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migration and dispersal

Migration and dispersal

Migration and dispersal

We have looked at some of the factors which constrain organisms to a fundamental niche, and then within that to a realised niche. One of the important factors involved was the ability to reach favourable areas. This is achieved through migration and dispersal.

Definitions.

Migration is the movement of large numbers of a species from one place to another, usually leaving none behind. Well-known examples include locust swarms & bird migrations.

Dispersal is the spreading of individuals away from others, often parents or siblings, which are left behind in the original area. The movement of plant seeds is dispersal, as is the movement of mammals away from their social group.

To the individual organism actually moving, there is no distinction between migration and dispersal. Both are movements from an unfavourable situation towards the potentially favourable.

Patterns of migration.

Wide variation between different organisms. The general pattern is that the fundamental niche of the species includes two different types of habitats, each of which is favourable at different times. Migration moves the species between the two habitats. The variation comes in the time scale over which this occurs.

1. Daily movements.

most snails clump together in groups in humid microhabitats during the day, then migrate into foraging areas at night
planktonic algae sink to deep water in the night (where they accumulate nutrients like P) and then rise to surface in the day (where they photosynthesise).

2. Seasonal movements.

grazing animals of mountains move up in the summer and down in the valleys in winter. This movement is reflected in traditional livestock management in mountain regions.
amphibians move from an aquatic breeding area to a land feeding area. Each year they return to the same breeding ground and the pattern is repeated.

Some seasonal movements involve very long distances. In the northern hemisphere, we are familiar with a spring movement north, to where food is abundant in the summer, and an autumn movement south, to where food is abundant after the rainy season

Here the productivity of each habitat is seasonal. The low-productivity period is insufficient to support the population, so it has to migrate.

Some birds fly south to Britain in winter, e.g. some curlew, then north to Iceland, Norway and the Arctic in summer.

Animals other than birds show long-distance seasonal migration. Baleen whales feed in summer in the Antarctic, then breed in winter in tropical seas to the north. Caribou migrate between the tundra and the boreal forests.

Another way of classifying patterns of migration is to look at how many times an individual can make the journey (see diagram)

1. One way only.

These migrate in one direction, but never come back. Butterflies like the red admiral and painted lady are most familiar. Individuals fly here from the Mediterranean in summer, breed and die. Their offspring fly south in autumn, breed and die. Their offspring come back next summer.

2. One return journey.

These species are born in one habitat, migrate to another, then return to breed and die in the habitat where they were born. Best known in eels and salmon
Salmon are thought to recognise their birthplace by the smell of chemicals in the water. Each river has a chemical 'fingerprint' which the fish learns as a juvenile.
This pattern is also common in insects. Butterflies, for example, are born on the larval food plant, migrate away and then return to the same type of plant to lay eggs. Aquatic insects like the caddis fly spend the larval stage in water, migrate away as adults, then return to breed.

3. Multiple return journeys

These include most of the examples we looked at earlier - the daily migrations of plankton, or the seasonal movements of birds, whales etc. Most are made several times in a lifetime.

Video examples

If the technology is working, we can see some examples of migratory behaviour. Take notes - you will be expected to use these as examples. What we are looking for here are the different methods used by animals to find their way about.

Dispersal.

a) Dispersal can rely on chance, or involve exploration.
Plant seeds are dispersed without any control by individual seed
Animals are more variable.

  • Some freshwater invertebrates are dispersed without control
  • Aphids have little control against the wind, but they do control when to take off, when to fall out of the sky, and can make later, short flights if the first landing point is unsuitable.
  • Others can explore a number of sites before deciding.

b) Dispersal ability varies between species

In general, species of temporary habitats are better dispersers.
These are characteristic of the early stages of succession - they are colonisers, and good dispersal ability is one feature of their life-history strategy.

But all species need to have some dispersal ability
an area becomes crowded, juveniles have to move
nowhere is safe for ever: climate changes, for example, continents move.

c) Plant dispersal is normally by the wind, and shows a typical pattern.
Most fall close to the parent plant: diagram

Some plants are dispersed by animals, and here the pattern of dispersal depends on the behaviour of the animal.
Many woodland plants, for example, have seeds which catch on animals' coats, and these tend to be distributed along paths.
The interaction between the jay and the oak tree shows an interesting pattern. The more seeds each bird carries, the further it takes them.

d) Dispersal differs between sea and freshwater animals. (diagram)

In freshwater, the dispersal stage is usually the adult. The long-lived phase is the larva. Sometimes the dispersal stage is a resistant structure, like an egg, which is blown by the wind.
In the sea, the dispersal stage us normally the larva. The adult is the long-lived stage.
This may reflect the fact that marine habitats are continuous, whereas freshwater habitats are isolated, and require land rather than water dispersal.

e) There is variation within populations in dispersal ability.
Sex.

Female birds tend to disperse more than males, the opposite is true with mammals:

Sex which disperses

Male

 

Female

 

No difference

Birds

3

 

21

 

6

Mammals

45

 

5

 

15

In insects, males are normally the more active dispersers, to the extent that the females of some species are wingless.

Shape

Some species produce two forms of offspring - a non-dispersing and a dispersing form.

The desert annual Gymnarrhena micrantha. produces a few large seeds that germinate on the site where the adult lived. In wet years, it also produces a large number of small seeds which are wind-dispersed.

Aphids normally produce wingless offspring. When conditions are crowded or food quality low, however, winged offspring are produced.

Social status.

Has been argued that social factors may influence which individuals in small mammal populations disperse.
oppressed individuals may leave. This may occur when times are hard, and the subordinates in the system are driven out or leave because they have so few resources. dispersers may be those individuals which have interacted le
ast with siblings. This hypothesis proposes that social ties are what holds individuals to a place, and those with weakest ties are those that disperse.

f) Species sometimes disperse as clones rather than sexually produced offspring.

For example, many plants have underground stems that grow horizontally, and produce buds at a distance from the parent. Common in grasses, for example.
The resulting plants are genetically identical to the 'parent' - hence a clone.
Sometimes the interconnections between the plants decay, leaving a clone of separate units. In bracken, clones can cover several hectares and be more than a thousand years old.

Two dispersal strategies can be recognised amongst clonal dispersers.

Guerilla

Here the connections between units are long and thin, and often short lived. Units pop up in new places and disappear from old ones. Good examples are water plants like duckweed and Canadian pond weed, which spread very rapidly through clonal growth.

Phalanx

The connections are short and long-lasting. The clone expands slowly and maintains its starting base for a long time. The best examples are tussock grasses in dry places.

Summary.

  1. Migration: whole population movement
  2. Dispersal: some of population left behind
  3. Affect size of realised niche
  4. Migration is between two habitats. Occurs over different time scales. Different species can do a half migration, one, or several.
  5. Some species explore for new dispersal sites, some rely on chance.
  6. All species must disperse, some disperse more than others.
  7. Variation in dispersal occurs between species in different habitat types, between sexes, body forms and individuals of different social status.
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