PS 4-34
The effects of urbanization on state change mediated by predator-prey interactions
Historical state changes from grass-dominated to shrub-dominated vegetation have been common in desert grassland areas of the southwestern United States and northern Mexico, triggered by historical episodes of heavy livestock grazing. Several processes have been proposed to explain persistent shrub dominance, including small mammal herbivory. Small mammals may limit recovery of herbaceous plant species within shrublands unless predation risk limits small mammal populations or activity. Recent urbanization within shrub-dominated areas may reduce the activity of native predators specializing on small mammals, reducing predation risk, and resulting in relative increases in herbivory and granivory rates. To test this hypothesis, we compared herbivory rates and the abundance of small mammals across an urbanization gradient within Chihuahuan Desert areas of Las Cruces, NM, USA. All sites occurred within the same soils and shrub-dominated vegetation type. The gradient included four replicates each of three development classes: relatively high density suburban development (2.5 homes ha-1), low density exurban development (0.25 homes ha-1), and wildland (undeveloped). We used live-trapping to estimate rodent abundance and seedling-trays to measure herbivory rates.
Results/Conclusions
Herbivory rates were higher in the high density suburban developments than the other classes but did not differ between low density exurban and wildland areas. Small mammal live-trapping, however, did not indicate differences in rodent (kangaroo rat and pocket mouse) abundance among different levels of urbanization. This result suggests that changes in rodent abundance is not responsible for increased herbivory rates observed in high density suburban developments. We are currently using camera traps to determine if the activity of mesocarnivores (coyote, fox, and bobcat) decreases and activity of lagomorphs (cottontails and jackrabbits) increases with urbanization as predicted by our model. Our results indicate that the interaction of urbanization and herbivore activity may influence the recovery of herbaceous plant species within shrub-dominated states of the Chihuahuan Desert.