Apparent competition enemyfree space

Another reason for being cautious in our discussion of competition is the existence of what Holt (1977, 1984) has called

'apparent competition', and what others have called 'competition for enemy-free space' (Jeffries & Lawton, 1984, 1985).

Imagine a single species of predator or parasite that attacks two species of prey (or host). Both prey species are harmed by the enemy, and the enemy benefits from both species of prey. Hence, the increase in abundance that the enemy achieves by consuming prey 1 increases the harm it does to prey 2. Indirectly, therefore, prey 1 adversely affects prey 2 and vice versa. These two prey species attacked by a predator are, in essence, indistinguishable from two consumer species competing for a resource

Trophic level

Competition

(a) Interference: a direct interaction

Apparent competition

(b) Exploitation: indirect interaction, via a shared resource

(c) Indirect interaction, via a shared enemy

(d) Indirect interaction via other species on same trophic level

Natural enemies (E)

(herbivores, parasites, pathogens)

Consumers (C)

Limiting resources (R)

(light, water, minerals, vitamins, etc.)

C2 C

C2 C

P C3

Figure 8.15 In terms of the signs of their interactions, all of the following are indistinguishable from one another: (a) two species interfering directly (interference competition); (b) two species consuming a common resource (exploitation competition); (c) two species being attacked by a common predator ('apparent competition' for 'enemy-free space'); and (d) two species linked by a third which is a competitor of one and a mutualist of the other. (-), direct interactions;

(-----), indirect interactions; arrows indicate positive influences, circles indicate negative influences. (After Holt, 1984; Connell, 1990.)

interactions are summarized in Figure 8.15, which shows that from the point of view of the two prey species, the signs of the interactions are indistinguishable from those that would apply in the indirect interaction of two species competing for a single resource (exploitation competition). In the present case there appears to be no limiting resource. Hence, the term 'apparent competition'.

In an experiment involving a parasitoid (the ichneumonid wasp Venturia canescens) and two caterpillar hosts (Plodia interpunctella and Ephestia kuehniella), Bonsall and Hassell (1997) allowed free passage of the parasitoid between the host species but kept the hosts apart to avoid the possibility of resource competition between them. When the experimental chambers contained just a single host species together with the parasitoid, both the parasite and host persisted and exhibited damped oscillations in population size, tending towards a stable equilibrium (Figure 8.16). But when the system was run with both host species, the parasitoid had a greater impact on the species with the lower intrinsic rate of increase (E. kuehniella). This host showed increasing population oscillations and invariably went extinct. By means of their elegant experimental design, Bonsall and Hassell were able to demonstrate the effect of apparent competition in a situation where resource competition between the caterpillar species was ruled out.

While the term 'apparent competition' is entirely appropriate, it is sometimes useful to think of 'enemy-free space' as the limiting resource for which prey (or host) species compete. This is because the persistence of prey species 1 will be favored by avoiding attacks from the predator, which we know also attacks prey 2. Clearly, prey 1 can achieve this by occupying a habitat, or adopting a form or a behavioral pattern, that is sufficiently different from that of prey 2. In short, 'being different' (i.e. niche differentiation) will once again favor coexistence - but it will do so because it diminishes apparent competition or competition for enemy-free space.

A rare experimental demonstration of apparent competition for enemy-free space involves two groups of prey living on subtidal rocky reefs at Santa Catalina Island, California. The first comprises three species of mobile gastropods, Tegula aureotincta, T. eiseni and Astraea undosa; the second comprises sessile bivalves, dominated by the clam Chama arcana. Both groups were preyed upon by a lobster (Panulirus interruptus), an octopus (Octopus bimaculatus) and a whelk (Kelletia kelletii), although these predators showed a marked preference for the bivalves. In areas characterized by large boulders and much crevice space ('high relief') there were high densities of bivalves and predators, but only moderate densities of gastropods; whereas in low relief areas largely lacking crevice space ('cobble fields') there were apparently no bivalves, only a few predators but high densities of gastropods.

The densities of the two prey groups were inversely correlated, but there was little in their feeding biology to suggest that they were competing for a shared food resource. On the other hand, when bivalves were experimentally introduced into cobble-field areas, the number of predators congregating there increased, the mortality rates of the gastropods increased (often observably associated with lobster or octopus predation) and the densities of the gastropods declined (Figures 8.17a, b). Experimental manipulation of the (mobile) gastropods proved impossible, but cobble sites with high densities of gastropods supported higher densities of predators, and had higher mortality rates of experimentally added bivalves than did sites with relatively low densities of gastropods (Figure 8.17c). On the rare high relief sites without Chama bivalves, predator densities were lower, and gastropod densities higher, than was normally the case (Figure 8.17d). It seems clear that each prey group adversely affected the other through an evidence for apparent competition ... ... in two caterpillars sharing a parasitoid,

... in gastropods, bivalves and their predators . . .

Figure 8.16 Parasite-mediated apparent competition via a parasitoid wasp Venturia canescens that lays eggs in two caterpillar host species. The experimental setups are illustrated on the left and the population dynamics of the parasitoid (dashed black lines) and host species (host 1 Plodia interpunctella (orange lines); host 2 Ephestia kuehniella (black lines)) on the right. (a) When only a single host was present, the parasitoid and host coexisted with stable dynamics. (b) When the parasitoid had access to both hosts, host 2 showed diverging oscillations and went extinct. (From Hudson & Greenman, 1998, after Bonsall & Hassell, 1997.)

Ephestia Eggs

increased number of predators, and hence increased predator-induced mortality.

An experiment with a similar aim involved removing a common leaf-mining fly (Calycomyza sp.) and its host plant Lepidaploa tortuosa (Asteracea) in replicate sites in a tropical forest community in Belize, Central America. Other leaf-mining fly species that shared natural enemies (parasitoid wasps) with Calycomyza, but whose host plants were different, demonstrated reduced parasitism and increased abundance (a year later) in the removal sites than in the control sites (Morris et al., 2004). These results support predictions of apparent competition, involving a shared natural enemy, in a situation where interspecific competition among the fly species for host plants could not occur.

To complete the picture, there is another indirect interaction between two species that qualifies for the term 'apparent competition' (Figure 8.15d), where species 1 and 2 have negative impacts on one another, and species 2 and 3 have positive (mutualistic)

impacts (see Chapter 13). Species 1 and 3 then have indirect negative impacts on one another without sharing a common resource or, for that matter, a common predator. They exhibit apparent competition, although not for enemy-free space (Connell, 1990).

The examples mentioned so far concern apparent competition in animals. Connell (1990) carried out a particularly revealing reappraisal of 54 published plant examples of field experiments on 'competition', where the original authors had claimed to have demonstrated conventional interspecific competition in 50. A closer look revealed that, in many of these, insufficient information had been collected to distinguish between conventional competition and apparent competition; and in a number of others the information was available - but was ambiguous. For example, one study showed that removal of Artemisia bushes from a large site in Arizona led to much better growth of 22 species of herb than was observed in either undisturbed sites or sites where Artemisia was removed from narrow 3 m strips. This was originally interpreted in terms of greatly

... and in leaf-mining flies sharing parasitoids in a tropical forest reappraisal of plant competition

Figure 8.17 Evidence for apparent competition for predator-free space at Santa Catalina Island, USA. (a) Predator density (number per 10 m2, with standard errors) and gastropod mortality increased (number of 'newly dead' shells per site, with standard errors) when bivalves were added to gastropod-dominated cobble sites (colored bars) relative to controls (gray bars). (b) This led to a decline in gastropod density (standard error bars shown). (c) Predator density was higher (number per 10 m2, with standard errors) at high (colored bars) than at low (gray bars) gastropod-density cobble sites, both in the presence and absence of Chama. (d) Densities of predators were lower (number per 10 m2, with standard errors) and densities of gastropods higher (number per m2, with standard errors) at high-relief sites without Chama (colored bars) than at those with (gray bars). (After Schmitt, 1987.)

Competition For Predator Free Space

Figure 8.17 Evidence for apparent competition for predator-free space at Santa Catalina Island, USA. (a) Predator density (number per 10 m2, with standard errors) and gastropod mortality increased (number of 'newly dead' shells per site, with standard errors) when bivalves were added to gastropod-dominated cobble sites (colored bars) relative to controls (gray bars). (b) This led to a decline in gastropod density (standard error bars shown). (c) Predator density was higher (number per 10 m2, with standard errors) at high (colored bars) than at low (gray bars) gastropod-density cobble sites, both in the presence and absence of Chama. (d) Densities of predators were lower (number per 10 m2, with standard errors) and densities of gastropods higher (number per m2, with standard errors) at high-relief sites without Chama (colored bars) than at those with (gray bars). (After Schmitt, 1987.)

reduced exploitative competition for water in the former case (Robertson, 1947). However, the herbs in the larger site also experienced greatly reduced grazing pressure from deer, rodents and insects, for which the Artemisia bushes were not only a source of food but a place of shelter, too. The outcome is therefore equally likely to have resulted from reduced apparent competition.

This emphasizes that the relative neglect of apparent competition in the past has been unwarranted, but also re-emphasizes that the distinction is important within interspecific competition between pattern on the one hand, and process or mechanism on the other. In the past, patterns of niche differentiation, and also of increased abundance of one species in the absence of another, have been interpreted as evidence of competition too readily. Now we can see that such patterns can arise through a wide variety of processes, and that a proper understanding requires that we distinguish between them - not only discriminating between distinguishing pattern and process conventional and apparent competition, but also specifying mechanisms within, say, conventional competition (a point to which we return in Section 8.10).

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Responses

  • hagosa
    How is predator mediated competition different than apparent competition?
    6 years ago

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