Changes in positive associations among vertebrate predators at South Georgia during winter

Abstract

We studied positive associations among seabirds and marine mammals at South Georgia on research cruises during the Austral winters of 1985, 1991 and 1993 and found statistically significant differences. We collected data on abundance and distribution, providing a critical reference for sub-Antarctic conservation in anticipation of future environmental changes. We found significant changes in the abundance of 29% of species surveyed and a consequent change in species diversity. We postulate that the resulting altered community composition may have previously unanticipated population effects on the component species, due to changes in positive interactions among species which use each other as cues to the presence of prey. We found a near threefold reduction in spatial overlap among vertebrate predators, associated with warming sea temperatures. As the strength and opportunity for positive associations decreases in the future, feeding success may be negatively impacted. In this way, environmental changes may disproportionately impact predator abundances and such changes are likely already underway, as Southern Ocean temperatures have increased substantially since our surveys. Of course the changes we describe are not solely due to changing sea temperature or any other single cause—many factors are important and we do not claim to have removed these from consideration. Rather, we report previously undocumented changes in positive associations among species, and argue these changes may continue into the future, given near-certain continued increases in climate-related changes.