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Chapter 55 Ecosystems
Overview: Observing Ecosystems • An ecosystem consists of all the organisms living in a community, as well as the abiotic factors with which they interact • Ecosystems can range in size, but regardless of an ecosystem’s size, its dynamics involve two main processes: energy flow and chemical cycling • Energy flows through ecosystems (IN ONE DIRECTION) while matter cycles within them (IN ALL DIRECTIONS)
Concept 55.1: Physical laws govern energy flow and chemical cycling in ecosystems. • All living systems require a constant input of free energy, and organisms use free energy to maintain organization, grow and reproduce. • Ecologists study the transformations of energy and matter within their system, and use these studies to suggest the health of an ecosystem. • Laws of physics and chemistry apply to ecosystems, particularly energy flow.
Conservation of Mass • The law of conservation of mass states that matter cannot be created or destroyed • Chemical elements must therefore be continually recycled within ecosystems • Ecosystems are open systems, absorbing energy and mass and releasing heat and waste products
Fig. 55-4 Tertiary consumers Microorganisms and other detritivores Secondary consumers Primary consumers Detritus Primary producers Heat Key Chemical cycling Sun Energy flow
Concept 55.2: Energy and other limiting factors control 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. • The extent of photosynthetic production sets the spending limit for an ecosystem’s energy budget • The amount of solar radiation reaching the Earth’s surface limits photosynthetic output of ecosystems. • Only a small fraction of solar energy actually strikes photosynthetic organisms, and even less is of a usable wavelength.
Gross and Net Primary Production • Total primary production is known as the ecosystem’s gross primary production (GPP) • Net primary production (NPP) is GPP minus energy used by primary producers for respiration • Only NPP is available to consumers • Ecosystems vary greatly in NPP and contribution to the total NPP on Earth NPP = GPP - R
Fig. 55-6 · Net primary production (kg carbon/m2·yr) 0 1 3 2
Primary Production in Aquatic Ecosystems • In marine and freshwater ecosystems, both light and nutrients control primary production: • Depth of light penetration affects primary production in a lake or ocean • More than light, nutrients limit primary production in geographic regions of the ocean and in lakes • A limiting nutrient is the element that must be added for production to increase in an area • Nitrogen and phosphorous are typically the nutrients that most often limit marine and freshwater production
Fig. 55-7 EXPERIMENT Long Island Shinnecock Bay G F E C D Moriches Bay B Great South Bay Atlantic Ocean A RESULTS 30 Ammonium enriched Phosphate enriched 24 Unenriched control 18 Phytoplankton density (millions of cells per mL) 12 6 0 C B D A E G F Collection site
Primary Production in Terrestrial Ecosystems • In terrestrial ecosystems, temperature and moisture affect primary production on a large scale: • Tropical rainforests, with their warm, wet conditions that promote plant growth, are the most productive of all terrestrial ecosystems. • Low productivity terrestrial ecosystems are generally dry (deserts or the arctic tundra). • Temperate forest and grassland ecosystems have moderate climates and intermediate productivity.
Concept 55.3: Energy transfer between trophic levels is typically only 10% efficient. • Secondary production of an ecosystem is the amount of chemical energy in food converted to new biomass during a given period of time.
Trophic Efficiency and Ecological Pyramids • Trophic efficiency is the percentage of production transferred from one trophic level to the next • It usually ranges from 5% to 20% • Trophic efficiency is multiplied over the length of a food chain
Fig. 55-10 Tertiary consumers 10 J Secondary consumers 100 J Primary consumers 1,000 J Primary producers 10,000 J 1,000,000 J of sunlight
Fig. 55-11 Trophic level Dry mass (g/m2) Tertiary consumers 1.5 Secondary consumers 11 Primary consumers 37 Primary producers 809 (a) Most ecosystems (data from a Florida bog) Trophic level Dry mass (g/m2) Primary consumers (zooplankton) 21 Primary producers (phytoplankton) 4 (b) Some aquatic ecosystems (data from the English Channel)
Meadow Habitat: Occupies 50.2 km2. Primary Producer Biomass – distributed uniformly and totals 3200 kg/km2. What would be the most likely immediate result of a disturbance that reduced the primary producer’s biomass by 50% AND removed all rabbits and insects? Long term result?
How much carbon (in g/m2) is released into the atmosphere as a result of the metabolic activity of herbivores?
What % of the biomass in the forest community is tied up in the shrub layer?
Concept 55.4: Biological and geochemical processes cycle nutrients between organic and inorganic parts of an ecosystem. • Life depends on recycling chemical elements • Nutrient circuits in ecosystems involve biotic and abiotic components and are often called biogeochemical cycles • Gaseous carbon, oxygen, sulfur, and nitrogen occur in the atmosphere and cycle globally • Less mobile elements such as phosphorus, potassium, and calcium cycle on a more local level
Fig. 55-13 Reservoir B Reservoir A Organic materials available as nutrients Organic materials unavailable as nutrients Fossilization Living organisms, detritus Coal, oil, peat Respiration, decomposition, excretion Assimilation, photosynthesis Burning of fossil fuels Reservoir C Reservoir D Inorganic materials available as nutrients Inorganic materials unavailable as nutrients Weathering, erosion Minerals in rocks Atmosphere,soil, water Formation of sedimentary rock
Biogeochemical Cycles • 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
Fig. 55-14a Transport over land Solar energy Net movement of water vapor by wind Precipitation over land Precipitation over ocean Evaporation from ocean Evapotranspiration from land Percolation through soil Runoff and groundwater
Fig. 55-14b CO2 in atmosphere Photosynthesis Cellular respiration Photo- synthesis Burning of fossil fuels and wood Phyto- plankton Higher-level consumers Primary consumers Carbon compounds in water Detritus Decomposition
Fig. 55-14c N2 in atmosphere Assimilation Denitrifying bacteria NO3 – Nitrogen-fixing bacteria Decomposers Nitrifying bacteria Ammonification Nitrification NH3 NH4 NO2 – + Nitrogen-fixing soil bacteria Nitrifying bacteria
Fig. 55-14d Precipitation Geologic uplift Weathering of rocks Runoff Consumption Decomposition Plant uptake of PO43– Plankton Dissolved PO43– Soil Uptake Leaching Sedimentation
Decomposition and Nutrient Cycling Rates • Decomposers (detritivores) play a key role in the general pattern of chemical cycling • Rates at which nutrients cycle in different ecosystems vary greatly, mostly as a result of differing rates of decomposition • The rate of decomposition is controlled by temperature, moisture, and nutrient availability • Rapid decomposition results in relatively low levels of nutrients in the soil
Fig. 55-15 Ecosystem type EXPERIMENT Arctic Subarctic Boreal Temperate A Grassland Mountain G M D B,C P T H,I E,F S O L N U J K R Q RESULTS 80 70 U 60 R O Q K 50 T Percent of mass lost J P 40 S D N F 30 I C M L 20 H A B E G 10 0 –10 15 –15 –5 0 5 10 Mean annual temperature (ºC)
Fig. 55-16 (a) Concrete dam and weir (b) Clear-cut watershed 80 Deforested 60 40 20 Nitrate concentration in runoff (mg/L) Completion of tree cutting 4 3 Control 2 1 0 1965 1966 1967 1968 (c) Nitrogen in runoff from watersheds
Concept 55.5: Human activities now dominate most chemical cycles on Earth. • As the human population has grown, our activities have disrupted the trophic structure, energy flow, and chemical cycling of many ecosystems • In addition to transporting nutrients from one location to another, humans have added new materials, some of them toxins, to ecosystems • Disruptions that deplete nutrients in one area and increase them in other areas can be detrimental to ecosystem dynamics.
Fig. 55-18 – Contamination of Aquatic Ecosystems Winter Summer
Toxins in the Environment • Humans release many toxic chemicals, including synthetics previously unknown to nature • In some cases, harmful substances persist for long periods in an ecosystem • One reason toxins are harmful is that they become more concentrated in successive trophic levels • Biological magnification concentrates toxins at higher trophic levels, where biomass is lower
Fig. 55-20 Herring gull eggs 124 ppm Lake trout 4.83 ppm Concentration of PCBs Smelt 1.04 ppm Zooplankton 0.123 ppm Phytoplankton 0.025 ppm
Fig. 55-23 350 300 250 Ozone layer thickness (Dobsons) 200 100 0 ’60 ’80 1955 ’70 ’75 ’85 ’90 ’95 ’05 ’65 2000 Year
Fig. 55-24 Chlorine atom O2 O3 Chlorine ClO O2 ClO Cl2O2 Sunlight
Fig. 55-25 (a) September 1979 (b) September 2006