Year-round territoriality in long-lived birds: rethinking the concept of carrying capacity

Abstract

Adult African Black Oystercatchers Haematopus moquini are sedentary and territorial year round, with juveniles expressing strong natal philopatry. At four South African study sites (two mainland and two islands) breeding numbers more than doubled between the late 1970s/early 1980s and 2005 in response to improved protection and increased food availability, providing large-scale, natural experiments that could not have been simulated by manipulation. Oystercatcher population increases did not occur at constant rates – in each case breeding numbers remained stable for long periods both before and after periods of rapid population increases. At all sites, patterns of stability and change in breeding densities could be explained by (1) territorial adults reducing average territory size in a delayed response to improved resources, and (2) a resultant rapid influx of previously excluded sexually mature birds into new breeding territories. Our results bring into question whether a minimum territory size (and associated maximum breeding density) is ever reached as a result of competitor-induced territorial compression. Thus, for shorebird populations limited at high densities by interference competition, we suggest that it is most realistic to use equilibrium numbers (ultimately due to a balance between birth and death rates) as a basis to define carrying capacity.