2.5 TOP-DOWN OR BOTTOM-UP CONTROL OF FOOD WEBS

20.2.5 Top-down or bottom-up control of food webs?

answer this question we should recognize a distinction between

Why is the world green?

community- and species-level cascades (Polis, 1999). In the

former, the predators in a community, as a whole, control the

We have seen that trophic cascades are normally viewed ‘from

abundance of the herbivores, such that the plants, as a whole,

the top’, starting at the highest trophic level. So, in a three-level

are released from control by the herbivores. But in a species-level

cascade, increases in a particular predator give rise to decreases

trophic community, we think of the predators controlling the

abundance of the grazers and say that the grazers are subject to

in particular herbivores and increases in particular plants, with-

out this affecting the whole community. Thus, Schmitz et al. (2000),

‘top-down control’. Reciprocally, the predators are subject to

in apparent contradiction of the ‘all cascades are wet’ proposition,

bottom-up control (abundance determined by their resources):

a standard predator–prey interaction. In turn, the plants are also

reviewed a total of 41 studies in terrestrial habitats demonstrat-

ing trophic cascades; but Polis et al. (2000) pointed out that all of

subject to bottom-up control, having been released from top-down

control by the effects of the predators on the grazers. Thus, in a

these referred only to subsets of the communities of which they

trophic cascade, top-down and bottom-up control alternate as we

were part – that is, they were essentially species-level cascades.

move from one trophic level to the next.

Moreover, the measures of plant performance in these studies were

But suppose instead that we start at the other end of the food

typically short term and small scale (for instance, ‘leaf damage’

as in the lizard–spider–herbivore–seagrape example above) rather

chain, and assume that the plants are controlled bottom-up by com-

petition for their resources. It is still possible for the herbivores

than broader scale responses of significance to the whole com-

munity, such as plant biomass or productivity.

to be limited by competition for plants – their resources – and

for the predators to be limited by competition for herbivores. In

Polis et al. (2000) proposed, then, that community-level

this scenario, all trophic levels are subject to bottom-up control

cascades are most likely to occur in systems with the following

characteristics: (i) the habitats are relatively discrete and homo-

(also called ‘donor control’), because the resource controls the

abundance of the consumer but the consumer does not control

geneous; (ii) the prey population dynamics (including those of

the primary producers) are uniformly fast relative to those of their

the abundance of the resource. The question has therefore arisen:

consumers; (iii) the common prey tend to be uniformly edible;

‘Are food webs – or are particular types of food web – dominated

and (iv) the trophic levels tend to be discrete and species inter-

by either top-down or bottom-up control?’ (Note again, though,

actions strong, such that the system is dominated by discrete trophic

that even when top-down control ‘dominates’, top-down and

bottom-up control are expected to alternate from trophic level

chains.

If this proposition is correct, then community-level cascades

to trophic level.)

top-down, bottom-up

are most likely in pelagic communities of lakes and in benthic

Clearly, this is linked to the issues we

communities of streams and rocky shores (all ‘wet’) and perhaps

have just been dealing with. Top-down

and cascades

control should dominate in systems

in agricultural communities. These tend to be discrete, relatively

with powerful community-level trophic cascades. But in systems

simple communities, based on fast-growing plants often dominated

where trophic cascades, if they exist at all, are limited to the species

by a single taxon (phytoplankton, kelp or an agricultural crop).

This is not to say (as the Schmitz et al. (2000) review confirms)

level, the community as a whole could be dominated by top-down

that such forces are absent in more diffuse, species-rich systems,

or bottom-up control. Also, there are some communities that

tend, inevitably, to be dominated by bottom-up control, because

but rather that patterns of consumption are so differentiated that

consumers have little or no influence on the supply of their food

their overall effects are buffered. From the point of view of the

resource. The most obvious group of organisms to which this

whole community, such effects may be represented as trophic

trickles rather than cascades.

applies is the detritivores (see Chapter 11), but consumers of

nectar and seeds are also likely to come into this category (Odum

and phytoplankton. At the lowest nutrient concentrations, the snails

& Biever, 1984) and few of the multitude of rare phytophagous

were dominated by the smaller P. gyrina, vulnerable to predation,

and the predator gave rise to a trophic cascade extending to the

insects are likely to have any impact upon the abundance of their

primary producers. But at the highest concentrations, the snails

host plants (Lawton, 1989).

The widespread importance of top-

were dominated by the larger H. trivolvis, relatively invulnerable

down control, foreshadowing the idea of

why is the world

to predation, and no trophic cascade was apparent (Figure 20.6).

This study, therefore, also lends support to Murdoch’s proposi-

green? . . .

the trophic cascade, was first advocated

in a famous paper by Hairston et al.

tion that the ‘world tastes bad’, in that invulnerable herbivores gave

rise to a web with a relative dominance of bottom-up control.

(1960), which asked ‘Why is the world green?’ They answered,

Overall, though, we see again that the elucidation of clear patterns

in effect, that the world is green because top-down control pre-

in the predominance of top-down or bottom-up control remains

dominates: green plant biomass accumulates because predators

a challenge for the future.

keep herbivores in check. The argument was later extended to

systems with fewer or more than three trophic levels (Fretwell,

1977; Oksanen et al., 1981).