Assigned reading for Environmental Conservation
M. Stephens
You may substitute equivalent sections from Cunningham or another text.
You should read and annotate the texts. Although you do not need to memorize, you should be
familiar with the terms and concepts in each section, and be prepared to discuss the material and
ask questions in class.
Excerpt from Online Biology Book
http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookTOC.html
Adapted by M. Stephens (January 2011)
Chapter 56: Community and Ecosystem Dynamics
Community Density and Stability |
Communities are made up of species adapted to the conditions of that community.
Diversity and stability help define a community and are important in environmental
studies. Species diversity decreases as we move away from the tropics. Species
diversity is a measure of the different types of organisms in a community (also
referred to as species richness). Latitudinal diversity gradient refers to species richness
decreasing steadily going away from the equator. A hectare (100 acres) of tropical
rain forest contains 40-100 tree species, while a hectare of temperate zone forest
contains 10-30 tree species. In marked contrast, a hectare of taiga (subarctic forest
dominated by evergreen conifers) contains only a paltry 1-5 species! Habitat
destruction in tropical countries will cause many more extinctions per hectare than it
would in higher latitudes.
Environmental stability is greater in tropical areas, where a relatively stable/constant
environment allows more different kinds of species to thrive. Equatorial communities
are older because they have been less disturbed by glaciers and other climate changes,
allowing time for new species to evolve. Equatorial areas also have a longer growing
season.
Change in Communities Over Time | Back to Top
Biological communities, like the organisms that comprise them, can and do change
over time. Ecological time focuses on community events that occur over decades or
centuries. Geological time focuses on events lasting thousands of years or more.
Community succession is the sequential replacement of species by immigration of new
species and local extinction of older ones following a disturbance that creates
unoccupied habitats for colonization. The initial rapid colonizer species are the
Community and Ecosystem Dynamics_Online Biology Book p. 1 of 7
pioneer community. Eventually a climax community of more or less stable but
slower growing species eventually develops.
During succession productivity declines and diversity increases. These trends tend to
increase the biomass (total weight of living tissue) in a community. Succession occurs
because each community stage prepares the environment for the stage following it.
Primary succession begins with bare rock and takes a very long time to occur.
Weathering by wind and rain plus the actions of pioneer species such as lichens and
mosses begin the buildup of soil. Herbaceous (non-woody) plants, including the
grasses, grow on deeper soil and shade out shorter pioneer species. Pine trees or
deciduous trees eventually take root and in most biomes will form a climax community
of plants that are stabile in the environment.
Secondary succession occurs when an environment has been disturbed, such as by
fire, geological activity, or human intervention (farming or deforestation in most
cases). This form of succession often begins in an abandoned field with soil layers
already in place. Compared to primary succession, which must take long periods of
time to build or accumulate soil, secondary succession occurs rapidly. The
herbaceous pioneering plants give way to pines, which in turn may give way to a
hardwood deciduous forest (in the classical old field succession models developed in
the eastern deciduous forest biome).
Human activity (such as clearing a climax forest community to establish a farm field
consisting of a cultivated pioneering species, say corn or wheat) replaces climax
communities with simpler communities.
Communities are composed of species that evolve, so the community must also
evolve. Comparing marine communities of 500 million years ago with modern
communities shows modern communities composed of quite different organisms.
Disturbance of a Community
The basic effect of human activity on communities is community simplification, an
overall reduction of species diversity. Agriculture is a purposeful human intervention
in which we create a monoculture of a single favored (crop) species such as corn. Most
of the agricultural species are derived from pioneering communities.
Inadvertent human intervention can simplify communities and produce stressed
communities that have fewer species as well as a superabundance of some species.
Disturbances favor early successional (pioneer) species that can grow and reproduce
rapidly.
Community and Ecosystem Dynamics_Online Biology Book p. 2 of 7
Ecosystems and Communities | Back to Top
Ecosystems include both living and nonliving components. These living, or biotic,
components include habitats and niches occupied by organisms. Nonliving, or abiotic,
components include soil, water, light, inorganic nutrients, and weather. An organism's
place of residence, where it can be found, is its habitat. A niche is often viewed as
the role of that organism in the community, factors limiting its life, and how it
acquires food.
Producers, a major niche in all ecosystems, are autotrophic (able to
produce/synthesize their own food), usually photosynthetic, organisms. In terrestrial
ecosystems, producers are usually green plants. Freshwater and marine ecosystems
frequently have algae as the dominant producers.
Consumers are heterotrophic organisms that eat food produced by another
organism. Herbivores are a type of consumer that feeds directly on green plants (or
another type of autotroph). Since herbivores take their food directly from the producer
level, we refer to them as primary consumers. Carnivores feed on other animals (or
another type of consumer) and are secondary or tertiary consumers. Omnivores, the
feeding method used by humans, feed on both plants and animals. Decomposers are
organisms, mostly bacteria and fungi that recycle nutrients from decaying organic
material. Decomposers break down detritus, nonliving organic matter, into inorganic
matter. Small soil organisms are critical in helping bacteria and fungi shred leaf litter
and form rich soil.
Even if communities do differ in structure, they have some common uniting processes
such as energy flow and matter cycling, shown in Figure 17. Energy flows move through
feeding relationships. The term ecological niche refers to how an organism functions in
an ecosystem. Food webs, food chains, and food pyramids are three ways of representing
energy flow.
Producers absorb solar energy and convert it to chemical bonds from inorganic
nutrients taken from environment. Energy content of organic food passes up food
chain; eventually all energy is lost as heat, therefore requiring continual input.
Original inorganic elements are mostly returned to soil and producers; can be used
again by producers and no new input is required.
Figure 17. The flow of energy through an ecosystem. Image from Purves et al., Life:
The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and
WH Freeman (www.whfreeman.com), used with permission.
Community and Ecosystem Dynamics_Online Biology Book p. 3 of 7
Energy flow in ecosystems, as with all other energy, must follow the two laws of
thermodynamics. The first law states that energy is neither created nor destroyed,
but instead changes from one form to another (potential to kinetic). The second
law mandates that when energy is transformed from one form to another, some usable
energy is lost as heat. Thus, in any food chain, some energy must be lost as we move
up the chain.
The ultimate source of energy for nearly all life is the Sun. Recently, scientists
discovered an exception to this once unchallenged truism: communities of organisms
around ocean vents where food chain begins with chemosynthetic bacteria that
oxidize hydrogen sulfide generated by inorganic chemical reactions inside the Earth's
crust. In this special case, the source of energy is the internal heat engine of the
Earth (geothermal energy).
Food chains indicate who eats whom in an ecosystem and represent one path of
energy flow through an ecosystem. Natural ecosystems have numerous interconnected
food chains. Each level of producer and consumers is a trophic level. Some primary
consumers feed on plants and make grazing food chains; others feed on detritus.
The population size in an undisturbed ecosystem is limited by the food supply,
competition, predation, and parasitism. Food webs help determine consequences of
perturbations: if titmice and vireos fed on beetles and earthworms, insecticides that
killed beetles would increase competition between birds and probably increase
predation of earthworms, etc.
Community and Ecosystem Dynamics_Online Biology Book p. 4 of 7
The trophic structure (energy/food flows) of an ecosystem forms an ecological
pyramid. The base of this pyramid represents the producer trophic level. At the apex
is the highest level consumer, the top predator. Other pyramids can be recognized in
an ecosystem. A pyramid of numbers is based on how many organisms occupy each
trophic level. The pyramid of biomass is calculated by multiplying the average weight
for organisms times the number of organisms at each trophic level. An energy
pyramid illustrates the amounts of energy available at each successive trophic level.
The energy pyramid always shows a decrease moving up trophic levels because:





Only a certain amount of food is captured and eaten by organisms on the next
trophic level.
Some of food that is eaten cannot be digested and exits digestive tract as
undigested waste.
Only a portion of digested food becomes part of the organism's body; rest is
used as source of energy.
Substantial portion of food energy goes to build up temporary ATP in
mitochondria that is then used to synthesize proteins, lipids, carbohydrates, fuel
contraction of muscles, nerve conduction, and other functions.
Only about 10% of the energy available at a particular trophic level is
incorporated into tissues at the next level. Thus, a larger population can be
sustained by eating grain than by eating grain-fed animals since 100 kg of
grain would result in 10 human kg but if fed to cattle, the result, by the time
that reaches the human is a paltry 1 human kg!
A food chain is a series of organisms each feeding on the one preceding it. There are
two types of food chain: decomposer and grazer. Grazer food chains begin with algae
and plants and end in a carnivore. Decomposer chains are composed of waste and
decomposing organisms such as fungi and bacteria.
Food chains are simplifications of complex relationships. A food web is a more
realistic and accurate depiction of energy flow. Food webs are networks of feeding
interactions among species.
The food pyramid provides a detailed view of energy flow in an ecosystem. The first
level consists of the producers (usually plants). All higher levels are consumers. The
shorter the food chain the more energy is available to organisms.
Most humans occupy a top carnivore role. Only about 2% of all calories available
from producers ever reach the tissues of top carnivores. “Leakage” of energy occurs
between each feeding level. Most natural ecosystems therefore do not have more than
five levels to their food pyramids. Large carnivores are rare because there is so little
energy available to them atop the pyramid.
Community and Ecosystem Dynamics_Online Biology Book p. 5 of 7
Food generation by producers varies greatly between ecosystems. Net primary
productivity (NPP) is the rate at which producer biomass is formed. Tropical forests and
swamps are the most productive terrestrial ecosystems. Reefs and estuaries are the
most productive aquatic ecosystems. All of these productive areas are in danger from
human activity. Humans redirect nearly 40% of the net primary productivity and
directly or indirectly use nearly 40% of all the land food pyramid. This energy is not
available to natural populations.
Terms | Back to Top
biomass
biomes
carnivores
climax
community
community
community
simplification
community
stability
community
succession
consumers
desert
ecological time ecosystem
ecotones
energy flow
grasslands
herbivores
freshwater
biome
latitudinal
diversity
gradient
net primary
productivity
(NPP)
Food webs, food
chains, and food
pyramids
closed
community
marine biome matter cycling
niches
omnivores
primary
succession
producers
species
diversity
temperate forest
biome
open
community
secondary
succession
tropical rain
forests
tundra
taiga
FYI: Supplemental Resources (optional exploration on your own)




The Rain Forest Report Card Maps, images, morphed movies showing the effects of
deforestation, and more make this a site to see for further information about the rain
forests and their plight.
Manu: Peru's Hidden Rain Forest PBS documentary, part of the Living Edens series.
Links to animals, plants, and people of this area. Quite a nice resource, as are many of the
PBS websites!
Population Ecology This site, maintained by Alexi Sharov of the Department of
Entomology at Virginia Tech provides a great start to the study of population ecology.
Links to people, organizations, online lectures, and other items of interest are provided.
Planet Earth - a suite of interactive learning activities on ecology Aimed at high school
students and teachers this site offers a series of great activities that will allow application
of the concepts learned to real world problems, such as the Wolves of Yellowstone.
Community and Ecosystem Dynamics_Online Biology Book p. 6 of 7

Types of Deserts This part of a larger U.S. Geological Survey page provides additional
details on the types of deserts and related features.
All text contents ©1995, 2000, 2001, 2002, 2007, by M.J. Farabee. Use of the text for
educational purposes is encouraged. Email: mj.farabee@emcmail.maricopa.edu
Last modified: Tuesday May 18 2010
URL: http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookcommecosys.html
Community and Ecosystem Dynamics_Online Biology Book p. 7 of 7