The ecology of the polytypic freshwater turtle species, Emydura macquarii macquarii

  • David Judge

    Student thesis: Master's Thesis


    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.
    Date of Award2001
    Original languageEnglish
    SupervisorArthur Georges (Supervisor)

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