The invasion of islands by non-native rodents is a leading cause of extinction among native species worldwide. Rice rat (Oryzomys palustris sanibeli) populations on Sanibel Island are in decline. Their conservation status is likely due to anthropogenic changes including fire suppression and altered hydrology. However, the introduction of exotic black rats (Rattus rattus) may also negatively affect rice rats. Additionally, hispid cotton rats (Sigmodon hispidus insulicola), a native sympatric species that is more than double the size of rice rats, may also alter rice rats distributions. To understand how these species may be interacting it is important to consider their ecological differences. Cotton rats are almost exclusively found in Sanibel’s grasslands and occasionally in buttonwood forests, while black rats are more frequently encountered in mangrove and buttonwood forests. Rice rats are found in all three plant communities. Because their ranges overlap, and because both native species are candidates for listing under the Endangered Species Act, it is important to determine if these species are competing. To monitor distributions of each species, Sherman live-traps were set at 25 points on 36 grids. Grids were run for four consecutive nights in the summer and winter for 3 years starting summer 2015.
Results/Conclusions
We used an occupancy modeling framework to determine if the presence of black rats or cotton rats influenced the occupancy probability of rice rats. Our analysis revealed that rice rats were not affected by cotton rat (phi confidence interval: -2.69 – 0.49) or black rat (phi confidence interval: -1.081 – 1.624) occupancy, as both confidence intervals contained zero. While these results indicate that cotton rats and black rats may not influence the spatial distribution of rice rats on Sanibel Island, these results do not address the potential for competition to influence spatio-temporal partitioning of limited resources. Future analyses will address this topic.