SUMMARY

CONCEPT 20.1 Energy in ecosystems originates with primary production by autotrophs.

20.1.1 Explain how variation in the leaf area index influences gross primary production through both carbon gains and losses.

Gross primary production (GPP) is the total amount of carbon fixed by the autotrophs in an ecosystem. The GPP of a terrestrial ecosystem is determined by the rate of photosynthesis and the leaf area index.

20.1.2 Describe the relationship between gross primary production and net primary production and how this relationship can change during succession.

Net primary production (NPP) is equal to GPP minus autotrophic respiration.

NPP changes during succession because of changes in leaf area index and in the proportions of photosynthetic and nonphotosynthetic plant tissues.

20.1.3 Summarize why it is important to measure net primary production and describe some of the ways ecologists measure net primary production at large spatial scales.

Researchers have developed diverse approaches to measuring

NPP at different spatial and temporal scales.

20.1.4 Illustrate how the relationship between net primary production and net ecosystem exchange is influenced by the mass of autotrophs versus the mass of heterotrophs.

Net ecosystem exchange is the balance between carbon gained from photosynthesis and carbon lost from both autotrophs and heterotrophs.

CONCEPT 20.2 Net primary production is constrained by both physical and biotic environmental factors.

20.2.1 Contrast the direct and indirect influences of climate on the amount of terrestrial net primary production through its influence on photosynthesis and resource supply.

Variation in terrestrial NPP is associated with variation in temperature and precipitation, both of which affect resource availability and the types and abundances of plants.

20.2.2 Describe how plant allocation and growth rates can influence the relationship between climate and net primary production.

The intrinsic growth rates of different plant species influence spatial variation in NPP and its response to variation in resource availability.

20.2.3 List the nutrients that commonly limit the net primary production of freshwater and marine ecosystems.

NPP in aquatic ecosystems is controlled by the supply of

nutrients, particularly phosphorus and nitrogen.

CONCEPT 20.3 Global patterns of net primary production reflect climate constraints and biome types.

20.3.1 Compare the net primary production of terrestrial and marine ecosystems at a global scale by considering their spatial coverage to evaluate their overall contribution to global net primary production.

Terrestrial and oceanic NPP contribute nearly equal proportions of global NPP.

20.3.2 Predict which biomes contribute the greatest amount of net primary production at a global scale based on the climatic factors that control net primary production.

The majority of terrestrial NPP occurs in the tropics.

Although zones of upwelling and coastal zones have the highest rates of NPP₁ the open ocean accounts for the majority of oceanic NPP because of its larger area.

Differences in NPP among terrestrial biomes reflect differences in leaf area index and in the length of the growing season.

CONCEPT 20.4 Secondary production is generated through the consumption of organic matter by heterotrophs.

20.4.1 Describe how the diet of a heterotroph can be determined by observation and isotopic analysis.

Heterotrophs derive energy from the consumption of live or dead

organic matter.

Heterotroph diets can be determined by measuring and comparing the ratios of stable isotopes in the tissues of the feeding organism and in those of its potential food sources.

20.4.2 Evaluate how ingestion, egestion, and respiration influence the amount of net secondary production.

Net secondary production is the energy ingested by heterotrophs minus the energy used in respiration and egested in feces and urine.

REVIEW QUESTIONS

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2. Plants allocate the energy they acquire through photosynthesis to different functions, including growth and metabolism. The allocation to growth can go preferentially to particular organs, such as leaves, stems, roots, or flowers. How would you expect the allocation of energy among plant organs to change as the amount of terrestrial ecosystem NPP increased? Explain why you would expect the allocation pattern you describe.

3. Some ecologists interested in the underlying factors that control variation in NPP in tundra measured the growth of plants at several locations over multiple years. They also measured air and soil temperatures, wind speed, solar radiation, and soil moisture. When they analyzed all of their data, they found that the best correlation between NPP and any of the physical environmental factors they had measured was with soil temperature. The researchers concluded that NPP in tundra is controlled by the effect of soil temperature on root growth. Is this conclusion correct?

4. What are some of the benefits and drawbacks associated with measuring NPP using (a) harvest techniques and (b) remote sensing?

HONE YOUR PROBLEM-SOLVING SKILLS

In addition to examining the cave bear food preferences described in this chapter, Hilderbrand and colleagues wanted to see whether grizzly bears living near coastal areas relied more heavily on fish in their diet than bears from inland areas that did not have access to salmon (Hilderbrand et al. 1996). They measured the isotopic composition of bone and hair samples from grizzly bears from the western United States killed before 1931, prior to the construction of dams that restricted salmon runs up the Columbia River basin. Meat is more enriched in ¹⁵N than plants, so bears with a carnivorous diet have higher ratios of ¹⁵N∕¹⁴N, expressed as a higher δ ¹⁵N, than bears with a diet of plant tissues.

1. Using the data in the table, construct a graph that tests the hypothesis that grizzly bears of coastal areas consume more fish than bears from inland areas. Use δ ¹³C on the x-axis (going from more negative to less negative) and δ ¹⁵N on the y-axis.

2. The availability of salmon for consumption by bears has declined since 1931 because of lower numbers of fish spawning in rivers.

a. What changes in isotopic composition would you expect if the bears compensated for the lower salmon availability by consuming a higher proportion of terrestrial plant material in their diet?

b. What changes would you expect in the isotopic composition if they compensated by eating more terrestrial mammals?

LIST OF KEY TERMS

biomass carnivores detritivores ecosystem gross primary production (GPP) herbivores

leaf area index

net ecosystem exchange (NEE) net primary production (NPP) Net secondary production omnivores primary production secondary production