Local and Landscape-Level Factors Influencing Black Tern Habitat Suitability

Abstract

Wetlands throughout eastern South Dakota were surveyed (1995-97) for foraging and nesting black terns (Chlidonias niger) to evaluate local and landscape factors influencing habitat suitability We surveyed 834 randomly selected, semipermanent, and seasonal wetlands that were stratified by physiographic domain, wet- land density, and wetland surface area. A discriminant function model was used in a geographic information system (GIS) to classify habitat suitability of all semipermanent wetlands in eastern South Dakota. We calcu- lated number of suitable, protected wetlands by combining wetlands with easement and fee-title tracts in the GIS. Black terns nested in 7.8% and foraged in an additional 17.9% of semipermanent wetlands. Significant variables in the discriminant function were wetland area, total semipermanent wetland area within the wetland complex, and grassland area in the upland matrix. Black terns were an area-dependent species that occupied large (f = 18.9 ha) wetlands located within high-density wetland complexes. Black terns typically occurred in wetlands within landscapes where <50% of upland grasslands were tilled. Classification rates were high (76- 100%), indicating that the model identified unsuitable wetlands using wetland area and landscape-level attri- butes. Characteristics of entire landscapes must be considered in habitat assessments because wetlands that do not correspond to landscape-scale habitat requirements may not be suitable despite favorable local condi- tions. Lower correct classification rates (22-78%) for occupied ponds indicated that suitability also is dependent on local conditions for wetlands which correspond to habitat requirements from a landscape perspective. Suitable black tern nest sites occurred within regenerating or degenerating wetlands where vegetation structure rather than species of vegetation dictated suitability of nest substrates. Wetland acquisition programs have protected 44% of wetlands suitable for black terns. Future wetland acquisitions should maintain the integrity of entire prairie landscapes in additional to attributes of individual wetlands.