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Chapter 38Community

and Ecosystem Ecology

A Community Contains Several Interacting Populations in the

Same Locale

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38.1 Competition can lead to resource partitioning

Competition is rivalry between populations for the same resources, such as light, space, nutrients, or mates Competitive exclusion principle - no two species

can occupy the same niche at the same time Ecological niche - role it plays in its community,

including its habitat (where the organism lives) and its interactions with other organisms and the environment

Resource partitioning decreases competition between the two species Character displacement is often viewed as evidence that

competition and resource partitioning have taken place

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Figure 38.1A Competition only occurs between two species of Paramecium when they are grown together

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Figure 38.1B Character displacement occurs in finches when they coexist, compared to when they exist separately

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Figure 38.1C Niche specialization occurs among five species of coexisting warblers

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Figure 38.1D Competition occurs between two species of barnacles

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38.2 Predator-prey interactions affect both populations

Predator-Prey Population Dynamics Do predators reduce population density of prey?

Population density of predator can be affected by the prevalence of prey

What causes a decrease in population size instead of the establishment of a steady population size? At least two possibilities account for the reduction:

Perhaps the biotic potential (reproductive rate) of the predator is so great that its increased numbers over consume the prey, and then as the prey population declines, so does the predator population

Perhaps the biotic potential of the predator is unable to keep pace with the prey and the prey population overshoots the carrying capacity and suffers a crash

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Figure 38.2A Predator-prey interaction between a snowshoe hare and a lynx

Insert FIGURE 38.2A Predator-prey interaction between a snowshoe hare and a lynx

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Prey Defenses and Mimicry

Prey Defenses Prey defenses are mechanisms that thwart the possibility

of being eaten by a predator Camouflage - ability to blend into the background Warning coloration tells the predator that the prey is potentially

dangerous Association with other prey

Mimicry - when one species resembles another that possesses an overt antipredator defense Example: Scarlet kingsnake mimicking venomous coral snake Viceroy butterfly mimicking the foul-tasting monarch butterfly

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Figure 38.2B Antipredator defenses

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Figure 38.2C Mimicry: All of these insects have the same coloration

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38.3 Parasitism benefits one population at another’s expense

Parasitism - similar to predation in that an organism, called the parasite, derives nourishment from another, called the host Example of symbiosis, an association in which one species is

dependent on other Example: Viruses, such as HIV, that reproduce inside human

lymphocytes are always parasitic Effects of parasites on health of host can range from

slightly weakening them to killing them In addition to nourishment, host organisms also provide

parasites with a place to live and reproduce, as well as a mechanism for dispersing offspring to new hosts

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38.4 Commensalism benefits only one population

Commensalism is a symbiotic relationship between two species in which one species is benefited and the other is neither benefited nor harmed Example: Spanish moss grow in the branches of

trees, where they receive light, but they take no nourishment from the trees

On closer examination commensalism is often mutualism or parasitism Amount of harm or good two species seem to do to

one another is dependent on what the investigator chooses to measure

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Figure 38.4 A clownfish living among a sea anemone’s tentacles

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APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES

38.5 Coevolution requires interaction between two species

Coevolution - evolutionary change in one species results in an evolutionary change in the other Organisms in symbiotic associations are especially

prone to the process of coevolution Also occurs between predators and prey

Example: Cheetah sprints forward to catch prey, and this behavior might be selective for those gazelles that jump high in the air

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Figure 38.5 Social parasitism in the cuckoo

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38.6 Mutualism benefits both populations

Mutualism - symbiotic relationship in which both members benefit Example: Bacteria that reside in the human intestinal tract acquire food,

but they also provide us with vitamins, molecules we are unable to synthesize for ourselves

Relationship between plants and their pollinators is a good example of mutualism Also examples of coevolution

Outcome of mutualism is an intricate web of species interdependencies critical to the community Example: Branches and cones of whitebark pine are turned upward,

meaning that the seeds do not fall to the ground when the cones open Birds called Clark’s nutcrackers eat the seeds of white bark pine trees

and store them in the ground Clark’s nutcrackers are critical seed dispersers for the trees

Cleaning symbiosis - symbiotic relationship in which crustaceans, fish, and birds act as cleaners for a variety of vertebrate clients Large fish in coral reefs line up at cleaning stations and wait their turn to

be cleaned by small fish that even enter the mouths of the large fish38-19

Figure 38.6A Clark’s nutcrackers store and disperse the seeds of whitebark pine trees

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Figure 38.6B Cleaning symbiosis occurs when small fish clean large fish

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A Community Develops and Changes Over Time

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APPLYING THE CONCEPTS—HOW SCIENCE PROGRESSES 38.7 The study of island

biogeography pertains to biodiversity

Would you expect larger coral reefs to have a greater number of species (species richness) than smaller reefs? Model of island biogeography - explains and

predicts the effects of distance from mainland and size of an island on community diversity

Biodiversity Model of island biogeography suggests that the larger

the conserved area, the better the chance of preserving more species If the environment has patches, it has a greater number of

habitats—and thus greater diversity One way to introduce patchiness is through stratification

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Figure 38.7A Island distance from mainland and rate of species migration

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Figure 38.7B Island size and rate of species extinction

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Figure 38.7C Equilibrium model of species richness

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38.8 During ecological succession, community composition

and diversity change When comparing various communities, composition and

diversity are examined Diversity of a community goes beyond composition because it

includes not only a listing of species but also the abundance of each species

Composition and diversity of a community can change over time due to various disturbances Disturbances can range in severity from a storm blowing down a

patch of trees, to a volcanic eruption Ecological Succession - series of species

replacements in a community following a disturbance Primary succession occurs in areas where no soil is present,

such as following a volcanic eruption or a glacial retreat Secondary succession begins in areas where soil is present

First species to begin secondary succession are called pioneer species; usually plants that tend to invade disturbed areas

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Figure 38.8A Secondary succession in a conifer plantation

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Models of Succession Climax-pattern model of succession, which suggests that

succession in a particular area will always lead to the same type of community Climax community

Climate, in particular, determines whether a desert, a type of grassland, or a particular type of forest results

Soil conditions might also affect the results Facilitation model - each stage facilitates the invasion and

replacement by organisms of the next stage Inhibition model predicts that colonists hold onto their space and

inhibit the growth of other plants until the colonists die Tolerance model predicts that different types of plants can colonize

an area at the same time Most outstanding characteristic of natural communities is their

dynamic nature Each successional stage has its own mix of plants and animals, and

if a sample of all stages is present, community diversity is greatest

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Figure 38.8B Secondary succession in a cultivated cornfield

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An Ecosystem Is a Community Interacting with the

Physical Environment

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38.9 Ecosystems have biotic and abiotic components

Abiotic components include resources, such as sunlight and inorganic nutrients, and conditions, such as type of soil, water availability

Biotic components of an ecosystem are influenced by the abiotic components, as when the force of the wind has affected the growth of a tree

Biotic Components of an Ecosystem Autotrophs require only inorganic nutrients and an

energy source to produce organic nutrients for their own use and for all the other members of a community Called producers because they produce food Photoautotrophs, also called photosynthetic organisms,

produce most of the organic nutrients for the biosphere 38-32

Heterotrophs

Heterotrophs need a preformed source of organic nutrients Called consumers because they consume food Herbivores are animals that graze directly on plants

or algae Carnivores feed on other animals Scavengers feed on the dead remains of animals

and plants that have recently begun to decompose Detritus refers to organic remains in the water and

soil that are in the final stages of decomposition Bacteria and fungi, including mushrooms, are the

decomposers that use their digestive secretions to chemically break down dead organic matter

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Figure 38.9 Niche specifications of plants compared to animals

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38.10 Energy flow and chemical cycling characterize ecosystems

Energy flow begins when producers absorb solar energy, and chemical cycling begins when producers take in inorganic nutrients from the physical environment Producers make organic nutrients (food) directly for themselves and

indirectly for the other populations of the ecosystem Energy flows through an ecosystem via photosynthesis because, as

organic nutrients pass from one component of the ecosystem to another a portion of those nutrients is used as an energy source

Eventually energy dissipates into the environment as heat First law of thermodynamics states that energy cannot be created

(or destroyed) Why ecosystems are dependent on a continual outside source of

energy Second law states that, with every transformation, some energy is

degraded into a less available form, such as heat For example, because plants carry on cellular respiration, only 55%of

the original energy absorbed by plants is available to an ecosystem38-35

Figure 38.10A Energy flow and chemical cycling in an ecosystem

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Figure 38.10B Energy balances for an herbivore

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38.11 Energy flow involves food webs

The various interconnecting paths of energy flow may be represented by a food web, a diagram that describes trophic (feeding) relationships

Trophic Levels Organisms are linked to one another in a straight line,

according to feeding relationships, or who eats whom Diagrams that show a single path of energy flow in an

ecosystem are called food chains

Trophic level is composed of organisms that occupy the same position within a food web or chain

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Figure 38.11 Grazing food web (top) and detrital food web (bottom)

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38.12 Ecological pyramids are based on trophic levels

Ecological pyramid - graphic representation of the number of organisms, the biomass, or the relative energy content of the various trophic levels in an ecosystem Ecological pyramids are helpful for explaining energy

loss in an ecosystem, but they oversimplify energy flow Most likely, a pyramid based on the number of organisms in

each trophic level wouldn’t work Example: Each tree would contain numerous caterpillars, so

there would be more herbivores than autotrophs

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Figure 38.12 Ecological pyramid based on the biomass content of bog populations could also be used to represent an energy pyramid

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38.13 Chemical cycling includes reservoirs, exchange

pools, and the biotic community Biogeochemical cycles - pathways by which chemicals

circulate through ecosystems involve both living (biotic) and nonliving (geologic) components Phosphorus cycle is a sedimentary cycle; the chemical is

absorbed from the soil by plant roots, passed to heterotrophs, and eventually returned to the soil by decomposers

Carbon and nitrogen cycles are gaseous, meaning that the chemical returns to and is withdrawn from the atmosphere as a gas

Reservoir is a source normally unavailable to producers, such as the carbon present in calcium carbonate shells on ocean bottoms

Exchange pool is a source from which organisms do generally take chemicals, such as the atmosphere or soil Chemicals move along food chains in a biotic community,

perhaps never entering an exchange pool 38-42

Figure 38.13 Model for chemical cycling

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38.14 The phosphorus cycle is sedimentary

Phosphorus, trapped in oceanic sediments, moves onto land after a geologic upheaval On land, the very slow weathering of rocks places

phosphate ions in the soil Some of these become available to plants, which use

phosphate to make ATP, and nucleotides that become DNA and RNA

Human Activities and the Phosphorus Cycle Human beings boost the supply of phosphate by

mining phosphate ores for producing fertilizer and detergents. This results in eutrophication (overenrichment) of waterways.

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Figure 38.14 The phosphorus cycle

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38.15 The nitrogen cycle is gaseous Ammonium (NH4

+) Formation and Use Nitrogen fixation occurs when nitrogen gas (N2) is converted to ammonium

(NH4+), that plants can use

Cyanobacteria in aquatic ecosystems and some free-living bacteria in soil are able to fix atmospheric nitrogen

Nitrate (NO3−) Formation and Use

Plants also use nitrates (NO3−) as source of nitrogen

Production of nitrates during the nitrogen cycle is called nitrification Formation of Nitrogen Gas

Denitrification is the conversion of nitrate back to nitrogen gas, which then enters the atmosphere

Denitrifying bacteria living in the anaerobic mud of lakes, bogs, and estuaries carry out this process as a part of their own metabolism

Human Activities and the Nitrogen Cycle Humans significantly increase transfer rates in nitrogen cycle by producing

fertilizers from N2

Fertilizer, which also contains phosphate, runs off into lakes and rivers and results in an overgrowth of algae and rooted aquatic plants

Acid deposition occurs because nitrogen oxides (NOx) and sulfur dioxide (SO2) enter the atmosphere from the burning of fossil fuels

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Figure 38.15 The nitrogen cycle

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38.16 The carbon cycle is gaseous In the carbon cycle, organisms in both terrestrial and aquatic

ecosystems exchange carbon dioxide (CO2) with the atmosphere CO2 in the atmosphere is the exchange pool for the carbon cycle

Reservoirs Hold Carbon Living and dead organisms contain organic carbon and serve as one of

the reservoirs for the carbon cycle Some 300 MYA, plant and animal remains were transformed into coal,

oil, and natural gas, the materials we call fossil fuels Human Activities and the Carbon Cycle

More CO2 is being deposited in the atmosphere than is being removed due to burning of fossil fuels and destruction of forests to make way for farmland

Greenhouse gas - allows solar radiation to pass through but hinder the escape of heat back into space, called the greenhouse effect

The greenhouse gases are contributing significantly to an overall rise in the Earth’s ambient temperature, a trend called global warming

Global climate has already warmed 0.6°C since Industrial Revolution Earth’s temperature may rise 1.5–4.5°C by 2100 if emission rates continue

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Connecting the Concepts:Chapter 38

Community ecology is concerned with how populations of different species interact with each other Number of individuals in each population is influenced by

negative interactions such as competition, predation, and parasitism

Positive interactions such as mutualism are also fairly common in nature (especially for plants) and are presumed to increase or maintain population sizes

Important recent discoveries about communities is that they are highly dynamic

Some ecologists research movement of energy and nutrients through communities Physical environment has a large influence on energy flow and

chemical cycling38-49

Our study of communities must include the abiotic environment. Human activities also influence the operation of ecosystems Example: Burning fossil fuels and trees is adding carbon

dioxide to the atmosphere Carbon dioxide and other greenhouse gases

allow sun’s rays to pass through, but they absorb and reradiate heat back to Earth, which is leading to global warming

Transfer rates in both the phosphorus and nitrogen cycles are affected when we produce fertilizers and detergents

Nitrogen and phosphorus runoff causes eutrophication in aquatic ecosystems The resulting pollution brought on by human activities affects

the functioning of the biosphere, the largest ecosystem of all

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