Woody plant encroachment is a widespread process of land cover change from grassland to shrubland which has the potential to impact faunal communities around the globe. This process may be driven by multiple factors including overgrazing, fire management, warmer temperatures, changes in nutrient deposition rates, and CO2 direct effects. Though recognized as a threat to human populations through its detrimental effects on rangeland quality and carrying capacity, little is known about the impact that woody plant encroachment may have on native mammals, especially predators. This study assesses the bottom-up effects of woody plant encroachment on an abundant, top predator in an area where shrubs have spread into a grassland area over the past century. The specific objective is to determine whether woody plant encroachment has led to a shift in coyote (Canis latrans) feeding ecology. Noninvasive sampling (scat surveys) of the coyote population and vegetation surveys were carried out in grassland and shrubland sites at the Sevilleta National Wildlife Refuge (NWR) in New Mexico. Genetic and stable isotope analyses were used to assess individual identity of the coyotes and determine the percentage of coyote diet that came directly or indirectly from C3 (shrub) vs. C4 (grass) plants.
A total of 297 scat samples were collected from sites in grassland and shrubland habitats at the Sevilleta NWR. Roughly two thirds of the samples were identified as coyote based on a mitochondrial DNA species test. Microsatellite analysis of these coyote samples indicated that there were at least 45 individuals at the study site. Vegetation surveys confirmed that percent woody vegetation cover was significantly higher at shrubland than grassland sites. Coyotes are generalist feeders and coyote scat samples have carbon isotopic signatures (δ13C values) similar to C3 shrubs (-27 0/00) for individuals using shrublands and C4 grasses (-140/00) for individuals using grasslands. This indicates that the recent change from grassland to shrubland habitat has had an impact on coyote feeding ecology and that the base of the coyote food chain has shifted from grasses to shrubs. Such dietary flexibility may allow generalist species like the coyote to adjust to the many changes, including shifts in prey density and diversity and modification of habitat structure, associated with woody plant encroachment.