Living organisms face innumerable pressures arising from the environment but despite these challenges many have been able to survive. The ability to adapt to change has determined the pool of species that now are populating our planet however it seems that many may be experiencing problems in adapting to change. The debate whether the current high rate of extinctions is due to natural or anthropogenic factors is in vogue throughout the scientific community, however a consensus of opinion does not exist. Clearly this knowledge is vital for the implementation of successful conservation strategies and we propose that studying invasive species will help us to understand what is needed for successful adaptation and the factors that enable species to prosper.
The first strategy that an organism employs in adapting to its environment involves adjustment to the physiological and cellular responses that it already possesses. The Hypothalamus-Pituitary-Adrenal axis (HPA) is one of the critical pathways in which homeostasis is maintained during stress and this critical pathway is conserved among vertebrate species. Using the cane toad (Rhinella marina) as the model, this thesis investigates the stress response in amphibians exposed to different environments. This thesis also investigates the interaction of HPA axis with other physiological and biological responses regarded as important during the process of adaptation.
This thesis provides evidence that different environments are able to promote changes in the stress response of cane toads that will be reflected in adrenocortical function and adrenal histomorphometry. Animals from the front line of dispersion in Australia (WA) showed the lowest levels of corticosterone (B) elevation in response to an adrenal challenge. Furthermore toads from native (MX) and established populations in Australia (QL) did not show marked differences in their stress response. This evidence is consistent with the idea that once a group finds an appropriate niche similar to its native environment, the response converges in a single best reactive response. Finally a group of toads maintained in captivity for one year (AH) showed the highest levels of total and free B after adrenocorticotrophic hormone (ACTH) administration, and this group also had the largest cortical cell area. These changers are associated with chronic stress and are similar to other published reports for species in captivity.
Another important finding was that glucocorticoids (GCs) in male cane toads interact with indices of energetic status and immune competence. It was also observed that these interactions predict changes in lungworm (Rhabdias sp.) load intensity. Corticosterone was found interacting with spleen size with increasing parasite loads. The interaction reveals that despite high levels of parasites, toads with low levels of B will have larger spleen sizes. Besides B, testosterone is also associated with parasite loads. Both hormones indirectly affect parasite loads by affecting variables related to energy resources. The influence of testosterone on fat reserves predicts that animals with low fat reserves and high levels of testosterone should have higher parasite intensity than toads with higher fat reserves, but the same levels of testosterone. On the other hand mediation analysis was used to determine the extent to which hormone levels directly or indirectly affect parasite loads. The analysis shows that the access to energy is a strong mediator of the effect of B and testosterone in parasite loads. This suggests that the negative effect of B and testosterone upon free fatty acids (FFA) influence significantly the intensity of lungworm parasites in male cane toads.
The effect of selected agrochemicals (fenitrothion and atrazine) on binding of steroid hormones to plasma globulins was also investigated in an amphibian (cane toad) and a mammalian (rat) model. Agrochemicals were shown to indirectly disrupt the stress response by changing the free:total ratio of corticosterone in plasma. The potential effect was examined of atrazine and fenitrothion no-observed-effect-levels (NOEL) on the binding of corticosterone (B) to corticosterone-binding-globulin (CBG) in both the amphibian and mammalian model. Competition studies showed that both atrazine and fenitrothion NOEL are able to compete with B for CBG binding sites in amphibian and rat plasma.
In conclusion, dispersing toads and those captured from the wild and kept in captivity display significant adjustments in endocrine and immune function during adaptation. However if they can survive and they become established in the new environment, no differences can be shown in the stress response compared with toads in native populations. Corticosterone can affect parasite loads by interacting with the immune system and energy reserves, and anthropogenic stressors such as agrochemicals can indirectly disrupt the stress response by changing the free:total ratio of corticosterone in plasma.
These studies were conducted on a readily available amphibian, the cane toad, however the findings are most likely also applicable to native amphibian species, many of which are critically endangered.