A genetic assessment of the human‐facilitated colonization history of black swans in Australia and New Zealand

Abstract

Movement of species beyond their indigenous distribution can fundamentally alter the conservation status of the populations involved. If introductions are human-facilitated, introduced species could be considered pests. Characterizing the colonization history of introduced species can therefore be critical to formulating the objectives and nature of wildlife management strategies. The black swan (Cygnus atratus) is native to Australia but is considered a reintroduced species in New Zealand, where the endemic population was reported extinct during the 19th century. After the reintroduction of a small number of individuals from Australia, the New Zealand population expanded unexpectedly rapidly, which was attributed to simultaneous waves of migration fromAustralia. An alternative, but hitherto unformalized, hypothesis is that local extant populations remained and admixed with introduced individuals. To contribute to our understanding of the reintroduction history of the species, we investigated dispersal patterns and demographic histories of seven populations from Australia and New Zealand, using population genetic inferences from a microsatellite dataset. Our results on genetic structure, dispersal rates, and demographic histories provide mixed evidence on the origin of New Zealand black swans. The hypothesis that reintroduced individuals mixed with remaining local individuals and that the subsequent dramatic population expansion may have been due to genetic rescue of the inbred indigenous population cannot be discarded and needs further investigation. K E Y W O R D Sconservation, Cygnus atratus, kakianau, pest species, phylopatry

Native or not? Ancient DNA rejects persistence of New Zealand's endemic black swan: A reply to Montano et al.

“…Along similar lines, the microsatellite “structure” analysis presented by Montano et al. (; their Figure 2) also appears to show a clear genotypic affinity between New Zealand and south‐eastern Australian C. atratus , with no evidence whatsoever for a unique or evolutionarily significant lineage persisting within New Zealand Cygnus (in apparent stark contrast to the results of their coalescent analyses). Additionally, there is no mtDNA or morphological evidence for C. sumnerensis having persisted in New Zealand beyond the earliest phase of Polynesian settlement.…”

“…By contrast, we suspect that the surprising and discordant conclusions of Montano et al. () reflect the limitations of coalescent modelling of major demographic and range shifts using small numbers of microsatellite loci. Such modelling analyses may struggle to capture the mutation dynamics of fast‐evolving loci, and also to reflect the rapid changes in population size associated with anthropogenic founder effects.…”

“…The findings of Montano et al. () contrast strongly with historic records documenting the absence of black swans at the time of European settlement of New Zealand, an absence that presumably underpinned the decision to introduce C. atratus from Australia to New Zealand (Thomson, ). The conclusions of Montano et al.…”

“…The conclusions of Montano et al. () are also at odds with newly published genetic and morphometric comparisons of modern versus ancient New Zealand black swans (Rawlence et al., ), which show that, at the time of first human contact, New Zealand was home to a unique swan taxon ( C. sumnerensis , Pouwa), which was subsequently extirpated and replaced by modern (Australian) C. atratus .…”

“…Overall, Montano et al. () conclude “the New Zealand population appears to be at least partly derived from a natural, and not a feral, population; thus, it may constitute an Evolutionary Significant Unit.”…”