Extinction vulnerability in two small, chronically inbred populations of Chatham Island black robin Petroica traversi

Abstract

New Zealand ecologists and wildlife managers have assumed traditionally that threatened insular endemic bird species are less susceptible to the predicted genetic consequences of declines and inbreeding. Conservation has relied on treating deterministic extinction pressures to trigger population recoveries, with few measures taken to minimise stochastic genetic threats to long-term viability. This study tested that assumption empirically by exam-ining extinction vulnerability in two small, critically threatened Chatham Island black robin populations protected from external threats on habitat-restricted islands but not man-aged purposefully for genetic health. Analysis of the factors eliminating the species’s original island populations indicated loss through habitat change and mammalian predation within perhaps no more than two to three decades of human contact. Loss was undoubtedly hastened by intrinsic extinction-proneness (low reproductive output, forest-interior preference, sensitivity to habitat quality, intolerance of open spaces), greater in black robins than in other Petroica species in New Zealand and Australia. Low productivity in ancestral survivors prevented spontaneous recovery after translocation to better habitat, requiring intensively assisted breeding to raise population sizes quickly. Strong growth in response to management did not suggest a moribund species but post-management growth rates were significantly slower. Analysis of demographic data from 1980-81 to 2001-02 showed breeding effort to be comparable in all populations but breeding failures of varying severity at different life-history stages affected natural recovery in each. The worst affected suf-fered from briefer adult life-spans and more severe failure late in the breeding cycle. Generally colder sub-canopy air temperatures suggested adverse environmental pressure on this population. Extant populations are highly inbred and severely depleted genetically fol-lowing successive bottlenecks (prolonged historically) and recovery from a single-pair event between 1979-80 and 1982-83. At current small sizes, accumulating levels of relat-edness are anomalously high. Contrary to perceptions that black robin recovery indicates normal fitness, viability was shown to be threatened by inbreeding depression expressed cumulatively in the breeding cycle as low juvenile production and survival, thus aggravating a principal vulnerability. Declines did not result during the study period and effects were limited to reproductive fitness costs in black robin fathers only. There was no evi-dence of harmful effects on survival probability or from maternal inbreeding but an emerging kinship effect on yearling production is possible. The restricted costs in such highly inbred populations (relative to less inbred congeners) suggest historical purging of genetic load. Work is needed to clarify this and the extent to which inbreeding could reduce re-sponses to new extinction pressures. Recent population declines may elevate levels of in-breeding further. Genetic management is strongly recommended, including habitat expan-sion to maximise population sizes and prevent further cross-breeding with a sympatric congener. Management and monitoring recommendations apply valuable lessons derived from a critique of past practices. In summary, findings give qualified support to the hy-pothesis of reduced susceptibility to inbreeding depression in threatened insular endemic birds. Findings do not support the assumption implicit in black robin management historically and in New Zealand wildlife management practice generally that stochastic genetic threats are unimportant risks to viability.