Primary Producers and Consumers

Material flow in ecosystems incorporates more than the fluxes between inorganic nutrient pools and organisms capable of accessing these pools. In addition, material fluxes between organisms, like those discussed in previous entries, coalesce with important ecosystem-scale consequences. One stoichiometric characteristic that is consistently important across scales of biological organization is the contrast between first-order consumers (i.e., organisms that obtain all or part of their energy requirements from consuming primary producer biomass or detritus) and the primary producers they feed upon. As discussed in prior sections, primary producers tend to have higher carbon content and lower N and P content than their consumers. The extent of elemental differences between primary producers and firstorder consumers depends on the ecosystem considered. In freshwater and marine pelagic systems where primary production is dominated by algae and cyanobacteria, differences between producers and first-order consumers in body nitrogen and phosphorus concentrations are smaller than the differences observed in benthic and terrestrial ecosystems dominated by vascular plants.

Despite this heterogeneity, the trend for higher body nitrogen and phosphorus concentrations in first-order consumers relative to their diet is consistent across ecosystems. In this article, we are less interested in the causes for these differences (e.g., differences in nutrient home-ostasis) than in their implications. The first implication is that the potential exists for first-order consumers to be nutrient rather than carbon (C) limited if food C:nutrient ratios provide carbon in excess of the consumer's energy requirements. In this situation, we expect an increase in the N or P concentrations in the producer tissue to cause higher growth rates in the consumer. This hypothesis has been validated with many experimental manipulations including microbial, invertebrate, and vertebrate firstorder consumers. The second implication is that nutrient limitation of first-order consumer growth should be greater in terrestrial than in aquatic ecosystems, since C:nutrient imbalances are greatest where vascular plants dominate primary production.

Given these two implications, what kind of patterns of matter flow in ecosystems can we expect? One expectation is that as we move from ecosystems composed of primary producers with poorer nutrient quality (lower body nitrogen and phosphorus concentrations) to those with greater N and P content, we should observe higher rates of herbivory (i.e., intake of producer biomass by herbivores) and first-order detritivory (i.e., intake of producer detritus by detritivores) and resulting decomposition (Figure 2). That should be the case whether we compare aquatic ecosystems only, terrestrial ecosystems only, or we include both types of ecosystems in the comparison. In addition, because aquatic first-order consumers should have faster growth rates than their terrestrial counterparts, we should also observe higher rates of herbivory and first-order detritivory in aquatic than in terrestrial ecosystems. The hypothesized higher herbivory and first-order detritivory rates in ecosystems

Concentrations of nitrogen and phosphorus in primary producer +

Growth rates of first-order consumers +

Buildup of biomass and demand for nutrients and metabolites +

Herbivory and first-order detritivory rates

Accumulation of producer biomass and detritus

Figure 2 Dependence of growth rates of first-order consumers on nitrogen and phosphorus concentrations in the diet and expected patterns of herbivory, first-order detritivory, and accumulation of producer biomass and detritus across ecosystems.

composed of richer primary producers suggests another important trend; because losses of producer biomass and detritus would be larger for richer producers than for poorer producers due to promoted herbivory and decomposition, we could also expect smaller pools of producer biomass and detritus in ecosystems composed of richer producers.

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