You know from Chapter 56 that food webs can be constructed to represent the interactions and energy transfer that occur between different species and trophic levels. For example, the food web presented in Figure 56.3 shows the connections among primary producers and consumers in Yellowstone National Park. While the arrows indicate the trophic connections among species, they do not show how much energy is actually being transferred up the food web and converted into secondary production. We will now consider the factors important to secondary production.

You know from Key Concept 8.1 that during energy transfer, some energy is lost as “unusable” to the system (second law of thermodynamics) and thus is unavailable to do work. For food webs, the consequence of this loss is that only part of the primary production consumed by heterotrophs is converted into heterotroph biomass. What factors are important to this movement and loss of energy from primary to secondary production? Net secondary production, or the amount of biomass obtained from the consumption of other organisms, depends on how much plant tissue is consumed (consumption efficiency), how much of the consumed food can actually be digested versus released as feces and urine (assimilation efficiency), and how much of the digested food is used in metabolic activities and released as CO₂ through respiration versus stored as biomass (production efficiency) (Figure 57.8). We can think of production efficiency as the percentage of energy stored in assimilated food that is used to produce new biomass.

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Question

The consumption and assimilation efficiencies are lower for herbivores because primary producers have structural and chemical defenses that make them harder to consume and digest than animals.

Consumers can vary dramatically in production efficiency. For example, endotherms have much lower production efficiencies than ectotherms (Table 57.1). Because endotherms have to maintain high body temperatures, they have higher metabolic rates than ectotherms, and thus have less energy left over to devote to growth and reproduction. Moreover, endotherms vary in production efficiency depending on body size and metabolism. Larger mammals have lower metabolic rates and higher production efficiencies compared with smaller mammals and birds, whose greater surface area-to-biomass ratios result in more heat loss and higher metabolic rates (see Key Concept 39.5).

Because production efficiency affects the growth and reproduction of consumers, changes in the quality and quantity of food can lead to population declines. Research on Steller sea lions in the Gulf of Alaska and the Aleutian Islands offers a case in point. Sea lion populations declined by 85 percent over a 25-year period starting in the early 1970s. Surveys of sea lion individuals showed that they were smaller and had lower birth rates compared with individuals before the decline, suggesting that food was a limiting factor. Prior to the decline, sea lions mostly fed on herring, rich in fats, along with lesser amounts of cod and pollock. But when herring populations declined in the 1970s, the sea lion diet consisted mostly of cod and pollock. The energy per gram of biomass of pollock and cod is roughly half that of herring, so sea lions would have needed to eat twice as much pollock and herring to make up for the loss in herring calories. While plenty of fish were available, consumption of a greater percentage of inferior food represented a decline in production efficiency, which in turn translated into a decline in sea lion populations.