COS 78-5 - Reintroduced megaherbivores indirectly shape small-mammal activity patterns

Background/Question/Methods

Megaherbivores are increasingly recognized as an important component of restoration ecology, given their large role in shaping habitat structure, plant community composition, and ecosystem function. Given these important effects, it is plausible that megaherbivore reintroduction may have unintended indirect consequences that ripple across the food web. For example, grazing by bison (Bison bison) in restored tallgrass prairies thins the plant canopy. This change in habitat structure increases the moonlight reaching ground level, which is an important cue of predation risk that shapes activity patterns of many nocturnal species, such as small mammals. Using 8 years of live-trapping data, we asked whether reintroduced megaherbivores indirectly altered moonlight avoidance by small mammals in tallgrass prairies.

Results/Conclusions

Across the lunar cycle, Peromyscus maniculatus activity decreased 25% (new moon: 4.66 ± 1.01 captures per 100 trap nights; full moon: 3.57 ± 0.86 captures per 100 trap nights), whereas Microtus ochrogaster activity doubled (new moon: 1.06 ± 0.47 captures per 100 trap nights; full moon: 2.54 ± 1.21 captures per 100 trap nights). Critically, these strong responses to moonlight were only observed in plots with bison, where plants intercepted 20% less light. Additionally, we equipped a subset of traps with temperature sensors to estimate trap-entry time. While M. ochrogaster were more active on bright nights, most activity occurred before moonrise or after moonset, avoiding periods of bright moonlight. We conclude that megaherbivores play an unappreciated but important indirect role in tallgrass prairies by inducing behavioral shifts in other animal species. Because overlap in activity patterns can predict the likelihood of predator-prey encounters, such activity shifts have important implications for trophic interactions throughout restored prairie food webs. Additional work to understand interspecific and intraspecific variation in responses to moonlight may improve efforts to forecast changes in community assembly due to restoration and land-use change.