The effects of urbanization on trophic interactions in a desert landscape
Trophic systems can be affected through top-down (predators) and bottom-up (resources) impacts. Human activity can alter trophic systems by causing predators to avoid areas (top-down) or by providing increased resources through irrigation and decorative plants that attract herbivores and their predators (bottom-up). Both processes may constrain the recruitment of herbaceous plants. We examined the effect of increasing urbanization within Las Cruces, NM, on a trophic system that included mammalian predators, mammalian prey, and herbaceous plants within natural shrubland vegetation. We tested the hypotheses that urbanization could cause a top-down effect in which predator abundance is reduced in more dense exurban areas, or urbanization could cause a bottom-up effect by providing food and water resources to support increased prey and predators. To test these hypotheses, we compared natural herbaceous plant recruitment, herbivory and granivory rates, rodent abundance, and lagomorph and carnivore activity across an urbanization gradient in Las Cruces, NM. The gradient included four replicates each of three development levels: high density exurban (0.40 ha lots), low density exurban (1.20 ha lots), and wildland (undeveloped). We used seedling and foraging trays to measure herbivory and granivory rates, live-trapping to estimate rodent abundance, and wildlife cameras to measure lagomorph and carnivore activity.
Supporting both top-down and bottom-up hypotheses, we found increased rates of herbivory on seedlings and decreased herbaceous plant recruitment in the most dense exurban areas. Patterns of overall rodent (kangaroo rat and pocket mouse) abundance, seed consumption rates, and jackrabbit activity did not vary with urbanization. However, we found increased cottontail activity in dense exurban areas. We also found increased predator (coyote) activity in dense exurban areas compared to wildlands, supporting the hypothesis that the effect of urbanization on plant recruitment is mediated by a bottom-up effect. Our results indicate that urbanization can have important indirect effects on natural vegetation.