The avifauna and its diversity in Sugarcreek Reserve : component trends over a summer and implications for sanctuary management

Despite the plethora of literature on the theoretical implications of species diversity and the measurement and expression of diversity in ecological communities, sample size dependence and controversy over the relationships between the diversity components have invalidated comparisons between studies and obscured ecologically meaningful conclusions. In this study, the avifauna of an internally heterogeneous 228 ha nature reserve (Sugarcreek Reserve) in southwestern Ohio was censused from May 3l to August 9, 1978, to discern trends in species composition and diversity components over the breeding and postbreeding seasons. Three hour fixed strip censuses, averaging three censuses per week, were conducted each week over the ll-week study period. Species diversity was calculated by the Shannon formula on both a discrete daily basis and on a cumulative basis over the span of the study. It was concluded that species diversity variables show distinctly different trends and relationships to one another when analyzed cumulatively from when analyzed discretely. Although the discrete measurement system is useful for illuminating component trends over time, the cumulative system allows quantification of sample size effects and is more likely to generate predictive relationships between diversity variables. The association iv

between species richness and species diversity was found to be significantly positive in both the discrete and the cumulative data, but the correlation is stronger in the cumulative data. The relationship between evenness and species diversity is directionally opposite in the two data sets. While the discrete measurement system found diversity and evenness to be positively correlated, with parallel variation attributable to the occasional occurrence of large monospecific flocks, the cumulative system found diversity and evenness to be negatively correlated. Opposing trends result from the addition of rare species to the sample with increasing sample size, which lowers the evenness value while increasing diversity. While there was no significant correlation between evenness and richness in the discrete data, a negative correlation in the cumulative data is again related to the addition of rare species to the sample. Null hypotheses predicting constancy of diversity components over the summer were supported by the discrete data. Significant increases in both cumulative species richness and cumulative species diversity were found as cumulative sample size increased over the study period. The increase in species diversity was significant only up to the second week of study, while richness increased significantly up to the fourth week. Cumulative species evenness decreased significantly with increasing cumulative sample size up to the third week of study. Approximately 42 hours of observation (14 three-hour censuses) with a cumulative sample size of 3030 was deemed sufficient to accurately assess true breeding bird composition and diversity of the study area.

Sugarcreek Reserve, with 6l species assumed to be breeding, contains a more diverse avifauna than comparable temperate areas for which diversities have been reported in the literature. This high bird species diversity results from high internal habitat heterogeneity and edge effect, the comparatively large size of the study area, and the recent insularization and concomitant supersaturation of the reserve as surrounding habitat has given way to intensive agriculture and housing subdivision and thus become unsuitable for many species. Future management of Sugarcreek Reserve should emphasize preservation of extinction-prone species assemblages rather than high habitat and species diversity per se. Such policy is most conducive to the long-term maintenance of regional diversity in a deteriorating landscape.