Longitudinal population demography of the allied rock-wallaby, Petrogale assimilis

Delean, J. Steven C. (2007) Longitudinal population demography of the allied rock-wallaby, Petrogale assimilis. PhD thesis, James Cook University.

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The allied rock-wallaby, Petrogale assimilis, is a medium-sized, monomorphic, continuously-breeding macropodid marsupial that lives in rocky habitats in the climatically unpredictable wet-dry tropics of north-eastern Australia. Long-term capture-recapture records of individually marked P. assimilis from an isolated population inhabiting Black Rock, a sandstone escarpment in western Queensland, were used to investigate the population demography of the species over 12 years.

In natural populations, both extrinsic environmental variation and intrinsic density-dependence contribute to variability in demographic fitness components. Changes in these fitness components, and trade-offs between them, determine the dynamics of populations. Almost no information is available on the temporal variability of the demographic fitness components and their relationship with rates of population change for kangaroos and wallabies. This research provides the first long-term study of a macropod species that is based on individually-marked animals, which are required to estimate such fitness components and their temporal variation. A major aim of this research was to determine the impact of variability in the fitness components on the population dynamics of this macropod species, and to compare these results with those available for other long-lived herbivores. The demographic components of the life history of P. assimilis at Black Rock were investigated, along with the interrelationships between long-term temporal variability in these components and intrinsic and extrinsic factors, and individual fitness. The long-term fitness of the population and the influence of temporal covariation between the demographic components on variability in population fitness were also examined.

The growth relationship between the size and age of P. assimilis at Black Rock was determined using various nonlinear models within a mixed-effects framework. The age and individual variation in growth of animals of unknown age were estimated from measurements of body size. Parametric nonlinear models did not fit the growth pattern, but semi-parametric spline models adequately described the growth pattern and accounted for individual variation. Differences in growth between the sexes were small, suggesting no strong sexual size dimorphism in P. assimilis. Growth rates were highest during pouch development, were lower during the weaning period, and decreased dramatically after weaning. The age structure of the population of P. assimilis at Black Rock varied substantially over the study period.

The log-transformed relationships between body mass and various body size measures were nonlinear and the head length was the best predictor of body mass of P. assimilis at Black Rock. Indices of body condition were calculated from the residuals of mixed-effects models that estimated the form of the size-mass relationship using splines. Substantial variation in body condition was explained by annual and seasonal variability and lagged rainfall, as well as variation between individual animals. Variation in body condition was not associated with sex or age-class, and did not depend on the lactation status of females. The estimated index of body condition appeared to represent the nutritional status of individual P. assimilis at Black Rock and was used as a predictor of variation in demographic rates associated with individual fitness.

Directional goodness-of-fit tests for Cormack-Jolly-Seber capture-recapture models showed that individual P. assimilis at Black Rock had a trap happy response to capture. The likelihood of recapture varied over time and depended on whether animals were captured on the previous sampling occasion. Recapture probability was generally very high, and was lower for adult females not marked as young than for other females marked as young and for all males, regardless of age at marking. Similar effects on recapture probability were identified using generalised linear mixed models, though the time-dependent effect could be simplified to a year by season interaction plus the effect of trapping effort. There was strong agreement between estimates of population size based on different methods. Population size fluctuated substantially over the study with periods of consistent increase and decline, and showed evidence of rapid population recovery from relatively low numbers under positive environmental conditions.

Support for the Trivers-Willard hypothesis (TWH), which states that if the costs of reproduction differ between the sexes then the offspring sex ratio will vary depending on the parent’s ability to allocate resources, was evaluated. The sex ratio at birth was equal and not correlated with mother’s age, body mass or body condition. Sex ratio varied seasonally; being female-biased in the mid- to late-dry season and male-biased in the late dry and early wet seasons. Survival from birth to pouch emergence was correlated with environmental conditions, depending on sex, in the direction consistent with TWH; male survival was higher than female survival under good conditions and lower under poor conditions. Also, survival was higher for pouch young of heavier mothers or mothers in good condition. Survival to pouch emergence was also density-dependent. In support of TWH, the sex ratio at pouch emergence was male-biased under good environmental conditions and was male-biased for mothers in good condition or heavier mothers. Mothers in good condition produced offspring in good condition, satisfying an assumption of TWH, and higher mass at pouch emergence resulted in improved survivorship to weaning.

Survival from pouch emergence to weaning was higher for females than males, and was positively correlated with environmental conditions. Females had higher survival to weaning than males in the hotter seasons, but there was no sex difference in survival in the cooler seasons. Survival was also higher for females than for males born to lighter mothers, but there was no difference between the sexes born to heavier mothers. Male-biased sex ratios at weaning were observed under good environmental conditions and for mothers with higher body mass, providing support for TWH at this life history stage. The body mass of offspring at weaning was correlated with maternal body mass, satisfying an assumption of TWH, and offspring mass was also higher under high rainfall conditions. Patterns of variability in the sex ratio and pre-weaning survival of P. assimilis at Black Rock were influenced by a combination of: (1) adaptive allocation of resources between the sexes depending on maternal condition; and (2) non-adaptive extrinsic modification associated with environmental stochasticity, allowing mothers to respond quickly in a variable environment to maximise their current reproduction and future survival.

Male P. assimilis matured later and were heavier at maturity than females. Early maturity was favoured for females but not for males under high rainfall conditions, independent of size at maturity. Age at maturity was density-dependent for males only, with delayed maturity at lower population sizes. Maturation was delayed for both sexes when body mass at weaning was low relative to individuals with high weaning mass.

Subadult male survival was lower than for females, and survival of both sexes increased under high rainfall conditions. Adult survival was higher than subadult survival, independent of sex, and survival of older adults was lower than that of prime-aged adults, indicating senescence. Survival increased under positive climatic conditions and was density-dependent, with lower survival at high population sizes. Males had lower survival to sexual maturity than females, and increased body mass improved survival for both sexes. Population growth rate was most sensitive to changes in prime-aged adult survival, as is typical for longer-lived vertebrates. The temporal variance in the juvenile survival stages was much higher than in adult survival. Therefore, consistent with other longer-lived, iteroparous vertebrates, the impact of relative changes to adult survival on population growth were much greater than changes of the same magnitude to either survival in the juvenile stages, or to the fecundity rate.

There was a negative relationship between temporal variation in the demographic rates and their sensitivities, indicating that the more variable vital rates had low effects on long-term population growth. The most variable demographic rates covaried suggesting that common factors contributed to temporal variability. Covariation in survival between the juvenile and pre-maturity life history stages accounted for most of the variability in the population growth rate, indicating survivorship in the juvenile stages was more important than adult survival in determining changes in the population growth rate of P. assimilis at Black Rock. The survival patterns of different life history stages were variable for P. assimilis at Black Rock, and responded differently to environmental variability and population density. Density-dependent and density-independent limiting factors primarily acted on the juvenile survival components of the life history, and covariation among these fitness components influenced the dynamics of the population. Survival of prime-aged adult P. assimilis at Black Rock was much less sensitive to these limiting factors, showed the highest elasticity, and appeared to be buffered against temporal variability. These results add to growing evidence from natural populations that natural selection may favour traits that are highly buffered against environmental variability.

Item ID: 2010
Item Type: Thesis (PhD)
Keywords: demography, petrogale assimilis, allied rock wallaby, wet-dry tropics, Black Rock, Queensland, macropods, population dynamics, age, growth, sex ratio, survival, body-mass, maturation, life histories, maturity, environmental stochiasticity, climate variability, longevity, weaning, reproduction, capture-recapture method, density-dependence, fitness
Date Deposited: 01 Dec 2008 00:19
FoR Codes: 06 BIOLOGICAL SCIENCES > 0606 Physiology > 060603 Animal Physiology Systems @ 0%
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