SITE-DEPENDENT REGULATION OF POPULATION SIZE: REPLY

Abstract

Site-dependent regulation has been described as a process by which the demographic rates of territorial birds decline as breeding sites of progressively lower quality are occupied by a growing population. Proponents regard this phenomenon as regulatory and refer to "strong negative feedback" generated by the increasing occupancy of lower-quality sites. We offer an alternative explanation of population equilibrium that obtains without heterogeneity in site suitability and even when all sites yield recruits in excess of replacement levels. This mechanism, proposed almost a century ago by the Irish naturalist C. B. Moffat, is based on cohort size restriction imposed by passive environmental limits to site occupancy. There are two such limits: a site-serviceability limit and a recruitment-wave limit. The former involves the rejection of low-quality sites by individuals pursuing higher fitness in becoming floaters. The recruitment-wave limit derives from the restricted extent to which sites producing surplus recruits can augment those failing to meet that criterion.