The documented decline of amphibian populations over the past two decades has increased
attention towards amphibian conservation. Much of this attention has been focused on testing
hypotheses as to the causal factors of these declines, however providing convincing data to
support any of these hypotheses has proved difficult. The testing of these hypotheses and the
implementation of endangered species recovery programs has been restricted by a lack of
knowledge of the ecology and population demography of amphibian species that have
suffered dramatic declines.
This thesis presents aspects of the research phase of the recovery program for the Southern
Corroboree Frog, Pseudophryne corroboree, a species that declined to very low numbers
during the early 1980's. In particular, this research aimed to determine the distribution,
abundance, population dynamics and demography of this rare species. A complete
reassessment of the conservation status of P. corroboree was undertaken and the nature of the
persistence of this species across the landscape was analysed. Temporal trends in abundance
and its relationship with population size were also investigated. Early life-history
survivorship and recruitment to metamorphosis were studied at the scale of individual nest
sites and populations, and the adult male population age structure and annual mortality were
investigated using skeletochronology.
The shout/response survey technique was used to survey and monitor the number of breeding
male P. corroboree during this study. This method was found to provide consistent results
when the surveys were conducted over a short (two week) period during the peak breeding
season in January. Neither time of day, nor the number of males present at a pool, was found
to influence the level of responsiveness of male P. corroboree to the shout/response
technique. Variation in the number of responding males to the shout/response technique
through the breeding season, assessed at a single site over two seasons, was unimodal with the
peak responding period occurring during the last two weeks of January during both the 1998
and the 1999 breeding seasons.
A systematic survey covering 213 sites across the entire historic distribution of P. corroboree
found this species to be persisting at 79 sites. The majority of these sites were in the northwestern
portion of the species former range, around the Jagungal Wilderness area, while no
extant sites were found in the south-eastern portion of the species former range in the Smiggin
Holes and Perisher Blue ski resorts area. The overall abundance of males at persistent sites
was extremely low, with 92 percent of sites having fewer than ten responding males. Only
one site was found to support greater than fifty responding males. A logistic regression
analysis found the persistence of P. corroboree to be associated with increased number of
pools within a site, decreased distance to nearest extant population and geographic position
(latitude and longitude) in the landscape. While annual variation was observed in the number
of breeding males for individual sites, there was no overall trend for an increase or decrease in
the number of males, regardless of population size. The average annual extinction rate for
local populations was five percent during this study, with those populations becoming extinct
having very few breeding males (between one and three) during the previous season.
Embryonic and tadpole survivorship was monitored for individual nests at three sites across
three years. Recruitment to metamorphosis for P. corroboree was characterised by high
variation in survivorship between nest sites, populations and years, while overall recruitment
for nest sites was skewed towards lower survivorship. Average nest survivorship to
metamorphosis across all sites and years was ten percent but the skewed nature of this
survivorship meant that the majority of nest sites attained very low or no survivorship. The
low proportion of nest sites that did attain high survivorship provided the greatest contribution
to overall recruitment. The levels of embryonic and tadpole mortality observed in this study
would be providing a considerable contribution to the regulation of current population sizes.
The greatest level of early life-history mortality was observed during the late autumn/winter
egg and tadpole stage, with high survivorship during the summer and early autumn egg stage
and the post-winter tadpole stage. The estimated sex ratio for seven populations, based on the
number of eggs within male nest sites, indicated that for most populations, regardless of
population size, there was a greater proportion of females to males. In general, the estimated
sex ratio of smaller populations showed greater annual variation and had a lower average
number of females to males than the single large population.
Tadpole surveys conducted across remnant populations during both 1998 and 1999 found
recruitment to metamorphosis to be very low for the majority of populations. A third of all
populations during both years attained no recruitment to metamorphosis, with those
populations that did attain recruitment typically having fewer than 20 tadpoles. While sites
with more frogs generally recruited more tadpoles, there was no strong relationship between
population size and the number of tadpoles recruited per male at the scale of either pool or
site. There was also no significant difference in recruitment levels between the two years.
Tadpole surveys across breeding pools within the single large population also found very low
tadpole abundance. There was no strong relationship between the number of male frogs at a
pool and the number of tadpoles per male and there was no significant difference in tadpole
abundance between the two years. Based on the low density of males at pools and sites
(typically less than five), and the skewed nature of nest survivorship identified from
monitoring individual nest sites, it seems likely that both deterministic and stochastic factors
are influencing recruitment levels in remnant populations of P. corroboree.
This study determined that adult male P. corroboree could be accurately aged using the
technique of skeletochronology, and this technique was used to determine the adult male
population age structure for three populations. The results indicated that adult male
P. corroboree can reach sexual maturity from metamorphosis in three years, but the majority
of individuals take four years. The oldest individual identified in this study was nine years
old from metamorphosis. The adult male age structure at the single large site showed very
little annual variation, whereas the two smaller populations showed highly pulsed age
structures from one year to the next. The size of adult males was found to be a poor predictor
of age. Annual adult male survivorship, calculated by following cohorts from one year to the
next, was 55 percent. Based on this calculation of annual adult male survivorship, it seems
likely that the initial decline in P. corroboree involved increased levels of adult mortality.
The results of this study indicate that the persistence of. corroboree in the wild is precarious
in the short-term. For this reason, it is recommended that efforts be undertaken to secure this
species ex situ. Attempts to increase population numbers in the wild would greatly benefit
from determining the factor(s) that have caused the decline in this species, however, failure to
do so should not preclude field experimental management aimed at developing technique to
increase the size of remnant populations. This is because it is likely that small population
stochasticity is contributing to the current regulation of population size and it is possible that
the factors that caused the decline in this species cannot be removed from the environment.
| Identifer | oai:union.ndltd.org:ADTP/219042 |
| Date | January 2000 |
| Creators | Hunter, David, n/a |
| Publisher | University of Canberra. Resource, Environmental & Heritage Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright David Hunter |
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