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Response of juvenile growth rate to experimental removal of nest predators in a long-lived vertebrate. Ecology (under review)

R-J Spencer, F. J. Janzen and M.B. Thompson. 2005

Examining the phenotypic and genetic underpinnings of life-history variation in long-lived organisms is central to the study of life-history evolution. Juvenile growth and survival are often densitydependent in reptiles and theory predicts the evolution of slow growth in response to low resources (resource-limiting hypothesis), such as under densely populated conditions. However, rapid growth is predicted when exceeding some critical body size reduces the risk of mortality (mortality hypothesis). Here we present results of paired large-scale, 5-year field experiments to identify causes of variation in individual growth and survival rates of an Australian turtle prior to maturity. To distinguish between these competing hypotheses, we reduced nest predators in two populations (retained a control population) to create variation in juvenile density by altering recruitment levels. We also conducted a complementary split-clutch field transplant experiment to explore the impact of incubation temperature (25C or 30C), nest predator level (low or high) and clutch on juvenile growth and survival. Juveniles in high-density populations were not resource limited, growing more rapidly than young turtles in the control populations. Our experiments also revealed a remarkably long-term impact of the thermal conditions experienced during embryonic development on growth of turtles prior to maturity. Moreover, this thermal effect was manifested in turtles approaching maturity rather than in turtles closer to hatching, and was dependent on the population density in the post-hatching rearing environment. This apparent phenotypic plasticity in growth is complemented by our observation of a strong, positive genetic correlation between individual growth in the experimental and control populations (rG = >+0.77). Thus, these Australian pleurodiran turtle populations have the impressive capacity to acclimatize plastically to major demographic perturbations, as well as enjoy the longer-term potential to evolve adaptively. These findings suggest that long-lived organisms possess two means for responding to substantial changes to population structure to maintain viability.


Population parameters and life table analysis of two co-existing freshwater turtles: are the Bellinger River turtle populations threatened? Wildlife Research

Sean J. Blamires, Ricky-John Spencer, Peter King, and Michael B. Thompson. 2005

Two species of freshwater turtle co-exist in the Bellinger River, Elseya georgesi is common but limited to the Bellinger River, whereas Emydura macquarii is widespread but rare in the Bellinger River. The Bellinger River population of E. macquarii has been proposed as a distinct subspecies, so may be endangered. Survivorship, fecundity, growth, size, and age, were determined for El. georgesi and the finite rate of increase (l) was estimated by a life table analysis using mark-recapture data from surveys between 1988-2004. These parameters were compared to those of well studied populations of E. macquarii to assess if modelling El. georgesi could serve as a surrogate for estimating the influences of these demographic parameters on l in E. macquarii in the Bellinger River. We estimated that approximately 4500 El. georgesi inhabit the study area, and despite a strongly biased size distribution towards large individuals, the population is increasing, lambda=1.15, in the best case scenario, or slightly decreasing, lambda=0.96, in the worst case scenario. Comparing El. georgesi with the Bellinger River and other E. macquarii populations suggest that E. macquarii grows faster, attains greater maximum size, has a greater clutch size and a higher fecundity than El. georgesi. Hence, El. georgesi does not serve as a good surrogate to determine influences on lambda in E. macquarii.

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Adult free zones in Australian small mammal populations: Response of Australian natives rodents to reduced cover. Austral Ecology.


Spencer R-J, Cavanough V.C., Baxter G.S., and Kennedy M.S. 2005.


Cover provides shelter, food, nesting opportunities, and protection from predators. The behavioural response of small mammals to reduced cover has been well documented. However, very little is known about the effect of cover on community and population dynamics. Australian small mammals generally inhabit extremely dynamic ecosystems, where cover and food supplies are greatly affected by fire. Species are described as early or late seral specialists, generally returning to a disturbed area once their habitat requirements are met. Habitat requirements have loosely been interpreted as cover and food supply, however, these factors are not mutually exclusive and few studies have attempted to determine the driving factors behind small mammal succession. In this study, we manipulated specific aspects of cover in the eucalypt forests of Fraser Island and show that the behaviour and population dynamics of small mammals were greatly affected. A reduction of cover from grass-trees (Xanthorrhoea johnsonii) did not affect small mammal species composition, however, the abundance and size structure distribution of the dominant species (Rattus fuscipes) decreased. Patch use by rodents also decreased after cover was reduced. Rattus fuscipes must trade-off remaining in an environment with increased risk of predation or disperse to an area with greater cover but increased competition. Juveniles dominated (>60%) populations of R. fuscipes after cover was reduced, however, size distributions of control sites were relatively more even (<25% juvenile). While adult R. fuscipes are either killed by predators or disperse to other areas, juveniles that remained or immigrated to an area of reduced cover gained a selective advantage over control sites, because reduced competition with adults increased body condition of juvenile R. fuscipes.

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Effects of Prescribed Fire and Fire History on the Structure and Composition of Mixed Forests on Fraser Island, Australia. Austral Ecology


Ricky-John Spencer  and Greg S. Baxter. 2005


Few generalities exist about the impact of disturbance regimes on ecosystem responses. Fire s are integral to the healthy functioning of most ecosystems and are often poorly understood in policy and management, however, the relationship between vegetation composition and habitat structure is intrinsically linked, particularly after fire. The aim of this study was to test whether the variability of habitat structure or vegetation composition and abundance in forests at a regional scale can be explained in terms of fire frequency using historical data and experimental prescribed burns. We tested this hypothesis in mixed eucalypt forests of Fraser Island. Fraser Island dunes show progressive stages in plant succession, including changes in floristics and structure and increasing biomass (east), followed by stages of decline as access to nutrients decreases (west). This retrogressive succession over a small area provides an opportunity to test for the relative affects of fire frequency and nutrient availability as regulating factors of forest communities, under relatively controlled conditions. We found that fire frequency was not a good predictor of vegetation composition or abundance across dune systems; rather, its affects we re dune specific. In contrast, habitat structure was strongly influenced by fire frequency, independent of dune system. A dense understorey occurred in frequently burnt areas, whereas infrequently burnt area s had a more even distribution of vegetation heights. Vegetation communities were at similar composition and abundances within six months of a fire and frequently burnt areas remain in a mature early successional state, with only relatively minor or subtle secondary successional stages. These early successional ecosystems were characterised by low diversity and frequently burnt areas on the east coast are dominated by Pteridium. Greater midstorey canopy cover in low frequency areas reduces light penetration and allows other species to compete more effectively with Pteridium. Our results strongly indicate that frequent fires on the Island have resulted in a decrease in relative diversity through dominance of several species. Prescribed fire represents a powerful management tool to shape habitat structure and complexity of Fraser Island forests.
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Clustering of related individuals in a population of the Australian lizard, Egernia frerei. Moleular Ecology.


Susan J. Fuller, C. Michael Bull, K. Murray and R. J. Spencer (2004)


Stable social aggregations are rarely recorded in lizards, but have now been reported from several species in the Australian scincid genus Egernia. Most of those examples come from species using rock crevice refuges that are relatively easy to observe. For many other Egernia species that occupy other habitats and are more secretive, it is harder to gather the observational data needed to deduce social structure. Instead we used genotypes at six polymorphic microsatellite DNA loci of 229 individuals of E. frerei, trapped in 22 sampling sites over 3,500 ha of eucalypt forest on Fraser Island, Australia. Each sampling site contained 15 trap locations in a 100 x 50 m grid. We estimated relatedness among pairs of individuals and found relatedness was higher within than between sites, and that female relatedness within sites was higher than male relatedness, or between males and females. Within sites we found that juvenile lizards were highly related to other juveniles and to adults trapped at the same location, or at adjacent locations, but relatedness decreased with increasing trap separation. We interpret the results as suggesting high natal philopatry among juvenile lizards and adult females. This result is consistent with stable family group structure previously reported in other rock dwelling Egernia species, and suggests that social behaviour in this genus is not habitat driven.

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A comparative study of environmental factors that affect nesting in Australian and North American freshwater turtles. Journal of Zoology.


Kenneth D. Bowen, Ricky-John Spencer and Fredric J. Janzen. 2005.


The timing of reproductive events is critical for fitness, and these events are often linked to weather and climate. Weather components are thought to influence the nesting behaviour of freshwater turtles, but to date there have been few quantitative studies and no comparative studies. We compared the environmental cues used by nesting Australian (Emydura macquarii and Chelodina expansa) and North American (Chrysemys picta) freshwater turtles, and quantified the differences in weather between days with and without nesting activity within the nesting season. We also characterised the diel time of nesting for each species. The results suggest that nesting behaviour is related to warm air and water temperatures in C. picta and to rainfall in E. macquarii and C. expansa. Chrysemys picta primarily nests in the afternoon and evening, E. macquarii is a crepuscular nester, and C. expansa nests diurnally. While changes in life history resulting from climate change are difficult to predict, we suggest that an increase in the number of El Nino events may have adverse effects on the two Australian species, whereasincreases in environmental temperature may expand the number of nesting opportunities for C. picta.

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