Direct impacts of invasive species are numerous and well documented, but indirect effects of invaders are less understood. Invasive plants are known to alter habitat structure, which can indirectly alter animal activity, however, it is virtually unexplored how such effects vary temporally. Here, we used a long-term experimental invasion to ask if an invasive grass (cogongrass; Imperata cylindrica) altered native small mammal foraging and how this indirect effect varied through time. We placed trays with one-hundred native longleaf pine (Pinus palustris) seeds in invaded and uninvaded reference plots for four consecutive nights every two months from February 2019 to April 2020 (eight total sampling periods). Longleaf pine is critical to biodiversity conservation in the Southeast USA and has been reduced to a small fraction of its historic range. Thus effects of invasion on longleaf pine communities could have multiple ramifications for biodiversity. To quantify changes in vegetation through time, we measured percent vegetation cover and light penetration at ground level for all plots at each sampling period. We hypothesized that cogongrass would provide more cover year-round than reference plots, resulting in more rodent activity and seed consumption in invaded than reference plots, across more sample months.
Results/Conclusions
Our results show cogongrass plots maintained over 75% vegetation cover throughout the year, while reference plot cover dropped as low as 45%, and provided significantly less cover than invaded plots in February, April, and June. Overall, significantly more seeds were consumed in invaded than reference plots (Wilcoxon rank sum test, p = 0.005), with a median seed consumption per sampling night of 88% and 61%, respectively. Mixed effect models revealed that, while multiple variables influenced seed consumption, invasion indirectly drove significant increases in seed consumption during the spring and fall months (February, April and October). Notably, increased granivory in October coincides with longleaf pine seed drop, suggesting that indirect impacts of cogongrass invasion on granivory could substantially reduce seed success in longleaf pine communities. Although more research is needed to understand the long-term impacts of these indirect effects, our findings suggest they could alter plant community composition and have cascading effects on biodiversity and varying ecological processes. More generally, our findings highlight the complexity of invasion impacts and show that indirect impacts can change through time and seasonally alter patterns in wildlife behavior.