Distribution

Introduction to the world-wide distribution of extant Anurans:

Both time and continental drifting has had a major effect on the separation and distribution of anuran groups. Older groups tend to have more fragmented distributions, with some being separated by large distances as the continents drifted apart and they may also be locally restricted to patchy populations, resulting from competition by younger, more advanced groups. Younger groups are normally endemic and restricted to a single region, as they have not had enough time to establish wide distributions. Therefore, the widest distributions are found in those intermediate groups that have had time to spread and successfully adapt to a number of habitats, but still remain successful. (Porter 1972).

The distribution of land has significantly shaped the current distribution of anurans across the globe, particularly how this land lies in relation to the different climatic regions, with temperature and precipitation being the most influential. Because of their permeable skin anurans are always at threat from desiccation, so are generally limited to habitats where they can obtain fresh water. They are also ectothermic and so are particularly sensitive to environmental conditions, with the compromise between thermal and hydric balance reducing the ability to thermoregulate behaviourally. There are, however, some exceptions to this and there is great variation as to how tolerant some species are. As a result some arid or cold areas function as filters, allowing some species in, but blocking many others. Despite their potentially limiting need for water and ectothermy, anurans occupy a large variety of aquatic, semi-aquatic, terrestrial and arboreal habitats, ranging from the tropics to inside the arctic circle and from wet to totally arid conditions. This has been achieved by a number of behavioural, physiological and reproductive specialisations. (Porter 1972). (Pough 1996).

World Distribution of Families

They can be found on every continent, except for Antarctica, as well as on most continental islands. The majority and largest diversity of anurans are found in the tropics, which receives uniformly high temperatures and humidity levels all year round, along with very high levels of rainfall. But species are also found within the arctic circle in Europe and Alaska, as well as the southern tips of South America and New Zealand. Generally there are no real sharply defined regions for families and there is broad overlapping between the tropical and temperate regions of Africa and Asia, as well as overlapping through Central America between North and South America. One area though, where there is little overlapping, is across Wallace’s Line, through the Indo-Australian archipelago. Families common to both the New and Old worlds are found in greater numbers in the northern hemisphere, because of the more extensive inter-linking of continents. (Porter 1972). (Pough 1996).

Distribution at the species population level is restricted by various combinations of climate, competition and predation, along with specialisations that a species may have developed for a particular habitat. Over the vast majority of anuran evolution, these habitats have been determined by such natural events as plate tectonics, ice ages and volcanic activity. Now human management is increasingly dictating the habitat structure of the world, heavily through habitat alteration for agriculture, or urbanisation. All of which is having an impact on distributions and creating major threats for some.