The effect of carnivores

The rocky intertidal zone also provided the location for pioneering work by Paine (1966) on the influence of a top carnivore on community structure.

The starfish Pisaster ochraceus preys on sessile filter-feeding barnacles and mussels, and also on browsing limpets and chitons and a small carnivorous whelk. These species, together with a sponge and four macroscopic algae, form predictable associations on rocky shores of the Pacific coast of North America. Paine removed all the starfish from a typical piece of shoreline about 8 m long and 2 m deep, and continued to exclude them for several years. At irregular intervals, the density of invertebrates and the cover of benthic algae were assessed in the experimental area and in an adjacent control site. The latter remained unchanged during the study. Removal of P. ochraceus, however, had dramatic consequences. Within a few months, the barnacle Balanus glandula settled successfully. Later, barnacles were crowded out by mussels (Mytilus californianus), and eventually the site became dominated by the latter. All but one of the species of algae disappeared, apparently through lack of space, and the browsers tended to move away, partly because space was limited and partly due to lack of suitable food. Overall, the removal of starfish led to a reduction in the number of species from 15 to eight. The main influence of the starfish Pisaster appears to be to make space available for competitively subordinate species. It cuts a swathe free of barnacles and, most importantly, free of the dominant mussels that would otherwise outcompete other invertebrates and algae for space. Once again, there is exploiter-mediated coexistence. Note that this argument applies specifically to the primary space occupiers, such as mussels, barnacles and macroalgae. In contrast, the number of less conspicuous species associated with living and dead mussel shells would be expected to increase in the bed that develops after Pisaster removal (more than 300 species of animals and plants occur in mussel beds; Suchanek, 1992).

... and plant species traits predator-mediated coexistence on a rocky shore ...

100 200 300 400 500 Potential productivity (g m-2)

Main limiting factor

Soil resources

Canopy resources

Soil resources

Canopy resources

- '

---- Grazed

-

-Ungrazed

i i i i

i i

100 200 300 400 500 Potential productivity (g m-2)

100 200 300 400 500 Potential productivity (g m-2)

Figure 19.17 Relationship between annual above-ground productivity (measured in ungrazed subplots) and species richness in (a) ungrazed and (b) grazed subplots. Open symbols represent low productivity subplots (< 200 g dry matter m-2; hilltop, south-and north-facing slopes in all years plus wadi in the dry 1999 season). Closed symbols represent high productivity subplots (> 200 g dry matter m-2; wadi sites in years other than 1999). (c) Conceptual model of the relationship between productivity and species richness in grazed and ungrazed semiarid Mediterranean rangeland. (After Osem et al., 2002.)

Table 19.1 Area, distance to mainland and occurrence of breeding pairs of pygmy owls and three species of tit. (After Kullberg & Ekman, 2000.)

Experiments similar to those of Paine have been performed in the more challenging environment of hydrothermal vents at a depth of 2500 m in the eastern tropical Pacific Ocean (Micheli et al., 2002). Colonization of replicate recruitment substrates (10 cm basalt cubes) was monitored for 5 months at increasing distances from the vent in three sites in the presence and absence (exclusion cages) of predators (fish and crabs). In terms of reduced prey abundance (particularly two gastropods endemic to vents - the limpet Lepetodrilus elevatus and the snail Cyathermia naticoides), the effects of predation were strongest near the vent where productivity and the overall abundance of invertebrates were greatest. Species richness, which generally declined with distance from the vent, was usually lower in the presence of predators (but only statistically significantly so at the Worm Hole site - Figure 19.18). The reason for a lack of predator-mediated coexistence is unknown.

Turning now to terrestrial ecosystems, in a study of nine Scandinavian islands, pigmy owls (Glaucidium passer-inum) occurred on only four of the islands, and the pattern of occurrence of three species of passerine birds in the genus Parus showed a striking relationship with this distribution (Table 19.1). The five islands without the predatory owl were home to only one species, the coal tit (Parus ater). However, in the presence of the owl, the coal tit was always joined by two larger tit species, the willow tit (P. montanus) and the crested tit (P. cristatus). Kullberg and Ekman (2000) argue that the smaller coal tit is superior in exploitation competition for food. The two larger species, however, have an advantage via interference competition for foraging sites close to the trunk of trees where they are safer from predators; in other words the larger species are less affected than the coal tit by predation from the owl. It seems that the owl may be responsible for predator-mediated coexistence, by reducing the competitive dominance enjoyed by coal tits in its absence.

However, an increase in species richness with predation is by no means universal in terrestrial ecosystems. Spiller and Schoener (1998) reviewed a predator-mediated coexistence among passerine birds...

but not among communities of insects or spiders

Table 19.1 Area, distance to mainland and occurrence of breeding pairs of pygmy owls and three species of tit. (After Kullberg & Ekman, 2000.)

Island

Area (km2)

Distance to mainland (km)

Pygmy owl

Coal tit

Willow tit

Crested tit

Aland

970

50

+

+

+

+

Osel

3000

15

+

+

+

+

Dago

989

10

+

+

+

+

Karlo

200

7

+

+

+

+

Gotland

3140

85

+

Oland

1345

4

+

Bornholm

587

35

+

Hano

2.2

4

+

Visingso

30

6

+

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