The decline and extinction of amphibian species over the past three decades is widely acknowledged as one of the greatest biodiversity crises of modem time. Providing convincing data to support hypotheses about these declines has proved difficult, which has greatly restricted the development and implementation of management actions that may prevent further amphibian declines and extinctions from occurring. In this thesis, I present research that was undertaken as part of the recovery programs for the southern corroboree frog (Pseudophryne corroboree),and the Booroolong frog (Litoria booroolongensis); two species that underwent very rapid declines in distribution and abundance during the 1980's. More specifically, I investigated potential causal factors in the declines of both species using experimental and correlative studies, and examined the mechanisms by which one threatening process (chytridiomycosis) may be causing continued decline and extinction in P. corroboree. I also examined the implications of population dynamics for monitoring L. booroolongensis, and suggest a possible monitoring strategy that may reliably facilitate the implementation of recovery objectives for this species. I also tested one possible reintroduction technique aimed at preventing the continued decline and extinction of P. corroboree populations. In Chapters 2 and 3,I present the results from a series of experiments in artificial enclosures designed to examine whether the tadpoles of L. booroolongensis are susceptible to predation by co-occurring introduced predatory fish species; brown trout (Salmo trutta), rainbow trout (Oncorhynchus mykiss),European carp (Cyprinus carpio), redfin perch (Percafluviatilis), and mosquito fish (Gambusia holbrooki). I demonstrated that the tadpoles of L. booroolongensis, and a closely related species Litoria lesueuri, were palatable to non-native trout species, but not to two native predatory fish species, Gadopsis bispinosus and Galaxias olidus. A pond breeding frog species included in this experiment, Limnodynastes tasmaniensis, was palatable to both the native and non-native fish species. In a separate experiment I also demonstrated that the tadpole of L. booroolongensis is palatable to the three other introduced fish species examined in this study; C. carpio, P. fluviatilis, and G. holbrooki. In three of the experiments, the provision of rock within enclosures as a potential refuge habitat did not afford protection to L. booroolongensis tadpoles from predation by any of the five introduced fish species examined. While all the introduced fish species tested here did consume L. booroolongensis tadpoles, the results also suggested that chemical unpalatability might afford some level of protection against some of these fish species. Firstly, the addition of alternative prey items in one of the experiments reduced the proportion of tadpoles consumed, suggesting that L. booroolongensis may not be a preferred prey item. Secondly, the proportion of tadpoles consumed varied greatly among the different fish species examined, suggesting differing levels of palatability. Overall, this study supports previous research in suggesting that chemical unpalatability may be an important strategy for the tadpoles of riverine frog species in south-eastern Australia to avoid predation by native fish species, and that this strategy is less effective against introduced fish species. While L. booroolongensis currently persists in streams inhabited by a number of introduced fish species, this study supports the likelihood that these species are having a negative impact on populations of L. booroolongensis in the wild. In Chapter 4,I present the results of a study aimed at examining potential monitoring techniques for L. booroolongensis. The results of a mark-recapture exercise demonstrated that L. booroolongensis may exhibit large fluctuations in abundance from one year to the next, and through a prospective power analysis approach, I demonstrated that it would be difficult to confidently identify population trends of interest using either indices or estimates of abundance for this species. An assessment of the capacity to identify the presence or absence of L. booroolongensis using nighttime spotlight surveys demonstrated the high detectability of this species using this technique, at both the scale of 300-meter sections of stream and individual breeding areas (typically less than 10-meters of stream). This study suggests that the monitoring objectives of the L. booroolongensis recovery program would be most effectively achieved using presence/absence surveys at different scales. In Chapter 5,I present the results of a field survey aimed at determining the current distribution and habitat requirements of L. booroolongensis in the South West Slopes region of New South Wales. Of the 163 sites I surveyed across 49 streams, I located L. booroolongensis along 77 of these sites from 27 streams. Based on population and habitat connectivity, this study identified 18 populations of L. booroolongensis that are likely to be operating as independent populations. Twelve of these populations are not represented in conservation reserves, but rather occur along streams that flow through the agricultural landscape. A broad scale habitat analysis identified a positive relationship between extent of rock structures along the stream and the occurrence of L. booroolongensis, and a negative relationship between the proportion of canopy cover and this species' occurrence. At the breeding habitat scale, this study identified a positive relationship between the presence of breeding males and; number of rock crevices in the aquatic environment, extent of emergent rocks, and proportion pool. This analysis also detected a negative relationship between occupancy and water depth. These results confirm previous work suggesting the importance of rocky stream habitats to the persistence of L. booroolongensis, but also suggest how disturbance processes, such as increasing sedimentation and weed invasion, may reduce the suitability of rocky structures as breeding sites. In Chapter 6,I investigated current levels of amphibian chytrid fungus (Batrachochytrium dendrobatidis) infection in corroboree frog populations, and used retrospective screening of museum specimens to assess the possibility that this pathogen was implicated in the initial decline of the corroboree frogs. Using histology, I did not detect any B. dendrobatidis infections in corroboree frog populations prior to their decline, however using the same technique, moderate levels of infection were detected in post-decline populations of both species. Real-time PCR screening of skin swabs identified much higher overall infection rates in post-decline populations of P. corroboree (between 44% and 59%),while significantly lower rates of infection were observed in P. pengilleyi populations (14%). These results suggest that the initial and continued decline of the corroboree frogs may well be attributed to the emergence of B. dendrobatidis in populations of these species. In Chapter 7,I investigated how B. dendrobatidis may be causing the continued decline of P. corroboree through the presence of an abundant reservoir host for this pathogen. I found that populations of adult C. signifera in sub-alpine bogs carry high B. dendrobatidis infection rates (86%),but appear unaffected by this infection. An experiment involving the release of P. corroboree tadpoles into 15 natural pools resulted in metamorphs from seven of these pools testing positive for B. dendrobatidis, with all these individuals dying soon after metamorphosis. These results support the possibility that B. dendrobatidis infection in P. corroboree populations is being facilitated by the presence of large numbers of infected C. signifera in the shared environment. Chapter 8 presents the results of a population augmentation study for P. corroboree. I investigated the extent to which increasing recruitment to metamorphosis may result in population recovery in this species. This was undertaken by harvesting eggs from the field and rearing them through to mid stage tadpoles over the winter period prior to being released back to their natal ponds in spring. While I was able to increase recruitment to metamorphosis by an average of 20 percent, this did not result in a noticeable influence on the subsequent adult population size, as both manipulated and non-manipulated sites declined over the course of this study by an average of 80 percent. I observed a positive relationship between natural recruitment to a late tadpole stage and subsequent adult male population size, however there was considerable variation associated with this relationship. The relationship between recruitment and subsequent population size at the augmentation sites was consistent with the relationship observed at the non-manipulated sites. These results suggest that recruitment to metamorphosis may not be the most important life stage restricting the population recovery of P. corroboree, but that mortality during post-metamorphic stages may be more important in regulating current population size. Hence, further attempts to use captive rearing to increase P. corroboree populations in the wild should focus on the release of post-metamorphic frogs. Overall, this thesis demonstrates the value of quantitative research to the implementation and progress of threatened species recovery programs. While this research will specifically contribute to the recovery programs for L. booroolongensis and P. corroboree, it more broadly contributes to the understanding and capacity to respond to the concerning levels of amphibian extinctions currently occurring throughout the world.
|Date of Award||2007|
|Supervisor||Will Osborne (Supervisor) & Arthur Georges (Supervisor)|