Chapter Outline
INTRODUCTION Ecosystems Are the Most Complex Level of Biological Organization Include living and nonliving factors Transfer of energy is regulated, nutrients are cycled Earth is a closed system with respect to nutrients and chemicals System is open with respect to energy Ecosystems May Have Clearly Recognizable Boundaries Ecosystems change over time and become new ecosystems Changes are gradual and adapt to particular conditions Overall characteristics of populations adjust to the new conditions BIOGEOCHEMICAL CYCLES All Substances in Organisms Cycle Through Ecosystems fig 26.1 The bulk of these are not contained within the bodies of organisms Contained within the atmosphere: carbon, nitrogen and oxygen Contained within rocks: phosphorus, potassium and other minerals Substances are incorporated from nonliving sources into organisms Returned to non-living world through decomposition The Water Cycle fig 26.2 All life depends directly on the presence of water Energy from sun powers the evaporation of water into atmosphere Most falls back into the oceans or subsurface bodies of water 98% of earth`s water is free, only 2% is fixed All organisms require water to live Plants obtain water from the earth Animals drink water or obtain it by eating plants Water occurs as surface and ground water Aquifers are permeable saturated layers of rock, sand and gravel Ground water is an important reservoir of water Water table: the unconfined portion of ground water Ground water flows more slowly than surface water Rate of use is increasing enormously Many aquifers are threatened with depletion Pollution in groundwater is a serious problem The Carbon Cycle fig 26.3 Based on atmospheric carbon dioxide Synthesis of organic compounds fixes 700 billion metric tons yearly Accomplished by various photosynthesizers All heterotrophic nonphotosynthesizers depend on their activity Carbon dioxide released into atmosphere when organisms decompose Some carbon compounds are accumulated Cellulose is more resistant to breakdown May eventually be incorporated into fossil fuels or minerals 1 trillion metric tons of CO2 are dissolved in the ocean Fossil fuels contain 500 billion metric tons 600 to 1000 billion metric tons contained within organisms Processes of respiration and photosynthesis are roughly balanced Carbon dioxide increasing as a result of burning fossil fuels May be altering global climates The Nitrogen Cycle fig 26.4 Nitrogen gas constitutes 78% of the atmosphere Very little nitrogen is fixed in the soil, oceans and organisms Few organisms convert atmospheric nitrogen into biologically useful forms All are nitrogen-fixing bacteria Triple bond linking nitrogen atoms makes the gas very stable Process is enzyme catalyzed and utilizes ATP Some nitrogen-fixing bacteria are free-living Some form symbiotic relationships with plants Fix enough nitrogen to be of significance Plants can grow in soils with low amounts of nitrogen Bacteria and fungi rapidly decompose nitrogen-containing compounds Use products to synthesize own proteins, release excess as ammonium Process called ammonification Fixed nitrogen is lost to the atmosphere by denitrification The Oxygen Cycle Only the earth possesses significant quantities of free oxygen Free oxygen is a product of three billion years of photosynthesis Without continued photosynthesis, respiration would deplete all nonatmospheric oxygen in fifty years The Phosphorus Cycle fig 26.5 Most biogeochemical cycle reservoirs in minerals, not atmosphere Phosphates exist in the soil in only small amounts Are relatively insoluble and contained in only certain kinds of rocks Weather out of rocks, transported to oceans Brought up by natural uplift of land masses or by marine animals Form rich natural deposits of guano from sea birds Millions of tons of phosphates added to farm land each year Calcium dihydrogen phosphate is called superphosphate Made by treating calcium phosphate with sulfuric acid Biogeochemical Cycles Illustrated: Recycling in a Forested Ecosystem Evidence in studies of Hubbard Brook Experimental Forest Central stream of large temperate deciduous forest watershed Measure water and nutrient flow made through concrete weirs Conclusions: undisturbed watershed efficiently retained nutrients Instructive with regard to loss of rainforest area to crops Experimental felling of trees and shrubs in one of six watersheds Amount of water runoff increased by 40% Amount of nutrients lost was greatly increased Conclusion: fertility lost, danger of flooding increased THE FLOW OF ENERGY Ecosystems Contain Autotrophs and Heterotrophs Autotrophs capture light energy and manufacture own food Heterotrophs obtain organic molecules synthesized by autotrophs Energy captured is slowly released through metabolic processes Primary Productivity Primary productivity: amount of organic matter produced from solar energy per area per time Gross primary productivity: total amount of energy converted to organic compounds per area per unit time Net primary productivity: total amount of energy fixed per unit of time minus energy expended in organismal metabolic activities Biomass: total weight of all organisms living in the ecosystem Increases as a result of net productivity High net productivity in cornfield ecosystem High net productivity, low biomass in tropical rainforest ecosystem Comparative net primary productivity: biomass ratios Tropical forest and marshlands = 1500 to 3000 grams/year Temperate forest = 1100 to 1500 grams/year Dry deserts = 200 grams/year Estuaries, coral reefs, sugarcane fields = 3600 to 9100 grams/year Intertidal zone = 14,600 grams/year Trophic Levels Green plants convert 1% of the sun`s energy Less energy converted by the animals that eat plants Levels of consumers Primary consumers: herbivores, feed on green plants Secondary consumers: carnivores and parasites feed on herbivores Decomposers: break down matter accumulated in bodies of organisms Detrivores: live on dead organisms and cast-off parts of organisms fig 26.6 Such trophic levels exist in all complicated ecosystems Organisms from each level compose food chain Relationships are more accurately branching food webs fig 26.7 Some energy ingested is lost at each successive trophic level Much goes to heat production, some lost for digestion and work Less than 10% goes toward growth and reproduction Experimental studies of freshwater ecosystem fig 26.8 For each 1000 calories of energy fixed by photosynthesizers 150 calories transferred to small heterotrophs 30 calories of that transferred to smelt 6 calories of that transferred to trout (or humans) 1.2 calories from trout transferred to humans Organisms that have a vegetarian diet have more food energy available There are more individuals at lower trophic levels than at upper levels Diagrammatic representations of such relationships form pyramids fig 26.9 Biomass pyramids may occasionally be inverted Energy pyramids cannot be inverted ECOLOGICAL SUCCESSION Succession: Ecosystems Have a Tendency to Change from Simple to Complex Cleared land becomes occupied by larger and more diverse plants Small pond becomes filled with vegetation encroaching from the edges Types of Succession Secondary succession Occurs in areas once exhibiting life but disturbed in some manner Frequently initiated by humans Also result from fires or by abandoning agricultural fields fig 26.10 Primary succession Occurs in areas devoid of all life Areas after retreat of glaciers New volcanic islands Xerarch succession occurs on dry, barren rocks Hydrarch succession occurs in open water Oligotrophic lake is poor-in nutrients Eutrophic lake is rich in nutrients Oligotrophic lake may become eutrophic through succession Climax vegetation (climax community) Characteristic vegetation may be associated with climate of region fig 26.11 Term no longer useful as once presumed Climates keep changing Process of succession is very slow Nature of a region`s vegetation affected by human activities General Characteristics of Succession Increase in total biomass, decrease in net productivity Earlier stages more productive than later ones Agricultural systems not allowed to mature, productivity kept high More species in mature ecosystems than in immature ecosystems Number of heterotrophs increases faster than number of autotrophs Organisms at later stages are more specialized than earlier organisms