Increased Primary Production Can Reduce Food Chain Production

The idea that increased food abundance might have negative effects on consumers is a surprising stoichio-metric prediction. Numerous lab and field studies have tested predictions of stoichiometric theory for trophic interactions with a special emphasis on examining how light and nutrients can jointly regulate herbivore production. For example, a laboratory study involving Daphnia and a green alga showed that growth of Daphnia was maximal at intermediate light. At low light, algal biomass was limiting to Daphnia growth. At very high light intensities, algal P-content was limiting (Figure 8). Consistent with stoichiometric theory, the light intensity supporting maximal growth moved to higher light intensities as nutrient content in the system increased. In a longer-term study also involving Daphnia and a green alga (Figure 9), increased light intensity was shown to inhibit Daphnia population growth and trophic efficiency but eventually nutrient recycling by the Daphnia was able to increase algal nutrient content so that high grazer densities eventually were achieved. In a result that supports the idea that increased light intensity can result in grazer extinction, in this study one replicate vessel received especially high light intensity, which resulted in unusually high algal C:P ratio and a Daphnia population that was never able to increase in abundance and was undergoing significant decline at the end of the experimental period.

Several studies involving field mesocosms have also shown that manipulation of light-nutrient balance has a strong effect on secondary production in nutrient-limited ecosystems. For example, a field experiment in a P-limited Canadian lake reduced light intensity to mescosms by more than tenfold. After 30 days, seston C:P ratio had decreased significantly and zooplankton production was increased nearly fivefold. Thus, there is strong evidence that secondary production in food webs is strongly influenced not only by overall rates of ecosystem productivity ('food quantity') but also by the quality of that production as indexed by its nutrient content.

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