Factors limiting kererū (Hemiphaga novaeseelandiae) populations across New Zealand

Abstract

Kererū declined rapidly following European settlement in New Zealand, and they remain at a reduced density. We assessed three sources of information to test the hypothesis that predation by introduced mammals and abundance of food resources are the two major factors determining kererū abundance across New Zealand. First, we reviewed the literature on factors affecting the vital rates of kererū. This analysis showed that predation is the cause of most nest failures and deaths in kererū. Second, we examined data from a major database of bird sanctuary outcomes across New Zealand to evaluate long-term responses of kererū to intensive pest control at local scales. Kererū detections did not always increase following predator control, which suggests that food supply or forest area may be more important limiting factors at some sanctuaries. Third, to understand the factors underlying temporal and spatial kererū distribution patterns at a national scale, we assessed changes and patterns in kererū local occupancy through time using data from the 1969–1979 and 1999–2004 editions of the Atlas of Bird Distribution in New Zealand. Specifically, we asked (1) whether there have been declines in kererū occupancy between the two Atlases, and (2) how patterns of kererū occupancy relate to indigenous forest cover, temperature, urbanisation, and forest type (podocarp or beech dominated). Kererū occupancy remained stable across the North Island between the two time periods but declined in the South Island. Across both islands and time periods, kererū occupancy decreased significantly as forest cover decreased, which suggests that forest availability is still a major limiting factor across some parts of New Zealand. Overall, our results support previous studies showing that predation by introduced mammals is the primary limiting factor for kererū in forests, but also highlight the importance of forest area and food supply for kererū recovery.

Identifying global research and conservation priorities for Columbidae: a quantitative approach using random forest models

“…Although some columbid species flourish in landscapes modified by humans, such as the widespread Feral Pigeon, Columba livia domestica (Carlen et al, 2021), the Zenaida Dove, Zenaida aurita (Wolff et al, 2018), or the invasive Eurasian Collared-Dove, Streptopelia decaocto (Luna et al, 2018), most species rely on their natural habitats to persist. For instance, the occupancy of the Kereru, Hemiphaga novaeseelandiae, an endemic New Zealand Pigeon, has been affected by habitat loss resulting from European settlement in New Zealand (Carpenter et al, 2021). Following Walker (2007), the present study confirms that the loss of natural habitat is the greatest cause of extinction and populations decline in columbid species.…”

Total response models as a conceptual management framework for conserving vulnerable secondary prey

“…For some prey species, any degree of predator reduction may have some benefit (i.e., linear relationships), while for some vulnerable prey species there may be no response unless predator densities are reduced to very low levels. The empirical evidence for most secondary prey species supports the latter (Figure 2)-positive responses generally occur below threshold predator densities, but note that some prey populations show little or no response even at low predator densities (see Figure 2 in Binny et al, 2020;Figure 5 in Carpenter et al, 2021; Figure 2 to Figure 5 in Norbury et al, 2022;Figure 3 in Spencer et al, 2017). There are many explanations for this (see Doherty & Ritchie, 2017), including compensatory effects of other pest predators (e.g., Courchamp et al, 1999a;Norbury et al, 2013), or persistence of factors that are more limiting than predation, such as inadequate food supply or shelter (e.g., Fischer et al, 2020;Lavers et al, 2010).…”

“…In these cases, prey can potentially increase to density "c" or "c*" but note that densities below "b" or "b*" can still decline to extinction (in the case of Figure 3g) or to a low-density domain "a" (in the case of Figure 3i). These predictions are evident in the empirical predator density-impact functions for birds, lizards, and invertebrates in Figure 2-positive prey outcomes occur only when predator densities are reduced below a low critical level, but below this level some prey populations still fail to respond (as shown in Binny et al, 2020;Carpenter et al, 2021;Norbury et al, 2022;Spencer et al, 2017). While there are many potential mechanisms for this type of response (outlined earlier), the models in Figure 3 offer an alternative explanation based on total response predation theory.…”

“…Examples of empirically derived predator density‐impact functions for vulnerable secondary prey species, showing relationships between: percentage trap catch of brushtail possums ( Trichosurus vulpecula ) and percentage of pairs of North Island kōkako ( Callaeas cinerea ) fledging young (a) (modified from Innes et al, 1999); mouse printing rates of tracking tunnels and numbers of southern grass skinks ( Oligosoma polychroma ) (b) and ground wētā ( Hemiandrus spp.) (c) in pitfall traps (modified from Norbury et al, 2022); ship rat ( Rattus rattus ) tracking index and number of kererū in 5 min bird counts (d) (reproduced from Carpenter et al, 2021); and residual ship rat tracking index (RTI) and predicted rat control effect size on recovery rates of deeply endemic New Zealand bird species for each endemicity level 7 years after the onset of control (e) (solid lines, predicted mean effect size; marker sizes depict relative weight each study contributed to overall effect size; shading, 95% confidence band; dotted lines, 95% prediction interval upper and lower bounds) (reproduced from Binny et al, 2020, with permission from Wiley). Approximate predator thresholds are apparent where prey recruitment or population size rapidly increase where predator density indices are less than about 10%.…”

Global change explains reduced seeding in a widespread New Zealand tree: indigenous Tūhoe knowledge informs mechanistic analysis

“…Thus, failure of tawa to produce large fruit crops is especially problematic for kererū population recovery due to the absence of complementary podocarp fruit. Combined with this resource scarcity, losses of kererū nests and adults to mammalian predation (Carpenter et al, 2021) could result in reproduction slipping below levels necessary to achieve population stability or growth. The once thriving presence of kererū, particularly the autumn mega-flocks of 100-500 birds per flock, has significantly declined in the Tuawhenua forests (Lyver et al, 2008).…”