Successful island reintroductions of New Zealand robins and saddlebacks with small numbers of founders

Abstract

Populations established with a small number of founders are thought to have a high risk of extinction due to Allee effects, demographic stochasticity, inbreeding and reduced genetic variation. We tested whether the initial number of birds released was related to persistence in reintroductions of saddlebacks (Philesturnus carunculatus) and robins (Petroica australis) to New Zealand offshore islands. Data were analysed for 31 populations that had been observed for at least 3 years since reintroductions. The numbers released ranged from 5-188. Most of the populations (26) survived and grew, including five from less than 15 founders, and four out of the five extinctions were attributable to introduced mammalian predators. The number of individuals released did not significantly affect extinction probability. The ability of these small releases to establish populations can be attributed to the closed nature of the islands (allowing birds to find mates), low mortality rates following release and high growth rates at low density. Stochastic simulation models based on data from two reintroduced populations suggested that populations with four founders (two male, two female) would have a negligible chance of extinction through demographic stochasticity and would be able to grow even if there were high rates of egg failure through inbreeding.

Has the debate over genetics and extinction of island endemics truly been resolved?

“…Experience in New Zealand would tell us that it would not be in the presence of introduced predators, which ultimately inflict high mortality rates on adults and juveniles of endemic species resulting in negative population growth rates (Wilson, 2004). Furthermore, many populations of New Zealand endemics show the capacity to recover rapidly when predators are controlled or eradicated (Taylor, Jamieson & Armstrong, 2005). Instead, gradual habitat loss, range contraction and isolation are more likely situations under which a population would decline more gradually or remain small for a period of time.…”

Large mainland populations of South Island robins retain greater genetic diversity than offshore island refuges

“…Small offshore islands that were either never colonised by introduced mammals or were later restored (including eradication of mammals) represent natural sanctuaries for many bird species (Clout 2001). In the past decades, translocation of endangered species to these islands has been one of New Zealand's most common and successful conservation tools (Armstrong and Mclean 1995;Taylor et al 2005;Jamieson et al 2006). Most of New Zealand's translocated island populations thrive in the absence of mammalian predators, even though they were typically founded by a small number of individuals and are isolated by large water barriers that prevent immigration (Taylor et al 2005).…”

“…In the past decades, translocation of endangered species to these islands has been one of New Zealand's most common and successful conservation tools (Armstrong and Mclean 1995;Taylor et al 2005;Jamieson et al 2006). Most of New Zealand's translocated island populations thrive in the absence of mammalian predators, even though they were typically founded by a small number of individuals and are isolated by large water barriers that prevent immigration (Taylor et al 2005). The level of genetic diversity in newly founded populations is strongly influenced by the genetic variation of the source population from which individuals were translocated, with the most appropriate source material having high levels of genetic variability.…”

Long-distance translocations to create a second millerbird population and reduce extinction risk

“…The loss of 42% of the initial 2011 cohort could have been a critical problem, owing to the increased risk of a population or genetic bottleneck posed by such a large, early loss (Lambert et al, 2005;Taylor et al, 2005). Fortunately, however, at least three of those birds had produced at least one brood of fledglings, and so contributed their genes to the first generation produced on Laysan.…”

“…Assuming no major decline in the population, millerbirds removed for translocation to Laysan thus would either be non-breeders or be replaced rapidly in the breeding population on Nihoa (Richardson et al, 2006). We were therefore confident that removing 50 adult birds over two translocations in good years was unlikely to result in an adverse populationlevel impact on Nihoa, would provide a sufficient selection of mates for successful breeding on Laysan, and, as seen in other translocations of island birds, would suffice for population establishment (e.g., Komdeur, 1994;Taylor et al, 2005;Parker and Laurence, 2008;Reynolds et al, 2008;Parker, 2013). We also expected that this target number would capture the majority of the millerbirds' very limited genetic diversity and was a sufficient number to found a thriving population on Laysan (Taylor and Jamieson, 2008;Addison and Diamond, 2011;Wright et al, 2014).…”