Strong interactors and keystone species

Some species are more intimately and tightly woven into the fabric of the food web than others. A species whose removal would produce a significant effect (extinction or a large change in density) in at least one other species may be thought of as a strong interactor. Some strong interactors would lead, through their removal, to significant changes spreading throughout the food web - we refer to these as keystone species.

A keystone is the wedge-shaped block at the highest point of an arch that locks the other pieces together. Its early use in food web architecture referred to a top predator (the starfish Pisaster on a rocky shore; see Paine (1966) and Section 19.4.2) that has an indirect beneficial effect on a suite of inferior competitors by depressing the abundance of a superior competitor. Removal of the keystone predator, just like the removal of the keystone in an arch, leads to a collapse of the structure. More precisely, it leads to extinction or large changes in abundance of several species, producing a community with a very different species composition and, to our eyes, an obviously different physical appearance.

It is now usually accepted that keystone species can occur at other trophic levels (Hunter & Price, 1992). Use of the term has certainly broadened since it was first coined (Piraino et al., 2002), leading some to question whether it has any value at all. Others have defined it more narrowly - in particular, as a species whose impact is 'disproportionately large relative to its abundance' (Power et al., 1996). This has the advantage of excluding from keystone status what would otherwise be rather trivial examples, especially 'ecological dominants' at lower trophic levels, where one species may provide the resource on which a whole myriad of other species depend -for example, a coral, or the oak trees in an oak woodland. It is certainly more challenging and more useful to identify species with disproportionate effects.

Semantic quibbles aside, it remains important to acknowledge that while all species no doubt influence the structure of their communities to a degree, some are far more influential than why is the world green?...

... or is it prickly and bad tasting?

an influence of primary productivity?

what is a keystone species?

Figure 20.6 Top-down control, but only with low productivity. (a) Snail biomass and (b) plant biomass in experimental ponds with low or high nutrient treatments (vertical bars are standard errors). With low nutrients, the snails were dominated by Physella (vulnerable to predation) and the addition of predators led to a significant decline (indicated by *) in snail biomass and a consequent increase in plant biomass (dominated by algae). But with high nutrients, Helisoma snails (less vulnerable to predation) increased their relative abundance, and the addition of predators led neither to a decline in snail biomass nor to an increase in plant biomass (often dominated by macrophytes). (After Chase, 2003.)

Low nutrients

High nutrients

Helisoma Physella

Low nutrients i 30

High nutrients

J5 10

Macrophytes Algae

High

Low High Low + High + pred pred

Initial snail density and predator treatments

others. Indeed, various indices have been proposed to measure this influence (Piraino et al., 2002); for example, the 'community importance' of a species is the percentage of other species lost from the community after its removal (Mills et al., 1993). Also, recognizing the concept of keystone species and attempting to identify them are both important from a practical point of view because keystone species are likely to have a crucial role in conservation: changes in their abundance will, by definition, have significant repercussions for a whole range of other species. Inevitably, though, the dividing line between keystone species and the rest is not clear cut.

In principle, keystone species can occur throughout the food web. Jones et al. (1997) point out that it need not even be their trophic role that makes them important, but rather that they act as 'ecological engineers' (see Section 13.1). Beavers, for example, in cutting down a tree and building a dam, create a habitat on which hundreds of species rely. Keystone mutualists (Mills et al., 1993) may also exert influence out of proportion to their abundance: examples include a pollinating insect on which an ecologically dominant plant relies, or a nitrogen-fixing bacterium supporting a legume and hence the whole structure of a plant community and the animals reliant on it. Certainly, keystone species are limited neither to top predators nor consumers mediating coexistence amongst their prey. For example, lesser snow geese (Chen caerulescens caerulescens) are herbivores that breed in large colonies in coastal brackish and freshwater marshes along the west coast of Hudson Bay in Canada. At their nesting sites in spring, before the onset of above-ground growth of vegetation, adult geese grub for the roots and rhizomes of graminoid plants in dry areas and eat the swollen bases of sedge shoots in wet areas. Their activity creates bare patches (1-5 m2) of peat and sediment. Since there are few pioneer plant species able to recolonize these patches, recovery is very slow. Furthermore, in ungrubbed brackish marshes, intense grazing by high densities of geese later in the summer is essential in establishing and maintaining grazing 'lawns' of Carex and Puccinellia (Kerbes et al., 1990). It seems reasonable to consider the lesser snow goose as a keystone (herbivore) species.

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