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ENWC314 Chap 15 book notes

by: Eden Tinkelman

ENWC314 Chap 15 book notes ENWC314

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The ecological world view chapter 15 book notes
Anastasia Chirnside, Joanna York
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This 10 page Bundle was uploaded by Eden Tinkelman on Wednesday August 24, 2016. The Bundle belongs to ENWC314 at University of Delaware taught by Anastasia Chirnside, Joanna York in Fall 2016. Since its upload, it has received 7 views. For similar materials see COMP TERRESTRIAL/MARINE ECOLOG in Ecology at University of Delaware.


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Date Created: 08/24/16
ENWC314 Chapter 15 book notes  15.1 Solar Energy Fixed in Photosynthesis Sustains all Trophic Levels o there are two broad approaches we can take to the study of communities and ecosystems of plants and animals  this has been the approach of the last four chapters and can be considered a species-by-species-ecological approach to community and ecosystem dynamics  the second approach moves beyond the details of particular species and concentrates on the physics of ecosystems as energy machines and nutrient processors o Ecosystem Metabolism  The metabolism of ecosystems can be most easily understood by appreciating that it is the sum of the metabolism of individual animals and plants  Individual organisms require a continual input of new energy to balance losses from metabolism, growth and reproduction  There are 2 major ways in which organisms pick up energy and materials  Autotrophs pick up energy form the sun and materials from non-living sources; green plants are autotrophs  heterotrophs pick up energy and materials by eating living matter; herbivores are heterotrophs that live by eating plants, and carnivores are heterotrophs that live by eating other animals.  Communities are mixtures of autotrophs and heterotrophs  The ecosystem level of integration includes both the organisms and their abiotic environment and is a comprehensive level at which to consider the movement of energy and materials o Materials and Energy as Currency  The first step in the study of ecosystem metabolism is to determine the food of the community  Three measurements might be used to define relative importance of a species in an ecosystem:  Biomass—we would use the weight or standing crop of each species as a measure of importance; in a dynamic situation in which yield is important, we need to know how rapidly a community produces new biomass; when metabolic rates and reproductive rates are high, production may be very rapid, even from standing crop  Flow of chemical materials—we can view the ecosystem as a kind if super-organism, taking in food materials, using them and passing them out; all chemical materials can be recycled many times through the community  Flow of energy—we can view the ecosystem as an energy transformer that takes solar energy, fixes some of it in photosynthesis and transfers this energy from green plants through herbivores to carnivores; most energy flows through an ecosystem only once, and is not recycled, but is transformed to heat and ultimately lost to the system; only the continual input of new solar energy keeps the ecosystem operating  because most energy is not recirculated, it’s one of the easier variables to measure in an ecosystem  15.2 Green Plants Process the Sun’s Energy Under Limitations Imposed by Temperature, Moisture and Nutrients o the process of photosynthesis is the cornerstone of all life o green plants are responsible for primary production—the energy and materials produced by plants as a result of photosynthesis  the bulk of the Earth’s living biomass is green plants (99.9% by weight); only a small fraction of life consists of animals  photosynthesis is the process of transforming solar energy into chemical energy  H 2 + CO + 2TPC H O 6O12 6 2  Respiration is the opposite of photosynthesis:  O 2 C H6O12 A6P CO +H O 2 2 o At metabolic equilibrium, photosynthesis equals respiration, and this is called the compensation point o Photosynthesis and respiration are rate processes, and are always measured as amount of material or energy per unit of time o gross primary production—energy (or carbon) fixed in photosynthesis per unit time o net primary production—energy (or carbon) fixed in photosynthesis-energy (or carbon) lost by respiration per unit time o Eugene Odum was one of the key founders of production ecology; he recognized by the 1950’a that the measurement of production and energy flow through communities could provide ecological insight into how ecosystems work, and in particular how humans were affecting the energy flows in natural communities o Measuring Primary Production  For terrestrial plants, the direct way to measure primary production is to measure the change in CO or O 2 2 concentrations in the air around plants  The simplest indirect method of measuring primary production is the harvest method—biomass change in community  Net primary production in biomass may apply to the whole plant, or it may be specified as aerial production or root production  The net primary production in biomass may then be converted to energy by measuring the caloric equivalent of the material in a bomb calorimeter  In aquatic systems, primary production can be measured in the same general way as in terrestrial systems  Gas-exchange techniques can be applied to water volumes—usually oxygen release, rather than carbon dioxide uptake, is measured  In general, primary production is highest in the tropical rain forest and decreases progressively toward the poles  Productivity of the open ocean is very low, approximately the same as that of the arctic tundra  But because oceans occupy about 71% of the total surface of the Earth, total oceanic primary production adds up to about 46% of the overall production of the globe  Grassland and tundra areas are less productive than forests in the same general region o Efficiency of Primary Production  Efficiency of gross primary production (%)= (energy fixed by gross primary production/ energy in incident sunlight) *100%  The amount of solar radiation intercepted by the Earth is 21*10 24J per year, or about 8.1 J/cm /min.  Plants use only about .02% of this amount for photosynthesis, and most of the remaining energy is reflected back by the atmosphere or converted to heat  The energy in incident sunlight at the Earth’s surface is reduced by more than half by atmospheric reflection or absorption, but still only a small amount of this incident energy is used in primary production  The efficiency of gross primary production is higher in forests than in herbaceous communities or in crops  A great deal of energy is lost in converting solar radiation to gross primary production  Net primary production, which is the useful primary production for herbivores or humans, must therefore be even less efficient  The results of these losses is that for a broad range of terrestrial communities, about 1% of the sun’s energy during the growing season is converted into net primary production  15.3 Light, Temperature, Rainfall and Nutrients Limit Primary Productivity o Marine Communities  Light is the first variable one might expect to control primary production and the depth to which light penetrates in a lake or ocean is critical in defining the zone of primary production; water absorbs solar radiation very readily  Very high light levels can inhibit photosynthesis of green plants—this inhibition can be found in tropical and subtropical surface waters throughout the year  Primary production is relatively low in the surface waters (due to excessive light) and is highest in the warm waters near the surface at 10-30 m depth  If light is the primary variable limiting primary production in the ocean, there should be a gradient of productivity from the poles toward the equator  There is no gradient of production from the poles to the equator  Large parts of the tropics and subtropics are very unproductive  The most productive areas are coastal areas off the western side of Africa and North and South Africa  Nutrients appear to be the primary limitation on primary production in the ocean  Two elements—nitrogen and phosphorus—often limit primary production in the oceans  One of the striking generalities of many parts of the ocean is the very low concentrations of nitrogen and phosphorus in the surface layers where the phytoplankton live, whereas the deep water contains much higher concentrations of nutrients  But the importance of nitrogen as a limiting factor raises another dilemma because several large parts of the oceans contain high amounts of nitrate and low numbers of phytoplankton  One explanation for these oceanic regions is that they are communities dominated by top-down processes in which herbivores control plant biomass, and nutrients are always in excess  Alternatively, these could be bottom-up communities limited by some nutrient other than nitrogen or phosphorus  Iron is an essential component component of the photosynthetic machinery of the cyanobacteria that fix nitrogen in the oceans  The impact of iron on primary production is mainly through its impact on nitrogen fixation, so we have a sequence of potential limitations that operate in iron- poor parts of the ocean: o Ironcyanobacterianitrogen fixationphytoplankton  In most of the open oceans, light is always available for photosynthesis, but nitrogen is not  They found that nitrogen addition stimulated phytoplankton growth most strongly, followed closely by iron addition  These results are consistent with the conclusion that nitrogen and iron in the oceans are key limiting resources  Compared with the land, the ocean is very unproductive; the reason seems to be that fewer nutrients are available  Although the maximal rate of primary production in the sea may be the same as that on land, these high rates in the sea can be maintained for a few days only, unless upwelling enriches the nutrient content of the water  Areas of upwelling in the ocean are exceptions to the general rule of nutrient limitation  The largest area of upwelling occurs in the Antarctic Ocean, where cold, nutrient-rich, deep water comes to surface along a broad zone near the Antarctic continent  Chlorophyll concentration in the surface water can be estimated by spectral reflectance using blue/green ratios  Remote sensing allows marine ecologists to analyze large-scale production changes in the ocean without being limited to a few measurements made off a ship  Total primary production in the ocean is rarely limited by light, but by the shortage of nutrients, particularly nitrogen and iron, which are critical for plant growth  Phosphorus limitation of primary production is very rare in oceanic ecosystems o Freshwater Communities  In freshwater communities, the same limiting factors that operate in the ocean do not seem to operate  Solar radiation limits primary production on a day-to- day basis in lakes and, within a given lake, you can predict the daily primary productivity from the solar radiation  Temperature is closely linked with light intensity in aquatic systems and is difficult to evaluate as a separate factor  Nutrient limitations operate in freshwater lakes, and the great variety of lake ecosystems is associated with a great variety of potential limiting nutrients  Nutrients added to lakes directly in sewage or indirectly as run-off had increased algal concentrations and had shifted many lakes from phytoplankton communities dominated by diatoms or green algal to those dominated by blue-green algae; this is called eutrophication  The conclusion was simple: phosphorus is the limiting nutrient for phytoplankton production in the majority of freshwater lakes  One of the changes that often accompany eutrophication in lakes is that the blue-green algae tend to replace green algae  Blue-green algae are ‘nuisance algae’ because they become extremely abundant when nutrients are plentiful and form floating scum  Blue-green algae become dominant in the phytoplankton for several reasons: o They are not grazed heavily by zooplankton or fish o Zooplankton can often manipulate the large colonies and filaments of blue-green algae o Some species of blue-green algae also produce secondary chemicals that are toxic to zooplankton o Blue-green algae are also poorly digested by many herbivores o Many blue-green algae can fix atmospheric nitrogen, putting them at an advantage when nitrogen is relatively scarce  In eutrophication, more and more phosphorus is continually loaded into a lake so that nitrogen can become a limiting factor o The phytoplankton community in many temperate freshwater lakes therefore may have two broad configurations at which it can exist:  One dominated by green algae and one with high nutrient levels organized by competition and dominated by blue- green algae  Estuaries are mixtures of fresh water and salt water and are often heavily polluted with nutrients from sewage and industrial wastes  Estuaries are complex gradients of nutrient limitation in which added phosphorus and nitrogen from pollution can strongly affect primary production throughout the estuary o Terrestrial Communities  In terrestrial habitats, temperature ranges are much greater than in aquatic habitats, and the great variation in temperature from coastal to alpine or continental areas makes it possible to uncouple the solar radiation— temperature variable, which is so closely linked in aquatic systems  The availability of satellite data to estimate primary production on a global basis has made it relatively easy to obtain measurement data of primary production  On land, production is limited by temperature, water and nutrients in the soil  Within the climatic constraints dictated by temperature and rainfall, soil nutrients limit production  In unexploited virgin grassland or forest, all nutrients that the plants take up form the soil and hold in be stabilized (input=output), or the site would deteriorate over time  Terrestrial communities, especially forests, have large nutrient stores tied up in the standing crop of plants  In this way they differ from communities in the sea and fresh water  This concentration of nutrients in the standing vegetation has important implications for nutrient cycles in forest communities  If the community is stable, the input of nutrients should equal the output  15.4 Energy Fixed by Green Plants Flows Either to Herbivores or to Detritus, or is Lost in Respiration o the biomass of plants that accumulates in an ecosystem as a result of photosynthesis can eventually go in one of two directions: to herbivores or to detritus feeders  detritus is non-living, particulate organic matter and its associated microbial populations that result from decomposition o Efficiency of Secondary Production  Once plants have captured solar energy in primary production, this energy flows on to the rest of the food chain, leading to secondary production, or the aggregate of growth and reproduction in herbivores and carnivores  One measure that we can use to describe ecosystems involves transfers between trophic levels and is called trophic efficiency: o Trophic efficiency= net production at trophic level i +1/ net production at trophic level i  The energy not transferred is lost in respiration or to detritus  For aquatic ecosystems, trophic efficiencies vary from 2% to 24% and average 10.1%  Daniel Pauly and Villy Christensen aggregated all the data for the fisheries of the world and showed that on average 8% of global aquatic primary production was being used to produce the global fisheries catch  Many terrestrial systems are dominated by decomposers, and most of the energy in the system flows through the decomposer link in the food web  This loss is greatly reduced at higher trophic levels, so that most of the production of herbivores is taken by carnivores and only 10% flows directly into the decomposer food chain  The amount of herbivory varies in different ecosystems  Herbivores in aquatic ecosystems consume a higher fraction of the primary production than they do in terrestrial ecosystems  Thus we cab distinguish aquatic ecosystems, which are dominated by grazing, from terrestrial ecosystems, which are dominated by decomposers  The standing crop of vascular plants in terrestrial, marine and freshwater communities is reduced by herbivores  By excluding herbivores experimentally, we would expect a larger impact on the standing crop of plants in marine ecosystems compared with the results of the same experiment in terrestrial ecosystems  One consequence of low ecological efficiencies is that organisms at the base of the food web are much more abundant than those at higher trophic levels  Charles Elton recognized this in 1927, and linked it with the observation that predators are usually larger than the prey they consume  The result of these two processes is a pyramid of numbers or biomass that has been called an Eltonian pyramid in his honor o Productivity of Grazing Systems  One practical application about grazing system efficiencies arose over the question of whether or not to encourage game ranching in Africa  If game animals are more efficient, it would pay farmers to protect and harvest the native animals and use them for meat, rather than raising cattle or sheep  The history of human settlement on most continents has been a repeating sequence of the elimination of wild animals and replacement by domestic cattle and sheep, and during the 1950s Africa seemed next on the list  Against this background, Fraser Darling in 1960 proposed that, in many areas of Africa, game animals were more productive than domestic cattle and sheep, and hence a sustained yield of game would be more profitable than a sustained yield of cattle and sheep  African wildlife has evolved within its ecosystems for millions of years, and thus is uniquely adapted to the African environment  The diversity of of herbivores in Africa therefore should use the vegetation more efficiently and be more productive than a cattle or sheep monoculture would be  Thus African wildlife should attain a higher biomass than cattle or sheep on native African ranges  African wildlife should provide a higher sustained yield and higher net revenue to the African people  In all these African ecosystems, rainfall is the key limiting factor for primary production  Human-organized grazing systems are more productive than natural grazing systems because natural systems support predator populations, diseases and parasites, but cattle and sheep are protected from predators and inoculated against parasitic infections  Consequently, some of the factors causing mortality in natural populations are absent in farming systems  Game cropping is thus not an efficient way to provide cheap meat to low-income native peoples  From a conservation viewpoint, game cropping may have adverse impacts o The creation of a market for game-meat provides an outlet for illegal trade in these species o From an economic viewpoint, game farming in fenced areas can provide luxury products (meat) and services (tourism) to foreigners at a substantial economic advantage o But private game farms typically do not promote conservation of the entire community o The net result is that private game farms conserve only a few species in the ecosystem o Thus game ranching in Africa doesn’t serve wither the purposes of conservation or those of sustainable, high-yield agriculture for the African people


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