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Community Ecology – Chapter 54
Key terms
Community
Critical factor
Tolerance limit
Habitat
Ecological niche
Fundamental niche
Realized niche
Resource partitioning
Intraspecific competition
Interspecific competition
Predator
Parasite
Parasitoid
Herbivore
Crypsis
Catalepsis
Batesian mimicry
Mullerian mimicry
Masting
Symbiosis
Commensalism
Mutalism
Rhizobium
Mycorrhizae
Keystone species
Primary succession
Secondary succession
Pioneer species
Climax community
Tolerance
Facilitation
Inhibition
Key concepts
Community viewpoints
Principle (or Law) of Competitive Exclusion
Distinguishing types of interactions
Distinguishing types of predators
Intermediate disturbance hypothesis
Ecological succession
Outline of topics
54.1 Biological communities are composed of species that occur together.
Concepts of Communities
• The term community refers to the collection of species interacting in a particular area. (p. 1162)
• The individualistic concept of communities holds that a community is nothing more than an
aggregation of species co-occurring in one area, each responding independently to the environmental
gradient. (p. 1162)
• Each species has critical factors and associated tolerance limits that determine where it can live.
• The holistic concept of communities views communities as an integrated unit. (p. 1162)
• Recent studies support the individualistic community view rather than the holistic view. (pp. 11621163)
54.2 Interactions among competing species shape ecological niches.
Fundamental and Realized Niches
• An organisms habitat describes the place where an organism lives.
• An organism's niche is the sum total of all the ways it utilizes environmental resources. (p. 1164)
• Interspecific competition occurs when two species attempt to utilize the same limited resource. (p.
1164)
• A fundamental niche is the entire niche a species is capable of using, while the realized niche is the
actual portion a species occupies. (p. 164)
Gause and the Principle of Competitive Exclusion
• The principle of competitive exclusion states that if two species are competing for a limited resource,
the species that uses the resource more efficiently will eventually limit the other, at least locally. (p.
1165)
• Niche overlap can occur between two or more species, as long as the degree of overlap does not
lead to significant negative effects on one or more of the species. (p. 1165)
• Gause's theory predicts when two species coexist on a long-term basis, resources must not be
limiting, or else the niches differ by one or more resources. (p. 1165)
Resource Partitioning
• Sympatric species often reduce, or avoid, competition by living in different portions of the habitat or
by utilizing different resources. (p. 1166)
Detecting Interspecific Competition
• Although experimental studies can reveal the existence of interspecific competition, and field
experiments can often overcome some of the limitations inherent in laboratory conditions, both
methods have their limitations that must be overcome. (p. 1167)
54.3 Predation has ecological and evolutionary effects.
Predation and Prey Populations
• Predation provides strong selective pressures on prey populations. (p. 1168)
• Prey species often evolve better defenses against predators, and predators often then evolve better
means of circumventing such defenses. (p. 1168)
• Predators, herbivores, parasitoids and parasites are similar ecological interactions where one
participant in the interaction is harmed while the other benefits. They can be distinguished by the
degree of intimacy between predator and prey, as well as the degree of lethality between predator
and prey.
Prey Defenses
• Plants and animals have developed many mechanisms for protection against predators, including
morphological defenses, which are structures that deter predators, and chemical defenses, such as
secondary chemical compounds in plants. (p. 1169)
• Defensive coloration can be used either as aposematic (warning) coloring or cryptic coloring
(camouflage). (p. 1170)
• Behavioral defenses include intimidation displays and catalepsis.
• Batesian mimicry occurs when palatable insects resemble distasteful or toxic species and thus gain
some protection from predation as predators learn to avoid them. (p. 1171)
• Müllerian mimicry occurs when unrelated but distasteful species come to resemble one another. (p.
1171)
• Masting, or production of many offspring, saturates predators, allowing for some proportion to
survive.
• Some defenses may be inducible; that is, only appear when the predator is present. Inducible
defenses allow the reallocation of energy to other life history tasks.
54.4 Species within a community interact in many ways.
Coevolution and Symbiosis
• Symbiosis is a form of coevolution in which two or more kinds of organisms live together in a
complex, long-term relationship. (p. 1172)
Commensalism
• Commensalism occurs when the relationship between two species benefits one species and neither
harms nor benefits the other species. (p. 1173)
Mutualism
• Mutualism occurs when both species involved in the relationship benefit and can be obligate. (p.
1174)
Parasitism
• Parasitism is a special form of predation in which the parasite is usually much smaller than the prey,
and the two remain closely associated. Parasitism is harmful to the prey and beneficial to the
predator. (p. 1175)
• It is evolutionarily advantageous for the parasite to have a relatively low lethality or slow death if
lethal in order to pass on offspring.
• Recent studies have shown that some parasites can alter the behavior of their host in ways that
facilitate transmission between hosts. (p. 1175)
Interactions Among Ecological Processes
• Predators can prevent or greatly reduce competitive exclusion by reducing the numbers of
individuals of competing species. (p. 1176)
• Parasites may affect sympatric species differently and thus influence the outcome of interspecific
interactions. (p. 1176)
• Keystone species are those that have effects on community composition greater than their
abundance in the community and include ecosystem engineers. (p. 1177)
54.5 Ecological succession may increase the species richness of communities.
Succession
• Primary succession occurs when life-less substrate is gradually inhabited by organisms, which
change the environment. (p. 1178)
• Secondary succession occurs in an area where an existing community has been disturbed. (p. 1178)
• Succession causes progressive changes in the soil. (p. 1178)
• The processes of tolerance, facilitation and inhibition are critical in succession. (p. 1178)
• The concept of a climax community, where all communities are moving in a linear way towards one
“ultimate” community, has been replaced with a more cyclic nondirectional viewpoint where
disturbance is the norm and not the exception.
The Role of Disturbance
• Increasingly, scientists are recognizing communities as dynamic entities that change due to climatic
shifts, species invasions, and disturbance events. (p. 1180)
• The intermediate disturbance hypothesis predicts that communities experiencing moderate levels of
disturbance will have higher species richness than communities experiencing either smaller or greater
amounts of disturbance. (p. 1180)
Dynamics of Ecosystems – Chapter 55
Key terms
Ecosystem
Autotrophs
Heterotrophs
Primary producers
Consumer
Primary consumer
Secondary consumer
Tertiary consumer
Decomposers
Herbivore
Carnivore
Detritivore
Trophic level
Primary production
Secondary production
Ingestion efficiency
Assimilation efficiency
Production efficiency
Trophic cascade
Top-down
Bottom-up
Biodiversity
Species richness
Species evenness
Key concepts
Energy transfer/ecological pyramids
Trophic cascades
Causes of biodiversity patterns
Island biogeography theory
Outline of topics
55.1 Chemicals cycle within ecosystems.
• An ecosystem includes all of the organisms living in a particular place, plus the abiotic environment
in which they live. (p. 1184)
• Due to the Law of Conservation of Matter, all chemical elements are recycled in the environment.
The Water Cycle
• Water exhibits many properties, such as cohesion, polarity, and high specific heat that make life
possible as we know it on Earth.
• Water is found unevenly on Earth, and one-third of humans lack sanitary water.
• Solar energy drives the hydrologic cycle, where water continuously cycles between bodies of water,
land, organisms, and atmosphere, eventually making its way back to the oceans. (p. 1184)
• Ninety-seven percent of freshwater is stored in ice and glaciers and most of the liquid freshwater is
groundwater. (p. 1185)
• Cutting down forests disrupts the water cycle, at times leading to desertification, as lower amounts
of moisture are returned to the atmosphere. (p. 1185)
•Humans are using water for irrigation, domestic and other uses more quickly that it is being
replenished.
The Carbon Cycle
• About 10% of the carbon dioxide in the atmosphere is fixed annually by photosynthesis. (p. 1186)
• When the bodies of dead organisms decompose, microorganisms release carbon dioxide into the
atmosphere through respiration, freeing it for incorporation into other organisms. (p. 1186)
• Large amounts of carbon dioxide are also dissolved in the world's oceans. (p. 1186)
• Humans are altering the carbon cycle through burning of fossil fuels and clearing of forests.
The Nitrogen Cycle
• Seventy-eight percent of the atmosphere is relatively inert nitrogen gas.
• Nitrogen is an important component of proteins.
• Certain bacteria and fungi can fix atmospheric nitrogen and decompose nitrogen-containing
compounds into forms useful in biological processes. (p. 1187)
• Some nitrogen-fixing bacteria form symbiotic relationships with the roots of legumes and other
plants. (p. 1187)
• Humans are altering the nitrogen cycle through widespread use of nitrogen fertilizers.
The Phosphorus Cycle
• Phosphates are relatively insoluble and are present only in certain types of rocks. But, other than
nitrogen, phosphorus is the required nutrient most likely to be scarce enough to limit plant growth. (p.
1188)
• Phosphorus is important in storing and releasing biological energy in the compound ATP.
• Humans are altering the phosphorus cycle through use of mined phosphorus fertilizer.
The Sulfur Cycle
• Most sulfur is present in rocks and is released by volcanic activity and weathering.
•Humans are altering the sulfur cycle through burning of fossil fuels.
Biogeochemical Cycles Illustrated: Recycling in a Forest Ecosystem
• Deforestation in the Hubbard Brook Experimental Forest lead to increased loss of minerals in runoff
water from the ecosystem. (p. 1189)
55.2 Energy flows through ecosystems.
Trophic Levels
• Autotrophs (producers) manufacture their own food, while heterotrophs (consumers) must obtain
organic molecules that have been synthesized by autotrophs. (p. 1190)
• Energy can be neither created nor destroyed according to the First Law of Thermodynamics.
• Due to the Second Law of Thermodynamics, energy transfer is not 100% efficient. Once energy
enters an ecosystem, it is passed from one organism to another in the form of chemical bonds, and
much of the energy is converted to heat. (p. 1190)
• Ingestion efficiency, assimilation efficiency and production efficiency together reduce energy transfer
between trophic levels (feeding levels).
• Overall, an average of 10% of organic matter is transferred from one trophic level to the next. (p.
1190)
• Primary productivity describes the amount of organic matter produced from solar energy in a given
area during a given period of time, and is usually divided into gross primary productivity and net
primary productivity. (p. 1191)
• Secondary productivity refers to the rate of biomass production by heterotrophs. (p. 1191)
Energy in Food Chains
• Productivity varies among communities.
Ecological Pyramids
• Due to the loss of useful energy at each trophic level, higher trophic levels generally have fewer
individuals, biomass and energy than lower levels. (p. 1193)
• Some aquatic ecosystems have inverted biomass pyramids due to extremely rapid turnover of
biomass. (p. 1193)
• The loss of energy also places a limit on the number of top-level carnivores that can be supported in
a community. (p. 1193)
55.3 Interactions occur among different trophic levels.
Trophic Cascades
• Top-down effects are said to occur when the effect of one trophic level flows down to the next level.
(p. 1194)
• Humans have tested the trophic cascade hypothesis by removing top predators from the ecosystem
and the effect on the producers depends on whether a tertiary or secondary consumer is removed. (p.
1195)
• Bottom-up effects are said to occur when factors at the bottom of food webs cause ramifications at
higher trophic levels. (p. 1195)
• Neither top-down nor bottom-up effects are inevitable, for many reasons; for example, in some
cases, species may simultaneously operate on multiple trophic levels. (p. 1196)
55.4 Biodiversity promotes ecosystem stability.
Measuring Species Richness
• Biodiversity incorporates genetic, species and ecosystem diversity, though species diversity is most
often used.
• Species diversity metrics incorporate evenness and richness.
Effects of Species Richness
• One theory is that species-rich communities are more stable, more constant in composition, and
thus better able to resist disturbance. (p. 1197)
Patterns in Species Richness
• Some of the factors that determine species richness include ecosystem productivity, spatial
heterogeneity, climate, area, depth, predation, altitude, latitude and competition. (p. 1198-9)
Island Biogeography
• The equilibrium theory of island biogeography proposes that island species richness is a dynamic
equilibrium between colonization and extinction, which are influenced by island size and distance from
the mainland (p. 1200)
• As island size increases, extinction rates decrease and, as distance from the mainland increases,
immigration rates decrease.