Forces favoring aggregations in space and time

The simplest evolutionary explanation for the patchiness of populations is that organisms aggregate when and where they find resources and conditions that favor reproduction and survival. These resources and conditions are usually patchily distributed in both space and time. It pays (and has paid in evolutionary time)

6.3.1 Patchiness fine- and coarsegrained environments

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Time 1

Time 2

Time 1

Time 2

Figure 6.4 The 'grain' of the environment must be seen from the perspective of the organism concerned. (a) An organism that is small or moves little is likely to see the environment as coarse-grained: it experiences only one habitat type within the environment for long periods or perhaps all of its life. (b) An organism that is larger or moves more may see the same environment as fine-grained: it moves frequently between habitat types and hence samples them in the proportion in which they occur in the environment as a whole.

Figure 6.5 Changes in the density of a population of 13-year periodical cicadas in northwestern Arkansas in 1985, and changes in the percentage eaten by birds. (After Williams et al., 1993.)

Figure 6.5 Changes in the density of a population of 13-year periodical cicadas in northwestern Arkansas in 1985, and changes in the percentage eaten by birds. (After Williams et al., 1993.)

to disperse to these patches when and where they occur. There are, however, other specific ways in which organisms may gain from being close to neighbors in space and time.

An elegant theory identifying a selective advantage to individuals that aggregate with others was suggested by Hamilton (1971) in his paper 'Geometry for the selfish herd'. He argued that the risk to an individual from a predator may be lessened if it places another potential prey individual between itself and the predator. The consequence of many individuals doing this is bound to be an aggregation. The 'domain of danger' for individuals in a herd is at the edge, so that an individual would gain an advantage if its social status allowed it to assimilate into the center of a herd. Subordinate individuals might then be forced into the regions of greater danger on the edge of the flock. This seems to be the case in reindeer (Rangifer tarandus) and woodpigeons (Columba palum-bus), where a newcomer may have to join the herd or flock at its risky perimeter and can only establish itself in a more protected position within the flock after social interaction (Murton et al., 1966). Individuals may also gain from living in groups if this helps to locate food, give warning of predators or if it pays for individuals to join forces in fighting off a predator (Pulliam & Caraco, 1984).

The principle of the selfish herd as described for the aggregation of organisms in space is just as appropriate for the synchronous appearance of organisms in time. The individual that is precocious or delayed in its appearance, outside the norm for its population, may be at greater risk from predators than those conformist individuals that take part in 'flooding the market' thereby diluting their own risk. Amongst the most remarkable examples of synchrony are the 'periodic cicadas' (insects), the adults of which emerge simultaneously after 13 or 17 years of life underground as nymphs. Williams et al. (1993) studied the mortality of populations of 13-year periodic cicadas that emerged in northwestern Arkansas in 1985. Birds consumed almost all of the standing crop of cicadas when the density was low, but only 15-40% when the cicadas reached peak density. Predation then rose to near 100%

as the cicada density fell again (Figure 6.5). Equivalent arguments apply to the many species of tree, especially in temperate regions, that have synchronous 'mast' years (see Section 9.4).

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