L

Zone

Worm Hole 25 r

B/S Zone

Figure 19.18 Patterns of invertebrate species richness (worms in the order Vestimentifera (class Pogonophora), worms in the class Polychaeta, gastropods, bivalves and crustaceans) per implanted recruitment substrate after 5 months at three sites with two experimental treatments: (a) East Wall, (b) Biovent, and (c) Worm Hole. The results are presented for four zones whose boundaries are based on water temperature and dominant benthic invertebrates (at increasing distances from the hydrothermal vent: Vestimentiferan (V), bivalve (B), suspension-feeder (S) and periphery (P)). The two middle zones were combined at the Worm Hole site. Experimental treatments: □, uncaged; 0, caged to exclude mobile predators - fish and crabs. (After Micheli et al., 2002.)

number of studies involving birds preying upon grasshoppers, rodents upon carabid beetles and lizards upon web spiders and concluded that these predators usually reduced prey richness or had no effect. In their own study in the Bahamas, they censused spider populations at 2-month intervals for 4.5 years in enclosures (three replicates) containing or lacking lizards. Species richness was dramatically increased by the exclusion of lizards (mainly Anolis sagrei) at high and medium levels in the vegetation (Figure 19.19a). The lizards preyed preferentially upon rare species of spiders (Figure 19.19b), resulting in increased dominance of the already abundant Metapeira datona, a species whose relative invulnerability to predation is probably due to its small size and habit of living in a suspended retreat rather than in the middle of the web.

As was the case with grazers, the manner in which prey species richness responds to predation no doubt depends partly on predation intensity, partly on ecosystem productivity, and partly on the particular characteristics of the prey species. Once again we have seen increases in prey species richness where carnivores feed preferentially on competitively dominant prey (starfish feeding on mussels, pygmy owls feeding on coal tits) and a decrease where the preferred prey are competitively inferior (lizards feeding on spiders).

Another reason for contrasting effects of consumers on lower trophic levels relates to their prey-selection behavior. They seldom simply take potential prey species from a community in turn, bringing each to extinction before turning to the next. Selection is moderated by the time or energy spent in search for the preferred prey (see Chapter 9), and many species take a mixed diet. However, others switch sharply from one type of prey to another, taking disproportionately more of the most common acceptable types of prey. In theory, such behavior could lead to the coexistence of a large number of relatively rare species (a frequency-dependent form of exploiter-mediated coexistence). In this context, there is evidence that predation on the seeds of tropical trees is often more intense where the seeds are more dense (beneath and near the adult that produced them) (Connell, 1979); the herbivorous butterfly Battus philenor forms search images for leaf shape when foraging for its two larval host plants, and concentrates on whichever happens to be the more common (Rausher, 1978); the freshwater zoo-planktivorous fish Rutilus rutilus switches from large planktonic waterfleas, its preferred prey, to small sediment-dwelling waterfleas when the density of the former falls below about 40 per liter (Townsend et al., 1986); and piscivorous coral reef fish (Cephalopholis boenak and Pseudochromis fuscus) concentrate on highly abundant cardinal fish (mainly Apogon fragilis) when these are present, leaving recruits of many other fish species relatively unmolested (Webster & Almany, 2002). However, such frequency-

predator dietary preferences may modify the outcome frequency-dependent selection may sometimes enhance diversity

Controls • Lizards removed

Controls • Lizards removed

Low vegetation

10 100 Total number of individuals

dependent selection is not a general rule and may not be common. For one thing, some species are so highly specialized that switching is not an option - giant pandas are specialists on bamboo shoots and specialization in diet is equally extreme amongst many phytophagous insects. Moreover, in other cases a predator may be sustained by one prey type whilst exterminating others. This has been claimed for the introduced snake Boiga irregularis, on the small island of Guam, midway between Japan and New Guinea. Coincident with its arrival in the early 1950s, and its subsequent spread through Guam, most of the 18 native bird species have declined dramatically and seven are now extinct. Savidge (1987) argues that by including abundant small lizards in its diet, B. irregularis has maintained high densities whilst exterminating the more vulnerable bird species.

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