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Ch 53 Introduction, community ecology
• A biological community consists of interacting species, usually
living within a defined area.
• Biologists want to know how communities work, and how to
manage them in a way that will preserve species and create an
environment that people want to live in.
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Species Interactions
• Because the species in a community interact almost constantly, the
fate of a particular population may be tightly linked to the other
species that share its habitat.
• Biologists analyze interactions among species by considering the
effects on the fitness (survival, reproduction)
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Species Interactions
• There are four general types of interactions among species in a
community:
1. Competition occurs when individuals use the same
resources—resulting in lower fitness for both (/).
2. Consumption occurs when one organism eats or absorbs
nutrients from another, increasing the consumer’s fitness but
decreasing the victim’s fitness (+/).
3. Mutualism occurs when two species interact in a way that
confers fitness benefits to both (+/+).
4. Commensalism occurs when one species benefits but the
other species is unaffected (+/0).
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Three Themes
• As you analyze each type of species interaction, watch for three key
themes:
1. Species interactions may affect the distribution and
abundance of a particular species.
2. Species act as agents of natural selection when they interact.
In biology, a coevolutionary arms race occurs between
predators and prey, between parasites and hosts, and between
other types of interacting species.
3. The outcome of interactions among species is dynamic
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Competition
• Competition is a –/– interaction that lowers the fitness of the
individuals involved. When competitors use resources, those
resources are not available to help individuals survive better and
produce more offspring.
• Intraspecific competition occurs between members of the same
species.
– Because intraspecific competition for resources intensifies as a
population’s density increases, it is a major cause of densitydependent growth.
• Interspecific competition occurs when members of different
species use the same limiting resources.
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Using the Niche Concept to Analyze Competition
• Early work on interspecific competition focused on the concept of
the niche—the range of resources that the species is able to use or
the range of conditions it can tolerate.
• Interspecific competition occurs when the niches of two species
overlap.
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When One Species Is a Better Competitor
• The competitive exclusion principle, formulated by G. F. Gause,
states that it is not possible for species within the same niche to
coexist.
• The hypothesis was inspired by a series of experiments Gause did
with similar species of the unicellular pond-dweller Paramecium.
– Grown in separate cultures, both species exhibited logistic
growth.
– When the two species grew in the same culture together, only
one species exhibited logistic growth; the other species was
driven to extinction.
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When One Species Is a Better Competitor
• Asymmetric competition occurs when one species suffers a much
greater fitness decline than the other.
• In symmetric competition, each species experiences a roughly
equal decrease in fitness.
• If asymmetric competition occurs and the two species have
completely overlapping niches, the stronger competitor is likely to
drive the weaker competitor to extinction.
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When One Species Is a Better Competitor
• Gause’s experiments illuminated an important distinction:
1. A species’ fundamental niche is the resources it uses or
conditions it tolerates in the absence of competitors.
2. A species’ realized niche is the resources it uses or
conditions it tolerates when competition occurs.
• If asymmetric competition occurs and the niches of the two species
do not overlap completely, the weaker competitor will move from
its fundamental niche to a realized niche, ceding some resources to
the stronger competitor.
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Experimental Studies of Competition
• Joseph Connell performed a series of experiments to test the
competitive exclusion principle.
• Connell used a common experimental strategy in competition
studies—removing one of the competitors and observing the
response by the remaining species.
• Experimental evidence supports competitive exclusion of
Chthamalus barnacles from the lower intertidal zone by Balanus
barnacles.
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Fitness Trade-Offs in Competition
• The ability to compete for a particular resource is only one aspect
of an organism’s niche.
• If individuals are extremely good at competing for a particular
resource, they are probably less good at enduring drought
conditions, warding off disease, or preventing predation―there is a
fitness trade-off.
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Mechanisms of Coexistence: Niche Differentiation
• Because competition is a –/– interaction, there is strong natural
selection on both species to avoid it.
• The predicted eventual outcome is an evolutionary change in traits
that reduces the amount of niche overlap and the amount of
competition.
• This change in resource use is called niche differentiation or
resource partitioning.
• The change in species’ traits is called character displacement.
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Mechanisms of Coexistence: Niche Differentiation
• Peter and Rosemary Grant recently documented character
displacement in Galápagos finches.
• After a severe drought in 1977, selection favored larger beak size in
the medium ground finch, Geospiza fortis.
– Only those individuals with larger beaks were able to crack
open the fruits of their major food source, Tribulus cistoides.
• A second severe drought occurred in 2003; by this time the large
ground finch, Geospiza magnirostris, had become established on
the island.
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Mechanisms of Coexistence: Niche Differentiation
• The Grants’ measurements revealed that this time, only the
smallest-beaked G. fortis individuals survived.
• Data on feeding behavior indicated that G. magnirostris were
outcompeting G. fortis for Tribulus cistoides; only G. fortis that
could eat extremely small seeds efficiently could survive.
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Competition and Conservation
• One of the goals of conservation biology is to keep biological
communities intact.
• One of the major threats to communities is invasive species.
• Recent experiments have shown that communities that contain a
large number of different species are more resistant to invasion than
communities with a smaller number of species.
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Consumption
• Consumption is a +/– interaction that occurs when one organism
eats another.
• There are three major types of consumption:
1. Herbivory is the consumption of plant tissues by herbivores.
2. Parasitism is the consumption of small amounts of tissues
from another organism, or host, by a parasite.
3. Predation is the killing and consumption of most or all of
another individual (the prey) by a predator.
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Constitutive Defenses
• Constitutive or standing defenses are defenses that are always present and
include:
– Avoidance (hiding, with or without camouflage, or running, flying or
swimming away).
– Poison (many plants lace their tissues with compounds that are toxic to
consumers).
– Schooling and flocking behaviors that confuse predators.
– Fighting back, with the use of weaponry or toxins.
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Constitutive Defenses
• Some of the best-studied constitutive defenses involve mimicry—
the close resemblance of one species to another.
• There are two forms of mimicry:
1. Müllerian mimicry is the resemblance of two harmful prey
species.
2. Batesian mimicry is the resemblance of an innocuous prey
species to a dangerous prey species.
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Inducible Defenses
• Although constitutive defenses can be extremely effective, they are
expensive in terms of the energy and resources that must be
devoted to producing and maintaining them.
• Many prey species have inducible defenses—defensive traits
produced only in response to the presence of a predator.
• Inducible defenses are efficient energetically, but they are slow—it
takes time to produce them.
• For example, mussels have thicker shells and attach more strongly
to a substrate only in the presence of crabs.
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Why Don’t Herbivores Eat Everything?
• Biologists recently conducted a meta-analysis—they compiled the
results of more than 100 studies—and raised the question of why
herbivores don’t eat more of the available plant food.
• Biologists routinely consider two hypotheses to help answer the
question of why herbivores do not eat more of the food available:
1. The top-down control hypothesis suggests that predation or
disease limits herbivores.
2. The bottom-up limitation hypothesis suggests that plants
provide poor nutrition or are well-defended against herbivory.
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Why Don’t Herbivores Eat Everything?
• Cottonwood trees and two of their herbivores,
beavers and leaf beetles, provide an example of
both top-down and bottom-up controls on
herbivory.
• Top-down control, nitrogen limitation, and effective defense are all
important factors in limiting the impact of herbivory.
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Adaptation and Arms Races
• When consumers and prey interact over time, a coevolutionary
arms race begins.
– Consumers evolve traits that increase their efficiency.
– In response, prey evolve traits that make them unpalatable or
elusive.
– This leads to selection on consumers for traits that counter the
prey adaptation, and so on.
• An example is the interaction between the parasite Plasmodium (the
consumer) and their host humans (the prey).
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Adaptation and Arms Races
• Plasmodium are unicellular protists that cause malaria, which kills
at least a million people a year.
• Recent data suggest that humans and Plasmodium are locked in a
coevolutionary arms race.
– In West Africa, the HLA-B53 allele confers protection against
malaria by displaying a signal that induces an immune
response.
– Individuals with at least one HLA-B53 allele are better able
to fight malarial infections.
– However, some Plasmodium populations appear to have
evolved resistance to these defenses.
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Can Parasites Manipulate Their Hosts?
• In some instances, parasites do manipulate their hosts.
• For example, nematodes (roundworms) parasitize a species of treedwelling ants and lay eggs in the ant’s posterior-most body region,
causing it to appear red instead of the normal black color.
• Infected ants also hold the region up in a “flagging” posture,
making them look like berries; as a result birds are more likely to
feed on infected ants than uninfected ants.
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Can Parasites Manipulate Their Hosts?
• The nematodes can only complete their life cycle inside the birds,
before being shed in bird feces that are subsequently eaten by ants.
• Biologists suggest that these nematodes not only change the
appearance of the ants, they also manipulate their behavior, greatly
increasing the likelihood that the parasite will be transmitted to a
new host.
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Using Consumers as Biocontrol Agents
• Research on the dynamics of predator-prey interactions has given
biologists the ability to control pests by introducing predators or
parasites.
• In agriculture and forestry, the use of predators and parasites as
biocontrol agents is a key part of integrated pest management:
strategies to maximize crop and forest productivity while using a
minimum of insecticides or other types of potentially harmful
compounds.
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Mutualisms
• Mutualisms are +/+ interactions that
involve a wide variety of organisms and
rewards. Examples of mutualisms can be
found between:
– Flowering plants and their
pollinators.
– Mycorrhizal fungi and plant roots.
– Bacteria that fix nitrogen and certain
species of plants.
– Rancher ants and aphids.
– Farmer ants and fungi.
– Crematogaster ants and acacia trees.
– Cleaner shrimp and fish.
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The Role of Natural Selection in Mutualism
• Even though mutualisms benefit both species, the interaction does
not involve individuals from different species being altruistic.
• The benefits received in a mutualism are a by-product of each
individual pursuing its own self-interest by maximizing its ability to
survive and reproduce.
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Mutualisms Are Dynamic
• An experiment explored whether the relationship between ants and
treehoppers is mutually beneficial.
• The study found that the ants benefited by receiving food from the
treehoppers, and the treehoppers benefited because the ants kept
their predator, the jumping spiders, away.
– However, when jumping spider populations were low, the
treehoppers did not benefit.
• Because the costs and benefits of species interactions are fluid, an
interaction between the same two species may vary from parasitism
to mutualism to competition.
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Community Structure
• Research on species interactions usually focuses on just two species
at a time, but biological communities contain many thousands of
species.
• To understand how communities work, biologists explore how
combinations of many species interact.
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How Predictable Are Communities?
• Frederick Clements hypothesized that biological communities are stable,
integrated, and orderly entities with a highly predictable composition.
• Clements argued that communities develop by passing through a series of
predictable stages dictated by extensive interactions among species, and that this
development culminates in a stable final stage called a climax community.
• Henry Gleason, in contrast, contended that the community found in a particular
area is neither stable nor predictable.
• According to Gleason, it is largely a matter of chance whether a similar
community develops in the same area after a disturbance occurs.
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Experimental Tests
• A study of planktonic communities in experimental ponds showed
that identical communities do not develop in identical habitats.
Each pond had a unique species assemblage.
• The overall message of research on community structure suggests
that Clements’ position was too extreme; Gleason’s view is closer
to accurate.
• Although both biotic interactions and climate are important in
determining which species exist at a certain site, chance and history
also play a large role.
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How Do Keystone Species Structure Communities?
• Even though species are not predictable assemblages, the structure
of a community can change dramatically if a single species of
predator or herbivore is removed from or added to a community.
• A keystone species is a species that has a much greater impact on
the surrounding species than its abundance would suggest.
• For example, the sea star Pisaster is a keystone species in some
intertidal areas. When Pisaster was removed from experimental
areas, the number of species present and the complexity of the
habitat changed radically.
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Disturbance and Change in Ecological Communities
• Community composition and structure may change radically in
response to changes in abiotic and biotic conditions.
• A disturbance is any event that removes some individuals or
biomass from a community.
• The important feature of a disturbance is that it alters some aspect
of resource availability.
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Disturbance and Change in Ecological Communities
• The impact of disturbance is a function of three factors:
1. Type of disturbance.
2. Frequency of disturbance.
3. Severity of disturbance
• Most communities experience a characteristic type of disturbance,
and in most cases, disturbances occur with a predictable frequency
and severity.
– This is called a community's disturbance regime.
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Determining a Community’s Disturbance Regime
• Ecologists use two approaches to determine the pattern of
disturbance in a community:
1. Inference of long-term patterns from data obtained in shortterm analysis.
2. Reconstruction of the history of a particular site.
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The Importance of Understanding Disturbance Regimes
• Biologists determined the history of disturbance in a fire-prone
community by studying tree rings.
• The results of this study established that fires are quite frequent in
the community examined.
• Biologists are now better able to manage these forests by allowing,
monitoring, and controlling burns in them.
• To maintain communities in good condition, biologists have to
ensure that the normal disturbance regime occurs. Otherwise,
community composition changes dramatically.
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Succession
• Succession is the recovery, the development of communities, that
follows a severe disturbance.
• Primary succession occurs when a disturbance removes the soil
and its organisms, as well as organisms that live above the surface.
• Secondary succession occurs when a disturbance removes some or
all of the organisms from an area but leaves the soil intact.
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Succession
• Early successional communities are
dominated by species that are short lived
and small in stature, and that disperse their
seeds over long distances.
• Late successional communities are
dominated by species that tend to be long
lived, large, and good competitors for
resources such as light and nutrients.
• The specific sequence of species that
appears over time is called the successional
pathway.
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Succession
• Three factors determine the pattern and rate of species replacement
during succession at a particular time and place:
1. The particular traits of the species involved.
2. How the species interact.
3. Historical and environmental circumstances, such as the size
of the area involved and weather conditions.
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The Role of Species Traits
• Dispersal capability and the ability to withstand harsh conditions
are particularly important early in succession.
• Pioneering species, the first organisms to arrive at a newly
disturbed site, tend to be “weedy”; weeds are plants adapted for
growth in disturbed soils.
• Early successional species devote most of their energy to
reproduction and little to competitive ability.
• These species have good dispersal ability, being able to tolerate
severe abiotic conditions, and high reproductive rates.
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The Role of Species Interactions
• Once colonization has begun, succession depends more on how
species interact with each other.
• During succession, existing species can have one of three effects on
subsequent species:
1. Facilitation occurs when early-arriving species make
conditions more favorable for the arrival of certain later
species.
2. Tolerance happens when existing species do not affect the
probability that subsequent species will become established.
3. Inhibition occurs when the presence of one species inhibits
the establishment of another.
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A Case History: Glacier Bay, Alaska
• An extraordinarily rapid and extensive glacial recession is
occurring at Glacier Bay, and it has thus become an important site
for studying succession.
• Originally researchers found one successional pathway, but more
recent research has suggested that three successional pathways have
occurred in this area.
• Species traits and species interactions tend to make succession
predictable, whereas history and chance events contribute a degree
of unpredictability.
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Species Richness in Ecological Communities
• Species richness is the number of species present in a given
community.
• Species diversity is a weighted measure that incorporates a
species’ relative abundance, as well as its presence or absence.
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Predicting Species Richness
• The number of species is usually positively correlated with habitat
size. However, islands in the ocean have smaller numbers of
species than do areas of the same size on continents.
• The number of species present on an island is a product of just two
events: immigration and extinction.
• Robert MacArthur and Edward O. Wilson contended that the rates
of both of these processes should vary with the number of species
present on an island.
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Predicting Species Richness
• Immigration rates should decline as the number of species on the
island increases because:
– Individuals that arrive are more likely to represent a species
that is already present.
– Competition should prevent new species from becoming
established when many species are already present on an
island.
• Extinction rates should increase as species richness increases,
because niche overlap and competition for resources will be more
intense.
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The Role of Island Size and Isolation
• MacArthur and Wilson formulated the model called the theory of
island biogeography.
• Their theory makes two predictions—species richness should be
higher on:
1. Larger islands than smaller islands.
2. Nearshore islands versus remote islands.
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Applying the Theory
• The theory of island biogeography is important because:
1. It is relevant to a wide variety of island-like habitats such as
alpine meadows, lakes and ponds, caves, and habitats isolated
by human development.
2. It is relevant to species that have metapopulation structure.
3. It made specific predictions that could be tested.
4. It can help inform decisions about the design of natural
preserves.
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Global Patterns in Species Richness
• Biologists have long understood that large habitat areas tend to be
species rich, and the theory of island biogeography has been
successful in framing thinking about how species richness should
vary among island-like habitats.
• Researchers have had a much more difficult time explaining what
may be the most striking pattern in species richness:
• In the mid-1800s, biologists recognized that communities in the
tropics have more species than communities in temperate or
subarctic environments.
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The Latitudinal Gradient
Data compiled in the intervening years have confirmed the
existence of a strong latitudinal gradient in species diversity—for
communities as a whole as well as for many taxonomic groups.
•
To explain this pattern, biologists have had to consider two
principles:
1. The causal mechanism must be abiotic.
2. The species diversity of a particular area is the sum of four
processes: speciation, extinction, immigration, and
emigration.
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The Latitudinal Gradient
• Over 30 hypotheses have been suggested to explain the latitudinal gradient.
Among them:
1.
The high-productivity hypothesis proposes that high productivity
promotes high diversity.
2.
The energy hypothesis contends that high temperature increases
productivity and the likelihood that organisms can tolerate the physical
conditions in a region.
3.
The area and age hypothesis argues that the tropical regions have had
more time and space for speciation than other regions.
4.
The intermediate disturbance hypothesis states that regions with a
moderate type, frequency, and severity of disturbance should have high
species richness and diversity.
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