Chapter 43: Ecosystems

Crêpes for Breakfast, Pancake Ice for

           A.   At a glance, Antarctica is a barren continent of mostly ice and extremes of cold.

           B.   But it also is home to unique wildlife and it is fragile.

           C.   The nations of the world have finally come to some agreement on why and how it should be preserved.


   I.     The Nature of Ecosystems

           A.   Overview of the Participants

                  1.   Regions on the earth function as systems running on energy from the sun processed through various organisms.

                  2.   Primary producers are autotrophs that can capture sunlight energy and incorporate it into organic compounds.

                  3.   Consumers  are heterotrophs that feed on tissues of other organisms.

                          a.   Herbivores  eat plants.

                          b.   Carnivores  eat animals.

                          c.   Parasites  reside in or on living hosts and extract energy from them.

                         d.   Omnivores  eat a variety of organisms.

                  4.   Decomposers are also heterotrophs and include fungi and bacteria that extract energy from the remains or products of organisms.

                  5.   Detritivores  include small invertebrates that feed on partly decomposed particles of organic matter (detritus).

                  6.   An ecosystem is a complex of organisms interacting with one another and  with the physical environment.

                          a.   Ecosystems are open systems through which energy flows and materials are cycled.

                          b.   Ecosystems require energy and nutrient input and generate energy (usually as heat) and nutrient output.

           B.   Structure of Ecosystems

                  1.   Trophic (“feeding”) levels are a hierarchy of energy transfers, or bluntly stated, “Who eats whom?”

                          a.   Level 1 (closest to the energy source) consists of producers; level 2 comprises herbivores; and levels 3 and above are carnivores.

                          b.   Decomposers feed on organisms from all levels.

                  2.   Food chains cross-connect to form food webs.

                          a.   A sequence of who eats whom is called a food chain; example: algae ——> fish ——> fisherman ——> shark.

                          b.   Interconnected food chains comprise food webs  in which the same food resource is often part of more than one food chain.


II.     The Nature of Food Webs

           A.   How Many Energy Transfers?

                  1.   Energy that producers initially capture passes through no more than four or five transfers.

                  2.   Energy is lost at each transfer.

           B.   Two Categories of Food Webs

                  1.   Energy flows into ecosystems from the sun.

                          a.   Energy flows through ecosystems by way of grazing food webs, in which energy flows from plants to herbivores and then to carnivores.

                          b.   In detrital food webs it flows mainly from plants through decomposers and detritivores.

                  2.   Energy leaves ecosystems through heat losses generated by metabolism.


III.     Focus on Science:  Biological Magnification in Food Webs


IV.     Studying Energy Flow Through Ecosystems

           A.   What Is Primary Productivity?

                  1.   Primary productivity is the rate of photosynthesis for the ecosystem during a specified interval.

                  2.   Net  primary productivity is the rate of energy storage in plant tissues in excess of the rate of respiration by the plants themselves.

           B.   What Are Ecological Pyramids?

                  1.   Trophic structure can be diagrammed as a pyramid in which producers form a base for successive tiers of consumers above them.

                  2.   Pyramids can be of two basic types:

                          a.   A biomass pyramid makes provision for differences in size of organisms by using the weight of the members in each trophic level.

                          b.   An energy pyramid  reflects the trophic structure most accurately because it is based on energy losses at each level.


V.     Focus on Science: Energy Flow at Silver Springs


VI.     Biogeochemical Cycles—An Overview

           A.   Biogeochemical cycles influence the availability of essential elements in ecosystems.

                  1.   Elements are available to producers as ions.

                  2.   Nutrient reserves are maintained by environmental inputs and recycling activities.

                  3.   The amount of nutrients being cycled is greater than the amount entering or leaving.

                  4.   Environment inputs are by precipitation, metabolism, and weathering. Outputs are by runoff and evaporation.

           B.   There are three categories of biogeochemical cycles:

                  1.   In the hydrologic cycle, oxygen and hydrogen move as water molecules.

                  2.   In the atmospheric cycles, elements can move in the gaseous phase; examples include car­bon and nitrogen.

                  3.   In sedimentary cycles, the element does not have a gaseous phase; an example is phosphorus.


VII.   Hydrologic Cycle

           A.   Water is moved or stored by evaporation, precipitation, retention, and transportation.

           B.   Water moves other nutrients in or out of ecosystems.

                  1.   A watershed funnels rain or snow into a single river.

                  2.   Nutrients are absorbed by plants to prevent their loss by leaching.


VIII.  Sedimentary Cycles

           A.   A Look at the Phosphorus Cycle

                  1.   Phosphorus moves from land, to sediments in the seas, and back to the land in its long-term geochemical phase of the cycle.

                  2.   In the ecosystem phase, plants take up the phosphorus from the soil; it is then transferred to herbivores and carnivores, which excrete it in wastes and their own decomposing bodies.

           B.   Eutrophication

                  1.   Water enriched with phosphorus-containing fertilizers promotes dense algal blooms.

                  2.   Activities that increase the concentrations of dissolved nutrients can lead to eutrophication—the enrichment of any aquatic ecosystem.


IX.     Carbon Cycle

           A.   Carbon enters the atmosphere (where it exists as carbon dioxide) from aerobic respiration, fossil-fuel burning, and volcanic eruptions.

           B.   Carbon is removed from the atmosphere (and bodies of water) by photosynthesizers and shelled organisms.

           C.   Decomposition of buried carbon compounds millions of years ago caused the formation of fossil fuels.

           D.   Burning of fossil fuels puts extra amounts of carbon dioxide into the atmosphere, an occurrence that may lead to global warming—the greenhouse effect.


X.     Focus on Science:  From Greenhouse Gases to a Warmer Planet?


XI.     Nitrogen Cycle

           A.   Nitrogen is needed for synthesis of proteins and nucleic acids.

                  1.   It is abundant in the atmosphere (80 percent) but not in the earth’s crust.

                  2.   Of all the nutrients needed for plant growth, nitrogen is the scarcest.

           B.   Cycling Processes

                  1.   In nitrogen fixation, bacteria convert N2 to NH3, which is then used in the synthe­sis of proteins and nucleic acids which become incorporated into plant, then animal tissues.

                  2.   Decomposition and ammonification  occurs when bacteria and fungi decompose dead plants and ani­mals and release excess ammonia or ammonium ions.

                  3.   Nitrification  is a type of chemosynthesis where NH3 or NH4+ is converted to
NO2; other nitrifying bacteria use the nitrite for energy and release NO3.

           C.   Nitrogen Scarcity

                  1.   Although the soil is enriched by nitrogen-fixing bacteria, soil nitrogen is still scarce due to leaching, denitrification, and farming methods that emphasize synthetic fertilizers.

                  2.   Denitrification is the release of nitrogen gas to the atmosphere by the action of bacteria (NO2 and NO3 ——> N2).

           D.   Human Impact on the Nitrogen Cycle

                  1.   Air pollutants, including oxides of nitrogen, contribute to soil acidity.

2.   Heavy nitrogen applications not only are costly and are lost in runoff and harvested crops.