Spatial Structure and Fragmentation Effects of Grasshopper Sparrows in Mendon Ponds, New York

Abstract

Grasshopper Sparrows (Ammodramus savannarum) in Mendon, New York, were studied from 1997 to 1999, as part of a long-term study incorporating data from 1995 to 2000. This population of Grasshopper Sparrows represents a dynamic metapopulation responding to pressures associated with a decrease in the area of suitable breeding habitat patches, and an increase in habitat fragmentation. Ten fields occupied by Grasshopper Sparrows in the Mendon Ponds area ranged from 1.7 ha to 13.2 ha. From 1997 to 1999, Grasshopper Sparrows were mist-netted and banded, nests were monitored and vegetation analysis was conducted using the fixed-quadrat and Robel pole methods. Both fields with and without Grasshopper Sparrows were studied. A significant difference was found in the % bare ground between fields with and without Grasshopper Sparrows, with the lower value being found in fields with Grasshopper Sparrows. Robel pole measurements of fields with and without Grasshopper Sparrows showed the sparrows preferred fields with shorter, less dense vegetation. Large fields (>8 ha) showed a higher proportion of pairing and nesting success than did small (< 8 ha) fields. The overall proportion of successful nests in large fields averaged 66%, while the small fields' proportion of nest success only averaged 44%. Five fields suffered population extinction of Grasshopper Sparrows between 1995 and 1999, and two fields were colonized. Neither large nor small fields could be considered "source" fields – fields in which births outnumber deaths and which provide individuals to colonize empty fields within the metapopulation; however, large fields were more likely to persist than were smaller fields and produced more young than small fields. Chances of extinction increased with a decrease in the average number of males present, and fields which did not suffer population extinctions were significantly larger than fields which did suffer population extinctions. Changes in land use, as when fields are developed into subdivisions, and the transformation of early successional grasslands into old fields or forests, also affect the persistence of Grasshopper Sparrows. As suitable grassland becomes unsuitable through land-use change and succession, Grasshopper Sparrows abandon the fields. Results of this study suggest that the metapopulation of Grasshopper Sparrows in Mendon Ponds occupies highly fragmented, relatively small habitat patches, and must be replenished by individuals from other populations via long-distance migration.