Many people bemoan the fact that globalisation tends to homogenize human culture around the world; most of the world's cities now contain the same chains of fast-food outlets and coffee shops. There has been a dramatic fall in the number of distinct languages spoken around the world, which parallels the decline that has occurred in the diversity of plant and animal species.
In the natural world, a similar kind of homogenization is occurring as, deliberately or accidentally, humans have moved plants and animals from places where they occur naturally to places where they do not belong. European starlings and sparrows, for example, are now common throughout the world while, paradoxically, they are becoming less common in Britain.
In many instances, this has a harmful effect on indigenous wildlife, because the alien species often displace native species. The North American grey squirrel, for example, has forced Britain's native red squirrel into just a few tiny refuges. Many amphibians and reptiles now occur as aliens in many parts of the world, often posing a threat to native species. Many of these alien invasions have resulted from international trade in amphibians and reptiles.
The movement of animals from one country to another is one aspect of the exploitation by people of natural populations. Amphibians and reptiles have long been used by people for a variety of purposes, including food, as a source of medicines, as pets, for medical research and for teaching. The brightly-coloured poison-dart frogs of South and Central America get their name from the fact that indigenous people have used their skin toxins to coat the tips of the hunting darts; today, many of these beautiful frogs are collected to supply a lucrative global trade in exotic pets.
Natural populations may be subject to direct, local exploitation, which typically has a very long history. With globalisation, however, many species are now subject to indirect exploitation, being the start of a global supply chain that carries them far beyond their country of origin. Over 200 amphibian species are used locally as food, and nearly 70 as a source of medicines. The international pet trade involves over 300 amphibian species, most of them from the tropics and destined for Europe and the USA; about 80 of these species are now threatened by extinction.
In Britain, people have not traditionally used amphibians as food; we do not share the French taste for frog legs. Our native frogs have been heavily exploited, however, for scientific research and for teaching. Until quite recently, any student studying biology would dissect at least one frog during their education. This lead to such a decline in frogs in mainland Britain that animal suppliers had to import them from Ireland.
Another species widely used in scientific research in Britain is the South African clawed frog, also known as Xenopus. Huge numbers of these frogs were used until recently for human pregnancy testing. Somehow, some escaped from laboratories and a number of wild naturalised populations have become established in Britain. Xenopus does not seem to pose a direct threat to our native amphibians, or to other kinds of wildlife, as these populations have remained small and very restricted.
The same cannot be said of the American Bullfrog, a spectacularly large and very noisy animal whose tadpoles take two years to develop, reaching about 10cm in length. A few years ago bullfrog tadpoles were imported into Britain and were offered as creatures that people could release into their garden ponds.
The large, muscular eggs of bullfrogs make them especially attractive in the world's food markets, and bullfrog farms have been built in many parts of the world, notably in South America. Unfortunately, bullfrogs find it easy to escape from such farms and quickly become established in natural habitats. Bullfrogs threaten native amphibians in a variety of ways. The adults have a voracious appetite and will readily eat smaller, native frogs. Their tadpoles grow faster than, and to a much a larger size than those of native frogs and toads and thus out-compete with them for food.
Very recently, it has been found that bullfrogs living in southern Britain carry, but are not affected by the deadly skin disease, called chytridiomycosis, that has decimated amphibian populations throughout the world. It has yet to be determined whether this disease adversely affects our native amphibians.
Bullfrogs may not be the only species that has carried chytridiomycosis around the world. Xenopus can also carry the fungus causing this disease without being affected by it and it has been suggested that chytridiomycosis came 'out of Africa' in the substantial international trade in these frogs.
Many of the world's reptiles have long been exploited for a variety of purposes. The skin of lizards and snakes is used as an alternative to leather in the manufacture of shoes, handbags and belts. The shells of tortoises and turtles is used for decorations. Many species are a source of traditional medicines. Turtle eggs and various snake organs are used as a source of aphrodisiacs. In general, such forms of exploitation have not harmed natural populations as long as they are conducted locally, but they threaten the continued existence of species when they become the basis of international trade.
Since the 17th century there has been a steady trade of green turtles from the Caribbean to Europe to make turtle soup. The exploitation of Caribbean turtles is gradually being brought under control by international agreement; in 2008, for example, Cuba will begin a voluntary moratorium on turtle fishing. In the 1970s a trade developed in Mediterranean tortoises, shipped to northern European countries as pets. This has caused severe declines among natural populations and has been banned under European legislation.
The USA has been a major player in the global market in reptiles, as both an importer and an exporter. Reptiles are imported for food, to stock aquariums and zoos and, most importantly, as pets. A few years ago, this trade consisted of 2.5 million animals each year, nearly half of them green iguanas from Central and South America. The USA also exports around ten million animals each year, most of them red-eared sliders, turtles that are reared in farms. When they escape, red-eared sliders invade natural habitats and are a serious threat to native species in several parts of the world.
The situation in Asia is particularly serious, leading to what is called the Asian turtle crisis. A massive trade in both freshwater and marine turtles has built up with animals from many Asian countries being exported to China; in 1999 an estimated 8000 tons of turtle meat was imported into China. This has destroyed many populations and 67 of Asia's 90 turtle species are now classified as threatened with extinction; 18 species are critically endangered.
The effects of the exploitation of some species can be alleviated by breeding them in captivity. Crocodiles and alligators typically grow quickly and mature early in life, making it relatively easy to farm them and in some parts of the world the market for crocodile meat and skin is met in this way. Turtles, however, grow slowly and mature late in life, making captive breeding impractical and their future can only be assured by protecting them in the wild. This is particularly problematic for marine turtles, which range over vast areas of ocean, where they are frequently accidentally caught in fishing nets. Marine turtle populations are most vulnerable to losses of their eggs, which are laid on beaches, many of which are popular tourist destinations.