Minimising non-target impacts of anticoagulant rodenticide use for a highly susceptible species, the New Zealand lesser short-tailed bat (Mystacina tuberculata) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Manawatū, New Zealand
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Date
2019
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Massey University
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Abstract
Anticoagulant rodenticides may cause mortality in non-target wildlife. In New Zealand, broad-scale anticoagulant use is essential for wildlife conservation, but also poses risks to threatened species. In 2009, >100 endemic lesser short-tailed bats (Mystacina tuberculata) died during a rodent control operation. In this thesis I confirm that these bats were intoxicated with the first-generation anticoagulant rodenticide diphacinone, and present several lines of research investigating the route of exposure, the effectiveness of management changes at minimising exposure and mortalities and the potential consequences of such non-target impacts on bat population viability.
I used infra-red video cameras at non-toxic baits in wild and captive settings to determine that the route of exposure of the bats to diphacinone was most likely through ingestion of contaminated arthropods. In a field trial, analysis of communal guano deposits revealed that an alternative baiting method reduced but did not prevent exposure of bats to diphacinone. However, this exposure was subclinical, as prolongation of mean blood prothrombin time was not evident. Furthermore, mark-recapture analysis of passive integrated transponder (PIT) tagged bats indicated zero to negligible effect of exposure on population survival.
Despite this result, sublethal exposure to anticoagulants is of concern because of the unknown effects on bat fitness and reproduction. An abundance estimate using closed-population mark-recapture analysis revealed that the study population was small (c.780 adults) relative to other lesser short-tailed bat populations, and thus particularly vulnerable to reductions. I developed a model describing the population dynamics of the bats to explore the potential effects of chronic reduction of survival and productivity on population viability. While model projections highlighted the need to suppress rodents in bat habitat, they also demonstrated that small annual reductions in survival could threaten population persistence.
This study has contributed to bat conservation management in New Zealand and highlights the delicate balance that needs to be achieved between managing invasive vertebrates and protecting native species that are highly susceptible to vertebrate pesticides. Investigating the effects of sublethal exposure of bats to anticoagulant rodenticides should be a conservation priority as there are global implications for health and viability of other insectivorous bat species.
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Mystacina tuberculata, Diphacinone, New Zealand, Rodents, Control, New Zealand