| Omnivory, defined as feeding on more than one trophic level, is ubiquitous in natural ecosystems, and is shown to stabilize food webs. The study of omnivory in terrestrial ecosystems has a traditional focus on arthropods. However, omnivory is also common among vertebrate taxa. Here I studied an omnivorous vertebrate, the White-footed Mouse, Peromyscus leucopus, and investigated how energy and nutrient availability in the form of C:N stoichiometry affect omnivory in P. leucopus, and how trophic shifts in P. leucopus affect their population densities and the abundance and diversity of their prey. Using stable isotopes of carbon and nitrogen, I demonstrated that some degree of trophic mixing is generally maintained at the population level and is beneficial to individual performance in P. leucopus under field conditions. These results suggest that nitrogen limitation helps facilitate omnivory in P. leucopus. Furthermore, depending on the timing of food supplementation and the nutritional quality of the supplemental food, P. leucopus population density may not respond to food supplementation or may decrease after supplementation. Specifically, the results showed that supplementation of low-quality food during a physiologically stressful time had a negative effect on P. leucopus population density. This agrees with the finding that individuals perform better when they are able to include prey in the their diets. At the prey community level, I showed that arthropod abundance and species evenness change with trophic shifts in P. leucopus . When P. leucopus shifted towards feeding at higher trophic levels, arthropods became less abundant with an increase in species evenness. This study suggests that although population density of P. leucopus does not fluctuate with pulsed input of low-quality food, their trophic shifts along may affect prey abundance and community compositions. |
