Territoriality

Territoriality is one particularly important and widespread phenomenon that results in asymmetric intraspecific competition. It occurs when there is active interference between individuals, such that a more or less exclusive area, the territory, is defended against intruders by a recognizable pattern of behavior.

Individuals of a territorial species that fail to obtain a territory often make no contribution whatsoever to future generations. Territoriality, then, skews and other hierarchies asymmetry enhances regulation territoriality is a contest

Figure 5.28 Space preemption in a perennial, Anemone hepatica, in a Swedish forest. Each line represents one individual: straight for unramified ones, branched where the plant has ramified, bold where the plant flowered and broken where the plant was not seen that year. Group A were alive and large in 1943, group B alive and small in 1943, group C appeared first in 1944, group D in 1945 and group E thereafter, presumably from seedlings. (After Tamm, 1956.)

1956 1955 1954 1953 1952 1951 1950 1949 1948 1947 1946 1945 1944 1943

10 20 30 D

10 20 30 D

10 20 30 40 50

is a 'contest'. There are winners (those that come to hold a territory) and losers (those that do not), and at any one time there can be only a limited number of winners. The exact number of territories (winners) is usually somewhat indeterminate in any one year, and certainly varies from year to year, depending on environmental conditions. Nevertheless, the contest nature of territoriality ensures, like asymmetric competition generally, a comparative constancy in the number of surviving, reproducing individuals. One important consequence of territoriality, therefore, is population regulation, or more particularly, the regulation of the number of territory holders. Thus, when territory owners die, or are experimentally removed, their places are often rapidly taken by newcomers. For instance, in great tit (Parus major) populations, vacated woodland territories are reoccupied by birds coming from hedgerows where reproductive success is noticeably lower (Krebs, 1971).

Some have felt that the regulatory consequences of territori-ality must themselves be the root cause underlying the evolution of territorial behavior - territoriality being favored because the population as a whole benefitted from the rationing effects, which guaranteed that the population did not overexploit its resources (e.g. Wynne-Edwards, 1962). However, there are powerful and fundamental reasons for rejecting this 'group selectionist' explanation (essentially, it stretches evolutionary theory beyond reasonable limits): the ultimate cause of territoriality must be sought within the realms of natural selection, in some advantage accruing to the individual.

Any benefit that an individual does gain from territoriality, of course, must be set against the costs of defending the territory. In some animals this defense involves fierce combat between competitors, whilst in others there is a more subtle mutual recognition by competitors of one another's keep-out signals (e.g. song or scent). Yet, even when the chances of physical injury are minimal, territorial animals typically expend energy in patrolling and advertizing their territories, and these energetic costs must be exceeded by any benefits if territoriality is to be favored by natural selection (Davies & Houston, 1984; Adams, 2001).

Praw and Grant (1999), for example, investigated the costs and benefits to convict cichlid fish (Archocentrus nigrofasciatus) of defending food patches of different sizes. As patch size increased, the amount of food eaten by a patch defender increased (the benefit; Figure 5.29a), but the frequency of chasing intruders (the cost; Figure 5.29b) also increased. Evolution should favor an intermediate patch (territory) size at which the trade-off between costs and benefits is optimized, and indeed, the growth rate of defenders was greatest in intermediate-sized patches (Figure 5.29c).

On the other hand, explaining territoriality only in terms of a net benefit to the territory owner is rather like history always being written by the victors. There is another, possibly trickier question, which seems not to have been answered - could those individuals without a territory not do better by challenging the territory owners more often and with greater determination?

Of course, describing territoriality in terms ofjust 'winners' and 'losers' is an oversimplification. Generally, there are first, second and a range of consolation prizes - not all territories are equally valuable. This has been demonstrated in an unusually striking way in a study of oyster-catchers (Haematopus ostralegus) on the Dutch coast, where pairs of birds defend both nesting territories on the salt marsh and feeding territories on the mudflats (Ens et al., 1992). For some birds (the 'residents'), the feeding territory is simply an extension of the benefits and costs of territoriality not simply winners and losers

Figure 5.29 Optimal territory size in the convict cichlid fish, Archocentrus nigrofasciatus. (a) As patch (territory) size increased, the amount of food eaten by a territory defender (standardized z score) increased but leveled off at the largest sizes (solid line, linear regression: r2 = 0.27, P = 0.002; dashed line, quadratic regression: r2 = 0.33, P = 0.003). (b) As patch (territory) size increased, the chase rate of territory defenders increased (linear regression: r2 = 0.68, P < 0.0001). (c) As patch (territory) size increased, the growth rate of territory defenders (standardized z score) was highest at intermediate-sized territories (quadratic regression: r2 = 0.22, P = 0.028). (After Praw & Grant, 1999.)

Figure 5.29 Optimal territory size in the convict cichlid fish, Archocentrus nigrofasciatus. (a) As patch (territory) size increased, the amount of food eaten by a territory defender (standardized z score) increased but leveled off at the largest sizes (solid line, linear regression: r2 = 0.27, P = 0.002; dashed line, quadratic regression: r2 = 0.33, P = 0.003). (b) As patch (territory) size increased, the chase rate of territory defenders increased (linear regression: r2 = 0.68, P < 0.0001). (c) As patch (territory) size increased, the growth rate of territory defenders (standardized z score) was highest at intermediate-sized territories (quadratic regression: r2 = 0.22, P = 0.028). (After Praw & Grant, 1999.)

nesting territory: they form one spatial unit. For other pairs, however (the 'leapfrogs'), the nesting territory is further inland and hence separated spatially from the feeding territory (Figure 5.30a). Residents fledge many more offspring than do leapfrogs (Figure 5.30b), because they deliver far more food to them (Figure 5.30c). From an early age, resident chicks follow their parents onto the mudflats, taking each prey item as soon as it is captured. Leapfrog chicks, however, are imprisoned on their nesting territory prior to fledging; all their food has to be flown in. It is far better to have a resident than a leapfrog territory.

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