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).
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