Urbanization alters terrestrial herbivore composition but not abundance

Background/Question/Methods

Desert ecosystems are one of the fastest urbanizing areas on the planet. This rapid shift has the potential to alter the abundances and species richness of herbivore and plant communities. Herbivores, for example, are expected to be more abundant in cities due to the concentration of food resources and reduction in carnivore populations. Despite this assumption, previous research conducted in urban Phoenix has shown that top-down herbivory led to equally reduced biomass. Since there are no published data reporting the abundance and density of herbivores within and outside Phoenix, it is unclear if this insignificant difference in herbivory at rural and urban sites is due to unaltered herbivore populations, or altered activity levels that counteract abundance differences. Vertebrate herbivore populations were surveyed at four sites inside and four sites outside of the city core during fall 2014 and spring 2015 in order to determine whether abundances and diversity differ significantly between urban and rural sites. In order to census species composition and abundance at these sites, 100 Sherman traps and 16 larger wire traps that are designed to attract and capture small vertebrates such as mice, rats, and squirrels, were set at each site for two consecutive trap nights. 

Results/Conclusions

Results suggest that the commonly assumed effect of urbanization on herbivore abundance does not apply to small rodent herbivore populations in a desert city. Surprisingly, the data obtained during fall trapping sessions indicated that small rodent abundances were statistically similar inside and outside of the city. A significant difference between small rodent genus diversity, however, was observed, and highlights the fact that certain genera of small rodents dominate the urban sites. This significant difference was not found at the species level. Although abundances and species diversity were similar at the urban and rural locations, preliminary data also suggest that the sites support different small vertebrate herbivore assemblages, and species that varied greatly in size. Certain species were identified at all of the sites, while others were only found in rural or urban parks, suggesting that species have different levels of urban tolerance.