Key Concepts

1. Key Concepts • An ecosystem has four components: (1) the abiotic environment, (2) primary producers, (3) consumers, and (4) decomposers. These components are linked by the movement of energy and nutrients. • As energy flows from producers to consumers and decomposers in a food web, much of it is lost. The productivity of terrestrial ecosystems is limited by warmth and moisture; nutrient availability is the key constraint in aquatic ecosystems.

2. Key Concepts • To analyze nutrient cycles, biologists focus on the nature of the reservoirs where elements reside and the processes that move elements between reservoirs. • Nutrient addition by humans is increasing productivity and causing pollution. The burning of fossil fuels has led to rapid global warming; rapid ecological and evolutionary changes are being observed in response.

3. Ch 56: Introduction to ecosystem ecology • An ecosystem consists of the multiple communities of organisms that live in an area along with abiotic components such as the soil, climate, water, and atmosphere. • The biotic and abiotic components of an ecosystem are linked by flows of energy and nutrients.

4. How Does Energy Flow through Ecosystems? • A primary producer, or autotroph, is an organism that can synthesize its own food from inorganic sources. • Primary producers form the basis of ecosystems by transforming the energy in sunlight or inorganic compounds into the chemical energy stored in sugars. • Primary producers use this chemical energy for maintenance and/ or growth.

5. Net Primary Productivity (NPP) • Energy that is invested in new tissue or offspring is called net primary productivity (NPP). • NPP represents the amount of energy available to consumers and decomposers.

6. Why Is NPP So Important? • Consumers eat living organisms. Primary consumers eat primary producers; secondary consumers eat primary consumers; tertiary consumers eat secondary consumers, and so on. • Decomposers, or detritivores, feed on detritus, the waste products or dead remains of other organisms.

7. Trophic Structure • Biomass represents energy. • To describe energy flows, biologists identify distinct feeding levels in an ecosystem. • Organisms that obtain their energy from the same type of source occupy the same trophic level.

8. Food Chains and Food Webs • A foodchain connects the trophic levels in a particular ecosystem, and thus describes how energy moves from one trophic level to another. • The decomposer food chain is made up of species that eat the dead remains of organisms. • The grazing food chain is composed of the network of herbivores (primary consumers) and the organisms that eat herbivores (secondary consumers).

10. Food Chains and Food Webs • These two food chains often merge at the higher trophic levels. In addition, many consumers feed at multiple trophic levels. • Food chains are usually embedded in more complex food webs.

11. How Does Energy Flow through an Ecosystem? • Net primary productivity results in biomass, organic material that non-photosynthetic organisms can eat. • In all environments, the chemical energy in primary producers eventually moves to one of two types of organisms: primary consumers or primary decomposers. • The primaryconsumer is an herbivore. • Primarydecomposers, including bacteria, archaea, and fungi,consumedetritus.

12. Energy Transfer between Trophic Levels • All ecosystems share a characteristic pattern: The total biomass produced each year is greatest at the lowest trophic level and declines at higher levels. • This pattern occurs because only a fraction of the total energy consumed is used for growth and reproduction.

13. The Pyramid of Productivity • When graphing biomass produced at each trophic level, a pyramid of productivity, which reports productivity and efficiency, emerges. • Productivity is a rate, measured in units of biomass produced per unit of area each year. • Efficiency is a ratio—the fraction of biomass transferred from one trophic level to the next. • Biomass production at each trophic level varies widely among ecosystems, but in general, efficiency of biomass transfer between trophic levels is only about 10 percent.

15. Trophic Cascades and Top-Down Control • When a consumerlimits a prey population, biologists say that top-down control is occurring. • A trophic cascade occurs when changes in top-down control cause conspicuous effects two or three links away in a food web. • For example, the reintroduction of wolves in Yellowstone National Park has led to far-reaching changes in the food web.

17. How Do Species Interactions Cause Trophic Cascades? Wolves absent Wolves reintroduced, 1995 • Elk browsed aspen trees • no new recruitment • Elk browsed streamside willows • beavers nearly exterminated • preyed primarily on elk • aspen regrew • willow regrew • beaver pop. increased

18. How Do Species Interactions Cause Trophic Cascades? Wolves absent Wolves reintroduced, 1995 • Elk browsed aspen trees • no new recruitment • Elk browsed streamside willows • beavers nearly exterminated • preyed primarily on elk • aspen regrew • willow regrew • beaver pop. increased

19. How Do Species Interactions Cause Trophic Cascades? Wolves absent Wolves reintroduced, 1995 • Elk browsed aspen trees • no new recruitment • Elk browsed streamside willows • beavers nearly exterminated • preyed primarily on elk • aspen regrew • willow regrew • beaver pop. increased

20. Biomagnification • In cycling through a food web along with chemical energy, certain molecules increase in relative concentration as they are transferred between trophic levels, a phenomenon called biomagnification. • An example of biomagnification can be observed in toxaphene—an insecticide that was commonly used in the United States until its toxicity to humans and wildlife led to its ban in 1986. • Toxaphene is a POP—a persistent organic pollutant that undergoes biomagnification

21. Toxaphene in the Arctic • The most dramatic example of toxaphene biomagnification is occurring in the Arctic. • Toxaphene has never been used in the Arctic but has blown in on wind currents. • Concentrations of toxaphene increase dramatically at higher levels in the Arctic food chain. • The Inuit people native to the Arctic depend on fish and mammals that show high levels of toxaphene, for food.

23. Estrogen-Mimics • Some biomagnified organic compounds bind to receptors for the vertebrate sex hormone estradiol, which is an estrogen. • In laboratory experiments, high concentrations of these estrogen-mimicking substances “feminize” individuals—meaning that male gonads begin producing female-specific compounds or cell types. • Toxins such as estrogen-mimics are thus passed through food webs along with energy and nutrients. • These POPs are resulting in skewed birth ratios among some populations of people in the Arctic.

25. Global Patterns in Productivity • In general, NPP on land is much higher than it is in the oceans, as more light is available to drive photosynthesis on land than in marine environments. • The terrestrial ecosystems with highest productivity are located in the wet tropics.

26. Which Biomes Are Most Productive? • Tropical wet forests and tropical seasonal forests cover less than 5 percent of Earth’s surface but together account for over 30 percent of total NPP. • Among aquatic ecosystems, the most productive habitats are algal beds and coral reefs, wetlands, and estuaries. • Even though NPP per square meter is extremely low in the open ocean, this biome is so extensive in terms of area that its total production is high. • Humans are appropriating almost a quarter of the planet’s biomass.

31. What Limits Productivity? • Productivity is limited by any factor that limits the rate of photosynthesis. • These factors include temperature and the availabilities of water, sunlight, and nutrients. • Different limiting factors prevail in different environments.

32. A Terrestrial-Marine Contrast • In terrestrial environments, NPP is lowest in deserts and arctic regions. • NPP on land is also limited by nutrient availability—often nitrogen or phosphorus.

33. A Terrestrial-Marine Contrast • The productivity of marine habitats is higher along coastlines than in deepwater regions due to nutrient limitation. • Shallow water along coasts receives nutrients from rivers and ocean currents. • Both of these sources are absent in the surface waters of the open ocean. • In addition, nutrients found in organisms near the surface of the open ocean constantly fall to dark, deeper waters in the form of dead cells.

34. Iron-Fertilization Experiments • Trace elements such as zinc, iron, and magnesium are particularly rare in the open ocean. • Results of iron-fertilization experiments indicate that NPP in marine ecosystems is limited primarily by the availability of nutrients, and iron is particularly important in the open ocean. • Some researchers suggest that fertilizing the open oceans could increase NPP in these deserts enough to reduce atmospheric CO2 and slow global warming.

39. Things that Cycle through Ecosystems • Nutrients • Nitrogen • Carbon • Water

40. How Do Nutrients Cycle through Ecosystems? • Atoms are constantly reused as they move through trophic levels, but they also spend time suspended in air, dissolved in water, or held in soil. • A biogeochemical cycle is the path that an element takes as it moves from abiotic systems through organisms and back again.

41. Nutrient Cycling within Ecosystems • Nutrients are taken up from the soil by plants, assimilated into plant tissue. • Carbon enters primary producers as carbon dioxide from the atmosphere. • Live plant tissue is eaten: the nutrients pass to consumers; dead plant tissue: the nutrients pass to decomposers. • Nutrients that reside in plant litter, animal excretions, and dead animal bodies are used by bacteria, archaea, roundworms, fungi, and other primary decomposers.

42. Nutrient Cycling within Ecosystems • Microscopic decomposers and the carbon-containing compounds that they release combine to form what biologists call soil organic matter, a complex mixture of partially and completely decomposed detritus. • Completely decayed organic material is called humus. • Eventually, decomposition converts the nutrients in soil organic matter to an inorganic form. • Once this step is accomplished, the nutrients are available for uptake by plants, which highlights the cyclical nature of nutrient flow through ecosystems.

43. What Factors Control the Rate of Nutrient Cycling? • The decomposition of detritus most often limits the overall rate at which nutrients move through an ecosystem. • Until decomposition occurs, nutrients stay tied up in intact tissues. • The decomposition rate is influenced by two types of factors: • Abiotic conditions such as oxygen availability, temperature, and precipitation. • The quality of the detritus as a nutrient source for the fungi, bacteria, and archaea that accomplish decomposition.

44. What Factors Control the Rate of Nutrient Cycling? • In boreal forests, the uppermost part of the soil consists of partially decomposed detritus and organic matter because the cold and wet conditions limit the metabolic rates of decomposers, resulting in the buildup of organic matter. • On the other hand, that uppermost layer of soil is virtually absent in tropical wet forests, where conditions are so favorable for fungi, bacteria, and archaea that decomposition keeps pace with detrital inputs.

46. Sources of Nutrient Loss and Gain • Nutrients leave an ecosystem whenever biomass leaves. • If an herbivore eats a plant and moves out of the ecosystem before excreting the nutrients or dying, the nutrients are lost. • Nutrients leave ecosystems when flowing water or wind removes particles or inorganic ions and deposits them somewhere else. • Several human activities accelerate nutrient loss.

47. Sources of Nutrient Loss and Gain • For an ecosystem to function normally, nutrients that are lost must be replaced. There are three major mechanisms to replace lost nutrients: • Atoms that act as nutrients are released as rocks weather. • Nutrients can also blow in on soil particles or arrive as solutes in streams. • Nitrogen is added when nitrogen-fixing bacteria convert molecular nitrogen (N2) in the atmosphere to usable nitrogen in ammonium or nitrate ions.

48. An Experimental Study • To test the effect of vegetation removal on nutrient loss from an ecosystem, a study was done at Hubbard Brook. • The researchers chose two watersheds (areas drained by a single stream), removed all vegetation from one (the experimental treatment), and left the other undisturbed (the control). • The results of the study showed that nutrient losses from the deforested site were typically 10 times higher than they were from the control site.