Alteration of Community Composition and Ecosystem Dynamics through the Stoichiometry of Recycling

When we place these specific stoichiometric recycling effects into the context of complete, complex ecosystems, a variety of interesting dynamics result. For example, as we just saw, terrestrial plant species that produce low-nutrient litter will have a depressive effect on mineralization rates in their vicinity. A low mineralization rate will reduce primary productivity, lower plant biomass, and raise light:nutrient ratios. These conditions in turn will favor particular plant species that are good nutrient competitors. Because one factor that can improve a species' competitive ability for a given nutrient is a low content of that nutrient in its leaves, a cycle of positive feedback is favored: with good nutrient competitors altering nutrient cycling in their vicinity in such a way as to improve their chances of success against other species.

Shifts among consumer species differing in stoichiometry can also have ecosystem-level consequences. In freshwater plankton food chains, large-body-size, high-growth-rate Daphnia are favored under particular regimes of fish predation, for example, when there are many piscivores that deplete the planktivorous fish populations, releasing Daphnia from fish predation. Because Daphnia have a characteristic high-P stoichiometric signature, changes in fish predation regime can realign pelagic nutrient pools and fluxes. It has been observed that phytoplank-ton in the presence of abundant Daphnia grazers will be

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10 100 C:N detritus

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100 1000 C:P detritus

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Figure 5 The rate of breakdown of terrestrial detritus depends strongly on stoichiometry. The higher the C:N or C:P ratio, the slower the leaf litter breaks down (lower rate coefficient, k). As a consequence, the rate that nutrients are recycling back into the soil also depends on stoichiometry, with higher mineralization rates for detritus with low C:N or C:P.

10 100 C:N detritus

1000 10

100 1000 C:P detritus

10 000

Figure 5 The rate of breakdown of terrestrial detritus depends strongly on stoichiometry. The higher the C:N or C:P ratio, the slower the leaf litter breaks down (lower rate coefficient, k). As a consequence, the rate that nutrients are recycling back into the soil also depends on stoichiometry, with higher mineralization rates for detritus with low C:N or C:P.

relatively more P- than N-limited and N-fixation will be reduced. These examples illustrate the close connections between community structure and ecosystem nutrient fluxes that are easily understood by consideration of stoichiometric principles.

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