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In this article Tim Halliday looks at the possible effect of climate change on amphibians and reptiles
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 Some of the declining species have been replaced by lowland species that have extended their ranges upwards.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.
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Content last updated: 05/10/2007
About our author
Tim Halliday is professor of Biology at the Open University, where he has worked on newts, toads and frogs since 1977. Originally interested in the sexual behaviour and life history of amphibians, in 1991 he turned his attention to declining amphibian populations and extinctions worldwide.
From 1993 to 2005, Tim was the International Director of the IUCN Declining Amphibian Populations Task Force, an organisation that documented amphibian declines and sponsored research into their causes.
