Common terns (Sterna hirundo) as indicators of ecosystem response to urbanization in the Barnegat Bay Watershed region of New Jersey, 1982-2007

Abstract

While recording avian populations in Barnegat Bay for more than 30 years, it was observed that out of 34 nesting islands inhabited by common terns (Sterna hirundo); only 15 currently host colonies of common terns. Within the same period of time, the region has experienced intense urban development, especially in recent years. The intent of this research is to provide a method to investigate the spatial relationship of changes in encroaching urbanization to common tern populations. Since the success of coastal communities is dependent upon sustainable coastal urbanization, integrating correlations between land-use change and avian populations can provide information to use when establishing conservation sites or protected areas. Conservation of avian ecosystems requires identifying critical habitat areas that are affected by urbanization. The null hypothesis of this research was that long-term population variability does not result from encroaching urbanization on common tern habitats. If common terns are adversely affected by the direct and indirect effects of increasing urbanization in the area, it could be an indication of declining ecosystem health. These birds serve as excellent bioindicators of ecosystem health because they feed at high trophic levels of food chains within the ecosystems in Barnegat Bay. To find correlations between changing urbanization and populations, 25 years of population data of common terns nesting on salt marsh islands in Barnegat Bay, New Jersey from 1984-2006 was compared with satellite imagery for 1984, 1995, 2001 and 2006 using Geographic Information Systems (GIS). The island group experiencing the greatest percent change (40.6%) in distance from the nearest edge of urbanization to tern habitats, also experienced the greatest overall decline in Common Tern populations over a 25 year period. Distance was calculated using the Euclidean Distance, or straight-line distance, tool in Spatial Analyst using ArcGIS. The change in distance indicates an encroachment of urbanized land in the direction on common tern nesting and breeding areas. During the study period, populations of common terns did not experience a linear decline; however, there was a linear increase in urbanization. Long-term population variability may be due to indirect effects of land-use change including volatile weather conditions, predation, and recreational disturbances or dredging projects.