Climate Effects on Tritrophic Systems

It has been shown that Australian aphid populations may develop when favourable weather conditions occur, and their tolerance to two factors in their environment can be estimated from field data. However, determining the level of regulation of a species by natural enemies is more difficult.

Assume, for the sake of simplicity, that the climatic optimum of each species occurs at the midpoint of their response limits in all dimensions. Hypothetical time series of the two indices at some mythical site are shown in Fig. 16.6 as the margins of the shaded area, with the physiological limits of each species in the tritrophic system superimposed. In the first example (Fig. 16.6a), the overlaps of the physiological limits of all three trophic levels and site-specific indices are quite good. In the second (Fig. 16.6b), the physiological limits of the plant and herbivore coincide well with each other and with the site's climatic indices, but the overlap with the physiological limits of the predator is marginal. Figure 16.6c illustrates a much poorer degrees of climate matching. In all cases, however, without specific details about the biology of the interacting species during adverse periods, nothing can be said about regulation. The case represented by Fig. 16.6c would appear to be similar to that of cowpea aphid in Australia, where brief periods of favourability allow population build-up and high migratory capacity permits regional persistence, with natural enemies being of little importance throughout much of the area studied. To evaluate the level of regulation of a population, it is necessary to assess the tritrophic dynamics as modified by weather and other factors at equilibrium. This is explored below.

Fig. 16.6. The hypothetical physiological limits of poikilotherm species in a food chain to temperature and saturation deficit. The hypothetical indices for the location are the margins of the shaded area: (a) plant and herbivore and a well-adapted natural enemy; (b) the same as (a) but with a poorly adapted natural enemy; (c) the effects of shifts in weather at the site on relationships in (a).

Fig. 16.6. The hypothetical physiological limits of poikilotherm species in a food chain to temperature and saturation deficit. The hypothetical indices for the location are the margins of the shaded area: (a) plant and herbivore and a well-adapted natural enemy; (b) the same as (a) but with a poorly adapted natural enemy; (c) the effects of shifts in weather at the site on relationships in (a).

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