COMPARATIVE LIFE HISTORIES OF GEORGIA AND VIRGINIA COTTON RATS

Changes in native small mammal populations with removal of invasive ant

Efforts to remove invasive species may benefit native species, but the effects can be complex and unpredictable. Thus, studies of invasive-species removal provide important information for guiding management and providing insight about variation in post-removal impacts within the community. Using southern pine-grassland ecosystems as a model system, we hypothesized that removal of the long-established red-imported fire ant (Solenopsis invicta, hereafter RIFA) would positively influence altricial Peromyscus species, due to increased survival of young in the nest and thus increased recruitment to the population, but would not impact semi-precocial hispid cotton rats (Sigmodon hispidus), which are mobile more quickly after birth and thus at less risk of depredation by RIFA. We compared small mammal populations on sites treated with a granular insecticide (Extinguish Plus) to remove RIFA in southwestern Georgia, United States, from April 2018 to December 2019. As expected, we detected no difference in cotton rat recruitment. However, contrary to our prediction, the same was true for cotton mice (Peromyscus gossypinus) and oldfield mice (Peromyscus polionotus). We found RIFA removal increased survival both of cotton rats and cotton mice, increasing average population rate of change (λ) on treated sites during the study period. In contrast, we observed lower survival of oldfield mice, with similar λ estimates on treated and untreated sites, but low sample sizes were problematic for this species. Our results show that removal of invasive species can have positive impacts for native species, but both the magnitude of RIFA effects on small mammals and mechanisms by which impacts occur are complex.

Stomach nematodes of cotton rats: parasites, commensals, or mutualists?

We related presence and burden of stomach nematodes to body mass and reproductive allocation in hispid cotton rats (Sigmodon hispidus) from two long-running field studies in Virginia (1983–1984, n = 286; and 1988–1990, n = 425) and one from Georgia 1987–1989 (n = 459). Eighty percent of rats from the earlier Virginia sample were infected, with mean nematode mass of 1,311 mg. In the later samples, 23% (Virginia) and 33% (Georgia) were infected with mean nematode mass of 493 and 769 mg, respectively. Presence of nematodes was positively correlated with host body length for each sex in each sample. We used analysis of covariance to examine length-adjusted residuals for presence of nematodes and mass of nematodes for association with somatic and reproductive response variables. Both body and reproductive masses were either positively associated or not related to nematode presence in the two low-prevalence samples, and either negatively associated or not related to nematode presence in the high-prevalence sample. No relationships were detected between host mass and nematode mass per host in either sex in any sample. There was no effect of nematode presence on litter size of pregnant females, but there was a positive effect of nematode mass on litter size in Georgia. Recent theory provides several possible explanations for such neutral-to-positive effects of stomach nematodes on host fitness, including the evolution of host tolerance to the parasites, fecundity compensation by the hosts, and positive effects on host health via immune modulation.

Effects of prescribed fire, supplemental feeding, and mammalian predator exclusion on hispid cotton rat populations

Predation and food resources can strongly affect small mammal population dynamics directly by altering vital rates or indirectly by influencing behaviors. Fire may also strongly influence population dynamics of species inhabiting fire-adapted habitats because fire can alter food and cover availability. We used capture-mark-recapture and radio-telemetry studies to experimentally examine how supplemental feeding, mammalian predator exclusion, and prescribed fire affected survival, abundance, and reproduction of hispid cotton rats (Sigmodon hispidus) in southwestern Georgia, USA. Prescribed fire reduced survival, abundance, and rates of transitions to reproductive states. Food supplementation increased survival, transitions to reproductive states, and abundance, but was not sufficient to prevent post-fire declines in any of these parameters. Mammalian predator exclusion did not strongly affect any of the considered parameters. Our results show that fire strongly influenced cotton rat populations in our study site, primarily by reducing cover and increasing predation risk from non-mammalian predators.