Spatial ecology, habitat use, and the impacts of rats on chevron skinks (Oligosoma homalonotum) on Great Barrier Island : a thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Conservation Biology, Massey University, Auckland, New Zealand
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Date
2009
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Massey University
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Abstract
The chevron skink (Oligosoma homalonotum) is one of the largest, yet least observed
skink species in New Zealand. The species was thought to have once been widespread
in Northern New Zealand, however currently it is only found on Great Barrier and Little
Barrier Islands. Great Barrier Island is the apparent stronghold for the species although
it appears to be in decline there, despite a net increase in habitat. Recent studies have
increased the understanding of the general ecology of the species, however little is
known about the threats to the survival of this species.
This study had two main objectives; the first was to establish if rats are a threat to
chevron skinks, and the second was to increase current knowledge of the species
ecology. The research was undertaken in an area of extensive rodent control (Glenfern
Sanctuary) and an adjacent unmanaged reserve in Port Fitzroy, on Great Barrier Island
in 2008. The first objective of this study involved confirming that rat densities in the
treatment (Glenfern Sanctuary) were sufficiently different to allow meaningful
comparisons of chevron skink population characteristics between sites. This was
achieved by determining absolute rat densities using Zippin’s removal method at four
sites, and correlating these with a relative abundance measure (tracking rates) to give
confidence in the observed trends. Rat densities were high (1.94 - 3.00 rats ha-1) in the
control, and low (0.00 and 0.06 rats ha-1) in the treatment sites, and these correlated
well with tracking rates. In light of these clear differences between the treatment and
control, the population structure and condition of chevron skinks were compared
between sites. The population structure showed erosion of juvenile and sub-adult size
categories, which indicated differences in vulnerabilities between size categories.
Physical evidence of failed rat predation was also observed in adult skinks in the
unmanaged control, which confirmed that rats were interacting with chevron skinks.
Although the adults survived the attacks they suffered injuries including eye damage,
punctures, cuts and tail loss. Smaller skinks would be unlikely to survive such attacks
due to the severity and scale of the injuries, supporting the assertions of the population
structure that smaller skinks may be more vulnerable than adults. The extent of tail loss
was converted to a condition index to determine if failed rat predation was more
widespread in the population, than was observed by conspicuous injuries. This
condition index (body-tail condition index) was stable through all size categories in the
treatment, but significantly reduced in adults in the unmanaged sites. That there was no
reduction in the condition of smaller skinks in the unmanaged control sites despite high
rat densities suggests that interactions between rats and smaller skinks are fatal, and
thus not represented in the data.
Nine chevron skinks were radio-tracked to determine habitat use, home range and
ranging behaviour. Habitat use of chevron skinks was similar to a previous study and
demonstrated that trees, crevices and logs were important refuge sites. Chevron skinks
were more likely to be found at sites with trees, crevices and debris dams. Chevron
skink home ranges indicated that adults moved further away from streams than
previously anticipated at this time of year, and skinks demonstrated site fidelity. There
was also overlap in home ranges between individuals, and skinks with overlapping
home ranges shared common refuges. During flooding events, chevron skinks
exhibited an arboreal response that appears to be a behaviour specific to stream
associated animals, which allows them to avoid being taken by floodwaters.
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Keywords
Oligosoma homalonotum, Chevron skink, Ecology, Rat predation, Conservation biology