2.6 STRONG INTERACTORS AND KEYSTONE SPECIESMURDOCH (1966), IN PARTI...

20.2.6 Strong interactors and keystone species

Murdoch (1966), in particular, chal-

. . . or is it prickly

lenged these ideas. His view, described

and bad tasting?

by Pimm (1991) as ‘the world is prickly

Some species are more intimately and tightly woven into the

fabric of the food web than others. A species whose removal

and tastes bad’, emphasized that even if

would produce a significant effect (extinction or a large change

the world is green (assuming it is), it does not necessarily follow

that the herbivores are failing to capitalize on this because they

in density) in at least one other species may be thought of as a

strong interactor. Some strong interactors would lead, through

are limited, top-down, by their predators. Many plants have

their removal, to significant changes spreading throughout the

evolved physical and chemical defenses that make life difficult for

herbivores (see Chapter 3). The herbivores may therefore be com-

food web – we refer to these as keystone species.

A keystone is the wedge-shaped block at the highest point of

peting fiercely for a limited amount of palatable and unprotected

an arch that locks the other pieces together. Its early use in food

plant material; and their predators may, in turn, compete for scarce

web architecture referred to a top predator (the starfish Pisaster

herbivores. A world controlled from the bottom-up may still

on a rocky shore; see Paine (1966) and Section 19.4.2) that has an

be green.

Oksanen (1988), moreover, has argued that the world is not

indirect beneficial effect on a suite of inferior competitors by

depressing the abundance of a superior competitor. Removal of

always green – particularly if the observer is standing in the middle

the keystone predator, just like the removal of the keystone in

of a desert or on the northern coast of Greenland. Oksanen’s

contention (see also Oksanen et al., 1981) is that: (i) in extremely

an arch, leads to a collapse of the structure. More precisely, it leads

unproductive or ‘white’ ecosystems, grazing will be light because

to extinction or large changes in abundance of several species, pro-

there is not enough food to support effective populations of

ducing a community with a very different species composition

herbivores: both the plants and the herbivores will be limited

and, to our eyes, an obviously different physical appearance.

It is now usually accepted that key-

bottom-up; (ii) at the highest levels of plant productivity, in ‘green’

stone species can occur at other trophic

ecosystems, there will also be light grazing because of top-down

what is a keystone

levels (Hunter & Price, 1992). Use of the

species?

limitation by predators (as argued by Hairston et al., 1960); but

(iii) between these extremes, ecosystems may be ‘yellow’, where

term has certainly broadened since it

plants are top-down limited by grazers because there are insuffici-

was first coined (Piraino et al., 2002), leading some to question

ent herbivores to support effective populations of predators. The

whether it has any value at all. Others have defined it more narrowly

– in particular, as a species whose impact is ‘disproportionately

suggestion, then, is that productivity shifts the balance between

top-down and bottom-up control by altering the lengths of food

large relative to its abundance’ (Power et al., 1996). This has the

chains. This still remains to be critically tested.

advantage of excluding from keystone status what would other-

wise be rather trivial examples, especially ‘ecological dominants’

There are also suggestions that the

level of primary productivity may be

an influence of

at lower trophic levels, where one species may provide the

resource on which a whole myriad of other species depend –

primary productivity?

influential in other ways in determining

for example, a coral, or the oak trees in an oak woodland. It is

whether top-down or bottom-up control

certainly more challenging and more useful to identify species

is predominant. Chase (2003) examined the effect of nutrient

with disproportionate effects.

concentrations on a freshwater web comprising an insect pred-

Semantic quibbles aside, it remains important to acknowledge

ator, Belostoma flumineum, feeding on two species of herbivorous

that while all species no doubt influence the structure of their

snails, Physella girina and Helisoma trivolvis, in turn feeding on macro-

communities to a degree, some are far more influential than

phytes and algae within a larger food web including zooplankton

(b)(a)

Low nutrients

3

30

2

20

Figure 20.6 Top-down control, but only

*

* * *

with low productivity. (a) Snail biomass

1

10

and (b) plant biomass in experimental

Snail biomass (g tank–1)

ponds with low or high nutrient treatments

Plant biomass (g tank–1)

0

(vertical bars are standard errors). With

low nutrients, the snails were dominated

High nutrients

50

25

by Physella (vulnerable to predation)

and the addition of predators led to a

40

significant decline (indicated by *) in snail

15

biomass and a consequent increase in

plant biomass (dominated by algae).

But with high nutrients, Helisoma snails

Helisoma

Macrophytes

(less vulnerable to predation) increased

5

Physella

Algae

their relative abundance, and the addition

0 Low

of predators led neither to a decline in

High +

Low +

High

snail biomass nor to an increase in

pred

plant biomass (often dominated by

Initial snail density and predator treatments

macrophytes). (After Chase, 2003.)

others. Indeed, various indices have been proposed to measure

lesser snow geese (Chen caerulescens caerulescens) are herbivores that

this influence (Piraino et al., 2002); for example, the ‘community

breed in large colonies in coastal brackish and freshwater marshes

importance’ of a species is the percentage of other species lost

along the west coast of Hudson Bay in Canada. At their nesting

from the community after its removal (Mills et al., 1993). Also,

sites in spring, before the onset of above-ground growth of vegeta-

tion, adult geese grub for the roots and rhizomes of graminoid

recognizing the concept of keystone species and attempting

plants in dry areas and eat the swollen bases of sedge shoots

to identify them are both important from a practical point of

view because keystone species are likely to have a crucial role

in wet areas. Their activity creates bare patches (1–5 m

²

) of peat

in conservation: changes in their abundance will, by definition,

and sediment. Since there are few pioneer plant species able to

have significant repercussions for a whole range of other species.

recolonize these patches, recovery is very slow. Furthermore,

Inevitably, though, the dividing line between keystone species and

in ungrubbed brackish marshes, intense grazing by high densities

of geese later in the summer is essential in establishing and

the rest is not clear cut.

maintaining grazing ‘lawns’ of Carex and Puccinellia (Kerbes et al.,

In principle, keystone species can