We examined the potential for interspecific competition by investigating spatial partitioning of two species native to Sanibel Island; the Sanibel Island rice rat (Oryzomys palustris sanibeli) and insular hispid cotton rat (Sigmodon hispidus insulicola). Cotton rat densities on Sanibel Island are a degree of magnitude greater than rice rat densities. The size of cotton rats may give them a competitive advantage over rice rats. However, the omnivorous habits of rice rats, as opposed to the herbivorous habits of cotton rats, may minimize niche overlap. Additionally, the semiaquatic nature of rice rats may result in minimal interference between these species. We placed two baited camera traps at each of 18 locations in freshwater wetlands dominated by sand cord grass (Spartina bakeri) to determine the activity periods of each species. Cameras were active for four day periods during summer and winter of 2015. We extracted species, date, and time information from these photos for further analysis. We calculated time-to-encounter between the species and compared them to randomly assigned encounter times to determine if they deviated from a random distribution. This allowed us to investigate spatio-temporal avoidance.
Results/Conclusions
We did not find evidence of spatio-temporal avoidance (p = 0.52) during the first year of this study. However, due to lower than expected rice rat detection rates (observed at four of 18 locations in season one, and zero locations in season two) these results correspond to just a few individuals and may not represent the entire Sanibel Island rice rat population. To provide further insight into population-level spatio-temporal partitioning, we are currently processing and analyzing two additional years of data (four seasons). However, the sympatric nature of rice rats and cotton rats throughout their ranges may mean that they have developed other ways to minimize competition, including dietary changes.