Pluralism in community ecology

It would be wrong to replace one monolithic view of community organization (the overriding importance of competition and niche differentiation) with another (the overriding importance of forces such as predation and disturbance that make competition much less influential). Certainly, communities structured by competition are not a general rule, but neither necessarily are communities structured by any single agency. Most communities are probably organized by a mixture of forces - competition, predation, disturbance and recruitment - although their relative importance may vary systematically, with competition and predation figuring more prominently in communities where recruitment levels are high (Menge & Sutherland, 1987) and in less disturbed environments (Menge & Sutherland, 1976; Townsend, 1991).

In an elegant series of experiments, Wilbur (1987) investigated the interactions between competition, predation and disturbance as they influenced four species of frog and toad that occur in North American ponds. In the absence of predators, tadpoles of Scaphiopus holbrooki were competitively dominant whilst, at the opposite extreme, Hyla chrysoscelis had a very low competitive status (Figure 19.23a). The presence of predatory salamanders, Notophthalmus viridescens, did not alter the total number of tadpoles reaching metamorphosis, but relative abundances were shifted because S. holbrooki, the competitive dominant, was selectively eaten (Figure 19.23b). Finally, Wilbur subjected his tadpole communities, in the presence and absence of predators, to water loss, to simulate a natural drying regime (disturbance). The influence of competition was to slow growth and retard the

Figure 19.23 (a) Relative abundance of tadpoles of each of four species introduced at high density into ponds (initial), and the relative abundances of metamorphs at the end of the experiment (final). (b) Number of metamorphs of four species in the absence and presence of predatory salamanders, and in ponds which persist or which dry up 100 days into the experiment. (After Wilbur, 1987; Townsend, 1991a.)

persists dries

communities are not necessarily structured by a single biotic process physical conditions can moderate the effects of predators and parasites

Figure 19.23 (a) Relative abundance of tadpoles of each of four species introduced at high density into ponds (initial), and the relative abundances of metamorphs at the end of the experiment (final). (b) Number of metamorphs of four species in the absence and presence of predatory salamanders, and in ponds which persist or which dry up 100 days into the experiment. (After Wilbur, 1987; Townsend, 1991a.)

Figure 19.24 Extermination of a dense intertidal amphipod population by microphallid trematodes and the consequent changes in sediment characteristics and topography of the mudflat. (a) Mean density (± SE) of Corophium volutator. (b) Mean silt content (particles < 63 |lm) and substrate erosion. (c) The topography of the Corophium bed before the parasite-induced C. volutator die-off. (d) The topography of the flat a few months after the disappearance of amphipods. (After Mouritsen & Poulin, 2002 and Mouritsen et al., 1998. Reproduced by permission of K. Mouritsen)

Figure 19.24 Extermination of a dense intertidal amphipod population by microphallid trematodes and the consequent changes in sediment characteristics and topography of the mudflat. (a) Mean density (± SE) of Corophium volutator. (b) Mean silt content (particles < 63 |lm) and substrate erosion. (c) The topography of the Corophium bed before the parasite-induced C. volutator die-off. (d) The topography of the flat a few months after the disappearance of amphipods. (After Mouritsen & Poulin, 2002 and Mouritsen et al., 1998. Reproduced by permission of K. Mouritsen)

timing of metamorphosis, thus increasing the risk of desiccation in drying ponds. S. holbrooki had the shortest larval period and made up a greater proportion of metamorphs in the drying experiment without predators. The presence of predators ameliorated the impact of competition, allowing surviving tadpoles of several species to grow rapidly enough to metamor-phosize before the ponds dried up.

The consequences of parasitism may also be moderated by physical conditions. The mud snail Hydrobia ulvae and the amphipod Corophium volutator dominate the benthic macrofaunal community on intertidal mudflats in the Danish Wadden Sea. These two species serve as first and second intermediate hosts, respectively, to microphallid trematodes, with sandpipers (Calidris spp.) as definitive hosts. The trematode eggs are expelled in the bird's droppings and the detritus-feeding snails accidentally eat them. The parasite larvae hatch and reproduce inside the snail, releasing into the water on a daily basis vast numbers of swimming cercariae that seek out an amphipod. As a consequence of a temperature-dependent release of cercariae from the snails, the parasites cause intensity-dependent mortality in the amphipod hosts, which itself increases rapidly with increasing temperature (Mouritsen & Jensen,

1997). C. volutator normally increases rapidly during spring and summer, commonly achieving densities exceeding 80,000 individuals per m2 in early fall (Mouritsen et al., 1997). Because these amphipods make permanent U-shaped burrows that stabilize the substrate, and because of their patchy distribution, Corophium-dominated mudflats have a characteristic topography with a mosaic of elevated plateaux (high-density patches) and sediment depressions (low-density patches) (Mouritsen et al., 1998). In this state, the Corophium bed is very stable even during strong onshore gales. But during the spring of 1990, ambient temperatures were unusually high and so was the prevalence of microphallid infections in the snail population, resulting in a massive release of cercariae from the snails and, within 5 weeks, the complete collapse of the amphipod population (Figure 19.24a) (Jensen & Mouritsen, 1992). As the sediment-stabilizing amphipods disappeared, the plateaux of the former Corophium bed (which covered about 80 ha) were subject to significant erosion (Figure 19.24b). The characteristic mudflat topography eventually vanished (Figure 19.26c, d) with dramatic consequences for many other mudflat macroinvertebrates, including species of nemertinea, polychaeta, gastropoda, bivalvia and crustacea.

We began this chapter by noting the diversity of ways in which a single species can affect communities and ecosystems. It would be wrong to finish it with the impression that competition, predation and parasitism are the principal population interactions that determine community organization. Facilitation is also of major significance, though once again its significance varies with physical conditions. Thus, the presence of a canopy of the seaweed Ascophyllum nodosum at its upper intertidal boundary in communities in the Gulf of Maine reduced maximum daily rock temperatures by 5-10°C and evaporative losses by an order of magnitude, with positive outcomes for recruitment, growth and survivorship of a range of benthic organisms (Bertness et al., 1999). In fact, nearly half of the recorded population interactions in this zone were positive (facilitation) rather than negative (competitive or predatory). On the other hand, at A. nodosum's lower boundary, rather than ameliorating physical conditions (these are not so severe deeper in the intertidal zone) the seaweed canopy provided excellent conditions for herbivores and carnivores and consumer pressure was severe.

Positive interactions among terrestrial plant species have also been demonstrated in many communities (Wilson & Agnew, 1992; Jones et al., 1994). Plants sometimes benefit their neighbors by reducing the likelihood of consumption by herbivores. Thus, Callaway et al. (2000) examined the role played by two competitively dominant and highly unpalatable plants, the physically defended thistle Cirsium obvalatum and the chemically defended Veratrum lobelianum. Both have invaded grazed meadows in the central Caucasus in the Republic of Georgia. Forty four percent (15/34) of all species in the study were rare (< 1.0% cover) in open meadow, but occurred at significantly higher covers under C. obvalatum and V. lobelianum (i.e. within a 60 X 60 cm plot containing one of the unpalatable species). Eight species were only found under an unpalatable species, and the communities associated with them had 78-128% more species in flower or fruit than in the open meadow sites. It seems that tasty species may avoid being eaten, and grow and reproduce better, if they associate with an unpalatable neighbor.

Finally, we have seen how the effects of predators and parasites are not restricted to their prey/hosts or even just those species with which they or their prey compete. Sometimes the effects extend beyond a single, or adjacent, trophic level to spread throughout the food web. This was the case, for example, for starfish (see Section 19.4.2), parasitized caddis larvae (see Section 19.5) and omnivorous crayfish (see Section 19.6). We turn our attention to the complex workings of whole food webs in the next chapter.

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