Within-lodge interactions between two ecosystem engineers, beavers (Castor canadensis) and muskrats (Ondatra zibethicus)

Ecosystem engineers are frequently observed to increase local biodiversity through their effects on resource flows. While promotion of successional processes and increased biodiversity may occur without direct interaction between ecosystem engineers and sympatric species, many cases exist where interactions are common. Under such conditions, it is unclear whether direct interactions serve to facilitate or inhibit coexistence of ecosystem engineers and the species attempting to use engineered habitats. We used remote videography within lodges of an ecosystem engineer, beavers (Castor canadensis), to quantify the taxonomic diversity of lodge use by non-beaver fauna and to characterize interactions between beavers and a second engineering species that commonly uses beaver-manipulated habitats, muskrats (Ondatra zibethicus). Beaver lodges were used by eleven types of vertebrates and invertebrates. Although no increased aggression was displayed by resident beavers towards intruding muskrats, the temporally partitioned patterns of muskrats' and beavers' entrances and exits to and from lodges, respectively, and altered behavior among both species during cohabitation, indicates that lodge use by muskrats represents an exploitative behavior as opposed to a mutualistic or even commensalistic relationship. We hypothesize that the ecological similarities between these species promotes competitive interactions, and the observed relationship highlights the tradeoffs faced by ecosystem engineers wherein constructed objects intended to exclude competitors are simultaneously associated with habitat modifications that promote the persistence of those same competitors.