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Chapter 54

BIOL 1030 Chapter 54: Chapter 54 Ecosystems

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Biological Sciences
BIOL 1030
Scott Kevin

Chapter 54 Ecosystems Lecture Outline 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. • The dynamics of an ecosystem involve two processes that cannot be fully described by population or community processes and phenomena: energy flow and chemical cycling. • Energy enters most ecosystems in the form of sunlight. • It is converted to chemical energy by autotrophs, passed to heterotrophs in the organic compounds of food, and dissipated as heat. • Chemical elements are cycled among abiotic and biotic components of the ecosystem. • Energy, unlike matter, cannot be recycled. • An ecosystem must be powered by a continuous influx of energy from an external source, usually the sun. • Energy flows through ecosystems, while matter cycles within them. Concept 54.1 Ecosystem ecology emphasizes energy flow and chemical cycling • Ecosystem ecologists view ecosystems as transformers of energy and processors of matter. • We can follow the transformation of energy by grouping the species in a community into trophic levels of feeding relationships. Ecosystems obey physical laws. • The law of conservation of energy states that energy cannot be created or destroyed but only transformed. • Plants and other photosynthetic organisms convert solar energy to chemical energy, but the total amount of energy does not change. • The total amount of energy stored in organic molecules plus the amounts reflected and dissipated as heat must equal the total solar energy intercepted by the plant. • The second law of thermodynamics states that some energy is lost as heat in any conversion process. • We can measure the efficiency of ecological energy conversions. • Chemical elements are continually recycled. • A carbon or nitrogen atom moves from one trophic level to another and eventually to the decomposers and back again. Trophic relationships determine the routes of energy flow and chemical cycling in ecosystems. • Autotrophs, the primary producers of the ecosystem, ultimately support all other organisms. • Most autotrophs are photosynthetic plants, algae or bacteria that use light energy to synthesize sugars and other organic compounds. • Chemosynthetic prokaryotes are the primary producers in deep-sea hydrothermal vents. • Heterotrophs are at trophic levels above the primary producers and depend on their photosynthetic output. • Herbivores that eat primary producers are called primary consumers. • Carnivores that eat herbivores are called secondary consumers. • Carnivores that eat secondary producers are called tertiary consumers. • Another important group of heterotrophs is the detritivores, or decomposers. • They get energy from detritus, nonliving organic material such as the remains of dead organisms, feces, fallen leaves, and wood. • Detritivores play an important role in material cycling. Decomposition connects all trophic levels. • The organisms that feed as detritivores form a major link between the primary producers and the consumers in an ecosystem. • Detritivores play an important role in making chemical elements available to producers. • Detritivores decompose organic material and transfer chemical elements in inorganic forms to abiotic reservoirs such as soil, water, and air. • Producers then recycle these elements into organic compounds. • An ecosystem’s main decomposers are fungi and prokaryotes. Concept 54.2 Physical and chemical factors limit primary production in ecosystems • The amount of light energy converted to chemical energy by an ecosystem’s autotrophs in a given time period is an ecosystem’s primary production. An ecosystem’s energy budget depends on primary production. • Most primary producers use light energy to synthesize organic molecules, which can be broken down to produce ATP. • The amount of photosynthetic production sets the spending limit of the entire ecosystem. • A global energy budget can be analyzed. • Every day, Earth is bombarded by approximately 1023 joules of solar radiation. • The intensity of solar energy striking Earth varies with latitude, with the tropics receiving the greatest input. • Most of this radiation is scattered, absorbed, or reflected by the atmosphere. • Much of the solar radiation that reaches Earth’s surface lands on bare ground or bodies of water that either absorb or reflect the energy. • Only a small fraction actually strikes algae, photosynthetic prokaryotes, or plants, and only some of this is of wavelengths suitable for photosynthesis. • Of the visible light that reaches photosynthetic organisms, only about 1% is converted to chemical energy. • Although this is a small amount, primary producers produce about 170 billion tons of organic material per year. • Total primary production in an ecosystem is known as gross primary production (GPP). • This is the amount of light energy that is converted into chemical energy per unit time. • Plants use some of these molecules as fuel in their own cellular respiration. • Net primary production (NPP) is equal to gross primary production minus the energy used by the primary producers for respiration (R): NPP = GPP - R • To ecologists, net primary production is the key measurement, because it represents the storage of chemical energy that is available to consumers in the ecosystem. • Primary production can be expressed as energy per unit area per unit time, or as biomass of vegetation added to the ecosystem per unit area per unit time. • This should not be confused with the total biomass of photosynthetic autotrophs present in a given time, which is called the standing crop. • Primary production is the amount of new biomass added in a given period of time. • Although a forest has a large standing cross biomass, its primary production may actually be less than that of some grasslands, which do not accumulate vegetation because animals consume the plants rapidly. • Different ecosystems differ greatly in their production as well as in their contribution to the total production of the Earth. • Tropical rain forests are among the most productive terrestrial ecosystems. • Estuaries and coral reefs also are very productive, but they cover only a small area compared to that covered by tropical rain forests. • The open ocean has a relatively low production per unit area but contributes more net primary production than any other single ecosystem because of its very large size. • Overall, terrestrial ecosystems contribute two-thirds of global net primary production, and marine ecosystems contribute approximately one-third. In aquatic ecosystems, light and nutrients limit primary production. • Light is a key variable controlling primary production in oceans, since solar radiation can only penetrate to a certain depth known as the photic zone. • The first meter of water absorbs more than half of the solar radiation. • If light were the main variable limiting primary production in the ocean, we would expect production to increase along a gradient from the poles toward the equator, which receives the greatest intensity of light. • There is no such gradient. • There are parts of the ocean in the tropics and subtropics that exhibit low primary production, while some high-latitude ocean regions are relatively productive. • More than light, nutrients limit primary production in aquatic ecosystems. • A limiting nutrient is an element that must be added for production to increase in a particular area. • The nutrient most often limiting marine production is either nitrogen or phosphorus. • In the open ocean, nitrogen and phosphorous levels are very low in the photic zone but are higher in deeper water where light does not penetrate. • Nitrogen is the nutrient that limits phytoplankton growth in many parts of the ocean. • This knowledge can be used to prevent algal blooms by limiting pollution that fertilizes phytoplankton. • Some areas of the ocean have low phytoplankton density despite their relatively high nitrogen concentrations. • For example, the Sargasso Sea has a very low density of phytoplankton. • Nutrient-enrichment experiments showed that iron availability limits primary production in this area. • Marine ecologists carried out large-scale field experiments in the Pacific Ocean, spreading low concentrations of dissolved iron over 72 km2 of ocean. • A massive phytoplankton bloom occurred, with a 27-fold increase in chlorophyll concentration in water samples from test sites. • Why are iron concentrations naturally low in certain oceanic areas? • Windblown dust from the land delivers iron to the ocean, and relatively little dust reaches the central Pacific and Atlantic Oceans. • The iron factor in marine ecosystems is related to the nitrogen factor. • When iron is limiting, adding iron stimulates the growth of cyanobacteria that fix nitrogen. • Phytoplankton proliferate, once released from nitrogen limitation. • Iron --> cyanobacteria --> nitrogen fixation--> phytoplankton production • In areas of upwelling, nutrient-rich deep waters circulate to the ocean surface. • These areas have exceptionally high primary production, supporting the hypothesis that nutrient availability determines marine primary production. • Areas of upwelling are prime fishing locations. • Nutrient limitation is also common in freshwater lakes. • Sewage and fertilizer pollution can add nutrients to lakes. • Additional nutrients shifted many lakes from phytoplankton communities dominated by diatoms and green algae to communities dominated by cyanobacteria. • This process is called eutrophication and has a wide range of ecological impacts, including the loss of most fish species. • David Schindler of the University of Alberta conducted a series of whole lake experiments that identified phosphorus as the nutrient that limited cyanobacteria growth. • His research led to the use of phosphate-free detergents and other water quality reforms. In terrestrial ecosystems, temperature and moisture are the key factors limiting primary production. • Tropical rain forests, with their warm, wet conditions, are the most productive of all terrestrial ecosystems. • By contrast, low-productivity ecosystems are generally dry (deserts) or dry and cold (arctic tundra). • Between these extremes lie temperate forest and grassland ecosystems with moderate climates and intermediate productivity. • These contrasts in climate can be represented by a measure called actual evapotranspiration, which is the amount of water annually transpired by plants and evaporated from a landscape. • Actual evapotranspiration increases with precipitation and with the amount of solar energy available to drive evaporation and transpiration. • On a more local scale, mineral nutrients in the soil can play a key role in limiting primary production in terrestrial ecosystems. • Primary production removes soil nutrients. • A single nutrient deficiency may cause plant growth to slow and cease. • Nitrogen and phosphorus are the soil nutrients that most commonly limit terrestrial production. • Scientific studies relating nutrients to terrestrial primary production have practical applications in agriculture. • Farmers can maximize crop yields with the right balance of nutrients for the local soil and type of crop. Concept 54.3 Energy transfer between trophic levels is usually less than 20% efficient • The amount of chemical energy in consumers’ food that is converted to their own new biomass during a given time period is called the secondary production of an ecosystem. • We can measure the efficiency of animals as energy transformers using the following equation: • production efficiency = net secondary production / assimilation of primary production • Net secondary production is the energy stored in biomass represented by growth and reproduction. • Assimilation consists of the total energy taken in and used for growth, reproduction, and respiration. • Production efficiency is thus the fraction of food energy that is not used for respiration. • This differs among organisms. • Birds and mammals generally have low production efficiencies of between 1% and 3% because they use so much energy to maintain a constant body temperature. • Fishes have production efficiencies of around 10%. • Insects are even more efficient, with production efficiencies averaging 40%. • Trophic efficiency is the percentage of production transferred from one trophic level to the next. • Trophic efficiencies must always be less than production efficiencies because they take into account not only the energy lost through respiration and contained in feces, but also the energy in organic material at lower trophic levels that is not consumed. • Trophic efficiencies usually range from 5% to 20%. • In other words, 80–95% of the energy available at one trophic level is not transferred to the next. • This loss is multiplied over the length of a food chain. • If 10% of energy is transferred from primary producers to primary consumers, and 10% of that energy is transferred to secondary consumers, then only 1% of net primary production is available to secondary consumers. • Pyramids of net production represent the multiplicative loss of energy in a food chain. • The size of each block in the pyramid is proportional to the new production of each trophic level, expressed in energy units. • Biomass pyramids represent the ecological consequences of low trophic efficiencies. • Most biomass pyramids narrow sharply from primary producers to top-level carnivores because energy transfers are so inefficient. • In some aquatic ecosystems, the pyramid is inverted and primary consumers outweigh producers. • Such inverted biomass pyramids occur because the producers—phytoplankton—grow, reproduce, and are consumed by zooplankton so rapidly that they never develop a large standing crop. • They have a short turnover time, which means they have a small standing crop biomass compared to their production. • turnover time = standing crop biomass (mg/m2) / production (mg/m2/day) • Because the phytoplankton replace their biomass at such a rapid rate, they can support a biomass of zooplankton much greater than their own biomass. • Because of the progressive loss of energy along a food chain, any ecosystem cannot support a large biomass of top-level carnivores. • With some exceptions, predators are usually larger than the prey they eat. • Top-level predators tend to be fairly large animals. • As a result, the limited biomass at the top of an ecological pyramid is concentrated in a small number of large individuals. • In a pyramid of numbers, the size of each block is proportional to the number of individuals present in each trophic level. • The dynamics of energy through ecosystems have important implications for the human population. • Eating meat is an inefficient way of tapping photosynthetic production. • Worldwide agriculture could feed many more people if humans all fed as primary consumers, eating only plant material. Herbivores consume a small percentage of vegetation: the green world hypothesis. • According to the green world hypothesis, herbivores consume relatively little plant biomass because they are held in check by a variety of factors, including predators, parasites, and disease. • How green is our world? • 83 × 1010 metric tons of carbon are stored in the plant biomass of terrestrial ecosystems. • Herbivores annually consume less than 17% of the total net primary production. • The green world hypothesis proposes several factors that keep herbivores in check: • Plants have defenses against herbivores. • Nutrients, not energy supply, usually limit herbivores. • Animals need certain nutrients that plants tend to supply in relatively small amounts. • The growth and reproduction of many herbivores are limited by availability of essential nutrients. • Abiotic factors limit herbivores. • Temperature and moisture may restrict carrying capacities for herbivores below the level that would strip vegetation. • Intraspecific competition can limit herbivore numbers. • Territorial behavior and competitive behaviors may reduce herbivore population density. • Interspecific interactions check herbivore densities. • Parasites, predators, and disease limit the growth of herbivore populations. • This applies the top-down model of community structure. Concept 54.4 Biological and geochemical processes move nutrients between organic and inorganic parts of the ecosystem • Chemical elements are available to ecosystems only in limited amounts. • Life on Earth depends on the recycling of essential chemical elements. • Nutrient circuits involve both biotic and abiotic components of ecosystems and are called biogeochemical cycles. • There are two general categories of biogeochemical cycles: global and regional. • Gaseous forms of carbon, oxygen, sulfur, and nitrogen occur in the atmosphere, and cycles of these elements are global. • Elements that are less mobile in the environment, such as phosphorus, potassium, calcium, and trace elements generally cycle on a more localized scale in the short term. • Soil i
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