Ks. Mccann et al., TROPHIC CASCADES AND TROPHIC TRICKLES IN PELAGIC FOOD WEBS, Proceedings - Royal Society. Biological Sciences, 265(1392), 1998, pp. 205-209
Prey-dependent models, with the predation rate (per predator) a functi
on of prey numbers alone, predict the existence of a trophic cascade.
In a trophic cascade, the addition of a top predator to a two-level fo
od chain to make a three-level food chain will lead to increases in th
e population size of the primary producers, and the addition of nutrie
nts to three level chains will lead to increases in the population num
bers at only the first and third trophic levels. In contrast, ratio-de
pendent models, with the predation rate (per predator) dependent on th
e ratio of predator numbers to prey, predict that additions of top pre
dators will not increase the population sizes of the primary producers
, and that the addition of nutrients to a three level food chain will
lead to increases in population numbers at all trophic levels. Surpris
ingly, recent meta-analyses show that freshwater pelagic food web patt
erns match neither prey-dependent models (in pelagic webs, 'prey' are
phytoplankton, and 'predators' are zooplankton), nor ratio-dependent m
odels. In this paper we use a modification of the prey-dependent model
-incorporating strong interference within the zooplankton trophic leve
l-that does yield patter ns matching those found in nature. This zoopl
ankton interference model corresponds to a more reticulate food web th
an in the linear, prey-dependent model, which lacks zooplankton interf
erence. We thus reconcile data with a new model, and make the testable
prediction that the strength of trophic cascades will depend on the d
egree of heterogeneity in the zooplankton level of the food chain.