Indirect impacts of R. cathartica invasion on small mammal seed predators

Background/Question/Methods

Invasive plant species can drastically affect ecosystem functions, as well as plant and animal communities. Many structural changes in ecosystems can arise from direct impacts of invaders on native species. Although not often investigated, these changes in habitat structure might generate novel indirect effects in ecological communities because the activities of foraging animals are often affected by habitat structure. For example, exotic shrubs plants might alter rates of seed predation by providing a structural refuge for small mammal seed consumers, or by altering the structure litter layer.  If so, these changes in seed predation could alter the composition of native plant communities and possibly facilitate greater invasion. We examined whether common buckthorn (Rhamnus cathartica) alters habitat characteristics, such as a decreased leaf litter layer and increased cover to influence native plant seed survival via choices in seed predation by small mammals. We completed seed removal studies in the UW Arboretum in two different habitats; one forest invaded with R. cathartica and the other was un-invaded (n=20, 10 plots in each habitat). Along with R. cathartica seeds, the native seeds used were: Acer rubrum, Acer saccharum, Prunus serotina, and Quercus rubra. The experiment ran for three weeks. 

Results/Conclusions

We found that there were seed removal differences by habitat between the invaded and un-invaded habitats, as well as seed removal differences by species. Total seed removal was significantly higher in the R. cathartica invaded forest than the non-invaded forest. Leaf litter depth was significantly higher in the non-invaded forest, while canopy cover did not differ between the two habitats. Across both invaded and un-invaded habitats, P. serotina and Q. rubra removal was the highest, while A. saccharum and A. rubrum were the lowest. All species except Q. rubra were removed significantly more in the invaded habitat. Removal of P. serotina and A. rubrum had a positive significant relationship with increasing leaf litter depth. P. serotina and A. saccharum removal had a negative significant relationship with increasing canopy cover. There was no relationship between number of woody stems in the two plot types and removal rates. Removal rates differed between invaded and un-invaded habitat suggesting that R. cathartica produces a change in seed removal rates. These differences could influence the survival of native seeds, and in turn the way that plant communities are established. Understanding these changes can assist with restoration efforts through management and seed additions.