Density or crowding

Of course, the intensity of intraspecific competition experienced by an individual is not really determined by the density of the population as a whole. The effect on an individual is determined,

Figure 5.5 (a) The fecundity (seeds per plant) of the annual dune plant Vulpia fasciculata is constant at the lowest densities (density independence, left). However, at higher densities, fecundity declines but in an undercompensating fashion, such that the total number of seeds continues to rise (right). (After Watkinson & Harper, 1978.) (b) Fecundity (eggs per attack) in the southern pine beetle, Dendroctonus frontalis, in East Texas declines with increasing attack density in a way that compensates more or less exactly for the density increases: the total number of eggs produced was roughly 100 per 100 cm2, irrespective of attack density over the range observed (•, 1992; •, 1993). (After Reeve et al., 1998.) (c) When the planktonic crustacean Daphnia magna was infected with varying numbers of spores of the bacterium Pasteuria ramosa, the total number of spores produced per host in the next generation was independent of density (exactly compensating) at the lower densities, but declined with increasing density (overcompensating) at the higher densities. Standard errors are shown. (After Ebert et al., 2000.)

Pasteuria Spp

rather, by the extent to which it is crowded or inhibited by its immediate neighbors.

One way of emphasizing this is by noting that there are actually at least three different meanings of 'density' (see Lewontin & Levins, 1989, where details of calculations and terms can be found). Consider a population of insects, distributed over a population of plants on which they feed. This is a typical example of a very general phenomenon - a population (the insects in this case) being distributed amongst different patches of a resource (the plants). The density would usually be calculated as the number of insects (let us say 1000) divided by the number of plants (say 100), i.e. 10 insects per plant. This, which we would normally call simply the 'density', is actually the 'resource-weighted density'. However, it gives an accurate measure of the intensity of competition suffered by the insects (the extent to which they are crowded) only if there are exactly 10 insects on every plant and every plant is the same size.

Suppose, instead, that 10 of the plants support 91 insects each, and the remaining 90 support just one insect. The resource-weighted density would still be 10 insects per plant. But the average density experienced by the insects would be 82.9 insects per plant. That is, one adds up the densities experienced by each of the insects (91 + 91 + 91 ... + 1 + 1) and divides by the total number of insects. This is the 'organism-weighted density', and it clearly gives a much more satisfactory measure of the intensity of competition the insects are likely to suffer.

However, there remains the further question of the average density of insects experienced by the plants. This, which may be referred to as the 'exploitation pressure', comes out at 1.1 insects per plant, reflecting the fact that most of the plants support only one insect.

What, then, is the density of the insect? Clearly, it depends on whether you answer from the perspective of the insect or the plant - but whichever way you look at it, the normal practice of calculating the resource-weighted density and calling it the 'density' looks highly suspect. The difference between resource-and organism-weighted densities is illustrated for the human population of a number of US states in Table 5.1 (where the 'resource' is simply land area). The organism-weighted densities are so much larger than the usual, but rather unhelpful, resource-weighted densities essentially because most people live, crowded, in cities (Lewontin & Levins, 1989).

The difficulties of relying on density to characterize the potential intensity of intraspecific competition are particularly three meanings of density

Table 5.1 A comparison of the resource- and organism-weighted densities of five states, based on the 1960 USA census, where the 'resource patches' are the counties within each state. (After Lewontin & Levins, 1989.)

Resource-weighted

Organism-weighted

State

density (km"2)

density (km"2)

Colorado

44

6,252

Missouri

159

6,525

New York

896

48,714

Utah

28

684

Virginia

207

13,824

acute with sessile, modular organisms, because, being sessile, they compete almost entirely only with their immediate neighbors, and being modular, competition is directed most at the modules that are closest to those neighbors. Thus, for instance, when silver birch trees (Betula pendula) were grown in small groups, the sides of individual trees that interfaced with neighbors typically had a lower 'birth' and higher death rate of buds (see Section 4.2); whereas on sides of the same trees with no interference, bud birth rate was higher, death rate lower, branches were longer and the form approached that of an open-grown individual (Figure 5.6). Different modules experience different intensities of competition, and quoting the density at which an individual was growing would be all but pointless.

Thus, whether mobile or sessile, different individuals meet or suffer from different numbers of competitors. Density, especially resource-weighted density, is an abstraction that applies to the population as a whole but need not apply to any of the individuals within it. None the less, density may often be the most convenient way of expressing the degree to which individuals are crowded - and it is certainly the way it has usually been expressed.

Figure 5.6 Mean relative bud production (new buds per existing bud) for silver birch trees (Betula pendula), expressed (a) as gross bud production and (b) as net bud production (birth minus death), in different interference zones. These zones are themselves explained in the inset. •, high interference; e, medium; o, low. Bars represent standard errors. (After Jones & Harper, 1987.)

Betula Pendula What Fertiliser

density: a convenient expression of crowding

Figure 5.6 Mean relative bud production (new buds per existing bud) for silver birch trees (Betula pendula), expressed (a) as gross bud production and (b) as net bud production (birth minus death), in different interference zones. These zones are themselves explained in the inset. •, high interference; e, medium; o, low. Bars represent standard errors. (After Jones & Harper, 1987.)

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Responses

  • j
    What is the difference between crowding and density?
    2 years ago
  • MARILYN
    What is the difference between crowding and population density?
    2 years ago

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