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Chapter 5
Community Processes:
Species Interactions &
Succession
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Outline 5
1. The Ecological Niche
roles, fundamental & realized niche, generalists vs.
specialists
2. Some General Types of Species
native, non–native, indicator, & keystone species
3. Types of Species Interactions
4. Competition & Predation
principle of competitive exclusion, predator–prey
relations
5. Symbiotic Species Interactions
parasitism, mutualism, & commensalism
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Outline 5
6. Characteristics of Populations
size, density, dispersion, age structure, dynamics
7. Population Dynamics & Carrying Capacity
growth limits, exponential vs. logistic growth, carrying capacity
8. Reproductive Strategies & Survival
r– vs. K–strategists; survivorship curves
9. Succession
primary & secondary succession
10. Island Biogeography
11. Stability & Sustainability
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•
•
•
•
•
•
Realized niche
Resources
Specialist
Conditions
Generalist
Fundamental
niche
• all the physical attributes
of the environment
• parts of the environment
used by an organism.
• the full niche of a spp.
that could be used.
• the portion of the niche
that is actually used by a
spp.
• Species with broad niche
• Species with narrow
niche
1. The Ecological Niche
• niche:
• conditions
• resources:
• habitat,
• the many physical attributes of the
environment, though not consumed, that
influence biological processes &
population growth, e.g., temperature,
salinity, acidity;
• the role that an organism plays in an
ecosystem, defined by the range of
conditions & resources within which an
organism can live;
• the actual place an organism lives.
• substances or parts of the environment
used by an organism & consumed or
otherwise made unavailable to other
organisms , e.g., food, water, & nesting
sites for animals; water, nutrients, &
solar radiation for plants;
Fundamental vs. Realized Niche
• fundamental niche: the full range of conditions &
resources that an organism could theoretically use
in the absence of competition with other species.
• realized niche: the portion of the fundamental
niche that an organism actually occupies; actual
range of conditions & resources that an organism
uses.
• niche overlap between species leads to competition;
• competition causes organisms to not be able to
occupy the full fundamental niche;
Generalists vs. Specialists
• generalists have broad
niches,
• examples of generalists:
cockroaches, coyotes,
dandelions, humans;
• generalists may have
advantage when
environmental conditions
change (e.g., weedy species
such as dandelions in
disturbed habitat);
• specialists have narrow
niches:
• examples of specialists:
spotted owls, which require
old–growth forests in the
Pacific Northwest; giant
pandas, which eat primarily
bamboo in bamboo forests of
China;
• whereas specialists may have
advantage when
environmental conditions are
more constant (e.g., many
species of tropical rain
forest).
Types of Species – Indicator Species
• indicator species: species that serve as early
warnings that a community or ecosystem is
being damaged:
• example: decline of migratory songbirds in
North America indicates loss & fragmentation
of habitat in mesoAmerica & South America;
• example: presence of trout in mountain streams
is an indicator of good water quality;
• example: presence of spotted owls is indicator
of healthy old–growth forest.
Types of Species – Keystone Species
• keystone species: species that play a critical role in an
ecosystem:
• example: sea otters are keystone species because they
prevent sea urchins from depleting kelp beds;
• example: dung beetles are keystone species because
they remove, bury, & recycle animal waste;
• example: beavers are keystone species because they
build dams & create habitat for a diverse community of
species (bluegill fish, muskrats, herons, ducks…).
• "The loss of a keystone species is like a drill
accidentally striking a power line. It causes lights to go
out all over."
–– E.O. Wilson
3. Types of Species Interactions
• major types of biotic interactions:
• interspecific competition: when two or more
species use the same limited resource (food, space,
etc.) and adversely affect each other
• example: fire ants and native species of ants in
North America; fire ants sharply are better
competitors & sharply reduce populations of up to
90% of native species.
• predation: members of one species (predator) feed
on another species (prey);
• example: lion feeding on gazelle.
Types of Species Interactions
• major types of biotic interactions (continued):
• symbiosis: a long–lasting relationship in which
species live together in intimate association:
• parasitism: one organism (parasite) lives on part of
another organism (host), e.g., flea living on a dog
• mutualism: two species interacting in a way that
benefits both, e.g., lichens consist of algae & fungi
that benefit each other (in this example can't live
apart);
• commensalism: one organism benefits from another,
but neither helps nor harm that other organism, e.g.,
epiphyte growing on a tree (epiphyte benefits & tree
not effected, unless there are many epiphytes).
4. Competition & Predation
• Interspecific competion results because of
niche overlap = overlap in requirements for
limited resources.
• Types of Competition:
• interference competition: one species limits
another species' access to a resource; e.g.,
hummingbirds defending feeding territories.
• exploitation competition: competing species
both have access to a limited resource, but
one exploits the resource more quickly or
efficiently.
Principle of Competitive Exclusion
G.P. Gausse, in a classical
experiment (1934), showed
that two species with
identical niches can not
coexist indefinitely.
This is called the principle of
competitive exclusion.
Note that when grown
together, Paramecium
aurelia outcompetes
Paramecium caudatum.
Resource Partitioning
Species with similar resource
requirements can coexist
because they use limited
resources at different times,
in different ways, or in
different places.
For example, specialized
feeding niches of various
birds of coastal wetland
enable coexistence of many
species.
Resource Partitioning
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Resource Partitioning
Five species of insect–eating warblers are able to
coexist in spruce forest of Maine. Each species
minimizes competition with others for food by spending
at least half its feeding time in a distinct portion of
spruce trees (shaded areas); each also consumes
somewhat different insect species.
Fig. 9–5
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Resource Partitioning
Fig. 9–5 (continued)
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Character Displacement
Over many years coexisting
species with similar
niches tend to evolve
physical & behavioral
adaptations to minimize
competition.
For example on islands
where they co–ocurr,
species of Darwin's finch
have evolved different
bill sizes & eat different
size prey.
Predators & Prey
What features characterize them?
Predators tend to evolve characteristics for
efficient capture of prey (keen eyesight,
speed, etc.).
Prey tend to evolve chacteristics to avoid
being eaten (camoflauge, chemical
defenses, behaviors that startle predators,
etc.).
5. Symbiotic Species Interactions
• Parasitism can be viewed as a special type of predation
wherein the parasite:
• 1) is usually smaller than the prey,
• 2) remains closely associated with the prey over time, &
• 3) rarely kills its host.
• Endoparasites
• live inside their host, e.g., tapeworm living in the gut of a
mammal; plasmodium living inside a vertebrate & causing
malaria.
• Ectoparasites
• live outside their host, e.g., mosquito feeding on the blood
of mammal; lamprey attaching to outside of a host fish (see
Fig. 9–13).
Mutualism
• Mutualism involves a relationship in which two
interacting species benefit.
• obligatory mutualism results when two organisms can
not live without each other;
• example: in lichens an algae provides photosynthesis & a
fungi provides a home for the algae;
• example: Rhizobium bacteria, in legume plant root
nodules, fix nitrogen & legume provides carbohydrates &
home;
• example: termites have gut organism that can digest
cellulose.
• In other mutualisms the organisms can live apart, but
there is strong mutual benefit in the relationship;
• example: flowering plants & their pollinators, plant gets
pollinated, pollinator gets nectar or pollen to eat;
Mutualism
There are many more classic examples of mutualism.
example: oxpeckers, a type of bird, feeds on the parasitic tics of
various large mammals in Africa, such as the black rhinoceros
(see Fig. 9–14);
example: mycorrhizal fungi live in the roots of various plants; the
fungus gets carbohydrates & the plant gets better absorption of
nutrients by the fungal mat that extends beyond the roots (see
Fig. 9–15);
example: the clownfish in the coral reefs of Australia lives among
the tentacles of sea anemones; the clownfish gains protection
from the stinging tentacles & food scraps when the anemone
feeds; the anemone gains protection from various fish that feed
on sea anemones (see Fig. 9–16);
example: certain species of stinging ants live in acacias; the ants
get a home and food in the form of nectar; the acacias get
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protection from various herbivores.
Commensalism
Commensalism involves a symbiotic relationship in
which one species beneifits while another is neither
helped not harmed to a significant degree.
example: redwood sorrel, a small herbaceous plant,
benefits from growing in the shade of tall redwoods, but the
redwoods are not affected;
example: epiphytes (such as orchids & bromeliads) that
grow on trunk & branches of trees in the tropical rain forest
gain a favorable place to live; whereas, at least when
epiphytes are not overly abundant, the tree is not affected
(see Fig. 9–17).
Note that if epiphytes become sufficiently abundant to block light, the tree
can be negatively affected, and this becomes an example of competition.
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1. Characteristics of Populations
• population
dynamics
• population size
• population density
• Dispersion
• age structure
• . is the number of individuals in a
population at a given time;
• is the number of individuals per unit
area in terrestrial ecosystems or per
unit volume in aquatic ecosystems;
• is the proportion of individuals in
each age group (e.g.,
prereproductive, reproductive, &
postreproductive) of a population.
• is the spatial patterning individuals;
• Changes in population size, density,
dispersion, & age distribution are
known as
Characteristics of Populations
What is the difference between clumped, uniform &
random dispersion?
Fig. 10–2
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2. Population Dynamics & Carrying Capacity
• Population size is governed by births, deaths,
immigration, and emigration:
• [Population Change] =
• [Births + Immigration] – [Deaths + Emigration]
• If the number of individuals added by births &
immigration are balanced by those lost by deaths &
emigration then there is zero population growth;
• populations vary in their capacity for growth, also
known as biotic potential;
• the intrinsic rate of growth (r) is the rate at which a
population will grow if it had unlimited resources.
Population Dynamics
What are
the most
important
factors that
tend to
increase or
decrease
population
size?
Fig. 10–3
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Carrying Capacity
• There are always limits to population growth in
nature.
• carrying capacity (K) is the number of
individuals that can be sustained in a given space;
• the concept of carrying capacity is of central
importance in environmental science;
• if the carrying capacity for an organism is
exceeded, resources are depleted, environmental
degradation results, & the population declines.
Exponential vs. Logistic Growth
• What’s the difference
between Exponential
& Logistic Growth?
• Exponential growth
occurs when resources
are not limiting.
• Logistic growth occurs
when resources
become more and
more limiting as
population size
increases.
Exponential Population Growth
• Exponential growth occurs
when resources are not
limiting.
• during exponential growth
population size increases
faster & faster with time;
• currently the human
population is undergoing
exponential growth;
• exponential growth can not
occur forever because
eventually some factor
limits population growth.
Logistic Population Growth
• Logistic population growth
occurs when the population
growth rate decreases as
the population size
increases.
• note that when the
population is small the
logistic population growth
curve looks like
exponential growth;
• over time, the population
size approaches a carrying
capacity (K).
Exceeding the Carrying Capacity
During the mid–1800s sheep populations exceeded the carrying capacity
of the island of Tasmania. This "overshoot" was followed by a
"population crash". Numbers then stabilized, with oscillation about
the carrying capacity.
Exceeding the Carrying Capacity
Reindeer introduced to a small island off of Alaska in the
early 1900s exceeded the carrying capacity, with an
"overshoot" followed by a "population crash" in which the
population was totally decimated by the mid–1900s.
Population Curves in Nature
Natural populations display a broad diversity of population curves.
Stable populations are relatively constant over time.
Cyclic curves are often associated with seasons or fluctuating resource
availability.
Irruptive curves are characteristic of species that only have high
numbers for only brief periods of times (e.g., seven–year cicada).
Population Curves in Nature
Population cycles for the snowshoe hare & Canadian
lynx are believed to result because the hares
periodically deplete their food, leading to first a crash
of the hare population & then a crash of the lynx
population.
3. Reproductive Strategies & Survival
• Organisms can be divided into two categories of
"strategies" for reproduction & survival:
• r–strategist species,
• tend to live in recently disturbed (early
successional) environments where resources are not
limiting; such species tend to have high intrinsic
rates of growth (high r);
• K–strategist species
• tend to live in environments where resources are
limiting (later succession) & tend to have lower
intrinsic rates of growth and characteristics that
enable them to live near their carry capacity
(population size near K).
r–Strategist Species
Characteristics of r–
strategists, including
production of many
small & unprotected
young, enable these
species to live in places
where resources are
temporarily abundant.
These species are typically
"weedy" or
opportunistic.
K–Strategist Species
Characteristics of
K–strategists,
including
production of few
large & well
cared for young,
enable these
species to live in
places where
resources are
limited.
These species are
typically good
competitors.
Survivorship Curves
• Three kinds of
curves:
• late loss (usually
K–strategists), in
which high
mortality is late in
life;
• constant loss
(such as
songbirds), in
which mortality is
about the same for
any age;
• early loss (usually
r–strategists), in
which high
mortality is early
in life.
6. Succession
succession: gradual & fairly predictable change in
species composition with time.
• some species colonize & become more abundant;
• other species decline or even disappear.
two kinds of succession:
• primary succession involves the gradual
establishment of biotic communiites in an area where
no life existed before;
• secondary succession involves the gradual
reestablishment of biotic communiites in an area
where a biotic community was previously present.
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Primary Succession
Primary succession occurs with time in lifeless areas.
• examples include succession newly formed islands &
succession after the retreat of a glacier;
• typically lichens & mosses first colonize bare rock;
• latter small herbs & shrubs colonize;
• finally tree species colonize;
• the first species to colonize are termed pioneer
species;
• the progression of species that colonize with time are
commonly termed early, mid, & late successional
species.
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Primary Succession
Generalized
physical
appearance
showing the
types, relative
sizes, and
stratification of
plant species in
various
terrestrial
communities or
ecosystems.
Fig. 9–18
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Primary Succession
Primary
succession
over several
hundred years
on bare rock
exposed by a
retreating
glacier on Isle
Royal in
northern Lake
Superior.
Fig. 9–19
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Primary Succession
Greatly
simplified view
of primary
succession in
a newly
created pond
in a temperate
area. Nutrient
rich bottom
sediment is
shown in dark
brown.
Fig. 9–20a
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Primary Succession
Fig. 9–20 b, c, & d
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Secondary Succession
Secondary succession occurs where the natural
community of organisms has been disturbed, removed,
or destroyed.
• example: "old field succession" in eastern North
America, where agricultural fields go through
succession from herbaceous plants, to shrubs &
early successional trees, to mid–successional forest,
to oak–hickory forest;
• according to the classic view, succession proceeds
until an area is occupied by a climax community,
however recent views recognize that succession is
influenced by variability & chaotic events such that a
single climax is not predictable.
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Secondary Succession
Secondary
succession
over 150–200
years in an
abandoned
farm field in
North
Carolina.
Fig. 9–21
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Secondary Succession
Successional changes in the animal community
accompany successional changes in the plant
community.
Fig. 9–22
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Disturbance
What is the role of disturbance in succession?
• disturbance: a discrete event that disrupts an
ecosystem or community;
• examples of natural disturbance: fires, hurricanes,
tornadoes, droughts, & floods;
• examples of human–caused disturbance: deforestation,
overgrazing, plowing;
• disturbance initiates secondary succession by
eliminating part or all of the existing community, & by
changing conditions & releasing resources.
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Mechanisms of Succession
Both primary & secondary succession are driven by
three mechanisms:
• facilitiation: a process by which an earlier
successional species makes the environment suitable
for latter successional species; e.g., legumes fixing
nitrogen can enable later successional species;
• inhibition: a process whereby one species hinders
the establishment & growth of other species; e.g.,
shade of late successional trees inhibits the growth of
early successional trees;
• tolerance: a process whereby later successional
species are unaffected by earlier successional
species.
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Changes During Succession
During succession species diversity & stratification
tend to increase, while growth rates & primary
productivity tend to decrease.
Fig. 9–23
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Ecosystem Changes During Succession
Characteristic
Plant size
Species diversity
Trophic structure
Early Succession
small
low
mostly producers
Ecological niches
few, more
generalized
low
Late Succession
large
high
mixture of producers,
consumers, &
decomposers
many, more
specialized
high
low
high
high
low
simple
low
complex
high
low
high
Community
organization (# links)
Biomass
Net Primary
Productivity
Food web
Efficiency for nutrient
cycling
Efficiency of energy
use
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7. Island Biogeography
In the species
equilibrium model of
island biogeography
(developed by
Robert MacArthur &
Edward O. Wilson)
the number of
species on an island
is determined by the
balance between
immigration &
extinction.
Fig. 9–24
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Island Biogeography
Small islands are
expected to have
lower immigration
rates & higher
extinction rates, &
hence less species
than large islands.
Fig. 9–24
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Island Biogeography
Far islands are
expected to have
lower immigration
rates, & hence less
species than near
islands.
Fig. 9–24
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Island Biogeography
The model of island biogeography has been widely
applied in conservation biology by viewing the
landscape as composed of habitat islands
separated by an ocean of degraded or unsuitable
habitat modified by human activity.
• large habitat patches tend to have more species;
• habitat patches that are near larger intact habitat
areas tend to have more species;
• these principles can be applied to land preservation
& management efforts.
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8. Stability & Sustainability
Stability has three aspects:
• inertia (or persistence): the ability of a system to resist
being disturbed or altered;
• constancy: the abilty of a living system to maintain a
certain size or state;
• resilience: the ability of a living system to recover after a
disturbance;
Signs of poor health or stressed ecosystems:
•
•
•
•
•
•
decrease in primary productivity;
increased nutrient losses;
decline or extinction of indicator species;
increased populations of pests or disease organisms;
decline in species diversity;
presence of contaminants.
Through an understanding of ecology we can grapple with what
it means to have sustainable ecosystems.
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