Short-Term Dynamics of Cavity-Nesting Bird Communities in Disjunct Floodplain Habitats

Abstract

We studied turnover rates in cavity-nesting bird communities during 1983 and 1984 in 34 cottonwood-willow (Populus-Salix) habitat fragments along the North Platte River in southeastern Wyoming. Our principal goal was to determine whether the equilibrium theory of island biogeography was applicable to this system. We tested two hypotheses: (1) equilibrial turnover of cavity-nesting bird species does not occur in disjunct floodplain habitats, and (2) no significant relations exist between absolute turnover rate (ATR) and habitat-fragment features. Paired-t tests and a one-sample sign test corroborated equilibrial turnover for the group of fragments collectively, but examination of the hypothesis at the level of individual fragments did not support the equilibrium model. Simpleand partialregression coefficients indicated ATR was positively associated with fragment size, the reverse of what equilibrium theory predicts; ATR, however, was negatively related to distance to nearest streamside habitat fragment (analogous to degree of isolation), which is consistent with the theory. ATR was significantly related to five fragment features in a validated multiple regression model: amount of edge/hectare, size of fragment, frequencies of snag classes 4 (56 to 70 cm dbh) and 6 (>85 cm dbh), and the presence of palustrine wetland bordering each fragment. Short-term dynamics of cavity-nesting bird communities in the system we studied are not completely predictable from the equilibrium theory. Fragment features appear to be the important deterministic forces affecting species richness and dynamics, but stochastic immigration and extinction may also be operative. Application of equilibrium island-biogeographic theory to conserve the system we analyzed is inappropriate. Instead, recommendations that recognize the deterministic influences we have identified should be used.