Uncertainty of destination may be reduced if an active agent of dispersal is involved. The seeds of many herbs of the woodland
Figure 6.2 (a) The density of wind-dispersed seeds from solitary trees within forests. The studies had a reasonable density of sampling points, there were no nearby conspecific trees and the source tree was neither in a clearing nor at the forest edge. (b) Observed long-distance dispersal up to 1.6 km of wind dispersed seeds from a forested source area. (After Greene & Calogeropoulos, 2001, where the original data sources may also be found.)
floor have spines or prickles that increase their chance of being carried passively on the coats of animals. The seeds may then be concentrated in nests or burrows when the animal grooms itself. The fruits of many shrubs and lower canopy trees are fleshy and attractive to birds, and the seed coats resist digestion in the gut. Where the seed is dispersed to is then somewhat less certain, depending on the defecating behavior of the bird. It is usually presumed that such associations are 'mutualistic' (beneficial to both parties - see Chaper 13): the seed is dispersed in a more or less predictable fashion; the disperser consumes either the fleshy 'reward' or a proportion of the seeds (those that it finds again).
There are also important examples in which animals are dispersed by an active agent. For instance, there are many species of mite that are taken very effectively and directly from dung pat to dung pat, or from one piece of carrion to another, by attaching themselves to dung or carrion beetles. They usually attach to a newly emerging adult, and leave again when that adult reaches a new patch of dung or carrion. This, too, is typically mutualis-tic: the mites gain a dispersive agent, and many of them attack and eat the eggs of flies that would otherwise compete with the beetles.
Many other animals cannot be said to explore, but they certainly control their settlement ('stopping', see Section 6.1.1) and cease movement only when an acceptable site has been found. For example, most aphids, even in their winged form, have powers of flight that are too weak to counteract the forces of prevailing winds. But they control their take-off from their site of origin, they control when they drop out of the windstream, and they make additional, often small-scale flights if their original site of settlement is unsatisfactory. In a precisely analogous manner, the larvae of many river invertebrates make use of the flowing column of water for dispersing from hatching sites to appropriate microhabitats ('invertebrate drift') (Brittain & Eikeland, 1988). The dispersal of aphids in the wind and of drifting invertebrates in streams, therefore, involves 'discovery', over which they have some, albeit limited, control.
Other animals explore, visiting many sites before returning to a favored suitable one. For example, in contrast to their drifting larvae, most adults of freshwater insects depend on flight for upstream dispersal and movement from stream to stream. They explore and, if successful, discover, suitable sites within which to lay their eggs: starting, moving and stopping are all under active control.
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