Effects of exurban development on trophic interactions in a desert landscape

Mechanisms of ecosystem change in urbanizing landscapes are poorly understood, especially in exurban areas featuring residential or commercial development set in a matrix of modified and natural vegetation. We asked how development altered trophic interactions and ecosystem processes in the matrix. We examined the effect of varying degrees of exurban development (housing density) on a trophic system that included an apex mammalian predator (coyote, Canis latrans), mammalian herbivores (lagomorphs and rodents), and herbaceous plants. We tested the hypothesis that plant recruitment would be negatively affected by exurban development due either to increases in herbivores associated with increased resource availability (a bottom-up effect) or to a reduction in predators that avoid humans (a top-down effect). In Las Cruces, New Mexico, USA, four replicate sites were located in each of three urbanization levels: high density exurban, low density exurban, and wildland dominated by Chihuahuan Desert vegetation. Seedling trays measured herbivory rates, live trapping estimated abundance of pocket mice and kangaroo rats, and remotely-triggered wildlife cameras estimated the activity of lagomorphs and coyotes. Increased herbivory on seedlings and decreased herbaceous plant recruitment were observed in high density exurban areas. Overall rodent abundance, seed consumption rates, and activity of the lagomorph Lepus californicus did not vary with urbanization level. Activity by another lagomorph, Sylvilagus audubonii, and coyotes was highest in dense exurban areas, consistent with a bottom-up effect. Exurban development can have important indirect effects on trophic interactions occurring in adjacent, untransformed ecosystems. Similar to earlier studies, such effects in the Chihuahuan Desert may be mediated by bottom-up processes associated with anthropogenic inputs.