Tidal diurnal and seasonal movements

Individuals of many species move en masse from one habitat to another and back again repeatedly during their life. The timescale involved may be hours, days, months or years. In some cases, these movements have the effect of maintaining the aggregation and the selfish herd

Figure 6.6 Density-dependent dispersal. (a) The dispersal rates of newly hatched blackfly (Simulium vittatum) larvae increase with increasing density. (Data from Fonseca & Hart, 1996.) (b) The percentage of juvenile male barnacle geese, Branta leucopsis, dispersing from breeding colonies on islands in the Baltic Sea to non-natal breeding locations increased as density increased. (Data from van der Juegd 1999.) (After Sutherland et al., 2002.)

Figure 6.6 Density-dependent dispersal. (a) The dispersal rates of newly hatched blackfly (Simulium vittatum) larvae increase with increasing density. (Data from Fonseca & Hart, 1996.) (b) The percentage of juvenile male barnacle geese, Branta leucopsis, dispersing from breeding colonies on islands in the Baltic Sea to non-natal breeding locations increased as density increased. (Data from van der Juegd 1999.) (After Sutherland et al., 2002.)

organism in the same type of environment. This is the case in the movement of crabs on a shoreline: they move with the advance and retreat of the tide. In other cases, diurnal migration may involve moving between two environments: the fundamental niches of these species can only be satisfied by alternating life in two distinct habitats within each day of their lives. For example, some planktonic algae both in the sea and in lakes descend to the depths at night but move to the surface during the day. They accumulate phosphorus and perhaps other nutrients in the deeper water at night before returning to photosynthesize near the surface during daylight hours (Salonen et al., 1984). Other species aggregate into tight populations during a resting period and separate from each other when feeding. For example, most land snails rest in confined humid microhabitats by day, but range widely when they search for food by night.

Many organisms make seasonal migrations - again, either tracking a favorable habitat or benefitting from different, complementary habitats. The altitudinal migration of grazing animals in mountainous regions is one example. The American elk (Cervus elaphus) and mule deer (Odocoileus hemionus), for instance, move up into high mountain areas in the summer and down to the valleys in the winter. By migrating seasonally the animals escape the major changes in food supply and climate that they would meet if they stayed in the same place. This can be contrasted with the 'migration' of amphibians (frogs, toads, newts) between an aquatic breeding habitat in spring and a terrestrial environment for the remainder of the year. The young develop (as tadpoles) in water with a different food resource from that which they will later eat on land. They will return to the same aquatic habitat for mating, aggregate into dense populations for a time and then separate to lead more isolated lives on land.

6.4.2 Long-distance migration

The most remarkable habitat shifts are those that involve traveling very long distances. Many terrestrial birds in the northern hemisphere move north in the spring when food supplies become abundant during the warm summer period, and move south to savannas in the fall when food becomes abundant only after the rainy season. Both are areas in which seasons of comparative glut and famine alternate. Migrants then make a large contribution to the diversity of a local fauna. Of the 589 species of birds (excluding seabirds) that breed in the Palaearctic (temperate Europe and Asia), 40% spend the winter elsewhere (Moreau, 1952). Of those species that leave for the winter, 98% travel south to Africa. On an even larger scale, the Arctic tern (Sterna paradisaea) travels from its Arctic breeding ground to the Antarctic pack ice and back each year -about 10,000 miles (16,100 km) each way (although unlike many other migrants it can feed on its journey).

The same species may behave in different ways in different places. All robins (Erithacus rubecula) leave Finland and Sweden in winter, but on the Canary Islands the species is resident the whole year-round. In most of the intervening countries, a part of the population migrates and a part remains resident. Such variations are in some cases associated with clear evolutionary divergence. This is true of the knot (Calidris canutus), a species of small wading bird mostly breeding in remote areas of the Arctic tundras and 'wintering' in the summers of the southern hemisphere. At least five subspecies appear to have diverged in the Late Pleistocene (based on genetic evidence from the sequencing of mitochondrial DNA), and these now have strikingly different patterns of distribution and migration (Figure 6.7).

birds

Figure 6.7 Global distribution and migration pattern of knots (Calidris spp.). Solid shading indicates the breeding areas; horizontally striped spots indicate the stop-over areas, used only during south- and northward migration; the cross-hatched spots indicate the areas used both as stop-over and wintering sites; and the vertically striped spots designate areas used only for wintering. The gray shaded corridors indicate proven migration routes; the broken-shaded corridors indicate tentative migration routes suggested in the literature. (After Piersma & Davidson, 1992.)

Global Distribution Pattern

Figure 6.7 Global distribution and migration pattern of knots (Calidris spp.). Solid shading indicates the breeding areas; horizontally striped spots indicate the stop-over areas, used only during south- and northward migration; the cross-hatched spots indicate the areas used both as stop-over and wintering sites; and the vertically striped spots designate areas used only for wintering. The gray shaded corridors indicate proven migration routes; the broken-shaded corridors indicate tentative migration routes suggested in the literature. (After Piersma & Davidson, 1992.)

Long-distance migration is a feature of many other groups too. Baleen whales in the southern hemisphere move south in summer to feed in the food-rich waters of the Antarctic. In winter they move north to breed (but scarcely to feed) in tropical and subtropical waters. Caribou (Rangifer tarandus) travel several hundred kilometers per year from northern forests to the tundra and back. In all of these examples, an individual of the migrating species may make the return journey several times.

Many long-distance migrants, however, make only one return journey during their lifetime. They are born in one habitat, make their major growth in another habitat, but then return to breed and die in the home of their infancy. Eels and migratory salmon provide classic examples. The European eel (Anguilla anguilla) travels from European rivers, ponds and lakes across the Atlantic to the Sargasso Sea, where it is thought to reproduce and die (although spawning adults and eggs have never actually been caught there). The American eel (Anguilla rostrata) makes a comparable journey from areas ranging between the Guianas in the south, to southwest Greenland in the north. Salmon make a comparable transition, but from a freshwater egg and juvenile phase to mature as a marine adult. The fish then returns to freshwater sites to lay eggs. After spawning, all Pacific salmon (Oncorhynchus nerka) die without ever returning to the sea. Many Atlantic salmon (Salmo salar) also die after spawning, but some survive to return to the sea and then migrate back upstream to spawn again.

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