Assessing the buffer effect of floaters by reinforcing local colonization in spatially structured populations

Ecological theory and recent empirical studies show that the persistence of spatially structured populations – those composed of spatially separated local populations connected by dispersal – may require the maintenance of pools of floaters (nonbreeding but sexually mature individuals) that rescue small local populations from extinction by replacing the lost breeders. Regardless of the rescue of local breeding populations, the persistence of spatially structured populations may also depend on sufficient numbers of colonizations that counteract local extinctions. Given their intrinsic potential mobility, floaters have been proposed to promote the persistence of spatially structured populations by enhancing the (re)colonization of empty habitat patches in fragmented habitats. However, robust empirical assessments of this hypothesis are lacking. Using a long‐term woodpecker study in fragmented habitats as a model framework, we addressed the largely neglected hypothesis that floaters enhance the colonization of local patches and, consequently, facilitate the persistence of spatially structured populations. The size of the entire spatially structured system (number of local populations) increased across years due to a slightly higher number of colonizations than extinctions, suggesting that an increase in colonization may enhance the long‐term persistence of this spatially structured population. After controlling for the number of patches occupied in previous years, the number of colonizations was positively related to the number of patches with floaters in previous seasons. Moreover, after controlling for potential confounding factors, patches with floaters were considerably more likely to be colonized in subsequent seasons than patches without floaters (0.079 vs. 0.008). Colonizations were often conducted through inter‐patch dispersal by floaters that established their territory in empty patches after a variable wandering period, which frequently led to delayed breeding beyond the age of sexual maturity. Altogether, these results provide empirical support for the hypothesis of an enhanced colonization mediated by floaters as a buffer mechanism for the long‐term persistence of spatially structured populations. The persistence of spatially structured populations (i.e. those composed of local populations connected by dispersal) depends on balanced colonization–extinction ratios. Using a long‐term woodpecker study with a positive colonization–extinction ratio, we