of those seeds.
The vulnerabilities of the two populations to control measures are therefore also different. The main options are three species of insect: two beetles and a fly (biological control of pests and weeds is discussed fully in Chapter 15). First, the thistle receptacle weevil, Rhinocyllus conicus, reduces seed set of thistles by around 30–35% in both Australia and New Zealand. Its release has been the most effective measure in controlling thistles in many parts of the world, but not, seemingly, in New Zealand. Second, the receptacle gallfly, Urophora solstitialis, reduces seed production by around 70% in Australia, though no estimates are available for New Zealand. And last, the root‐crown weevil, Trichosirocalus horridus, reduces plant growth by around 87% and thus affects both survival and fecundity. It is this species that has seemed to be most effective in Australia, and the elasticity analysis is entirely consistent with this, since growth and survivorship are relatively important there, compared with reproduction.
In New Zealand, by contrast, targeting seed production would seem from the elasticity analysis to be the most appropriate strategy, and the lack of success of R. conicus there is therefore likely to be a result not of inappropriate targeting but of the ineffectiveness of R. conicus (Shea & Kelly, 1998). This in turn suggests that the subsequent release of the gallfly should have been more successful, but sadly this seems not to have been the case either, perhaps because young gallfly larvae are themselves preyed upon by R. conicus larvae (Groenteman et al., 2011). As these authors remark, ‘Thirty‐five years into the biocontrol programme and three agents later, C. nutans is still a major weed in parts of New Zealand’. Elasticity analyses of population projection matrices can direct managers to a pest’s vulnerabilities, but they cannot conjure up effective biocontrol agents.
Chapter 5 Intraspecific Competition
5.1 Introduction
Organisms grow, reproduce and die (Chapter 4). They are affected by the conditions in which they live (Chapter 2) and by the resources that they obtain (Chapter 3). But no organism lives alone. Each, for at least part of its life, is part of a population of its own species.
a definition of competition
Competition can be defined as an interaction between individuals brought about by a shared requirement for a resource, leading to a reduction in the survivorship, growth and/or reproduction of at least some of the individuals concerned. Individuals of the same species have very similar requirements for survival, growth and reproduction, but their combined demand for a resource may exceed the immediate supply. The individuals then compete for the resource and at least some of them will become deprived. In this chapter we examine the nature of such intraspecific competition, its effects on the competing individuals and on populations of competing individuals.
Consider, initially, a simple hypothetical community: a thriving population of grasshoppers (all of one species) feeding on a field of grass (also of one species). To provide themselves with energy and material for growth and reproduction, grasshoppers must find and eat grass, but use energy in doing so. A grasshopper that finds itself at a spot where there is no grass because another grasshopper has eaten it must move on and expend more energy before it takes in food. The more grasshoppers there are, the more this will happen, increasing energy expenditure, decreasing the rate of food intake, and hence potentially decreasing its chances of survival and leaving it less energy for development and reproduction. Survival and reproduction determine a grasshopper’s contribution to the next generation. Hence, the more intraspecific competitors for food a grasshopper experiences, the less its likely contribution will be.
As far as the grass itself is concerned, an isolated seedling in fertile soil may have a very high chance of surviving to reproductive maturity. It will probably exhibit extensive modular growth and probably, therefore, eventually produce many seeds. However, a seedling that is closely surrounded by neighbours (shading it with their leaves and depleting the water and nutrients of its soil with their roots) will be very unlikely to survive, and if it does, will almost certainly form few modules and set few seeds.
We see immediately that the ultimate effect of competition on an individual is a decreased contribution to the next generation compared with what would have happened had there been no competitors. Intraspecific competition typically leads to decreased rates of resource intake per individual, and thus to decreased rates of individual growth or development, or perhaps to decreases in the amounts of stored reserves or to increased risks of predation. These may lead, in turn, to decreases in survivorship and/or decreases in fecundity, which together determine an individual’s reproductive output.
5.1.1 Exploitation and interference
exploitation
In many cases, competing individuals do not interact with one another directly. Rather, they deplete the resources that are available to each other. Grasshoppers may compete for food, but a grasshopper is not directly affected by other grasshoppers, but rather by the level to which they have reduced the food supply. Likewise, two grass plants may compete, and each may be adversely affected by the presence of close neighbours, but this is most likely to be because their resource depletion zones overlap – each may shade its neighbours from the incoming flow of radiation, and water or nutrients may be less accessible around the plants’ roots than they would otherwise be. The data in Figure 5.1, for example, show the dynamics of the interaction between single‐celled algal species, diatoms, and one of the resources they require, silicate. As diatom density increases over time, silicate concentration decreases until both reach a steady state in which there is less resource available for the many than there had been previously for the few. This type of competition – in which competitors interact only indirectly, through their shared resources – is termed exploitation.
Figure 5.1 In exploitation competition, resource levels decline as population density increases. Dynamics over time of populations of the freshwater diatoms (a) Cyclotella pseudostelligera and (b) Fragilaria crotonensis and of the silicate which is one of their key resources. The diatoms consume silicate during growth and the populations of diatoms stabilise when the silicate has been reduced to a very low concentration.
Source: After Descamps‐Julien & Gonzalez (2005).
interference
In 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 5.8.4) 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
