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    Introduced mammalian predators influence demography and trait variation of a New Zealand stag beetle
    (Springer Nature, 2024-08-01) Grey L; Trewick SA; Johnson SL
    Abstract: Most species co-evolve with their predators and develop strategies to avoid predation. This is not possible when a novel predator invades an environment. Native residents must quickly adapt to their new predator or face local extinction. Intense competition for mating opportunities exerts significant selective pressure that can drive the evolution of exaggerated structures across taxa. However, these elaborate traits can elevate the risk of predation for some organisms. In the present study, we observe the effect that rats have had on a population of endemic New Zealand stag beetles, Geodorcus helmsi. Rats in Rakiura | Stewart Island often prey on stag beetles, taking them to a sheltered area to eat them and discard any uneaten parts of the beetle, namely the head and mandibles. We compared the head size, mandible size and numbers of predated and non-predated male and female beetles in three sites in Rakiura | Stewart Island that differ in their abundance of mammalian predators. We found that the population demography and the size of the beetles differed significantly between sites. Additionally, we determined whether predated beetles were larger than non-predated beetles, across multiple years, regardless of site. We found that overall the predated specimens were larger than the non-predated beetles. The trends found here suggest that exaggeration of the male mandibles increases the predation risk of these iconic beetles, acting as a limit to mandible size. Implications for insect conservation: Our results show for the first time the effect that novel predators can have on a population of animals with exaggerated sexually selected traits. The presence of novel predators can cause a shift in both population demography and trait distribution.
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    The behavioural response of a small prey animal to the odour cues of familiar predators in a New Zealand model system : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Zoology at Massey University, Manawatu, New Zealand
    (Massey University, 2012) Carter, Gaylynne Marie
    There is a rich ethological literature examining the effects of predators upon the behaviour of their prey. There are also apparent discrepancies in results reported particularly with respect to wild versus domesticated or captive subject species, and especially amongst Rodentia where predator avoidance behaviours predominate. Using New Zealand as a model system, the aims of this thesis were to determine how prey animals respond behaviourally to a familiar predator odour; to evaluate the role of predator exposure in prey behaviour; and to test whether captive-derived results are applicable in wild populations. I undertook experiments under three different conditions: the standard laboratory Y-maze choice test, an enclosure test, and a field test. In each experimental scenario the prey (ship rats) were initially presented with odour treatments derived from a familiar predator (stoat), familiar non-predator (possum) and novel non-predator (chinchilla). The rats’ behaviour was then video recorded for analysis of either avoidance or non-avoidance behaviour. The captive enclosure rats and the free-living rats exhibited clearly definable non-avoidance responses to stoat odour, but in the Y-maze the same captive rats did not exhibit a consistent response. I hypothesized that the non-avoidance behaviours represented a form of predator inspection behaviour. Further captive enclosure and field experiments aimed to test whether the non-avoidance response to stoat odour was specific or generalized. Under both conditions the most marked non-avoidance behaviour was in response to male stoat and secondarily to the mixed sex cat odour. My research concluded with a snap trap experiment pairing stoat-odour lure and no-odour snap traps, yielding an overwhelming capture success in the stoat-odour lured traps. Significant developments in the study of predator-prey behaviour resultant from this research include the development of a captive experimental protocol that generates comparable field results, the description of non-avoidance responses to predator odour, and the reporting of predator inspection a behaviour previously unreported in ship rats and rarely reported within Rodentia. Future studies should aim to verify the non-avoidance behaviours in other rat species as a first step to determining whether this is a ship rat specific behaviour, or if its lack of expression in other rodents is an experimental artefact.
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    Black-fronted terns and banded dotterels : causes of mortality and comparisons of survival : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Palmerston North, New Zealand
    (Massey University, 2002) Keedwell, Rachel Jane
    1. The braided rivers of the South Island, New Zealand, provide feeding and breeding habitat for a range of endemic bird species. Many of these species are entirely reliant on braided river habitat for breeding, but because of a reduction in habitat quantity and quality due to weed infestation, hydroelectric power development and water abstraction, and high predation rates by introduced mammals, the survival of many of these bird species is under threat. 2. Predation is the primary cause of mortality for most braided river bird species, but most studies have only measured the effects of predation at the egg stage. In this study. I monitored breeding success of banded dotterels (Charadrius bicinctus) and black-fronted terns (Sterna albostriata) on the Ohau River, South Island, from 1998-2000, and examined the effects of predation on both species. The black-fronted tern is an endangered species about which very little is known, so I also investigated aspects of its breeding biology. 3. I used time-lapse video cameras at 39 nests of banded dotterels and determined that nest monitoring did not affect nest survival rates, nor do predators use human scent trails to locate nests. I attached radio transmitters to 49 banded dotterel chicks to assess rates and causes of mortality, but although a minimum of 18% of chicks were killed by predators, the transmitters did not provide information on the relative importance of the different predator species. Hatching success at 338 banded dotterel nests was 68%, and 48% of hatched nests fledged at least one chick. 4. Hatching success for 1022 black-fronted tern nests was 50.2%, and a minimum of 27.6% of 897 chicks survived through to fledging. Cause of mortality was assessed for 148 chick, juvenile and adult terns, and I found that predators were responsible for 47% of deaths. Feral cats (Felis catus) and Norway rats (Rattus norvegicus) were probably responsible for 19% and 51% of predator related deaths, respectively. Mortality rates remained high immediately after fledging because of predation. Annual adult mortality was estimated at 88-92% but the associated confidence intervals were wide (57-99%). I document aspects of black-fronted tern ecology such as incubation period, fledging period, egg and chick weights, and develop a preliminary method for sexing adult black-fronted terns based on body measurements. I measured black-fronted tern chick growth and survival and found that hatching asynchrony results in lower growth and survival in second hatched chicks. I collated all existing data black-fronted tern populations and from these data estimate that the population size is less than 10 000 but conclude that better data on population size and trends are required. 5. Using population viability analysis models, I compare the survival of banded dotterels and black-fronted terns on the braided rivers. Higher productivity and shorter generation times, rather than any behavioural differences, are probably the key factors that result in stronger population growth for banded dotterels, despite both species being subject to similar levels of predation. Simulations indicate that predator control could be the most effective way to increase black-fronted tern survival. 6. This study provides the first comprehensive record of black-fronted tern breeding biology. I provide evidence that the black-fronted tern population is almost certainly in decline and the species urgently needs further research. In addition, the interrelationships between rabbit (Oryctolagus cunniculus) abundance, predator abundance and nest predation rates are poorly understood and urgently need attention in order to better manage braided river communities and ensure the survival of black-fronted terns and other vulnerable bird species.