The timing of animal activity has important consequences for population dynamics, patterns of community diversity, and disease transmission. Invasive plants often increase the magnitude of native animal activity (e.g., seed removal) by reducing predation risk, but despite links between predation risk and activity timing, the effect of invasive plants on the temporal distribution of consumer activity is not often considered. Moreover, indirect effects of invasive plants on animal behavior likely exhibit strong seasonal dynamics. Leaves of invasive plants are present earlier in the spring and persist later into autumn compared to native deciduous woody plants, providing a dense refuge from predators during a time of year where no such refuge historically existed. However, most studies investigating the indirect effects of invasive shrubs on animal behavior occur in the summer. To address these knowledge gaps, we compared the activity timing of a native small mammal (Peromyscus leucopus) in plots invaded by a non-native shrub (buckthorn, Rhamnus cathartica) and plots with invasive shrubs removed by a) estimating the time at which small mammals entered live-traps and b) comparing timestamps on mouse photographs taken by infrared motion-activated cameras.
Results/Conclusions
Cover was always greatest in plots with buckthorn, but buckthorn provided the most cover in spring and autumn. Correspondingly, P. leucopus were almost three times more likely to be captured in buckthorn (14.31 ± 2.08 captures/100 trap nights, mean ± s.e.) than plots with buckthorn removed (5.31 ± 2.08 captures/100 trap nights) in spring and autumn. Buckthorn invasion also influenced the timing of small-mammal activity: despite no difference in mean trap-entry time, variance in trap-entry time was more than 5 times greater in buckthorn plots. Data collected from camera traps confirmed an increase in the variance of activity time for mice in buckthorn plots in spring and fall, but not summer. These strong seasonal effects of buckthorn suggest that, because most field research is conducted in the summer, the indirect effects of invasive plants on animal behavior may be generally underestimated. By demonstrating that buckthorn changed not only the magnitude of small-mammal activity, but its temporal variation, our results illustrate a previously undiscussed consequence of the novel predation refuge provided by invasive shrubs. These changes in temporal variation could change the likelihood of encounter with conspecifics and predators, potentially altering the strength of disease transmission, competition, and other ecological interactions.