Chapter 54 Ecosystems

Overview: Ecosystems, Energy, and Matter An ecosystem consists of all the organisms living in a community As well as all the abiotic factors with which they interact

Ecosystems can range from a microcosm, such as an aquarium To a large area such as a lake or forest Figure 54.1

Regardless of an ecosystem’s size Its dynamics involve two main processes: energy flow and chemical cycling Energy flows through ecosystems While matter cycles within them

Ecosystem ecologists view ecosystems Concept 54.1: Ecosystem ecology emphasizes energy flow and chemical cycling Ecosystem ecologists view ecosystems As transformers of energy and processors of matter

Ecosystems and Physical Laws The laws of physics and chemistry apply to ecosystems Particularly in regard to the flow of energy Energy is conserved But degraded to heat during ecosystem processes

Trophic Relationships Energy and nutrients pass from primary producers (autotrophs) To primary consumers (herbivores) and then to secondary consumers (carnivores)

Energy flows through an ecosystem Entering as light and exiting as heat Figure 54.2 Microorganisms and other detritivores Detritus Primary producers Primary consumers Secondary consumers Tertiary consumers Heat Sun Key Chemical cycling Energy flow

Nutrients cycle within an ecosystem

Decomposition Decomposition Connects all trophic levels

Detritivores, mainly bacteria and fungi, recycle essential chemical elements By decomposing organic material and returning elements to inorganic reservoirs Figure 54.3

Primary production in an ecosystem Concept 54.2: Physical and chemical factors limit primary production in ecosystems Primary production in an ecosystem Is the amount of light energy converted to chemical energy by autotrophs during a given time period

Ecosystem Energy Budgets The extent of photosynthetic production Sets the spending limit for the energy budget of the entire ecosystem

The Global Energy Budget The amount of solar radiation reaching the surface of the Earth Limits the photosynthetic output of ecosystems Only a small fraction of solar energy Actually strikes photosynthetic organisms

Gross and Net Primary Production Total primary production in an ecosystem Is known as that ecosystem’s gross primary production (GPP) Not all of this production Is stored as organic material in the growing plants

Net primary production (NPP) Is equal to GPP minus the energy used by the primary producers for respiration Only NPP Is available to consumers

Different ecosystems vary considerably in their net primary production And in their contribution to the total NPP on Earth Lake and stream Open ocean Continental shelf Estuary Algal beds and reefs Upwelling zones Extreme desert, rock, sand, ice Desert and semidesert scrub Tropical rain forest Savanna Cultivated land Boreal forest (taiga) Temperate grassland Tundra Tropical seasonal forest Temperate deciduous forest Temperate evergreen forest Swamp and marsh Woodland and shrubland 10 20 30 40 50 60 500 1,000 1,500 2,000 2,500 5 15 25 Percentage of Earth’s net primary production Key Marine Freshwater (on continents) Terrestrial 5.2 0.3 0.1 4.7 3.5 3.3 2.9 2.7 2.4 1.8 1.7 1.6 1.5 1.3 1.0 0.4 125 360 3.0 90 2,200 900 600 800 700 140 1,600 1,200 1,300 250 5.6 1.2 0.9 0.04 22 7.9 9.1 9.6 5.4 0.6 7.1 4.9 3.8 2.3 65.0 24.4 Figure 54.4a–c Percentage of Earth’s surface area (a) Average net primary production (g/m2/yr) (b) (c)

Overall, terrestrial ecosystems Contribute about two-thirds of global NPP and marine ecosystems about one-third Figure 54.5 180 120W 60W 0 60E 120E North Pole 60N 30N Equator 30S 60S South Pole

Primary Production in Marine and Freshwater Ecosystems Both light and nutrients are important in controlling primary production

The depth of light penetration Light Limitation The depth of light penetration Affects primary production throughout the photic zone of an ocean or lake

More than light, nutrients limit primary production Nutrient Limitation More than light, nutrients limit primary production Both in different geographic regions of the ocean and in lakes

A limiting nutrient is the element that must be added In order for production to increase in a particular area Nitrogen and phosphorous Are typically the nutrients that most often limit marine production

Nutrient enrichment experiments Confirmed that nitrogen was limiting phytoplankton growth in an area of the ocean EXPERIMENT Pollution from duck farms concentrated near Moriches Bay adds both nitrogen and phosphorus to the coastal water off Long Island. Researchers cultured the phytoplankton Nannochloris atomus with water collected from several bays. Figure 54.6 Coast of Long Island, New York. The numbers on the map indicate the data collection stations. Long Island Great South Bay Shinnecock Bay Moriches Bay Atlantic Ocean 30 21 19 15 11 5 4 2

Figure 54.6 (a) Phytoplankton biomass and phosphorus concentration (b) Phytoplankton response to nutrient enrichment Great South Bay Moriches Bay Shinnecock Starting algal density 2 4 5 11 30 15 19 21 24 18 12 6 Unenriched control Ammonium enriched Phosphate enriched Station number (millions of cells per mL) Phytoplankton 8 7 3 1 Inorganic phosphorus (g atoms/L) (millions of cells/mL) CONCLUSION Since adding phosphorus, which was already in rich supply, had no effect on Nannochloris growth, whereas adding nitrogen increased algal density dramatically, researchers concluded that nitrogen was the nutrient limiting phytoplankton growth in this ecosystem. Inorganic phosphorus RESULTS Phytoplankton abundance parallels the abundance of phosphorus in the water (a). Nitrogen, however, is immediately taken up by algae, and no free nitrogen is measured in the coastal waters. The addition of ammonium (NH4) caused heavy phytoplankton growth in bay water, but the addition of phosphate (PO43) did not induce algal growth (b).

Experiments in another ocean region Showed that iron limited primary production Table 54.1

The addition of large amounts of nutrients to lakes Has a wide range of ecological impacts

In some areas, sewage runoff Has caused eutrophication of lakes, which can lead to the eventual loss of most fish species from the lakes Figure 54.7

Primary Production in Terrestrial and Wetland Ecosystems In terrestrial and wetland ecosystems climatic factors Such as temperature and moisture, affect primary production on a large geographic scale

The contrast between wet and dry climates Can be represented by a measure called actual evapotranspiration

Actual evapotranspiration Is the amount of water annually transpired by plants and evaporated from a landscape Is related to net primary production Figure 54.8 Actual evapotranspiration (mm H2O/yr) Tropical forest Temperate forest Mountain coniferous forest Temperate grassland Arctic tundra Desert shrubland Net primary production (g/m2/yr) 1,000 2,000 3,000 500 1,500

Live, above-ground biomass On a more local scale A soil nutrient is often the limiting factor in primary production Figure 54.9 EXPERIMENT Over the summer of 1980, researchers added phosphorus to some experimental plots in the salt marsh, nitrogen to other plots, and both phosphorus and nitrogen to others. Some plots were left unfertilized as controls. RESULTS Experimental plots receiving just phosphorus (P) do not outproduce the unfertilized control plots. CONCLUSION Live, above-ground biomass (g dry wt/m2) Adding nitrogen (N) boosts net primary production. 300 250 200 150 100 50 June July August 1980 N  P N only Control P only These nutrient enrichment experiments confirmed that nitrogen was the nutrient limiting plant growth in this salt marsh.

The secondary production of an ecosystem Concept 54.3: Energy transfer between trophic levels is usually less than 20% efficient The secondary production of an ecosystem Is the amount of chemical energy in consumers’ food that is converted to their own new biomass during a given period of time

Production Efficiency When a caterpillar feeds on a plant leaf Only about one-sixth of the energy in the leaf is used for secondary production Figure 54.10 Plant material eaten by caterpillar Cellular respiration Growth (new biomass) Feces 100 J 33 J 200 J 67 J

The production efficiency of an organism Is the fraction of energy stored in food that is not used for respiration

Trophic Efficiency and Ecological Pyramids Is the percentage of production transferred from one trophic level to the next Usually ranges from 5% to 20%

Pyramids of Production This loss of energy with each transfer in a food chain Can be represented by a pyramid of net production Figure 54.11 Tertiary consumers Secondary Primary producers 1,000,000 J of sunlight 10 J 100 J 1,000 J 10,000 J

One important ecological consequence of low trophic efficiencies Pyramids of Biomass One important ecological consequence of low trophic efficiencies Can be represented in a biomass pyramid

Most biomass pyramids Show a sharp decrease at successively higher trophic levels Figure 54.12a (a) Most biomass pyramids show a sharp decrease in biomass at successively higher trophic levels, as illustrated by data from a bog at Silver Springs, Florida. Trophic level Dry weight (g/m2) Primary producers Tertiary consumers Secondary consumers Primary consumers 1.5 11 37 809

Certain aquatic ecosystems Have inverted biomass pyramids Figire 54.12b Trophic level Primary producers (phytoplankton) Primary consumers (zooplankton) (b) In some aquatic ecosystems, such as the English Channel, a small standing crop of primary producers (phytoplankton) supports a larger standing crop of primary consumers (zooplankton). Dry weight (g/m2) 21 4

Number of individual organisms Pyramids of Numbers A pyramid of numbers Represents the number of individual organisms in each trophic level Figure 54.13 Trophic level Number of individual organisms Primary producers Tertiary consumers Secondary consumers Primary consumers 3 354,904 708,624 5,842,424

The dynamics of energy flow through ecosystems Have important implications for the human population Eating meat Is a relatively inefficient way of tapping photosynthetic production

Worldwide agriculture could successfully feed many more people If humans all fed more efficiently, eating only plant material Trophic level Secondary consumers Primary producers Figure 54.14

The Green World Hypothesis According to the green world hypothesis Terrestrial herbivores consume relatively little plant biomass because they are held in check by a variety of factors

Most terrestrial ecosystems Have large standing crops despite the large numbers of herbivores Figure 54.15

The green world hypothesis proposes several factors that keep herbivores in check Plants have defenses against herbivores Nutrients, not energy supply, usually limit herbivores Abiotic factors limit herbivores Intraspecific competition can limit herbivore numbers Interspecific interactions check herbivore densities

Nutrient circuits that cycle matter through an ecosystem Concept 54.4: Biological and geochemical processes move nutrients between organic and inorganic parts of the ecosystem Life on Earth Depends on the recycling of essential chemical elements Nutrient circuits that cycle matter through an ecosystem Involve both biotic and abiotic components and are often called biogeochemical cycles

A General Model of Chemical Cycling Gaseous forms of carbon, oxygen, sulfur, and nitrogen Occur in the atmosphere and cycle globally Less mobile elements, including phosphorous, potassium, and calcium Cycle on a more local level

A general model of nutrient cycling Includes the main reservoirs of elements and the processes that transfer elements between reservoirs Figure 54.16 Organic materials available as nutrients Living organisms, detritus unavailable Coal, oil, peat Inorganic Atmosphere, soil, water Minerals in rocks Formation of sedimentary rock Weathering, erosion Respiration, decomposition, excretion Burning of fossil fuels Fossilization Reservoir a Reservoir b Reservoir c Reservoir d Assimilation, photosynthesis

All elements Cycle between organic and inorganic reservoirs

Biogeochemical Cycles The water cycle and the carbon cycle Figure 54.17 Transport over land Solar energy Net movement of water vapor by wind Precipitation over ocean Evaporation from ocean Evapotranspiration from land Percolation through soil Runoff and groundwater CO2 in atmosphere Photosynthesis Cellular respiration Burning of fossil fuels and wood Higher-level consumers Primary Detritus Carbon compounds in water Decomposition THE WATER CYCLE THE CARBON CYCLE

Water moves in a global cycle Driven by solar energy The carbon cycle Reflects the reciprocal processes of photosynthesis and cellular respiration

The nitrogen cycle and the phosphorous cycle Figure 54.17 N2 in atmosphere Denitrifying bacteria Nitrifying Nitrification Nitrogen-fixing soil bacteria bacteria in root nodules of legumes Decomposers Ammonification Assimilation NH3 NH4+ NO3 NO2  Rain Plants Consumption Decomposition Geologic uplift Weathering of rocks Runoff Sedimentation Plant uptake of PO43 Soil Leaching THE NITROGEN CYCLE THE PHOSPHORUS CYCLE

Most of the nitrogen cycling in natural ecosystems Involves local cycles between organisms and soil or water The phosphorus cycle Is relatively localized

Decomposition and Nutrient Cycling Rates Decomposers (detritivores) play a key role In the general pattern of chemical cycling Figure 54.18 Consumers Producers Nutrients available to producers Abiotic reservoir Geologic processes Decomposers

The rates at which nutrients cycle in different ecosystems Are extremely variable, mostly as a result of differences in rates of decomposition

Vegetation and Nutrient Cycling: The Hubbard Brook Experimental Forest Is strongly regulated by vegetation

Long-term ecological research projects Monitor ecosystem dynamics over relatively long periods of time The Hubbard Brook Experimental Forest Has been used to study nutrient cycling in a forest ecosystem since 1963

The research team constructed a dam on the site To monitor water and mineral loss Figure 54.19a (a) Concrete dams and weirs built across streams at the bottom of watersheds enabled researchers to monitor the outflow of water and nutrients from the ecosystem.

In one experiment, the trees in one valley were cut down And the valley was sprayed with herbicides Figure 54.19b (b) One watershed was clear cut to study the effects of the loss of vegetation on drainage and nutrient cycling.

Nitrate concentration in runoff Net losses of water and minerals were studied And found to be greater than in an undisturbed area These results showed how human activity Can affect ecosystems Figure 54.19c (c) The concentration of nitrate in runoff from the deforested watershed was 60 times greater than in a control (unlogged) watershed. Nitrate concentration in runoff (mg/L) Deforested Control Completion of tree cutting 1965 1966 1967 1968 80.0 60.0 40.0 20.0 4.0 3.0 2.0 1.0

As the human population has grown in size Concept 54.5: The human population is disrupting chemical cycles throughout the biosphere As the human population has grown in size Our activities have disrupted the trophic structure, energy flow, and chemical cycling of ecosystems in most parts of the world

In addition to transporting nutrients from one location to another Nutrient Enrichment In addition to transporting nutrients from one location to another Humans have added entirely new materials, some of them toxins, to ecosystems

Agriculture and Nitrogen Cycling Agriculture constantly removes nutrients from ecosystems That would ordinarily be cycled back into the soil Figure 54.20

Nitrogen is the main nutrient lost through agriculture Thus, agriculture has a great impact on the nitrogen cycle Industrially produced fertilizer is typically used to replace lost nitrogen But the effects on an ecosystem can be harmful

Contamination of Aquatic Ecosystems The critical load for a nutrient Is the amount of that nutrient that can be absorbed by plants in an ecosystem without damaging it

When excess nutrients are added to an ecosystem, the critical load is exceeded And the remaining nutrients can contaminate groundwater and freshwater and marine ecosystems

Sewage runoff contaminates freshwater ecosystems Causing cultural eutrophication, excessive algal growth, which can cause significant harm to these ecosystems

Combustion of fossil fuels Acid Precipitation Combustion of fossil fuels Is the main cause of acid precipitation

North American and European ecosystems downwind from industrial regions Have been damaged by rain and snow containing nitric and sulfuric acid Figure 54.21 4.6 4.3 4.1 Europe North America

By the year 2000 The entire contiguous United States was affected by acid precipitation Figure 54.22 Field pH 5.3 5.2–5.3 5.1–5.2 5.0–5.1 4.9–5.0 4.8–4.9 4.7–4.8 4.6–4.7 4.5–4.6 4.4–4.5 4.3–4.4 4.3

Environmental regulations and new industrial technologies Have allowed many developed countries to reduce sulfur dioxide emissions in the past 30 years

Toxins in the Environment Humans release an immense variety of toxic chemicals Including thousands of synthetics previously unknown to nature One of the reasons such toxins are so harmful Is that they become more concentrated in successive trophic levels of a food web

In biological magnification Toxins concentrate at higher trophic levels because at these levels biomass tends to be lower Figure 54.23 Concentration of PCBs Herring gull eggs 124 ppm Zooplankton 0.123 ppm Phytoplankton 0.025 ppm Lake trout 4.83 ppm Smelt 1.04 ppm

In some cases, harmful substances Persist for long periods of time in an ecosystem and continue to cause harm

Atmospheric Carbon Dioxide One pressing problem caused by human activities Is the rising level of atmospheric carbon dioxide

Rising Atmospheric CO2 Due to the increased burning of fossil fuels and other human activities The concentration of atmospheric CO2 has been steadily increasing Figure 54.24 CO2 concentration (ppm) 390 380 370 360 350 340 330 320 310 300 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 1.05 0.90 0.75 0.60 0.45 0.30 0.15 0.15  0.30  0.45 Temperature variation (C) Temperature CO2 Year

How Elevated CO2 Affects Forest Ecology: The FACTS-I Experiment The FACTS-I experiment is testing how elevated CO2 Influences tree growth, carbon concentration in soils, and other factors over a ten-year period Figure 54.25

The Greenhouse Effect and Global Warming The greenhouse effect is caused by atmospheric CO2 But is necessary to keep the surface of the Earth at a habitable temperature

Increased levels of atmospheric CO2 are magnifying the greenhouse effect Which could cause global warming and significant climatic change

Depletion of Atmospheric Ozone Life on Earth is protected from the damaging effects of UV radiation By a protective layer or ozone molecules present in the atmosphere

Satellite studies of the atmosphere Suggest that the ozone layer has been gradually thinning since 1975 Figure 54.26 Ozone layer thickness (Dobson units) Year (Average for the month of October) 350 300 250 200 150 100 50 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005

The destruction of atmospheric ozone Probably results from chlorine-releasing pollutants produced by human activity Figure 54.27 1 2 3 Chlorine from CFCs interacts with ozone (O3), forming chlorine monoxide (ClO) and oxygen (O2). Two ClO molecules react, forming chlorine peroxide (Cl2O2). Sunlight causes Cl2O2 to break down into O2 and free chlorine atoms. The chlorine atoms can begin the cycle again. Sunlight Chlorine O3 O2 ClO Cl2O2 Chlorine atoms

Scientists first described an “ozone hole” Over Antarctica in 1985; it has increased in size as ozone depletion has increased (a) October 1979 (b) October 2000 Figure 54.28a, b