Exploitation and interference

In many cases, competing individuals do not interact with one another directly. Instead, individuals respond to the level of a resource, which has been depressed by the presence and activity of other individuals. The grasshoppers were one example. Similarly, a competing grass plant is adversely affected by the presence of close neighbors, because the zone from which it extracts resources (light, water, nutrients) has been overlapped by the 'resource depletion zones' of these neighbors, making it more difficult to extract those resources. In such cases, competition may be described as a definition of competition exploitation

Figure 5.1 Intraspecific competition amongst cave beetles (Neapheanops tellkampfi). (a) Exploitation. Beetle fecundity is significantly correlated (r = 0.86) with cricket fecundity (itself a good measure of the availability of cricket eggs - the beetles' food). The beetles themselves reduce the density of cricket eggs. (b) Interference. As beetle density in experimental arenas with 10 cricket eggs increased from 1 to 2 to 4, individual beetles dug fewer and shallower holes in search of their food, and ultimately ate much less (P < 0.001 in each case), in spite of the fact that 10 cricket eggs was sufficient to satiate them all. Means and standard deviations are given in each case. (After Griffith & Poulson, 1993.)

Neapheanops

Figure 5.1 Intraspecific competition amongst cave beetles (Neapheanops tellkampfi). (a) Exploitation. Beetle fecundity is significantly correlated (r = 0.86) with cricket fecundity (itself a good measure of the availability of cricket eggs - the beetles' food). The beetles themselves reduce the density of cricket eggs. (b) Interference. As beetle density in experimental arenas with 10 cricket eggs increased from 1 to 2 to 4, individual beetles dug fewer and shallower holes in search of their food, and ultimately ate much less (P < 0.001 in each case), in spite of the fact that 10 cricket eggs was sufficient to satiate them all. Means and standard deviations are given in each case. (After Griffith & Poulson, 1993.)

exploitation, in that each individual is affected by the amount of resource that remains after that resource has been exploited by others. Exploitation can only occur, therefore, if the resource in question is in limited supply.

In many other cases, competition takes the form of interference. Here individuals interact directly with each other, and one individual will actually prevent another from exploiting the resources within a portion of the habitat. For instance, this is seen amongst animals that defend territories (see Section 5.11) and amongst the sessile animals and plants that live on rocky shores. The presence of a barnacle on a rock prevents any other barnacle from occupying that same position, even though the supply of food at that position may exceed the requirements of several barnacles. In such cases, space can be seen as a resource in limited supply. Another type of interference competition occurs when, for instance, two red deer stags fight for access to a harem of hinds. Either stag, alone, could readily mate with all the hinds, but they cannot both do so since matings are limited to the 'owner' of the harem.

Thus, interference competition may occur for a resource of real value (e.g. space on a rocky shore for a barnacle), in which case the interference is accompanied by a degree of exploitation, or for a surrogate resource (a territory, or ownership of a harem), which is only valuable because of the access it provides to a real resource (food, or females). With exploitation, the intensity of competition is closely linked to the level of resource present and the level required, but with interference, intensity may be high even when the level of the real resource is not limiting.

In practice, many examples of competition probably include elements of both exploitation and interference. For instance, adult cave beetles, Neapheanops tellkampfi, in Great Onyx Cave, Kentucky, compete amongst themselves but with no other species and have only one type of food - cricket eggs, which they obtain by digging holes in the sandy floor of the cave. On the one hand, they suffer indirectly from exploitation: beetles reduce the density of their resource (cricket eggs) and then have markedly lower fecundity when food availability is low (Figure 5.1a). But they also suffer directly from interference: at higher beetle densities they fight more, forage less, dig fewer and shallower holes and eat far fewer eggs than could be accounted for by food depletion alone (Figure 5.1b).

5.1.2 One-sided competition

Whether they compete through exploitation or interference, individuals within a species have many fundamental features in common, using similar resources and reacting in much the same way to conditions. None the less, intraspecific competition may be very one sided: a strong, early seedling will shade a stunted, late one; an older and larger bryozoan on the shore will grow over a smaller and younger one. One example is shown in Figure 5.2. The overwinter survival of red deer calves in the resource-limited population on the island of Rhum, Scotland (see Chapter 4) declined sharply as the population became more crowded, but those that were smallest at birth were by far the most likely to die. Hence, the ultimate effect of competition is interference

Deer Rhum Island Mortality Curve

far from being the same for every individual. Weak competitors may make only a small contribution to the next generation, or no contribution at all. Strong competitors may have their contribution only negligibly affected.

Finally, note that the likely effect of intraspecific competition on any individual is greater the more competitors there are. The effects of intraspecific competition are thus said to be density dependent. We turn next to a more detailed look at the density-dependent effects of intraspecific competition on death, birth and growth.

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Responses

  • asmara
    What is exploitation interference?
    7 months ago
  • Marko Gloeckner
    What is exploitation in ecology?
    6 months ago

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