Temporal variation in bird assemblages: how representative is a one-year snapshot?

Abstract

Bird assemblages generally are no longer regarded as stable entities, but rather as fluctuating in response to many factors. Australia’s highly variable climate is likely to result in a high degree of dynamism in its bird assemblages, yet few studies have investigated variation on an inter-annual temporal scale. We compared two year-long samples of the bird assemblages of a series of highly fragmented buloke Allocasuarina luehmannii (Casuarinaceae)woodland remnants in south-eastern Australia, the first sample taken in 1994–1995 and the second in 2001–2002. Bird densities were almost three times higher in the second period than in the first. Mean species richness also was significantly higher. Species richness of each individual site was unrelated between the two years. Minimum species turnover was 63% and was higher, on average, for migratory and nomadic than for sedentary species. Therefore, site-level bird assemblage composition was markedly different between the two survey periods and, on average, the assemblage composition of each site bore greater resemblance to those of other sites in the same year than to that of the same site in the other survey period. Most species changed substantially in their distribution among remnants between the two periods. The change in distribution of most species did not differ significantly from that expected if the species had redistributed at random among the sites. This suggests that although the remnant vegetation of the area is highly fragmented with minimal interpatch connectivity, bird movements among remnants must be relatively frequent. Interannual variability in Australian bird assemblages may be higher than is commonly recognized. In such dynamic systems, we must be cautious when extrapolating from the findings of short-term studies to longer temporal scales, especially in relation to conservation management. A greater understanding of the processes driving distributional patterns is likely to enable better predictions of species’ responses to habitat change.