Species become migratory when it gives them adaptive benefits. The genetic basis of migration means that it is slow to respond to environmental changes.
Animals migrate when they need different habitats for different stages in their life-cycle. This behaviour has evolved (is evolving) and has a genetic basis, leading to variability in the ‘migration strategies’ of individuals. Presumably individuals who ‘choose’ not to follow their species normal migration pattern will not breed as successfully as those who do when things remain ‘normal’, but if environmental conditions change then these aberrant individuals might well be more fortunate and go on to initiate new migration patterns.
Environmental Changes
When the environment changes on a ‘geological’ timescale species will have the time for natural selection to alter their migration behaviour, but problems arise if the changes are too rapid. Mankind is affecting the environment in many ways (see ‘Human Effects’), and often changes are too fast to allow species to adapt.
- Monarch Butterflies – typically migrate from California and Mexico to as far north as Canada each year to breed. They do this because the north is too cold for the adults to overwinter but is a better place for the caterpillars. Several generations pass the summer in these northern habitats, but only those individuals who successfully migrate south before the cold weather survive into the next year. They need to feed on nectar during their long migrations and therefore need to pass through suitable habitats.
- The Red Admiral – shows a less extreme type of migration than the Monarch. Some individuals will stay and overwinter in the northern part of their range, while others will move south as winter approaches. Some years those that stay in the north will survive and get an ‘early start’ in the following spring, in other years they will die. If the climate continues to warm up we can expect these non-migratory individuals to become more reproductively successful and eventually the species might become ‘non-migratory’.
- Salmon and Allis Shad – need shallow freshwater for breeding and the open sea to grow as adults. Changes in the estuaries they need to cross during their migration can have disastrous effects, as can any deterioration of the watercourses they pass through or problems at their spawning grounds.
Navigation
While it is true that some mammals can learn migration routes from their elders invertebrates and fish can not. Migration (and therefore navigation during migration) is instinctive. In other words navigation must have a genetic basis and take many generations to change by evolution. Invertebrate navigation must depend on environmental ‘clues’ which remain constant over very long periods of time: such as magnetic fields; position of the sun, moon and stars; direction of prevailing winds etc. Exactly how these are used is poorly understood.
Other articles by John Blatchford
