Download Chapter 4 Summary

Chapter 4 Summary
4-1 The Nature of Ecology
Ecology is a study of the connections among organisms and their living and nonliving
environment. Prokaryotic and eukaryotic organisms are composed of cells.
Organisms may reproduce by asexual reproduction or sexual reproduction. Organisms that
reproduce sexually are classified as members of the same species if they can interbreed.
Members of a species that reside in the same area at the same time constitute a population. In
most natural populations, individuals differ slightly, a result of genetic diversity. A population
normally lives in a particular habitat, within a distribution or range. Populations of many species
make up a community. An ecosystem is a community and its nonliving environment.
4-2 The Earth's Life-Support Systems
The lower portion of the earth's atmosphere is the troposphere. The next layer is the stratosphere.
The portions of the earth's atmosphere in which organisms exist—the lower atmosphere, the
hydrosphere, and the lithosphere—make up the biosphere.
Life on the earth is sustained by three interconnected factors: the one-way flow of energy from
the sun through the biosphere and back into space, matter cycling, and gravity.
4-3 Ecosystem Components
Biologists have classified the terrestrial portion of the biosphere into biomes. Marine and
freshwater portions of the biosphere are divided into aquatic life zones.
Abiotic components of an ecosystem are physical and chemical factors that influence living
organisms. Each population has a range of tolerance to various abiotic factors and its tolerance
limits determine its abundance and distribution. The law of tolerance describes the parameters
within which species can survive. The number of organisms in a population can be affected by a
single limiting factor, as described in the limiting factor principle. Important limiting factors
include dissolved oxygen (DO) content, temperature, sunlight, nutrient availability, and salinity.
Most producers or autotrophs capture sunlight energy and make carbohydrates by way of
photosynthesis. Some producers carry out chemosynthesis. All other organisms in an ecosystem
are consumers or heterotrophs. Omnivores feed on both plants and animals while decomposers or
detritivores eat detritus. Matter recycling and one-way energy flows ensure that there is little or
no waste in natural ecosystems.
Most organisms release energy by aerobic respiration, which requires oxygen. Some get energy
by anaerobic respiration or fermentation.
Kinds of biological diversity or biodiversity include genetic diversity, species diversity,
ecological diversity, and functional diversity.
4-4 Energy Flow in Ecosystems
Organisms get the food and nutrients they need by participating in a food chain. Various food
chains can link together to form a food web. Each organism in an ecosystem can be assigned to a
trophic level in its food chain or food web. Each trophic level contains a certain amount of
biomass. The transfer of energy between these levels has a certain ecological efficiency.
Food chains can be represented as a pyramid of energy flow.
4-5 Primary Productivity of Ecosystems
Gross primary productivity is the rate at which producers use photosynthesis to make more
biomass. It varies across the earth.
The planet's net primary productivity (NPP) ultimately limits the number of consumer organisms
(including humans) that can survive on the earth. Humans now use, waste, or destroy about 27%
of the earth's total NPP and 40% of the NPP of the planet's terrestrial ecosystems. This share is
expected to increase and thus threaten the habitats and food supplies of other species.
4-6 Soils
Soil is a renewable resource but takes many years to form. Soils are arranged in soil horizons,
which can be viewed by examining a soil profile. Fertile soils contain a large proportion of
humus. Water works its way through the soil by infiltration. Minerals and organic matter are
dissolved and carried by water through leaching. Soil texture, a factor in soil porosity, is affected
by the proportions of clay, sand, and silt in the soil. Porosity determines soil permeability.
4-7 Matter Cycling in Ecosystems
Nutrients, atoms, ions, and molecules are continuously cycled in nutrient cycles or
biogeochemical cycles. The hydrologic cycle or water cycle collects, purifies, and distributes the
earth's water. Water travels around the earth in the form of water vapor. Condensation nuclei are
necessary for precipitation to occur. Other cycles are the carbon cycle, the nitrogen cycle, the
phosphorus cycle, and the sulfur cycle. Human activities are altering these cycles.
4-8 How Do Ecologists Learn about Ecosystems?
Ecologists use field research and laboratory research to gather data. They use systems analysis to
develop mathematical models that simulate ecosystems and generate predictions.