Ecosystems 54 BIOLOGICAL SCIENCE FOURTH EDITION SCOTT FREEMAN

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. © 2011 Pearson Education, Inc.

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. © 2011 Pearson Education, Inc.

Introduction • 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. © 2011 Pearson Education, Inc.

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. © 2011 Pearson Education, Inc.

Why Is NPP So Important? • Energy that is invested in new tissue or offspring is called net primary productivity (NPP). • NPP results in biomass—organic material that nonphotosynthetic organisms can eat. • NPP represents the amount of energy available to consumers and decomposers. © 2011 Pearson Education, Inc.

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. The four components of an ecosystem are linked by the movement of energy and nutrients. © 2011 Pearson Education, Inc.

Solar Power Transforms Incoming Energy to Biomass • The Hubbard Brook Forest is a model ecosystem for studying energy and nutrient flow. • For this ecosystem, researchers calculated the gross primary productivity—the total amount of photosynthesis in a given area and time period. • They also calculated the gross photosynthetic efficiency— the efficiency with which plants use the total amount of energy available to them. © 2011 Pearson Education, Inc.

Solar Power Transforms Incoming Energy to Biomass • Results showed that only a small fraction of the energy consumed by a primary consumer is used for secondary production, the production of new tissue by primary consumers. Most of the energy is used for maintenance. • The general pattern for the Hubbard Brook ecosystem is typical of ecosystems around the globe. © 2011 Pearson Education, Inc.

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. © 2011 Pearson Education, Inc.

Food Chains and Food Webs • A food chain 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). © 2011 Pearson Education, Inc.

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. • Food webs are a compact way of summarizing energy flows and documenting the complex trophic interactions that occur in ecosystems. © 2011 Pearson Education, Inc.

Energy Flow to Grazers versus Decomposers • The amount of energy consumed by primary consumers versus primary decomposers varies enormously among habitats. • In a forest, much of the biomass is tied up in indigestible wood and is not transferred to other organisms until it decays. – Thus, you have to understand decomposers if you want to understand energy flow in a forest. • The percentage of NPP that gets consumed in marine habitats, where most of the primary production is done by algae, is much higher. © 2011 Pearson Education, Inc.

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 primary consumer is an herbivore. • Primary decomposers, including bacteria, archaea, and fungi, consume detritus. © 2011 Pearson Education, Inc.

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. The amount of biomass produced at each subsequent trophic level must be less than the amount at the previous level. © 2011 Pearson Education, Inc.

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. © 2011 Pearson Education, Inc.

Energy Flow BLAST Animation: Energy Flow © 2011 Pearson Education, Inc.

Trophic Cascades and Top-Down Control • When a consumer limits 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. – Removing certain species from an ecosystem often has analogous changes. © 2011 Pearson Education, Inc.

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. © 2011 Pearson Education, Inc.

What Is a POP? • Toxaphene persists in the environment without breaking down into harmless substances. It is a POP—a persistent organic pollutant that undergoes biomagnification. • Even if toxaphene is present at extremely low concentration in primary producers, it is sequestered at much higher concentration in primary consumers. The effect is magnified at each succeeding trophic level. • In many cases, concentrations get high enough in secondary and tertiary consumers to poison them. © 2011 Pearson Education, Inc.

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. © 2011 Pearson Education, Inc.

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. • These POPs are resulting in skewed birth ratios among some populations of people in the Arctic. • Toxins such as estrogen-mimics are thus passed through © 2011 Pearson Education, Inc.

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. • Marine productivity is highest along coastlines, and it can be as high near the poles as it is in tropics. © 2011 Pearson Education, Inc.

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. © 2011 Pearson Education, Inc.

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. © 2011 Pearson Education, Inc.

A Terrestrial-Marine Contrast • In terrestrial environments, NPP is lowest in deserts and arctic regions. This observation suggests that the overall productivity of terrestrial ecosystems is limited by a combination of temperature and availability of water and sunlight. • NPP on land is also limited by nutrient availability—often nitrogen or phosphorus. © 2011 Pearson Education, Inc.

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. © 2011 Pearson Education, Inc.

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 CO 2 and slow global warming. © 2011 Pearson Education, Inc.

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. © 2011 Pearson Education, Inc.

Nutrient Cycling within Ecosystems • Nutrients are taken up from the soil by plants and assimilated into plant tissue. • Carbon enters primary producers as carbon dioxide from the atmosphere. • If the plant tissue is eaten, the nutrients pass to consumers; if the plant tissue dies, 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. © 2011 Pearson Education, Inc.

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. © 2011 Pearson Education, Inc.

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: 1. Abiotic conditions such as oxygen availability, temperature, and precipitation. 2. The quality of the detritus as a nutrient source for the fungi, bacteria, and archaea that accomplish decomposition. © 2011 Pearson Education, Inc.

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. © 2011 Pearson Education, Inc.

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. © 2011 Pearson Education, Inc.

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: 1. Atoms that act as nutrients are released as rocks weather. 2. Nutrients can also blow in on soil particles or arrive as solutes in streams. 3. Nitrogen is added when nitrogen-fixing bacteria convert molecular nitrogen (N 2) in the atmosphere to usable nitrogen in ammonium or nitrate ions. © 2011 Pearson Education, Inc.

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. © 2011 Pearson Education, Inc.

Global Biogeochemical Cycles To understand how biogeochemical cycling works on a global scale, researchers focus on three fundamental questions: 1. What are the nature and size of the reservoirs where elements are stored for a period of time? 2. How fast does the element move between reservoirs, and what processes are responsible for moving elements from one compartment to another? 3. How does one biogeochemical cycle interact with another biogeochemical cycle? © 2011 Pearson Education, Inc.

The Global Water Cycle • The global water cycle begins with the evaporation of water out of the ocean and precipitating back into it. – Evaporation exceeds precipitation. • When this water vapor moves over the continents, it is augmented by water transpired by plants. – Precipitation falls on the continents. • The cycle is completed both by water that moves from the land to the oceans via streams and by groundwater—water that is found in soil. © 2011 Pearson Education, Inc.

The Global Water Cycle • Humans are affecting the water cycle in many ways. • One of the most direct ways is in groundwater storage and the replenishment of groundwater. • The water table, the upper limit of saturated soil, is dropping on every continent. © 2011 Pearson Education, Inc.

The Global Nitrogen Cycle • In the global nitrogen cycle, nitrogen is added to ecosystems in a usable form only when it is reduced, or “fixed”—converted from N 2 to NH 3. • The vast pool of molecular nitrogen (N 2) in the atmosphere is unavailable to plants because they can use nitrogen only in the form of ammonium (NH 4+) or nitrate (NO 3–) ions. • Nitrogen fixation results from lightning-driven reactions in the atmosphere and from enzyme-catalyzed reactions in bacteria that live in the soil and oceans. © 2011 Pearson Education, Inc.

The Global Nitrogen Cycle • Human-fixed nitrogen in the form of fertilizers, nitric oxide from burning fossil fuels, and cultivation of certain crops is having a major effect on the nitrogen cycle. • Adding nitrogen to terrestrial ecosystems usually increases productivity. But overfertilization with nitrogen cause anoxic “dead-zones” in aquatic ecosystems, and can lead to a decrease in species diversity in terrestrial ecosystems. © 2011 Pearson Education, Inc.

Nitrogen Cycle BLAST Animation: Nitrogen Cycle © 2011 Pearson Education, Inc.

The Global Carbon Cycle • The global carbon cycle involves the movement of carbon among terrestrial ecosystems, the oceans, and the atmosphere. • The ocean is by far the largest of these three reservoirs. • In both terrestrial and aquatic ecosystems, photosynthesis is responsible for taking carbon out of the atmosphere and incorporating it into tissue. • Cellular respiration, in contrast, releases carbon that has been incorporated into living organisms to the atmosphere, in the form of carbon dioxide. © 2011 Pearson Education, Inc.

The Global Carbon Cycle • Burning fossil fuels moves carbon from an inactive geological reservoir, in the form of petroleum or coal, to an active reservoir—the atmosphere. • When you burn gasoline, you are releasing carbon atoms that have been locked up in petroleum reservoirs for hundreds of millions of years. • Other human activities such as intensive agriculture and deforestation have changed the carbon cycle by adding large amounts of carbon dioxide to the atmosphere. © 2011 Pearson Education, Inc.

The Global Carbon Cycle • Carbon dioxide is a greenhouse gas—a gas that traps heat radiated from Earth and keeps it from being lost to space. • Increases in the amounts of greenhouse gases have the potential to warm Earth’s climate by increasing the atmosphere’s heat-trapping potential. © 2011 Pearson Education, Inc.

The Global Carbon Cycle Web Activity: The Global Carbon Cycle © 2011 Pearson Education,

Carbon Cycle BLAST Animation: Carbon Cycle © 2011 Pearson Education, Inc.

Global Warming • Earth is facing the most traumatic episode of environmental change in human history. The trauma has two sources: the massive loss of species and global warming. • Two factors are responsible for the human impacts on ecosystems: 1. The rapid increase in human population. 2. The rapid increase in human resource use. • Residents of industrialized countries, though relatively few in number, burn extraordinary quantities of fossil fuels and thus are largely responsible for global warming. © 2011 Pearson Education, Inc.

Understanding the Problem In explaining climate change, the chain of causation begins with the direct link between the CO 2 released by human fossil fuel use and increases in atmospheric CO 2. It continues with a link between high atmospheric CO 2 and increased trapping of infrared radiation that would otherwise leave the atmosphere; it concludes with recent and dramatic increases in average temperatures around the globe. © 2011 Pearson Education, Inc.

Understanding the Problem • The Intergovernmental Panel on Climate Change (IPCC) declared that evidence for global warming is unequivocal and that it is “very likely” due to human-induced changes in greenhouse gases. • Climate change has occurred throughout Earth’s history. What is unusual is the extremely rapid rate at which temperatures are changing currently. © 2011 Pearson Education, Inc.

Understanding the Problem • Although the temperature increases being observed are global averages, there is a lot of temporal and spatial variation. • However, averaged over the entire planet and over time, Earth is already much warmer than it was just a few decades ago, and is projected to get much more so. • The amount of this rise depends in part on whether CO 2 and other greenhouse gases continue to increase. To date, however, there is no indication that the rate of increase is slowing. © 2011 Pearson Education, Inc.

Greenhouse Effect BLAST Animation: Greenhouse Effect © 2011 Pearson Education, Inc.

Global Warming • The models currently being used suggest that average global temperature will undergo additional increases from approximately 1. 1ºC to 6. 4ºC (2. 0ºF– 11. 5ºF) by the year 2100. • The low number is based on models that assume no further increase in greenhouse gases over present levels; the high number is based on models that assume continued intensive use of fossil fuels and increases in greenhouse gases. © 2011 Pearson Education, Inc.

Positive and Negative Feedback • Positive feedback occurs when changes due to global warming result in a release of additional greenhouse gases, accelerating the warming trend. • Two types of positive feedback have been documented thus far: 1. A warmer and drier climate has increased the frequency of forest fires, which release CO 2, leading to more warming. 2. Traditionally, tundras sequester carbon in the form of soil organic matter. However, greenhouse gases that are trapped in permafrost could be released when tundras warm. © 2011 Pearson Education, Inc.

Positive and Negative Feedback • Negative feedback occurs when changes due to global warming result in increased uptake and sequestration of CO 2 and other greenhouse gases—meaning that global warming should be reduced. • Recent experiments have shown that the growth rates of several tree species and some agricultural crops increase in direct response to increasing atmospheric CO 2. Because CO 2 is required for photosynthesis, it can act as a fertilizer. © 2011 Pearson Education, Inc.

Impact on Organisms • Even though global temperatures have risen only slightly in comparison with projections for the next 50– 100 years, biologists have already documented dramatic impacts on organisms. • For example: 1. The geographic ranges of many organisms are changing. 2. The timing of seasonal events, or phenology, is changing in many biomes. © 2011 Pearson Education, Inc.

Impact on Organisms 3. The integrity of coral reefs, among the most productive and species-rich ecosystems in the world, is threatened by two issues associated with global warming: – Increased levels of atmospheric CO 2 are dissolving in ocean water and slowing the rate at which corals form. – Increases in water temperature cause reef-building corals to expel their photosynthetic algae, resulting in coral bleaching, which can kill the coral. 4. Rapid changes in climate may lead to extinction © 2011 Pearson Education, Inc.

Productivity Changes • Several of the changes that humans are inducing in biogeochemical cycles alter NPP. • NPP is increasing on land decreasing in the oceans. • The overall increase in terrestrial productivity is thought to be due to rising temperatures, increased rainfall in the tropics, and CO 2 fertilization—all factors that increase the rate of photosynthesis in plants. © 2011 Pearson Education, Inc.

Why Is NPP Declining in the Oceans? • The drop in marine productivity is likely due to changes in water density. • Water in the benthic zone is nutrient rich; this water is at 4°C year-round in large regions of the ocean. • When the temperature of surface water rises due to global warming, the water at the surface becomes even less dense than benthic water. • This prevents water currents from carrying nutrients from the benthic zone to the surface, where nutrients can spur the growth of photosynthetic bacteria and algae. © 2011 Pearson Education, Inc.

Productivity Changes • Increased productivity can be beneficial for certain ecosystems, but can also have negative effects. • Negative effects of increased productivity include a decline in species richness, anaerobic “dead zones, ” and harmful algal blooms. • On a global level, lower NPP in the ocean means that the productivity of the world’s fisheries may decline. • Overall, biologists don’t yet know whether changes in productivity will be beneficial or detrimental to ecosystems. © 2011 Pearson Education, Inc.