Analysis of Species Persistence in Food Webs

To assess the regulation of a species in any trophic level by the interaction of top-down and bottom-up forces, a realistic model of the system must be formulated and evaluated at equilibrium (see Gutierrez, et al. 1994, for an analysis of a tritrophic system based on equation 16.8). Simplifications of model equation 16.8 by Schreiber and Gutierrez (1998) gave qualitative predictions about field systems that have proved useful in exploring species invasions into existing food webs and assembly rules for species persistence. The model may incorporate the effects of variable weather on each trophic level, making it a valuable tool for evaluating the effects of climate change over a large geographical area. These results could also be displayed using GIS.

An example of this kind of food web analysis is that of the pea aphid (Acyrthosiphon pisum)/blue alfalfa aphid (A. kondoi) system in California lucerne (cf. Schreiber and Gutierrez, 1998). All of the major actors in the food web, except the coccinellids, are exotic. Pea aphid is currently controlled at very low levels by the fungal pathogen Pandora neoaphidis, which likely entered the system when blue alfalfa aphid was introduced (Gutierrez et al., 1990). This pathogen attacks blue alfalfa aphid at one-tenth of the rate of pea aphid and during a normal wet northern Californian winter it causes catastrophic mortality to pea aphid populations (Pickering and Gutierrez, 1991). In the absence of the pathogen, pea aphid out-competes blue alfalfa aphid during hot periods. Pea aphid is also attacked by two parasitoids, the host-specific Aphidius smithi and A. ervi . The latter also attacks the less preferred blue alfalfa aphid. Other aphidophagous predators (lady beetles, lacewings and others) attack both aphids in proportion to their occurrence, and are important in lowering aphid abundance but not in regulating their numbers. If rainfall decreased in California, a change in species dominance from blue alfalfa aphid to pea aphid would likely occur. Evidence for this was seen during years of drought when pea aphid numbers surged (Gutierrez et al, 1990). Pea aphid has effective natural enemies (principally A. smithi) that regulate its numbers in the absence of the pathogen (Bosch et al, 1966).

Using capital letters to represent the species just described, an assembly diagrams based on the Schreiber and Gutierrez (1998) model of this aphid system for wet and dry winter scenarios is shown in Fig. 16.7. The directional arrows indicate the sequence of when the different species might enter the system. In wet winters (Fig.16.7a), the tritrophic system A/B/E predominates starting from any sequence of introductions. However, if the winter is dry (Fig. 16.7b) and irrigation is available for plant growth, the system would be dominated by A/P/S. This outcome would be mediated by weather through the action of the pathogen and parasitoids on the dynamics of the aphids.

The key point here is that climate change in this and other systems may upset the balance between species in food webs, leading to new webs. It may lead to changes in the relative importance of species and hence regulation; it may cause the extinction of some species, and in the extreme result in an entirely new biota.

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