Long-term trends in population and community measures of colonial nesting waterbirds in Galveston Bay estuary

We examined diversity measures of the colonial-nesting waterbird community in Galveston Bay Estuary from 1973-1990 to determine if trends were apparent at three levels of ecological organization (i.e., community, assemblage, and population), each representing a different level of data aggregation. The community was dominated numerically by the Laughing Gull (Larus atricilla), a species with the lowest annual variability. In general, species that exhibited high annual variability also had low mean abundances (e.g., Anhinga (Anhinga anhinga) and Yellow-crowned Night-Heron (Nycticorax violaceus)). White Ibis (Eudocimus albus) and Sandwich Tern (Sterna sandvicensis) were unusual in that they were both highly variable and abundant. Trend analyses of the abundances of individual species showed that Roseate Spoonbill (Ajaia ajaja), Snowy Egret Egretta thula), and Black Skimmer (Rynchops niger) declined significantly. Neotropical Cormorant (Plialacrocorax brasilianus) and Sandwich Tern increased significantly, and the remaining 13 species showed no statistical change. A classification analysis identified five species assemblages. Two assemblages increased significantly in abundance, one assemblage decreased significantly, and two assemblages showed no change. At the community level, none of the indices we examined showed a significant change over the 18-year period. We hypothesized that loss of coastal marsh vegetation and the development of favorable feeding conditions outside of Galveston Bay have caused some of the population changes we identified. Our ability to identify trends was affected by the level of data aggregation. Community measures were less sensitive to change than were assemblage and population-level data