Seasonal changes in microhabitat use and niche overlap between Akodon azarae and Calomys laucha (Rodentia, Muridae) in agroecosystems of central Argentina

We examined seasonal changes in microhabitat use by the murid rodents Akodo n azarae and Calomys laucha in agroecosystems of the former pampa region of Argentina. In summer, trap data showed that both species had low densities and were almost equally distributed between the mature cropfields and-thei r surrounding weedy borders. Analysis of vegetation at the trap sites revea led no selection for microhabitats at the borders, whereas in the cropfield s both species shared preferences for covered microhabitats. In contrast,, winter data revealed a sharp habitat segregation, being Calomys numerically dominant in post-harvest cropfields and Akodon more abundant in borders. M oreover, there were clear differences in microhabitat selection at the bord ers, A. azarae occupying the more covered microhabitats and C. laucha the l ess covered. Breadth and overlap of niches were calculated using principal component analysis, in order to recognize interspecific competition and its influence on community niche space. Available evidence indicates that the structure of this rodent community is strongly influenced by seasonal chang es in habitat stucture and rodent abundance. The socially dominant A. azara e retains the best part of the available niche space, a fact becoming more evident under the harsh winter conditions.