Using derivative thresholds to resolve conflicting habitat management strategies

Abstract

Habitat management efforts for multiple species with contrasting habitat requirements can be challenging, particularly on small landscapes where large-scale heterogeneity may not be a viable solution. We sought to utilize derivative functions from species-response curves to identify thresholds along an environmental gradient for two species with differing responses to juniper cover. We estimated percentages of one-seed juniper (Juniperus monosperma) cover around loggerhead shrike (Lanius ludovicianus) and gray vireo (Vireo vicinior) nests in central New Mexico in 2016–2020 to better understand tolerance of juniper expansion for these two species of conservation concern. Species-response curves were derived from generalized additive mixed-effect models, with year as a random effect and a local regression smoother. Loggerhead shrike nests were more likely to occur in areas without junipers at the 170-m radius spatial scale. Nest-site selection declined as percent juniper cover increased. Gray vireo nests were most likely to be found in areas with approximately 11 % juniper cover at the 100-m radius spatial scale. Derivative functions were used to find juniper percentages where the rate of change in species' response was most negative for loggerhead shrikes and most positive for gray vireos. The rate of change in nest-site selection decreased the fastest at 13 % and increased the fastest at 4 % juniper cover for loggerhead shrike and gray vireo, respectively. These results suggest that maintaining juniper percentages between 4 and 13 % at relatively broad spatial scales would likely increase available nesting habitat both species. We demonstrate how derivative functions can be used to estimate the rate of change of species' response to environmental gradients and an application of such thresholds to resolve conflicting habitat management for two species of conservation concern.