Background/Question/Methods Interspecific interactions are often invoked to explain the dynamics and make up of small mammal communities. But experimental tests are rare, particularly in the Northeast United States. We experimentally manipulated small mammal communities in thirty forest fragments in the Hudson Valley, New York by either removing or adding white-footed mice (Peromyscus leucopus), Eastern chipmunks (Tamias straitus), or grey squirrels (Sciurus carolinensis). We use mark-recapture models to test the hypotheses that removing potential competitors increases (apparent) survival and activity levels (measured by probability of capture) of individuals of the remaining species, while adding competitors decreases survival and activity.

Results/Conclusions Evidence for strong interactions between these small mammal species is mixed. In many cases there was no detectible effect of either removal or addition of other species, although low capture rates in some forest fragments limit our statistical power. Rapid immigration into our removal sites and emigration or mortality of transplanted animals also muddied the water. But in some places interspecific interactions are clearly important. For instance, mice became more active (likely to be captured) when chipmunks were removed from several fragments. And (apparent) survival of chipmunks was reduced with increasing numbers of squirrels on addition sites. Intriguingly, when the mice were removed from one forest fragment, red-backed voles (Myodes gapperi) proceeded to invade.