Cottontails vs. vegetation: A more unpredictable interaction than we thought

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Georgina Fortis-Fernández, Yury Glebskiy and Zenón Cano-Santana, Departamento de Ecología y Recursos Naturales, Facultad de Ciencias, Universidad Nacional Autónoma de México, Mexico City, Mexico

Background/Question/Methods
Mammal herbivores, including rabbits, influence the structure of plant communities over time. Their effects may include changes in composition, richness, and diversity. Therefore, the plant community established as a result of the eruption of the Xitle volcano, approximately 1700 years ago, in today's declared natural reserve (Reserva Ecológica del Pedregal de San Ángel, Mexico City), may be likely to change due to the herbivory of the cottontail rabbit (Sylvilagus floridanus), which is the main consumer of biomass. Thus, the questions we are addressing are: 1) Does rabbit activity change the structure of plant communities? 2) Are these changes becoming more pronounced in time? 3) Will the composition of the plant community change in the absence of rabbits? To answer these questions, since 2017 we have studied ten paired plots, five of which allow rabbit activity (controls), and the other five do not allow it (exclusions). To assess the structure of the plant community, we used cover and species richness data, we also obtained biomass data. We calculated Chao 2 richness index, the Shannon-Wiener index and the diversity α, β, and γ.
Results/Conclusions
Our results show that exclusion of the cottontail did not affect the diversity estimators, the total coverage, and biomass of plants, however, the absence of the rabbit increased the cover of Poaceae plants, while the presence of the cottontail reduced the Poaceae biomass. Species richness was affected by the exclusion either positively or negatively depending on the particular plot and the year and, although a clear pattern for these changes has not been found, the differences between the plots become more evident over time. Our results show that the dominant Poaceae species are the most susceptible to the herbivory of S. floridanus, and the effects generated on their cover and biomass may produce changes in the overall community. These results indicate a positive effect of the cottontail on the richness as a controller of the dominant plant species. However, the cottontail is a selective consumer of some rare plants, causing negative effects on richness. In conclusion, we show that a single herbivorous mammal species, in the same ecosystem and in similar densities, can have contradictory effects on plant richness and, these effects depend on the microsite and the year of observation.