE) in each vegetation community.

Fig 4. Effects of habitat frgamentation (Frag_hab) and quantity of woodland vegetation cover (Hab_f) on reproductive output (R) of ferruginous pygmy-owls in northwest Mexico, 2001–2010.

The 9 territory patches shown all have high abundance of potential nest sites and were selected to illustrate effects. Black pixels (30-m) had ≥20% woody vegetation cover and were classified as woodland and gray pixels had <20% woody vegetation cover. Estimates of R are based on model 3 in Table 3.

Fig 5. Temporal variation and effects of weather on reproductive output of ferruginous pygmy-owls in northwest Mexico, 2001–2010.

Temperature and precipitation were measured at regional scales and estimates are from model 8 in Table 5.

Fig 6. Effects of conspecifics on reproductive output of ferruginous pygmy-owls in northwest Mexico, 2001–2010.

Conspecific density was measured around each focal patch each year and regional occupancy was measured as the proportion of patches occupied in each of 11 watershed regions in each year. Estimates of the effect of local conspecific density are based on model {Density} and estimates of the effect of regional occupancy are adjusted for the effects of local density from model {Density + Occ_region} in S4 Appendix.

Fig 7. Estimated reproductive output within individual territory patches occupied by ferruginous pygmy-owls in northwest Mexico, 2001–2010.

Patches are sorted in ascending order basis on the estimated habitat effects and only patches with ≥2 observations (n = 92) are shown. Upper figure shows predictions based on estimated habitat effects only (model 3, Table 3), and the middle and lower figures show estimates based on habitat and temporal factors, and habitat and conspecific density, respectively. In lower figures, diamonds are average reproductive output and horozontal lines across bars illustrate the range of estimates among years.

Fig 8. Interactive effects of important factors associated with different environmental components of habitat quality on reproductive output of ferruginous pygmy-owls in northwest Mexico, 2001–2010.

The hypothetical weather gradient in the top figure was standardized based on annual precipitation and mean maximum temperature during the brooding season so as to represent conditions that ranged from wet and cool to hot and dry. Estimates are based on the top-ranked models that included these interactions in Table 6.

Fig 9. Effects of vegetation community and fundamnetal habitat quality on density-dependent declines in reproductive output of ferruginous pygmy-owls in northwest Mexico, 2001–2010.

Habitat quality was classified as high (>3.0), moderate (>2.4–3.0), or low (0.9–2.4) based on patch-specific predictions of reproductive output from model 3 in Table 3. Slope parameters and SE are from least-squares regression. Estimates are based on the model {Habitat × Density + Weather} in Table 6 and S5 Appendix.