Community Ecology – Chapter 54

Key terms

Community / Mullerian mimicry
Critical factor / Masting
Tolerance limit / Symbiosis
Habitat / Commensalism
Ecological niche / Mutalism
Fundamental niche / Rhizobium
Realized niche / Mycorrhizae
Resource partitioning / Keystone species
Intraspecific competition / Primary succession
Interspecific competition / Secondary succession
Predator / Pioneer species
Parasite / Climax community
Parasitoid / Tolerance
Herbivore / Facilitation
Crypsis / Inhibition
Catalepsis
Batesian mimicry

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. 1162-1163)

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 / Secondary production
Autotrophs / Ingestion efficiency
Heterotrophs / Assimilation efficiency
Primary producers / Production efficiency
Consumer / Trophic cascade
Primary consumer / Top-down
Secondary consumer / Bottom-up
Tertiary consumer / Biodiversity
Decomposers / Species richness
Herbivore / Species evenness
Carnivore
Detritivore
Trophic level
Primary production

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 HubbardBrookExperimentalForest 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.