Community Ecology Processes, Models, and Applications

Kaunzinger and Morin (1998) may be a feature of
relatively unproductive systems.
The other approach used to evaluate links be-
tween population dynamics and food chain length
involves comparisons of the dynamics of species
that occur in chains of differing length. If the mod-
els are correct, population dynamics should be
more variable in longer chains, and that increased

variation should lead to higher values of temporal

variability for the same species embedded in longer

food chains. Lawler and Morin (1993) found that

the population dynamics of protists in relatively

simple laboratory food chains become more vari-

able with modest increases in food chain length.

Comparisons of the temporal variability of popula-

tions of the same bacterivorous protists in short

food chains in which the bacterivores were the

top predators, and in food chains that are just one

trophic level longer in which the bacterivores are

intermediate species preyed on by another preda-

tory protist, point to increased temporal variation

in abundance in the majority of longer food chains.

Increased temporal variation in abundance would

be consistent with longer return times in longer

food chains, as suggested by Pimm and Lawton

(1977).

There is also reason to suspect that energy and

population dynamics can interact in ways not di-

rectly considered by Pimm and Lawton (1977), as

described in earlier models by Rosenzweig (1971)

in the context of the so-called ‘paradox of enrich-

ment’. Rosenzweig found that a number of differ-

ent predator–prey models became increasingly

unstable as systems were made more productive –

a consequence of increasing rates of increase or

carrying capacities in the models. In this scenario,

adding energy to a simple food chain might desta-

bilize the system, and shorten the chain. Of course,

it is possible that the addition of another trophic

level to an energetically enhanced chain could

offset the destabilizing effects of enrichment,

though the findings of Pimm and Lawton (1977)

might argue against this. However, addition of

weakly interacting species to the food web has

been suggested to confer increased stability on

unstable systems (McCannet al.1998), so some

kinds of increased trophic complex may help to

offset the predicted destabilizing effects of enrich-

ment.

1.4.3 Unresolved issues

The relative contribution of foraging-based and

stability-based mechanisms to the connectance of

real food webs remains unresolved. Given the cen-

tral importance of connectance for determining

a

Nutrient level (g food/l)

(Log(N+1))/ml

(Log (N+1))/ml

(Log (N+1))/ml

b

c

9

8

7

6

5

3.5

Trophic level 1

Trophic level 2

Trophic level 3

3.0

2.5

2.0

1.5

1.0

0.5

0.0

1.0

0.6

0.4

0.2

0.0

0.001 0.01 0.1 1.0

Figure 1.7Effects of productivity on the abundance of
species occupying basal (a; bacteria, trophic level 1),
intermediate (b;Colpidium, trophic level 2) and top
(c;Didinium, trophic level 3) levels in simple linear food
chains. Three-level food chains persist only at higher levels
of productivity, as shown by the failure ofDidiniumto
persist at lower levels of productivity determined by
nutrient levels in experimental microcosms. Solid and
empty symbols in (b) show the abundance of the
intermediate trophic level in food chains with and without
the third trophic level, respectively. Reprinted with
permission from Macmillan Publishers Ltd:Nature395,
495–7, C.M.K. Kaunzinger and P. Morin,#1998.

THE TOPOLOGY OF ECOLOGICAL INTERACTION NETWORKS 21