CH 55 & 56 – Energy flow in Ecosystems

Overview: Ecosystems An ecosystem consists of all the organisms living in a community, as well as the abiotic (non-living) factors with which they interact Ecosystems range from a small, such as an aquarium, to a large, such as a lake or forest

Figure 55.2 Figure 55.2 A desert spring ecosystem. 3

Energy flows through ecosystems Matter cycles within them Ecosystem dynamics involve two main processes: energy flow and chemical cycling Energy flows through ecosystems Matter cycles within them Physical laws govern energy flow and chemical cycling in ecosystems Conservation of Energy (first law of thermodynamics) Energy enters from solar radiation and is lost as heat Conservation of matter - Chemical elements are continually recycled within ecosystems Ecosystems are open systems, absorbing energy and mass and releasing heat and waste products

Energy, Mass, and Trophic Levels Autotrophs build molecules themselves using photosynthesis or chemosynthesis as an energy source Heterotrophs depend on the biosynthetic output of other organisms

Energy and nutrients pass from primary producers (autotrophs) to primary consumers (herbivores) to secondary consumers (omnivores & carnivores) to tertiary consumers (carnivores that feed on other carnivores) Detritivores, or decomposers, are consumers that derive their energy from detritus Prokaryotes and fungi are important detritivores Decomposition connects all trophic levels

Microorganisms and other detritivores Secondary and tertiary consumers Figure 55.4 Sun Key Chemical cycling Energy flow Heat Primary producers Primary consumers Detritus Figure 55.4 An overview of energy and nutrient dynamics in an ecosystem. Microorganisms and other detritivores Secondary and tertiary consumers Arrows represent energy flow so they go from prey TO predator 7

Concept 55.3: Energy transfer between trophic levels is typically only 10% efficient Net Primary Production (NPP) is the amount of new biomass added in a given time period Only NPP is available to consumers Ecosystems vary greatly in NPP and contribution to the total NPP on Earth Limited by light, nutrients and other abiotic factors Secondary is the amount of chemical energy in food converted to new biomass

Production Efficiency When a caterpillar feeds on a leaf, only about one-sixth of the leaf’s energy is used for secondary production An organism’s production efficiency is the fraction of energy stored in food that is not used for respiration

Plant material eaten by caterpillar Figure 55.10 Plant material eaten by caterpillar 200 J 67 J Figure 55.10 Energy partitioning within a link of the food chain. Cellular respiration 100 J Feces 33 J Not assimilated Growth (new biomass; secondary production) Assimilated 10

Interesting Energy production facts: Birds and mammals have efficiencies in the range of 13% because of the high cost of endothermy Fishes have production efficiencies of around 10% Insects and microorganisms have efficiencies of 40% or more

Trophic Efficiency and Ecological Pyramids Trophic efficiency is the percentage of production transferred from one trophic level to the next It is usually about 10%, with a range of 5% to 20% Trophic efficiency is multiplied over the length of a food chain

Approximately 0.1% of chemical energy fixed by photosynthesis reaches a tertiary consumer A pyramid of net production represents the loss of energy at each level Tertiary consumers 10 J Secondary consumers 100 J Primary consumers 1,000 J Primary producers 10,000 J 1,000,000 J of sunlight

In a biomass pyramid, each level represents the dry mass of all organisms in each level Most biomass pyramids show a sharp decrease at successively higher trophic levels

Role of Humans in Energy flow: Dynamics of energy flow in ecosystems have important implications for the human population Eating meat is a relatively inefficient in terms of utilizing photosynthetic production Worldwide agriculture could feed many more people if humans ate only plant material Fossil fuels used to Produce foods

Biological and geochemical processes cycle nutrients and water in ecosystems Life depends on recycling chemical elements Nutrient cycles in ecosystems involve biotic and abiotic components and are often called biogeochemical cycles

Biogeochemical Cycles Gaseous carbon, oxygen, sulfur, and nitrogen occur in the atmosphere and cycle globally Less mobile elements include phosphorus, potassium, and calcium These elements cycle locally in terrestrial systems but more broadly when dissolved in aquatic systems

Burning of fossil fuels Formation of sedimentary rock Figure 55.13 Reservoir A Organic materials available as nutrients Reservoir B Organic materials unavailable as nutrients Fossilization Living organisms, detritus Peat Coal Oil Respiration, decomposition, excretion Burning of fossil fuels Assimilation, photosynthesis Reservoir D Inorganic materials unavailable as nutrients Reservoir C Inorganic materials available as nutrients Figure 55.13 A general model of nutrient cycling. Weathering, erosion Atmosphere Minerals in rocks Water Formation of sedimentary rock Soil 18

In studying cycling of water, carbon, nitrogen, and phosphorus, ecologists focus on four factors Each chemical’s biological importance Forms in which each chemical is available or used by organisms Major reservoirs for each chemical Key processes driving movement of each chemical through its cycle

The Carbon Cycle Carbon-based organic molecules are essential to all organisms Photosynthetic organisms convert CO2 to organic molecules that are used by heterotrophs Carbon reservoirs include fossil fuels, soils and sediments, solutes in oceans, plant and animal biomass, the atmosphere, and sedimentary rocks CO2 is taken up and released through photosynthesis and respiration Volcanoes and the burning of fossil fuels also contribute CO2 to the atmosphere

Burning of fossil fuels and wood Figure 55.14b CO2 in atmosphere Photosynthesis Photo- synthesis Cellular respiration Burning of fossil fuels and wood Phyto- plankton Consumers Figure 55.14 Exploring: Water and Nutrient Cycling Consumers Decomposition 21

Figure 55.UN03 Figure 55.UN03 Appendix A: answer to Concept Check 55.4, question 1 22

The Nitrogen Cycle Nitrogen is a component of amino acids, proteins, and nucleic acids The main reservoir of nitrogen is the atmosphere (N2), though this nitrogen must be converted to NH4+ or NO3– for uptake by plants, via nitrogen fixation by bacteria

Decomposition and sedimentation Figure 55.14c N2 in atmosphere Reactive N gases Industrial fixation Denitrification N fertilizers Fixation Runoff Dissolved organic N NO3– Terrestrial cycling N2 NH4+ NO3– Aquatic cycling Denitri- fication Figure 55.14 Exploring: Water and Nutrient Cycling Decomposition and sedimentation Assimilation Decom- position NO3– Fixation in root nodules Uptake of amino acids Ammonification Nitrification NH3 NH4+ NO2– 24

The Phosphorus Cycle Phosphorus is a major constituent of nucleic acids, phospholipids, and ATP Phosphate (PO43–) is the most important inorganic form of phosphorus The largest reservoirs are sedimentary rocks of marine origin, the oceans, and organisms Phosphate binds with soil particles, and movement is often localized

Wind-blown dust Geologic uplift Weathering of rocks Runoff Consumption Decomposition Plant uptake of PO43– Plankton Dissolved PO43– Uptake Leaching Figure 55.14 Exploring: Water and Nutrient Cycling Sedimentation Decomposition 26