The long-term effects of climate change on amphibians and reptiles are difficult to predict, but they are likely to be profound. Amphibians and reptiles are commonly described as being 'cold-blooded' but, in many ways, this is erroneous. Many are able to achieve a body temperature at least as high as a bird or a mammal, and to maintain that temperature over quite long periods of time.
The real distinction between the two types of animal is that, whereas birds and mammals generate their body heat internally, and rely on feathers or fur to keep it in, amphibians and reptiles derive their body heat directly or indirectly from the sun and have very limited mechanisms for retaining it. It follows that amphibians and reptiles can only live in places where it is sufficiently warm, at least for some of the time, to enable them to warm up their bodies sufficiently for them to be active enough to feed and mate. Any changes in the temperature of their environment, are likely to have a profound effect on their behaviour and thus on where they can live.
Climate change involves not just changes in temperature, but also changes in rainfall patterns, with more falling in some places, less in others. This may not greatly affect reptiles, which are generally emancipated from water, but is very significant for amphibians, which, because of their thin, moist skin and their unprotected eggs, are very dependent on wet conditions. It is generally assumed that climate change poses a threat to wild animals, but it is quite possible that some, including amphibians and reptiles, may prosper in a changed climate. The link between climate change and the viability of amphibian and reptile populations will only become clear over time; all over the world, long-term monitoring programmes are gathering data year by year that will resolve this issue.
Some amphibians in Britain, such as the Natterjack Toad and the Smooth Newt, appear to be breeding several weeks earlier in the year than they were 20 years ago, but this does not appear to be happening with other amphibians. Likewise, in the USA, climate change seems to be causing earlier breeding in some species, and in some locations, but not in others. In Brazil, the extinction of five frog species has been attributed to a series of severe frosts. In both Australia and Puerto Rico, dramatic declines in some frog populations seem to be due to prolonged drought.
Many species in Britain, notably plants, insects and birds, are responding to our milder climate by moving northwards and are now to be found in places in Scotland and northern England where they did not occur before. It remains to be seen whether any of our amphibians and reptiles, which have rather limited powers of dispersal, will do the same. Of great importance to amphibians is rainfall, as all our species depend on ponds in which to breed.
An ideal pond for our native amphibians is one which dries out in late summer, but which refills in the winter. Such a pond provides an aquatic environment for tadpoles and newt larvae to grow, develop and metamorphose into young adults in the summer. It is not suitable, however for fish, such as sticklebacks, which need permanent water, or for insects such as dragonflies and diving beetles, whose larvae, like sticklebacks, find amphibian tadpoles and larvae easy prey. Common toads and crested newts can breed successfully in permanent ponds, because their larvae are distasteful to many predators.
If the changing climate in Britain means more rain in winter, but less in summer, then it is very likely that that there will be more of the temporary ponds that seem to suit our amphibians best. In other parts of the world, however, there are clear signs that changing rainfall patterns are having a harmful effect on amphibians. In northwest USA, for example, reduced rainfall has meant that the ponds that frogs and salamanders breed in have very low water levels in spring. As a result, these animals are forced to lay their eggs close to the water surface, exposing them to higher levels of harmful ultra-violet radiation from the sun; this damages them and, in turn, makes them more susceptible to a common fungus that infects and destroys their eggs.
Very subtle effects of climate change, revealed only by meticulous, long-term research, may be implicated in the recent steady decline of Common Toads in southern Britain. Toads are long-lived animals that hibernate in winter, emerging in the spring to breed. Between 1983 and 2005, a population of toads in Dorset has experienced more frequent mild winters. During this period, female toads have been showing reduced survival and have laid fewer eggs annually. They are also in poorer condition, meaning that, for their size, individuals are not as heavy when they emerge from hibernation as they used to be. The reason for this seems to be related to the annual cycle of food availability and use in toads.
Toads do not feed during their brief breeding season. After breeding, they leave their breeding ponds and are active on land during the summer, feeding on an abundant supply of insects, worms, slugs, etc. This enables them to build up large fat reserves in their body by the time cold weather in autumn sends them into hibernation. In a cold winter, toads are inactive and do not draw on their fat reserves, and so emerge in spring with a lot of fat that can be converted into eggs. In a mild winter, however, the toads may become active at a time when there is little for them to eat, so that they have to draw on their fat reserves, leaving less for them to convert into eggs in spring; hence their poor condition and low fecundity.
As more information is gathered about declining amphibian populations around the world, it is becoming clear that species living at higher altitudes are more likely to be in decline than those at low altitudes. This may be related to climate change as many of the world's mountainous areas are experiencing less rainfall and warmer temperatures. In the Cloud Forest Reserve at Monteverde, in Costa Rica, several species of amphibians and reptiles have declined dramatically as conditions have become drier. Some of the declining species have been replaced by lowland species that have extended their ranges upwards. This bodes ill for those species which are confined to isolated peaks and mountain ridges; as conditions deteriorate for them, they have nowhere else to go.
Climate change and altitude also seem to be involved in the global epidemic of the disease chytridiomycosis that is wiping out amphibian populations throughout the world. This has a particularly devastating effect among amphibians that breed in high-altitude mountain streams and it is thought that climate change has brought about conditions that are favourable to the fungus that causes this disease.
In many parts of the world, climate change may have a profound effect on a variety of reptiles because of the unusual way in which the sex of their offspring is determined. In most animals, including humans, sex is genetically-determined; in humans, females have a pair of X chromosomes, males have one X and one Y chromosome. In many reptiles, however, sex is determined by very slight variations in the temperature at which an embryo develops in the egg, particularly during the middle third of its development.
In some turtles, eggs developing at a few degrees below 30o C become males, those developing a few degrees above 30o C become females. In many lizards, the reverse is true; males predominate at higher temperatures. In crocodiles, things are a bit more complicated. Lower and higher temperatures produce more females, intermediate temperatures produce more males. This phenomenon is called temperature-dependent sex determination, or TSD.
TSD has been extensively studied in the laboratory, where temperatures can be carefully controlled, but its consequences in nature, where temperatures vary a great deal, are much more difficult to determine. It is likely, however, that climate change, and other environmental changes could have profound effects on the long-term future of those reptiles that have TSD. For example, sea turtles lay their eggs on open beaches. If their nesting areas become shaded, either by the planting of trees or the construction of tall buildings, the sex of their offspring could be skewed one way or the other. A skewed sex ratio, especially if it involves a reduction in the number of females, can have a devastating effect on the long-term viability of a population.