LEPTASTERIAS. (AFTER BRIAND, 1983.)THE MOST OBVIOUS TESTABLE PREDI...

13, Leptasterias. (After Briand, 1983.)

The most obvious testable predic-

greater primary

tions stemming from this hypothesis

productivity supports

are, first, systems with greater primary

of trying to quantify the number of links. Food chain length has

more trophic

productivity (e.g. at lower latitudes)

levels? . . .

should be able to support a larger num-

been defined in various ways (Post, 2002), and in particular has

ber of trophic levels; and second, systems

sometimes been used to describe the number of species in the

chain, and sometimes (as here) the number of links. For instance,

where energy is transferred more efficiently (e.g. based on insects

starting with basal species 1 in Figure 20.14, we can trace four

rather than vertebrates) should also have more trophic levels. How-

ever, these predictions have received little support from natural

possible trophic pathways via species 4 to a top predator: 1– 4–

systems. For instance, an analysis of 32 published food webs in

11–12, 1– 4–11–13, 1–4–12 and 1– 4–13. This provides four food

habitats ranging from desert and woodland to Arctic lakes and

chain lengths: 3, 3, 2 and 2. Figure 20.14 lists a total of 21 further

tropical seas found no difference in the length of food chains

chains, starting from basal species 1, 2 and 3. The average of all

when 22 webs from low-productivity habitats (less than 100 g

the possible food chain lengths is 2.32. Adding one to this gives

of carbon m

year

) were compared with 10 webs from high-

us the number of trophic levels that can be assigned to the food

productivity habitats (greater than 1000 g m

year

). The median

web. Almost all communities described have consisted of between

two and five trophic levels, and most of these have had three or

food chain length was 2.0 in both cases (Briand & Cohen, 1987).

four. What sets the limit on food chain length? And how can

Moreover, a survey of 95 insect-dominated webs revealed first

we account for variations in length?

that food chains in tropical webs were no longer than those from

(presumably) less productive temperate and desert situations,

In addressing these questions, we

but also that these food chains composed of insects were no longer

will conform to a bias that has per-

parasites are usually

vaded investigations of food chain

ignored

than those involving vertebrates (Schoenly et al., 1991).

length – a bias in favor of predators and

On the other hand, a number of studies on a much smaller

against parasites. Thus, when a food chain is described as having

scale (e.g. in a group of streams; Townsend et al., 1998) or where

resource availability has been manipulated experimentally, have

four trophic levels, these would typically be a plant, a herbivore,

shown food chain length to decrease with decreased product-

Results like these may indicate that total energy is indeed

ivity, especially when the decreases take productivity below

important but is far more dependent on ecosystem size than

around 10 g carbon m

year

(Post, 2002). For example, in an

productivity per unit area. But they may mean, alternatively, that

experiment using water-filled containers as analogs of natural

ecosystem size affects food chain length by some other means

and available energy has no detectable effect (Post, 2002). One

tree-holes, a 10-fold or 100-fold reduction from a ‘natural’ level

of energy input (leaf litter) reduced maximal food chain length

possibility is that ecosystem size affects species richness (it

by one link, because in this simple community of mosquitoes,

certainly does so – see Chapter 21) and richer webs tend to

midges, beetles and mites, the principal predator – a chironomid

support longer chains. Unsurprisingly, richness and chain length

tend to be associated. Untangling causation from correlation is

midge Anatopynia pennipes – was usually absent from the less

productive habitats ( Jenkins et al., 1992). This suggests that

an important challenge.

the simple productivity argument may indeed apply in the least

productive environments (the most unproductive deserts, the

deepest reaches of caves). However, establishing this is likely to

prove difficult, since there are other reasons for expecting top