When populations experience localized mortality, net immigration from adjacent source areas often occurs (Pulliam 1988; McCullough 1996). This compensatory immigration can accelerate the restoration of populations partially or completely eliminated by disturbances of natural (Adams & Warren 2005) or anthropogenic origin (Greathouse, March & Pringle 2005; Albanese, Angermeier & Peterson 2009) and is critical to metapopulation viability (Pulliam 1988; Hanski 1998). Compensatory immigration can benefit harvesters by increasing local yield (McCullough 1996; Gell & Roberts 2003) but also limits the effectiveness of pest control (Nakata & Satoh 1994; Efford, Warburton & Spencer 2000) and reduces the benefits of protected areas (Woodroffe & Ginsberg 1998; Gundersen et al. 2001).
Compensatory immigration varies greatly among studies (Table S1), but there is very little information about the factors influencing its magnitude or impact on adjacent source populations. From a theoretical perspective, compensatory immigration should be favoured by optimal habitat selection, especially when associated with competition as suggested by the ideal free and ideal despotic distributions (Fretwell & Lucas 1969) and related models. Conversely, immigration may be constrained by factors affecting the detection of lower density patches and the ability to move towards them. The abundance of potential immigrants could affect compensatory immigration. In some situations, all individuals in a source area might be able to move, and in others, only the ones near the boundary, depending on the size of each area, the distance at which density changes or habitat vacancies can be detected (Nakata & Satoh 1994; Efford, Warburton & Spencer 2000) and the mobility of the species (Rakitin & Kramer 1996; Woodroffe & Ginsberg 1998; Albanese, Angermeier & Peterson 2009). Compensatory immigration may also increase with habitat quality in the depleted area relative to habitat quality in potential source areas because more individuals can be accommodated before density-dependent processes reduce habitat quality to that of the surrounding areas (Fretwell & Lucas 1969; MacCall 1990; Delibes, Ferreras & Gaona 2001). In addition, landscape connectivity (Taylor et al. 1993), the degree to which the distribution of habitat patches and the quality of the matrix between them allow organisms to detect available openings and move towards them (Kozakiewicz & Jurasińska 1989; Forcada et al. 2008), may increase compensatory immigration by allowing a greater proportion of the source population to move.
Most studies providing evidence for compensatory immigration (Table S1) did not have adequate replication to measure the effect of factors potentially influencing compensatory immigration and lacked spatial or temporal controls to estimate baseline immigration in the absence of localized mortality. Furthermore, most studies carried out a single massive removal and did not continue removal until immigration ceased, a procedure required to assess maximal compensatory immigration. The goals of this study were (i) to quantify compensatory immigration in response to localized mortality, (ii) to assess its impact on populations in adjacent unharvested locations and (iii) to assess the importance of potential immigrants, habitat quality and landscape connectivity as factors influencing the magnitude of compensatory immigration. We applied a repeated, small, localized removal of two damselfish species in their natural coral reef habitat until immigration ceased, and no individuals remained. We used seven sites, each consisting of a depleted area surrounded by an unharvested source area. Sites varied in the number and density of fish in the source area, habitat quality and landscape connectivity. We also monitored two control sites to estimate immigration in the absence of removal.