Environmental Problems, Their Causes, and Sustainability 1

Figure 4-20 Generalized pyramid of energy flow showing thedecrease in usable energy available at each succeeding trophic levelin a food chain or web. In nature, ecological efficiency varies from 2%to 40%, with 10% efficiency being common. This model assumes a10% ecological efficiency (90% loss of usable energy to the environment,in the form of low-quality heat) with each transfer from onetrophic level to another.10Tertiaryconsumers(human)Secondaryconsumers(perch)HeatDecomposersHeatHeat100Heat1,000Primaryconsumers(zooplankton)Heat10,000Usable energyavailable ateach tropic level(in kilocalories)Producers(phytoplankton)Figure 4-21 Annual energy flow (in kilocalories persquare meter per year) for an aquatic ecosystem inSilver Springs, Florida. (From Cecie Starr, Biology:Concepts and Applications, 4th ed., Brooks/Cole[Wadsworth] © 2000)Top carnivoresCarnivoresHerbivoresProducers213833,36820,810Decomposers/detritivores5,0604-5 PRIMARY PRODUCTIVITYOF ECOSYSTEMSHow Fast Can Producers Produce Biomass?A Very Important RateDifferent ecosystems use solar energy to produceand use biomass at different rates.The rate at which an ecosystem’s producers convertsolar energy into chemical energy as biomass is theecosystem’s gross primary productivity (GPP). Figure4-22 shows how this productivity varies across theearth.To stay alive, grow, and reproduce, an ecosystem’sproducers must use some of the biomass they producefor their own respiration. Net primary productivity(NPP) is the rate at which producers use photosynthesisto store energy minus the rate at which they use someof this stored energy through aerobic respiration asshown in Figure 4-23. In other words, NPP GPP R,where R is energy used in respiration. NPP is a measureof how fast producers can provide the food needed byconsumers in an ecosystem.Various ecosystems and life zones differ in theirNPP, as graphed in Figure 4-24 (p. 72). Looking at thisgraph, what are the three most productive and thethree least productive systems? Generally, would youexpect NPP to be higher at the equator than at theearth’s poles? Why? Despite its low net primary productivity,there is so much open ocean that it producesmore of the earth’s NPP per year than any of the otherecosystems and life zones shown in Figure 4-24.In agricultural systems, the goal is to increase theNPP and biomass of selected crop plants by addingwater (irrigation) and nutrients (mostly nitrates andphosphates in fertilizers). Despite such inputs, theNPP of agricultural land is not very high comparedwith that of other terrestrial ecosystems (Figure 4-24).How Does the World’s Net Rate of BiomassProduction Limit the Populations of ConsumerSpecies? Nature’s LimitsThe number of consumer organisms the earthcan support is determined by how fast producerscan supply them with energy found inbiomass.As we have seen, producers are the source of all foodin an ecosystem. Only the biomass represented byNPP is available as food for consumers and they useonly a portion of this. Thus the planet’s NPP ultimatelylimits the number of consumers (including humans) thatcan survive on the earth. This is an important lessonfrom nature.70 CHAPTER 4 Ecosystems: What Are They and How Do They Work?