Means of Dispersal
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[edit] Definition
Dispersal is of interest to biologists for two reasons; in understanding how species maintain their geographic distribution in the short term; and in understanding how they achieved their present distribution (see the main article on Biogeography).
We shall review some of the more popular methods of dispersal as they relate to both these issues.
[edit] Walking
Terrestrial animals can walk from place to place, or, in the case of snakes, slither. This does not mean that they can get anywhere on a given landmass, because there are still limitations of climate, terrain, and altitude: to a small mammal suited to temperate climates, 40 miles of desert is as effective a barrier as 4000 miles of ocean.
[edit] Swimming
Fish, frogs, seals, and so forth, can disperse by swimming. It does not, of course, follow that they can swim where they choose. Obligate freshwater fishes, of course, can't cross the oceans at all; nor can amphibians; obligate saltwater fish can't swim up rivers of fresh water. Then there are the usual restrictions of climatic zones and so forth, as with dispersal by walking.
We should note that there are some terrestrial mammals which can disperse by swimming over distances of a few miles: elephants, in particular, have often been observed to swim from shore to shore over distances of 10 miles or more.
[edit] Flying
With walking and swimming, there is one further constraint on long-range dispersal: an animal will not voluntarily leave a habitat it likes, or a migratory route it knows, for the unknown: it will not set off over terrain to which it is maladapted in search of a better life elsewhere.
The difference with flying is that a bird or bat or insect can be rapidly blown thousands of miles off course. Creatures in such a situation will, when they can, make for the nearest land; in the case of birds, these are the source of many of the rare sightings that so delight birdwatchers. Migratory birds in particular seem to be affected by such events, since it is hard for them, especially when their route lies across water, to hunker down and hang on to something in the event of stormy weather.
The result is that birds can and do end up thousands of miles from where they intended to be, having crossed territory that they would never have been willing to traverse.
[edit] Rafting
A terrestrial animal may be swept out to sea on a fallen tree or other raft of vegetable matter: The factors which affect its chances of survival are the time it takes before the raft washes up on some shore; food (how fortunate to be a fruit-eating species on a fruit-bearing tree!); and the ability to got without fresh water. This last factor is easy for reptiles, impossible for amphibians, and possible for some mammals over some periods of time, especially mammals that are adapted to alternate wet and dry seasons that have the good fortune to be set adrift at the start of the dry season.
Rafting is therefore something of a long shot, especially when you consider that, as with other means of dispersal, it takes at least two (or one pregnant female) to make a breeding population.
[edit] Dispersal by humans
Humans, of course, get everywhere, and we take our domesticated plants and animals with us, together with unwanted companions such as the house mouse.
[edit] Hitch-hikers
Animals and birds carry their parasites with them; they also carry the seeds of the plants they feed on in their stomachs, dispersing the seeds. Birds, in particular, allow plant species to jump large distances.
Some plants produce sticky or hooked seeds (burrs) which attach themselves to the coats of animals.
[edit] Passive airborne dispersal
Many plants have seeds adapted to disperse on the wind; the distance they travel is down to chance and the weather. The tiny spores of ferns, in particular, are well-adapted to get anywhere, and so ferns are often the first plants to return after a fire, just as they were also the first plants back to Krakatoa after its eruption.
Some small spiders practice airborne dispersal by what is called "parachuting": they let out a long line of silk and drift away on the breeze.
For the possibility of passive airborne dispersal by fish, see our main article on rains of fish. This cannot be very important as a means of dispersal for fish: they can hardly adapt to such a mode of dispersal as their usual method, because the waterspouts that suck up fish cannot be relied on in the same way as the breezes that bear dandelion seeds. Nor can this mode of transport act as a long-shot, long-range means of dispersal analogous to "rafting", because the tornadoes that cause rains of fish blow themselves out rapidly and couldn't carry the fish any great distance.[1]
[edit] Passive seaborne dispersal of seeds
The seeds of many plants can survive immersion in salt water for a time. Amongst the earliest experiments were those of Charles Darwin, who discovered that many seeds could tolerate salt water for some weeks[2][3]. Most seeds also, as he pointed out, sink after a little immersion in salt water: for this reason he concluded that such seeds could not be carried alone, but as part of a fallen tree or branch, that is, by rafting.
Some seeds, however, seem well adapted to seaborne dispersal, especially coconuts.
[edit] Continental drift
The relative motion of continental plates has aptly been compared to the rate at which fingernails grow. In the short term, then it cannot be considered a means of dispersal. In the longer term, say a million years or so, sitting on a continental plate in motion will get a species quite a distance, and such considerations are essential for long-term biogeography
