Legacy or colonization? Posteruption establishment of peregrine falcons (Falco peregrinus) on a volcanically active subarctic island

Abstract How populations and communities reassemble following disturbances are affected by a number of factors, with the arrival order of founding populations often having a profound influence on later populations and community structure. Kasatochi Island is a small volcano located in the central Aleutian archipelago that erupted violently August 8, 2008, sterilizing the island of avian biodiversity. Prior to the eruption, Kasatochi was the center of abundance for breeding seabirds in the central Aleutian Islands and supported several breeding pairs of peregrine falcons (Falco peregrinus). We examined the reestablishment of peregrine falcons on Kasatochi by evaluating the genetic relatedness among legacy samples collected in 2006 to those collected posteruption and to other falcons breeding along the archipelago. No genotypes found in posteruption samples were identical to genotypes collected from pre‐eruption samples. However, genetic analyses suggest that individuals closely related to peregrine falcons occupying pre‐eruption Kasatochi returned following the eruption and successfully fledged young; thus, a genetic legacy of pre‐eruption falcons was present on posteruption Kasatochi Island. We hypothesize that the rapid reestablishment of peregrine falcons on Kasatochi was likely facilitated by behavioral characteristics of peregrine falcons breeding in the Aleutian Islands, such as year‐round residency and breeding site fidelity, the presence of an abundant food source (seabirds), and limited vegetation requirements by seabirds and falcons.


Islands and supported several breeding pairs of peregrine falcons (Falco peregrinus).
We

| INTRODUCTION
How populations are founded and how communities reassemble following disturbances, such as volcanic eruptions, are affected by a number of factors, including the severity of disturbance, priority effects (dispersal and arrival order; Hoverman & Relyea, 2008), and availability of propagules (Mazzola et al., 2011) either from survivors (representing legacy biodiversity, Walker et al., 2013), or from coloniz ers (representing founder biodiversity). Most studies have examined these factors retrospectively (Fleischer, McIntosh, & Tarr, 1998;Percy et al., 2008;Ricklefs, 2010;Shaw, 1996;Yang, Bishop, & Webster, 2008), as opportunities to study community reassembly following major disturbances are rare. As colonization of newly sterilized areas can occur rapidly (e.g., plants on Krakatau, Thornton, 1984; birds on Surtsey, Petersen, 2009; predaceous flies on Kasatochi, Walker et al., 2013), catastrophic disturbance events that simplify community rela tionships via the elimination of most or all flora and fauna provide par ticularly useful opportunities to study deterministic versus stochastic processes influencing the reassembly of communities (Mazzola et al., 2011;Walker et al., 2013).
Initial founding events and the expansion of founding individuals across the landscape may profoundly influence later populations (Yang et al., 2008) and community structure in newly created habitats. For example, among closely related species in the Hawaiian Archipelago, there is generally a linear relationship between island age and genetic distance (sequential radiation), suggestive of rapid colonization follow ing island genesis, yet evidence of subsequent colonization was not observed (e.g., Fleischer et al., 1988;Percy et al., 2008;Shaw, 1996).
This pattern of colonization illustrates the importance of initial found ing events, as initial founders may impact the ability of subsequent dispersers to successfully colonize (Waters, Fraser, & Hewitt, 2012). Therefore, species and/or individuals that survived the eruption may play a pivotal role in the success of subsequent colonization attempts by other species or individuals (Franklin, 2005), ultimately impacting community assemblage posteruption (Walker et al., 2013). Establishment, or reestablishment, of terrestrial fauna and asso ciated food webs on islands following major disturbances (or new geological formation) can also depend upon the presence of plants for nesting substrate (i.e., birds on Anak Krakatau; Thornton, Zann, & Stephenson, 1990) and associated food base (i.e., prey for insec tivores/carnivores or vegetation for herbivores, frugivores, and nec tarivores; Walker et al., 2013). Thus, establishment of local breeding populations for certain species lags until a sufficient level of habitat development has occurred. This limitation, however, may favor rapid colonization and establishment of species that are not dependent upon terrestrial vegetation, such as seabirds and marine mammals.
Highly mobile animal species that rely on the marine environment for their food base should not be constrained in their ability to rapidly colonize or recolonize disturbed islands. The limited habitat require ments of seabirds are exemplified on Surtsey Island, which emerged off the south coast of Iceland in an extended series of eruptions. Only 2 weeks after the eruption began, gulls (Larus sp.) were observed landing on the island between eruption events (Gudmundsson, 1966;Petersen, 2009). Within 7 years of emergence of the island, 3 years after the cessation of volcanic activity, marine birds started nesting on the island (Fridriksson & Magnússon, 1992;Petersen, 2009). Similarly, marine birds increased in abundance following the eruption on San Benedicto Island, Islas Revillagigedo, Mexico, despite poor survival of seeds and vegetation (Ball and Gluscksman 1975). Therefore, highly vagile species characterized by minimal terrestrial habitat require ments should be early founders in newly sterilized areas and play a piv  (Drummond & Larned, 2007). In particular, least auklets (Aethia pusilla) and crested auklets (A. cristatella) nested in immense numbers (>200,000 individuals; Williams, Drummond, & Buxton, 2010). By the date of the eruption in 2008, many seabirds had finished breeding for the year, and most adults had left the island; it was assumed they survived to return the following year (Williams et al., 2010). Several other seabird species were still breeding and along with nonfledged young were entombed or perished otherwise in the eruption. Commander Islands (7) Attu (5) Buldir (5) Amchitka (13) Amatignak (1) Kasatochi ( (White, 1975;White, Emison, & Williamson, 1971). Given this general breeding site fidelity, it was unclear whether avian predators and their fledglings survived the eruption. However, the presence of peregrine falcons (almost exclusively predatory) and bald eagles (partial scaven gers) within the first year or two, respectively, following the eruption suggested that these species quickly recolonized, or at least utilized re sources on, posteruption Kasatochi. It is not known, however, whether individual birds that occupied pre eruption Kasatochi returned post eruption (e.g., represented legacy biodiversity), or whether they were new colonizers. Prior investigation of population and regional level genetic structuring suggests peregrine falcons occupying the Aleutian Islands show significant regional level structuring and are genetically distinguishable from peregrine falcons occupying habitats elsewhere in Alaska, but show lower levels of structuring across island groups (S. L. Talbot, unpublished data). Thus, levels of natal site fidelity (philo patry) are apparently not sufficiently high to isolate specific island populations, suggesting that individual peregrines occupying post eruption Kasatochi cannot necessarily be assumed to represent legacy biodiversity.
Testing for relative importance of in situ and ex situ survival vs. col onization following disturbances, such as volcanic eruptions, is often limited due to lack of historical information about predisturbance res idents (Walker et al., 2013). This limitation can be overcome if histor ical data are sufficient to distinguish colonizers from survivors (Yang et al., 2008). three adults and four juveniles) consisted of feathers collected from two of the four active eyries or within the territories of known pairs from perches and plucking stations in 2006. Therefore, our legacy sample size represents between 25% (n = 2/8) and 87.5% (n = 7/8) of the peregrine falcons breeding in 2006. We assumed that molted feathers came from individuals that were residents of Kasatochi and not peregrines from nearby islands or nonbreeders, given their breeding site fidelity and ter ritorial behavior (White, Clum, Cade, & Hunt, 2002). In addition, feather and egg shell membranes (n = 31), collected as part of a larger regional study from peregrine falcons breeding throughout the Aleutian Islands, were included to derive insight into the contributions of islands in the reestablishment of falcons on Kasatochi Island (Figure 1).

| Genetic relationship within years
Overall rxy values estimated for Kasatochi peregrines within years were negative (Table 1). In general, rxy values were more negative than val ues estimated from other islands in the Aleutian archipelago (Table 2) and the Aleutian Island peregrine falcons as a whole (rxy = −0.010, variance = 0.066

| Genetic relationship throughout the Archipelago
Increasing genetic differentiation with increasing geographic dis tance was not observed within the Aleutian Archipelago pre eruption

| DISCUSSION
We found no genetic evidence to indicate that individual peregrine  (Table 1) (1989;Zann & Darjono, 1992). Furthermore, peregrine falcons are only visitors to the volcanic islands of Motmot (emerged in 1968, Ball & Glucksman, 1975Schipper, Shanahan, Cook, & Thornton, 2001) and Islas Revillagigedo (erupted in 1952;Hahn, Hogeback, Römer, & Vergara, 2012) and con tinue to be absent from Surtsey Island (emerged in 1963;Petersen, 2009). Although other volcanic islands have source populations in rel atively close geographic proximity, peregrine falcons breeding in the Aleutian Islands are primarily nonmigratory with only some mid winter interisland movement and juvenile dispersal (White, 1975;White et al., 2002).  Ball & Glucksman, 1975), and Surtsey (Larus sp.; Petersen, 2009). Only a few (<50) breed ing birds (Anas superciliosa and Hirundo tahitica) are present on Motmot (Schipper et al., 2001). Among the Krakataus, community assembly was needed to facilitate the reintroduction of avian predators, such as the peregrine falcon. Zann and Darjono (1992)  waters, made up 69% of the prey (n = 548 total prey items; White, Emison, & Williamson, 1973). Pre eruption densities of crested and least auklets in waters surrounding Kasatochi Island did not change posteruption (Drew, Dragoo, Renner, & Piatt, 2010), and lack of crev ices and vegetation likely increased their susceptibility to predation by peregrine falcons (Figure 5; see figure 1 in Williams et al., 2010 is likely due to sample size limitations. These trends suggest early posteruption recruitment from displaced prior residents occupying nearby islands, augmented in subsequent years by immigrants from other islands. Dispersal propensity would likely be a beneficial evolu tionary strategy for species occupying this highly dynamic landscape. Therefore, species, such as peregrine falcons, that have rapidly col onized (or recolonized) novel habitats in this region may be predis posed to exhibit a metapopulation dynamic and possess (or evolved) characteristics that exploit this interplay of source and sink dynamics among neighboring islands. Indeed, source sink dynamics have also been characterized for mainland peregrine falcons (e.g., Kauffman, Pollock, & Walton, 2004;Wootton & Bell, 2014). The Aleutian Island archipelago is a geologically dynamic region and geologic and modern records indicate high levels of volcanic activity (Jicha, Scholl, Singer, Yogodzinski, & Kay, 2006;Miller et al., 1998). Eruptions occur approx imately every 21-80 years among volcanically active islands within the Andreanof Island group (Alaska Volcano Observatory 2013; Coats, 1950)