Cane toads are one of the most infamous amphibian invaders in the world, having been translocated from their native range in South America to many countries including Australia. The toads have been an ecological catastrophe for some Australian wildlife, but have provided an amazing study system for researchers. The species’ global invasion success has been extensively studied by multidisciplinary groups in a number of countries. That’s because the toads have exhibited an ability to change very rapidly to adapt to the new challenges they have encountered. The mechanisms involved in their success involve multiple aspects of biology – physiology, biochemistry, behaviour, ecology – and those elements combine and interact in subtle ways as to allow the survival of cane toads in locations that are very different from each other, and in some cases very different from the conditions under which the species evolved.
Physiological aspects are crucial, because the response to environmental stress triggers rapid adaptive physiological responses. In amphibians, the hydric balance response is especially critical because of the highly permeable skin, which plays a role not only in the movement of water, but also in the gas exchange (a large part of gas exchange in amphibians happens through cutaneous respiration). When an amphibian is placed in a novel environment, where local conditions of moisture and temperature are very different from the ones in its original environment, it must adapt to these new conditions to ensure survival. From an individual’s point, it’ll need an immediate response to each new challenge – and in a broader ecological sense, those responses need to be heritable to new generations so that they persist, enabling survival of the species in the novel location.
Many studies have assessed the skin permeability of amphibians in their natural habitats, in controlled experimental conditions and comparing different species. Those studies have established interspecific patterns of physiological response to environmental stress. Our new study takes this a step further, by comparing populations of the species in countries with very different environmental characteristics. We can ask how an invader rapidly changes its physiology to successfully invade such habitats.
In its original environment, where temperatures are comfortably warm and moisture is constant, there’s little need for a cane toad to be able to resist water loss: the risk of reaching fatal levels of dehydration is low. However, because of such low skin resistance to water loss, a toad in its native range must be able to regain water very quickly. For the populations that we studied in Hawai’i, one of the first locations where the species has been introduced outside of Central and South America, there’s much more variability in temperature and rainfall. We found that skin resistance to water loss was already greater than in the native range, although rehydration capacity was low. Interestingly, the cane toads in Hawai’i are highly associated with anthropogenically disturbed areas like golf courses, where mechanical irrigation and urban shelter is readily available.
The most significant change that we recorded occurred in animals from the Australian introduction. Skin resistance to water flow has increased greatly, as the animals are now exposed to a much harsher climate – temperatures reach much higher values, and seasonal drought is now a challenge they must face. The Australian animals are not as quick at rehydrating, perhaps because water bodies are less abundant, meaning it becomes more important to keep in the water you have, as you won’t be able to gain more water any time soon. Toad physiology is very variable within Australia, and further studies will be needed to clarify the delicate balance between environmental conditions, physiological adaptation, and behavioural modulation needed to colonize such a diverse range of habitats.