10.2.1. Direct evidence
In a pioneering study Holmes (1961, 1962) demonstrated experimentally that interspecies competition occurred between Hymenolepis diminuta and Moniliformis moniliformis (= dubius) in rats. Since then similar studies have confirmed that interspecies competition also occurs between other helminth species under experimental conditions (Mapes and Coop, 1971; Silver et al., 1980; Holland, 1984; Bates and Kennedy, 1990). The latter authors studied interactions between the acanthocephalans Pomphorhynchus laevis and Acanthocephalus anguillae in experimentally infected rainbow trout and found that the establishment of both species was unaffected by the presence of the other species. However, it was found that after establishment had occurred A. anguillae was numerically disadvantaged by the presence of P. laevis whereas the converse was not the case. This type of extreme, asymmetrical interaction should therefore be classed as ammensalism rather than competition. According to Bates and Kennedy (1990) the most tenable explanation for this phenomenon is that it is caused by an exclusion zone around individuals of P. laevis, possibly mediated by the host, in which the survival of A. anguillae is reduced. They therefore suggested that in rivers where P. laevis dominates fish infracommunities, interspecies competition, or ammensalism, might make it impossible for A. anguillae to become established. This suggestion can be extrapolated to other acanthocephala species as Kennedy (1985, 1992) showed that examples of co-occurrence of two or more acanthocephala species within freshwater localities in Britain were far fewer than expected.
10.2.2. Circumstantial evidence indicating interspecies competition
In order to evaluate this evidence it is necessary to consider the concept of the niche. The term niche is best considered as a multidimensional space, determined by a large number of physical and biotic variables, within which species exist (Hutchinson, 1957). Each dimension represents a factor that influences the biological fitness of the species. In this context it is best not to make a distinction between parameters which benefit the organism directly, such as food resources, and those which may harm it such as allelopathic compounds or predators. However, the parameters that are given most attention in parasite ecology are the species, age and sex of the host, microhabitats, time and nutrient requirements. Microhabitat selection may be based on long-term, genetically fixed behavioural patterns (habitat segregation) or on short-term, interactive site selection induced by the presence of a potential competitor. The circumstantial evidence indicating that interspecies competition may be occurring among fish parasite communities is discussed below.
Although 12 species of helminth parasites occurred in the trout only six, namely D. sagittata, C. farionis, C. metoecus, P. simile, N. rutili and C. truttae were commonly encountered and the present analysis is restricted to them. Two of these, D. sagittata and P. simile, which occur on the gills and in the urinary bladder, respectively, are completely isolated from the gut-inhabiting species and cannot therefore interact with them directly. It is also unlikely that cross-immunity will influence the survival of these spatially segregated species.
Competitive interactions are however possible between the four gut-inhabiting species. Although these are restricted to the region behind the stomach there is evidence that they have habitat preferences which are probably based on genetically fixed behavioural patterns (Figure 26). Thus, both C. metoecus and C. trutta occurred predominantly in the pyloric caeca region of the intestine, while C. farionis and N. rutili favoured the post-pyloric region. As the available surface area of the fish intestines is relatively small the mean densities of helminth parasites per square metre are relatively high. These were estimated to be 10 000, 10 000, 14 000 and 16 000 for 1-, 2-, 3- and 4-year-old trout, respectively. These values compare with maximum densities of 54 000, 46 000, 77 000 and 61 000 for 1-, 2-, 3- and 4-year-old trout, respectively. It is noteworthy that the parasite densities per square metre vary little with age despite the fact that the mean parasite abundance values (19.6, 24.4, 48.3 and 56.6 for 1-, 2-, 3- and 4-year-old fish, respectively) increase markedly with the age of the fish. This observation is attributable to the fact that the surface area of the gut increases as the fish grows. The fact that density changes are minimal with age suggests that space may be a limiting factor. These high densities, which are indicative of a resource-rich habitat where pre-digested food is continuously available, exceed those of free-living, benthic
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