An ecological study of Emydura macquarii macquarii in the south-east region
of Australia was conducted between October 1995 and March 1998. E. m. macquarii is
an abundant and widespread species of short-necked turtle that is highly variable in
morphology and related life history attributes. No study in Australia had previously
looked at geographic variation in biological traits in freshwater turtles, hence the level
of variation in E. m. macquarii had been poorly documented. The principal aims of this
study were to investigate the plasticity of life history traits across populations of E. m.
macquarii and to speculate on possible causes. A more intensive study was also
conducted on a rare and suspected declining population of E. m. macquarii in the
Nepean River to determine whether relevant management and conservation measures;
were required.
The study involved comparing various life history attributes between five
populations of E. m. macquarii (Brisbane River, Macleay River, Hunter River, Nepean
River and Murray River). The populations were specifically chosen to account for the
range of variation in body size within this subspecies. Body size (maximum size, size at
maturity, growth rates), population structures (sex ratios, age and size structures),
reproductive traits (clutch mass, clutch size, egg size, egg content, etc.) and other
attributes were collected for each population. Patterns of life history traits, both within
and among populations, were explored so that causes of variation could be sought.
Geographic variation in Body Size and other Related Life History Traits
Body size in E. m. macquarii differed markedly between populations. Females
ranged in maximum sizes (carapace length) of 180 mm in the Macleay River to over
300 mm in the Murray River. E. m. macquarii was sexually dimorphic across all
populations with females larger than males in all cases. Maximum body size was
positively related to the size at which a turtle matures. The size at maturity in turn was
positively related to juvenile growth rates. Age was a more important factor for males
in terms of timing of maturity whereas in females it was body size. Morphological
variation was not only great between populations, but also within populations.
Maximum body size was unrelated to latitude; hence it was inferred that habitat
productivity had the most important influence on geographic variation in body size.
Population structures also differed between populations. Sex ratios did not differ in the
Brisbane, Macleay and Murray Rivers. However, a male bias was present in the Nepean
River population and a female bias in the Hunter River. Juveniles were scarce in the
Brisbane and Macleay Rivers but numerous in the Nepean and Hunter Rivers.
Geographic Variation in Reproduction
There was large variation in reproductive traits across populations of E. m.
macquarii. Nesting season began as early as mid-September in the Brisbane River and
as late as December in the Hunter River, and continued until early January. Populations
in the Hunter and Murray Rivers are likely to produce only one clutch per season while
populations from the Macleay and Nepean Rivers can produce two, and on some
occasions, three clutches annually. The majority of females would appear to reproduce
every year.
Clutch mass, clutch size, and egg size varied greatly both within and among
populations. A large proportion of variation in reproductive traits was due to the effects
of body size. E. m. macquarii from large-bodied populations such as in the Brisbane
and Murray Rivers produced bigger eggs than small-bodied populations. Within a
population, clutch mass, clutch size, and egg size were all correlated with body size,
except the Nepean River. The variability of egg size was smaller in large-bodied
populations where egg size was more constant.
Not all variation in reproductive traits was due to body size. Some of this
variation was due to annual differences within a population. Reproductive traits within
a population are relatively plastic, most likely a result of changing environmental
conditions. Another source is the trade-off between egg size and clutch size. A negative
relationship was found between egg size and clutch size (except the Brisbane River).
Reproductive variation was also influenced by latitudinal effects. Turtles at lower
latitudes produces more clutches, relatively smaller clutch sizes, clutch mass and larger
eggs than populations at higher latitudes. Annual reproductive output is greater in
tropical populations because they can produce more clutches per year in an extended
breeding season.
Eggs that were incubated at warmer temperatures hatched faster and produced
smaller hatchlings. Incubation temperatures above 30�C increased egg mortality and
hatchling deformities, suggesting this is above the optimum developmental temperature
for E. m. macquarii. Hatchling size was positively related to egg size, hence hatchling
sizes was on average larger in the Murray and Brisbane rivers. However, population
differences remained in hatchling size after adjustments were made for egg size. For
example, hatchlings from the Hunter River were smaller than those from the Macleay
River despite the egg size being the same. These differences were most likely due to the
shorter incubation periods of hatchlings from the Hunter River.
Nepean River
The Nepean River population of E. m. macquarii is at the southern coastal limit
of its range. This is a locally rare population, which is believed to be declining. This
study aimed at determining the distribution, abundance, and population dynamics to
assess whether any conservation management actions were required. E. m. macquarii in
the Nepean River was mainly concentrated between Penrith and Nortons Basin,
although even here it was found at a very low density (10.6 - 12.1 per hectare). The
largest male caught was 227 mm while the largest female was 260.4 mm. Males
generally mature between 140 - 150 mm in carapace length and at four or five years of
age. Females mature at 185 -195 mm and at six to seven years of age.
Compared with other populations of E. macquarii, Nepean River turtles grow
rapidly, mature quickly, are dominated by juveniles, have a male bias and have a high
reproductive output. Far from being a population on the decline, the life history traits
suggest a population that is young and expanding. There are considered to be two
possible scenarios as to why the Nepean River population is at such a low density when it appears to be thriving. The first scenario is that the distribution of the population on
the edge of its range may mean that a small and fluctuating population size may be a
natural feature due to sub-optimal environmental conditions. A second scenario is that
the population in the Nepean River has only recently become established from dumped
pet turtles.
| Identifer | oai:union.ndltd.org:ADTP/218598 |
| Date | January 2001 |
| Creators | Judge, David, n/a |
| Publisher | University of Canberra. Resource, Environmental and Heritage Sciences |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | ), Copyright David Judge |
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