Chapter 55 Ecosystems

An ecosystem consists of all the organisms living in a community –As well as all the abiotic factors with which they interact –They can be very small or very large AquariumConiferous Forest

Energy Flow and Chemical Cycling 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 (energy transformations are inefficient…some energy is always lost as heat)

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 Microorganisms and other detritivores Detritus Primary producers Primary consumers Secondary consumers Tertiary consumers Heat Sun Key Chemical cycling Energy flow Detritus is dead organic material

Nutrient cycling Nutrients cycle within an ecosystem

Decomposition –Connects all trophic levels –Detritivores, mainly bacteria and fungi, recycle essential chemical elements –By decomposing organic material and returning elements to inorganic reservoirs

Decomposition

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

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 –And only ~1% of that is converted to chemical energy by photosynthesis –…still that’s a lot of energy

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 (The plants use some of the energy for the fuel of day-to-day living) Net primary production (NPP) –Is equal to GPP minus the energy used by the primary producers for respiration –It is the amount of new biomass added in a given time period –Only NPP is available to consumers

Different ecosystems vary considerably in their net primary production a nd in their contribution to the total NPP on Earth Percentage of Earth’s surface area (a) Average net primary production (g/m 2 /yr) (b) (c)

The open ocean contributes relatively little per unit of area…but there is an awful lot of area. Forested areas contribute a great deal per unit area 180  120  W 60  W 00 60  E120  E 180  North Pole 60  N 30  N Equator 30  S 60  S South Pole Regional annual NPP (light violet lowest  red highest)

Primary Production in Marine and Freshwater Ecosystems In marine and freshwater ecosystems –Both light and nutrients are important in controlling primary production

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

Nutrient Limitation More than light, nutrients limit primary production 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

The addition of large amounts of nutrients to lakes has a wide range of ecological impacts In some areas, sewage runoff h as caused eutrophication (overnourishment?) of lakes, which can lead to the eventual loss of most fish species from the lakes The overnourished upper lake has a tremendous cyanobacterial bloom Phosphorus is often the limiting nutrient for cyanobacterial growth. Oversupply leads to blooms

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

Actual Evapotranspiration 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 (warm and moist is better than cold and dry) Actual evapotranspiration (mm H 2 O/yr) Tropical forest Temperate forest Mountain coniferous forest Temperate grassland Arctic tundra Desert shrubland Net primary production (g/m 2 /yr) 1,000 2,000 3, ,0001,500 0

On a local scale –A soil nutrient is often the limiting factor in primary production 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/m 2 ) Adding nitrogen (N) boosts net primary production 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.

Secondary Production 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 The production efficiency of an organism –Is the fraction of energy stored in food that is not used for respiration Plant material eaten by caterpillar Cellular respiration Growth (new biomass) Feces 100 J 33 J 200 J 67 J 67J of the 100J of assimilated energy is used for respiration. 33J of 100J is new biomass…so the PE is 33% Nondigested materials aren’t figured into PE

Production efficiency Endotherms (birds and mammals) have low production efficiencies. In the range of 1- 3% –A lot of energy is used to maintain that constant body temp. Ectotherms have higher production efficiencies –Fish at ~10% –Insects at around 40%

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

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

Pyramids of Biomass Most biomass pyramids –Show a sharp decrease at successively higher trophic levels Trophic level Dry weight (g/m 2 ) Primary producers Tertiary consumers Secondary consumers Primary consumers

Certain aquatic ecosystems can have inverted biomass pyramids –Here the phytoplankton (algae) grow very rapidly and are very productive but their consumption by zooplankton holds the population size down 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/m 2 ) 21 4

Pyramids of Numbers A pyramid of numbers –Represents the number of individual organisms in each trophic level Trophic level Number of individual organisms Primary producers Tertiary consumers Secondary consumers Primary consumers 3 354, ,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 (processing the plant material through another food animal decreases efficiency of energy transfer) Trophic level Secondary consumers Primary consumers Primary producers Relative food energy available to the human population at different trophic levels

The Green World Hypothesis Most terrestrial ecosystems have large standing crops despite the large numbers of herbivores According to the green world hypothesis –Terrestrial herbivores consume relatively little plant biomass because they are held in check by a variety of factors

The green world hypothesis proposes several factors that keep herbivores in check –Plants have defenses against herbivores –Nutrients, not energy supply, usually limit herbivores (paucity of essential nutrients in their diet) –Abiotic factors limit herbivores (temperature, water availability) –Intraspecific competition can limit herbivore numbers (territoriality, competition, etc.) –Interspecific interactions check herbivore densities (predators, parasites, disease, etc)

Chemical Cycling 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

Biogeochemical Cycles The water cycle and the carbon cycle Transport over land Solar energy Net movement of water vapor by wind Precipitation over ocean Evaporation from ocean Evapotranspiration from land Precipitation over land Percolation through soil Runoff and groundwater CO 2 in atmosphere Photosynthesis Cellular respiration Burning of fossil fuels and wood Higher-level consumers Primary consumers 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

More Biogeochemical Cycles N 2 in atmosphere Denitrifying bacteria Nitrifying bacteria Nitrifying bacteria Nitrification Nitrogen-fixing soil bacteria Nitrogen-fixing bacteria in root nodules of legumes Decomposers Ammonification Assimilation NH 3 NH 4 + NO 3  NO 2  Rain Plants Consumption Decomposition Geologic uplift Weathering of rocks Runoff Sedimentation Plant uptake of PO 4 3  Soil Leaching THE NITROGEN CYCLE THE PHOSPHORUS CYCLE The nitrogen cycle and the phosphorous cycle

Decomposition and Nutrient Cycling Rates Decomposers (detritivores) play a key role in the general pattern of chemical cycling The rates at which nutrients cycle in different ecosystems are extremely variable, mostly as a result of differences in rates of decomposition Consumers Producers Nutrients available to producers Abiotic reservoir Geologic processes Decomposers

Human activities 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

Agriculture and Nitrogen Cycling Agriculture constantly removes nutrients from ecosystems t hat would ordinarily be cycled back into the soil

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

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

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 eutrophication, excessive algal growth, which can cause significant harm to these ecosystems

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 Europe North America Numbers indicate the average pH of precipitation in that area

By the year 2000 the entire contiguous United States was affected by acid precipitation (defined as precipitation less than pH 5.6) Field pH  – – – – – – – – – –4.4  4.3

Toxins in the Environment Humans release an immense variety of toxic chemicals –Including thousands of synthetics previously unknown to nature (Xenobiotic) One of the reasons such toxins are so harmful –Is that they become more concentrated in successive trophic levels of a food web In some cases, harmful substances –Persist for long periods of time in an ecosystem and continue to cause harm

In biological magnification –Toxins concentrate at higher trophic levels because at these levels biomass tends to be lower Concentration of PCBs Herring gull eggs 124 ppm Zooplankton ppm Phytoplankton ppm Lake trout 4.83 ppm Smelt 1.04 ppm Small concentrations of toxin spread among many individuals Fewer individuals each with high concentration of toxin

Atmospheric Carbon Dioxide The rising level of atmospheric carbon dioxide is human in origin

Rising Atmospheric CO 2 Due to the increased burning of fossil fuels and other human activities t he concentration of atmospheric CO 2 has been steadily increasing CO 2 concentration (ppm)  0.15  0.30  0.45 Temperature variation (  C) Temperature CO 2 Year

The Greenhouse Effect and Global Warming The greenhouse effect is caused by atmospheric CO 2 –But is necessary to keep the surface of the Earth at a habitable temperature Increased levels of atmospheric CO 2 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 s uggest that the ozone layer has been gradually thinning since 1975 Ozone layer thickness (Dobson units) Year (Average for the month of October)

The destruction of atmospheric ozone –Probably results from chlorine-releasing pollutants produced by human activity Chlorine from CFCs interacts with ozone (O 3 ), forming chlorine monoxide (ClO) and oxygen (O 2 ). Two ClO molecules react, forming chlorine peroxide (Cl 2 O 2 ). Sunlight causes Cl 2 O 2 to break down into O 2 and free chlorine atoms. The chlorine atoms can begin the cycle again. Sunlight ChlorineO3O3 O2O2 ClO Cl 2 O 2 O2O2 Chlorine atoms

Scientists first described an “ozone hole” over Antarctica in 1985 – it has increased in size as ozone depletion has increased