SUMMARY
There are four main factors due to which animals and plants are prevented from spreading to every possible area.
1. Climate. Animals are adapted to a combination of temperature and humidity that is affected by rainfall. Lower temperature prevents majority of reptiles from migrating northwards into the temperate areas.
Polar bear, penguins and a large number of mountain inhabiting species are adapted to cold climate and cannot come down to tropics and subtropics. Amphibians need high humidity not only for their survival but also for reproduction and hence cannot venture into areas of low rainfall. Majority of animals cannot cross or survive in deserts due to extremely low moisture and high temperature. That makes deserts effective physical barriers. Fishes, although adapted to live in aquatic environment, are clearly restricted to either marine or freshwater habitats apparently due to osmotic problems.
Very few migratory fishes can make use of both environments such as species of salmons and eels that migrate thousands of kilometres for reproduction. Low temperature of mountains prevents certain animals such as parrots from spreading to these areas.
2. Vegetation. Like animals plants are also sensitive to temperature and rainfall and they affect dispersal of animals because the latter depend on vegetation for food. Tropical areas support broad-leaved dense forests whereas in temperate areas only cold tolerant conifers can survive, each type harbours its distinctive fauna. Desert climate can support few plants and thereby few animals. Some animals can feed on many types of vegetation and hence can spread to larger areas but others are choosy and would not accept anything except for their specialised diet. For instance, giant panda feeds on bamboo shoots in China and Koala can live only on eucalyptus leaves in Australia. Such animals cannot survive outside their habitats.
3. Other animals. Different animals at different trophic levels make food chains which are interwoven in a complex food web. Such interactions among animals often restrict a particular species to migrate alone to other areas. Interaction between predator and prey, parasite and host and among commensals and competitors pose complex problems in an ecosystem and any immigrating exotic species can upset the balance in the native population.
Dingo dogs, placental cats and foxes are in a danger of exterminating native carnivores in Australia. When two species have similar ecological requirements, they become competitors and one of the species is generally exterminated and restricted to a very small area. British red squirrel has reduced its range after the introduction of American grey squirrel. Similarly extinction of Tasmanian wolf is attributed to the arrival of dingo dogs in Australia. Parasites generally have specific hosts and hence must migrate together to new areas. Predators and prey also show similar interactions.
4. Physical barriers. Barriers such as mountains, deserts, rivers and oceans physically stop animals from invading new areas even when environment is conducive to their survival. For land animals water is a barrier and for aquatic animals land. Fresh water fishes and amphibians cannot cross seas but amphibious reptiles such as tortoises, lizards and snakes, owing to their thick and impervious skin have crossed seas to reach distant islands far away from the mainland.
Climate and scarcity of vegetation makes deserts and mountains effective barriers rather than inability of animals to walk over them. Generally, rivers and lakes do not form effective barriers for most of the vertebrate species if they are good swimmers and usually they are, and rivers form a network of highways for migrating freshwater fishes. Distribution of land animals is therefore restricted by one of the environmental limiting factors.