Response of Avian Communities in Large-river Floodplains to Environmental Variation at Multiple Scales

Large-river floodplains have been subjected to profound human-caused alteration, but subsequent effects on landbirds in these habitats are largely unknown. To assess the extent to which landscape measures might serve as ecological indicators in these systems, we examined patterns of habitat use by forest birds along a 380-km stretch of the Wisconsin River, USA. We surveyed forest bird communities during the breeding season in 1999 and 2000 at 48 sites divided among six reaches of the 100-year floodplain. Several tree and bird species thought to be characteristic of floodplain forests in Wisconsin, based on earlier surveys, appear to have declined in abundance or were absent altogether, even in reaches located in a 150-km undammed stretch of the river. Bird species richness was similar among reaches, but overall abundance was notably lower at the two northernmost reaches. The most widespread and abundant birds were those that are typically associated with forest edge habitats, but several species associated with forest interior conditions were also relatively abundant. A canonical correspondence analysis based on a model derived from the entire pool of environmental variables indicated that most of the variation in avian community structure was accounted for by geographic variables describing latitudinal changes and distance from potential source habitats along the Mississippi River. Partial ordinations and univariate variance partitioning, however, showed confounding among environmental variable sets and revealed that local-habitat measures tended to explain somewhat more variation than geographic variables. Metrics describing landscape pattern and composition accounted for the least amount of independent variation among the three variables sets. Our results indicate that landscape measures are necessary, but not sufficient to describe patterns of habitat use by forest birds in the Wisconsin River floodplain. Moreover, our data suggest that, while maintaining habitat for forest birds is still possible in these areas, it may be more difficult to conserve species with an affinity for conditions that characterize floodplains. This study further demonstrates that, for some species, designations such as ‘‘forest interior’’ or ‘‘edge’’ are not always portable from one region to another or even over relatively short distances within a region.