Human‐mediated extirpation of the unique Chatham Islands sea lion and implications for the conservation management of remaining New Zealand sea lion populations

Abstract

While terrestrial megafaunal extinctions have been well characterized worldwide, our understanding of declines in marine megafauna remains limited. Here, we use ancient DNA analyses of prehistoric (<1450-1650 AD) sea lion specimens from New Zealand's isolated Chatham Islands to assess the demographic impacts of human settlement. These data suggest there was a large population of sea lions, unique to the Chatham Islands, at the time of Polynesian settlement. This distinct mitochondrial lineage became rapidly extinct within 200 years due to overhunting, paralleling the extirpation of a similarly large endemic mainland population. Whole mitogenomic analyses confirm substantial intraspecific diversity among prehistoric lineages. Demographic models suggest that even low harvest rates would likely have driven rapid extinction of these lineages. This study indicates that surviving Phocarctos populations are remnants of a once diverse and widespread sea lion assemblage, highlighting dramatic human impacts on endemic marine biodiversity. Our findings also suggest that Phocarctos bycatch in commercial fisheries may contribute to the ongoing population decline.

Did interaction between human pressure and Little Ice Age drive biological turnover in New Zealand?

“…3) at c. 1500 AD. In addition to genetic data from 185 previously sequenced ancient specimens (Boessenkool et al, 2009;Collins et al, 2014;Rawlence et al, 2016), new sequences recovered from 28 additional archaeological Phocarctos specimens revealed clear lineage turnover closely corresponding to the transition between early prehistoric (1280-1450 AD) and middle-to late-prehistoric (1450-1796 AD) periods (Fig. 3).…”

“…Additionally, as the prehistoric subantarctic ‘source’ populations of both Megadyptes and Phocarctos were substantial [long‐term N c of P. hookeri ‘subantarctic’ estimated to be c . 40,000–100,000 individuals (Collins et al ., ; Rawlence et al ., )], we infer that they likely produced sufficient numbers of vagrants to facilitate rapid northward expansion at this time (Boessenkool et al ., ; Rawlence et al ., ). In contrast to mainland lineage turnover around the onset of the LIA, there is no current evidence for contemporaneous (LIA‐associated) demographic shifts in New Zealand's subantarctic marine megafauna.…”

“…) went extinct within a few centuries of human settlement. Indeed, Bayesian modelling confirms that even moderate hunting pressure would have driven these endemic populations to extinction within a few centuries (Rawlence et al ., ). Subsequently, these extirpated temperate mainland populations were replaced by lineages previously restricted to cold subantarctic waters.…”

“…Notably, Polynesian horticulture in New Zealand was limited primarily to northern areas of the South Island (McGlone et al, 1994;Anderson, 1998;Jacomb et al, 2010;Smith & James-Lee, 2010;Smith, 2017)a limiting factor in sustaining permanent settlement in cold southern New Zealand. Additionally, as the prehistoric subantarctic 'source' populations of both Megadyptes and Phocarctos were substantial [long-term N c of P. hookeri 'subantarctic' estimated to be c. 40,000-100,000 individuals Rawlence et al, 2016)], we infer that they likely produced sufficient numbers of vagrants to facilitate rapid northward expansion at this time (Boessenkool et al, 2009;Rawlence et al, 2015a). In contrast to mainland lineage turnover around the onset of the LIA, there is no Figure 2 Extinction-colonization dynamics for mainland New Zealand Megadyptes penguins.…”

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

“…750 years ago), and the archipelago's naive fauna was subsequently devastated, with at least 58 endemic avian taxa having been driven to extinction over subsequent centuries (Holdaway, Worthy, & Tennyson, ; Tennyson & Martinson, ; Waters & Grosser, ). In addition to the loss of the iconic moa (Worthy & Holdaway, ), a number of large coastal bird and pinniped lineages were rapidly eliminated by hunting and later replaced by exotic lineages (e.g., Boessenkool et al., ; Collins et al., ; Rawlence et al., , ; Waters, Fraser, Maxwell, & Rawlence, ).…”