Community Ecology
Reading: Freeman, Chapter 50, 53
What is a community?
• A community is an assemblage of
plant and animal populations that
live in a particular area or habitat.
–Populations of the various species in
a community interact and form a
system with its own emergent
properties.
Pattern vs. Process
• Pattern is what we can easily observe
directly - vegetation zonation, species lists,
seasonal distribution of activity, and
association of certain species.
• Process gives rise to the patternherbivory, competition, predation risk,
nutrient availability, patterns of disturbance,
energy flow, history, and evolution.
• Community ecology seeks to explain the
underlying mechanisms that create, maintain, and
determine the fate of biological communities.
Typically, patterns are documented by observation,
and used to generate hypotheses about processes,
which are tested.
– Not all science is experimental. Hypotheses tests
can involve special observations, or experiments.
Emergent Properties of a Community
• Scale
• Spatial and Temporal Structure
• Species Richness
• Species Diversity
• Trophic structure
• Succession and Disturbance
• Scale is the size of a community.
• Provided that the area or habitat is
well defined, a community can be a
system of almost any size, from a
drop of water, to a rotting log, to a
forest, to the surface of the Pacific
Ocean.
• Spatial Structure is the way species are
distributed relative to each other.
• Some species provide a framework that
creates habitats for other species. These
species, in turn create habitats for others,
etc.
• Example: Trees in a rainforest are
stratified into several different levels,
including a canopy, several
understories, a ground level, and roots.
Each level is the habitat of a distinct
collection of species. Some places,
such as the pools of water that collect
at the base of tree branches, may
harbor entire communities of their
own.
• Temporal structure is the timing of the
appearance and activity of species. Some
communities, i.e., arctic tundra and the decay
of a corpse, have pronounced temporal species,
other communities have less.
• Example: Many desert plants and animals are
dormant most of the year. They emerge, or
germinate, in response to seasonal rains. Other plants
stick around year round, having evolved adaptations
to resist drought.
• Species Richness - is the number
of species in a community. Clearly,
the number of species we can
observe is function of the area of
the sample. It also is a function of
who is looking. Thus, species
richness is sensitive to sampling
procedure
• Diversity is the number of species in the
community, and their relative abundances.
• Species are not equally abundant, some
species occur in large percentage of samples,
others are poorly represented.
• Some communities, such as tropical
rainforests, are much more diverse than
others, such as the great basin desert.
• Species Diversity is often expressed using
Simpson’s diversity index: D=1-S (pi)2
Example Problem
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A community contains the following species:
Number of Individuals
Species A
104
Species B
71
Species C
19
Species D
5
Species E
3
What is the Simpson index value for this
Answer:
• Total Individuals= (104+19+71+5+3)=202
• PA=104/202=.51 PB=19/202=.09
• PC=71/202=.35
PD=5/202=.03
PE=3/202=.02
• D=1{(.51)2+(.09)2+(.35)2+(.03)2+(.02)2}
• D=1-.40=.60
Clicker Question
In the example above, what was the
species richness?
A. .60
B. 202 individuals
C. 5 species
D. .40
E. None of the above
Succession, Disturbance and
Change
• In terms of species and physical
structure, communities change with
time.
– Ecological succession, the predictable change in species
over time, as each new set of species modifies the
environment to enable the establishment of other species, is
virtually ubiquitous.
• Example; a sphagnum bog community
may persist for only a few decades before
the process of ecological succession
changes transform it into the surrounding
Black Spruce Forest.
• A forest fire may destroy a large area of
trees, clearing the way for a meadow.
Eventually, the trees take over and the
meadow is replaced.
• Disturbances are events such
as floods, fire, droughts,
overgrazing, and human activity
that damage communities,
remove organisms from them,
and alter resource availability.
Some Agents of Disturbance
• Fire
• Floods
• Drought
• Large Herbivores
• Storms
• Volcanoes
• Human Activity
Disturbance, Invasion, Succession
• Disturbance creates opportunities for new
species to invade an area and establish
themselves.
• These species modify the environment, and
create opportunities for other species to invade.
The new species eventually displace the
original ones. Eventually, they modify the
environment enough to allow a new series of
invaders, which ultimately replace them, etc.
• Invasion:
• Disturbance creates an ecological vacuum that
can be filled from within, from outside, or
both. For example, forest fires clear away old
brush and open up the canopy, releasing
nutrients into the soil at the same time. Seeds
that survive the fire germinate and rapidly
grow to take advantage of this opportunity. At
the same time, wind-borne and animaldispersed seeds germinate and seek to do the
same thing.
• The best invaders have good dispersal powers
and many offspring, but they are often not the
best competitors in the long run.
Succession
• Disturbance of a community is usually
followed by recovery, called ecological
succession.
• The sequence of succession is driven by the
interactions among dispersal, ecological
tolerances, and competitive ability.
– Primary succession-the sequence of species on
newly exposed landforms that have not previously
been influenced by a community, e.g., areas exposed
by glacial retreat.
– Secondary succession occurs in cases which
vegetation of an area has been partially or completely
removed, but where soil, seeds, and spores remain.
• Early in succession, species are generally
excellent dispersers and good at tolerating
harsh environments, but not the best
interspecific competitors.
• As ecological succession progresses, they are
replaced with species which are superior
competitors, (but not as good at dispersing
and more specialized to deal with the
microenvironments created by other species
likely to be present with them).
• Early species modify their environment in
such a way as to make it possible for the next
round of species. These, in turn, make their
own replacement by superior competitors
possible.
A climax community is a more
or less permanent and final stage
of a particular succession, often
characteristic of a restricted area.
 Climax communities are characterized
by slow rates of change, compared
with more dynamic, earlier stages.
 They are dominated by species
tolerant of competition for resources.
An Influential ecologist named F.E. Clements
argued that communities work like an
integrated machine. These “closed”
communities had a predictable composition.
According to Clements, there was only one true
climax in any given climatic region, which was
the endpoint of all successions.
Other influential ecologists, including Gleason,
hypothesized that random events determined
the composition of communities.
He recognized that a single climatic area could
contain a variety of specific climax types.
• Evidence suggests that for many
habitats, Gleason was right, many
habitats never return to their original state
after being disturbed beyond a certain
point.
– For example; very severe forest fires have
reduced spruce woodlands to a terrain of
rocks, shrubs and forbs.
• An incredibly rapid glacial retreat is occurring in
Glacier Bay, Alaska. In just 200 years, a glacier
that once filled the entire bay has retreated over
100km, exposing new landforms to primary
succession.
– Clements would have predicted that succession today
would follow the sequence of ecological succession
that has occurred in the past for other parts of
Alaska.
– In fact, three different successional patterns seem to
be occurring at once, depending upon local
conditions. Thus, Clements’ view of succession is
somewhat of an oversimplification.
Are Climax Communities Real?
Succession can take a long
time.
 For example, old-field succession may
require 100-300 years to reach climax
community. But in this time frame, the
probability that a physical disturbance (fire,
hurricane, flood) will occur becomes so
high, the process of succession may never
reach completion.
• Increasing evidence suggests that some
amount of disturbance and nonequilibrium
resulting from disturbance is the norm for most
communities.
• One popular hypothesis is that communities are
usually in a state of recovery from disturbance.
– An area of habitat may form a patchwork of
communities, each at different stages of ecological
succession. Thus, disturbance and recovery
potentially enable much greater biodiversity than is
possible without disturbance.
Are biological communities real
functional units?
• Do communities have a tightly prescribed organization
and composition, or are they merely a loose
assemblage of species?
• This is an unsolved problem in ecology.
• Clements argued that communities are stable,
functional units with a fixed composition-each
integrated part needs the others. Every area should
ultimately have the same species, given time.
• Gleason argued that their composition is unstable and
variable-they are more like assemblages of everything
that can live together in one place
The Kiddie Pool Experiment
• Jenkins and Buikema conducted an experiment to
see whether artificial ponds would develop
predictable assemblages of freshwater
microorganisms.
• -if this were the case, it would support the notion
that communities are real, integrated units.
• -They set up 12 identical “ponds” and filled them
with sterile water. Came back in year to study the
composition of the resulting communities.
• Result-the ponds had very different compositions of
species.
• Accidents of dispersal, and different dispersal
capabilities affected which species ended up in each
pond.
• The early arrival of certain competitors, and
predators greatly affected the ability of later species
to colonize later.
• -Gleason’s view was supported. Composition of
communities is dictated largely by chance and
history.
• Trophic structure is the hierarchy of feeding. It
describes who eats whom
• (a trophic interaction is a transfer of energy: i.e.,
eating, decomposing, obtaining energy via
photosynthesis).
• For every community, a diagram of trophic
interactions called a food web.
• Energy flows from the bottom to the top.
A Simple Food Web
Killer Whales
Sharks
Harbor Seals
Yellowfin Tuna
Mackerel
Cod
Halibut
Zooplankton
Unicellular Algae and Diatoms
Killer Whales
Harbor Seals
Mackerel
Zooplankton
Phytoplankton
One path
through a
food web is a
food chain.
• The niche concept is very important in
community ecology.
• A niche is an organism’s habitat and
its way of making a living.
• An organism’s niche is reflected by its
place in a food web: i.e, what it eats,
what it competes with, what eats it.
• Each organism has the potential to
create niches for others.
• Keystone species are disproportionately important
in communities.
• Generally, keystone species act to maintain species
diversity.
• The extinction of a keystone species eliminates the
niches of many other species.
• Frequently, a keystone species modifies the
environment in such a way that other organisms are
able to live, in other cases, the keystone species is a
predator that maintains diversity at a certain trophic
level.
Examples of Keystone Species
• California Sea Otters: This species preys upon sea
urchins, allowing kelp forests to become established.
• Pisaster Starfish: Grazing by Pisaster prevents the
establishment of dense mussel beds, allowing other
species to colonize rocks on the pacific coast
• “Mangrove” trees: Actually, many species of trees
are called mangrove trees. Their seeds disperse in
salt water. They take root and form a dense forest in
saltwater shallows, allowing other species to thrive
Trophic Cascades
• Species at one trophic level influence species at
other levels; the addition or subtraction of species
affects the entire food web.
– This causes positive effects for some species, and
negative effects for others. This is called a
trophic cascade. For instance, removing a
secondary consumer might positively affect the
primary consumers they feed upon, and
negatively affect the producers that are food for
primary consumers.
Top down vs. Bottom up
• Most biological communities have both top-down
and bottom-up effects on their structure and
composition.
– In a well known study of ponds by Matthew
Leibold, it was demonstrated that the biomass of
herbivores (zooplankton) was positively correlated
to the biomass of producers (algae), indicating a
top down effect.
– He intentionally introduced fish to some ponds,
The result was a decrease in zooplankton and
increase in producers, indicating a top down
effect.
Badly scanned from
Rose and Mueller (2006)
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Types of Interspecific
Interactions
Effect on
Species 1
Neutralism
Competition
Commensalism
Amensalism
Mutualism
Predation,
• Parasitism, Herbivory
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Effect on
Species 2
0
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Neutralism
• Neutralism the most common type of
interspecific interaction. Neither
population affects the other. Any
interactions that do occur are indirect or
incidental.
• Example: the tarantulas living in a
desert and the cacti living in a desert
Competition
• Competition occurs when organisms in the same
community seek the same limiting resource.
This resource may be prey, water, light,
nutrients, nest sites, etc.
• Competition among members of the same
species is intraspecific.
• Competition among individuals of different
species is interspecific.
• Individuals experience both types of
competition, but the relative importance of the
two types of competition varies from population
to population and species to species
“Styles” of Competition
• Exploitation competition occurs
when individuals use the same limiting
resource or resources, thus depleting
the amount available to others.
• Interference competition occurs
when individuals interfere with the
foraging, survival, or reproduction of
others, or directly prevent their physical
establishment in a portion of a habitat.
Some specific types of
competition
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Consumptive competition
Preemptive competition
Overgrowth competition
Chemical composition
Territorial competition
Encounter competition
Example of Interference
Competition
• The confused flour beetle, Triboleum
confusum, and the red flour beetle, Triboleum
castaneum cannibalize the eggs of their own
species as well as the other, thus interfering
with the survival of potential competitors.
• In mixed species cultures, one species always
excludes the other. Which species prevails
depends upon environmental conditions,
chance, and the relative numbers of each
species at the start of the experiment.
Outcomes of Competition
• Exploitation competition may cause the
exclusion of one species. For this to occur, one
organism must require less of the limiting
resource to survive. The dominant species must
also reduce the quantity of the resource below
some critical level where the other species is
unable to replace its numbers by reproduction.
• Exploitation does not always cause the exclusion
of one species. They may coexist, with a
decrease in their potential for growth. For this to
occur, they must partition the resource.
• Interference competition generally results in the
exclusion of one of the two competitors.
The Competitive Exclusion Principle
• Early in the twentieth century, two mathematical
biologists, A.J. Lotka and V. Volterra developed a
model of population growth to predict the
outcome of competition.
• Their models suggest that two species cannot
compete for the same limiting resource for long.
Even a minute reproductive advantage leads to
the replacement of one species by the other.
• This is called the competitive exclusion
principal.
Evidence for Competitive Exclusion.
• A famous experiment by the Russian
ecologist, G.F. Gausse demonstrated that
Paramecium aurellia outcompetes and
displaces Paramecium caudatum in mixed
laboratory cultures, apparently confirming
the principle.
• (Interestingly, this is not always the case.
Later studies suggest that the particular strains
involved affect the outcome of this interaction).
Other experiments...
• Subsequent laboratory studies on other
organisms, have generally resulted in
competitive exclusion, provided that the
environment was simple enough.
• Example: Thomas Park showed that,
via interference competition, the
confused flour beetle and the red flower
beetle would not coexist. One species
always excluded the other.
Resource Partitioning
• Species that share the same
habitat and have similar needs
frequently use resources in
somewhat different ways - so that
they do not come into direct
competition for at least part of the
limiting resource. This is called
resource partitioning.
• Resource partitioning obviates competitive
exclusion, allowing the coexistence of
several species using the same limiting
resource.
• Resource partitioning could be an
evolutionary response to interspecific
competition, or it could simply be that
competitive exclusion eliminates all
situations where resource partitioning does
not occur.
• One of the best known cases of resource
partitioning occurs among Caribbean anoles.
– As many as five different species of anoles may
exist in the same forest, but each stays restricted to
a particular space: some occupy tree canopies,
some occupy trunks, some forage close to the
ground.
– When the brown anole was introduced to Florida
from Cuba, it excluded the green anole from the
trunks of trees and areas near the ground: the
green anole is now restricted to the canopies of
trees:the resource (space, insects) has been
partitioned among the two species
– (for now at least, this interaction may not be stable in the
long run because the species eat each other’s young).
Character Displacement
• Sympatric populations of similar
species frequently have differences in
body structure relative to allopatric
populations of the same species.
• This tendency is called character
displacement.
• Character displacement is thought to
be an evolutionary response to
interspecific competition.
Example of Character Displacement
• The best known case of character displacement occurs
between the finches, Geospiza fuliginosa and
Geospiza fortis, on the Galapagos islands.
• When the two species occur together, G. fuliginosa
has a much narrower beak that G fortis. Sympatric
populations of G fuliginosa eats smaller seeds than G
fortis: they partition the resource.
• When found on separate islands, both species have
beaks of intermediate size, and exploit a wider variety
of seeds.
• These inter-population differences might have evolved
in response to interspecific competition.
Competition and the Niche
• An ecological niche can be thought of in terms of
competition.
• The fundamental niche is the set of resources and
habitats an organism could theoretically use under
ideal conditions.
• The realized niche is the set of resources and
habitats an organism actually used: it is generally
much more restricted due to interspecific competition
(or predation.)
Two organisms cannot occupy
exactly the same niche.
This is sometimes called Gausse’s
rule(although Gausse never put it exactly
that way).
-Experiments by Gausse (Paramecium), Peter Frank
(Daphnia), and Thomas Park (Triboleum) have
confirmed it for simple laboratory scenarios.
-This creates a bit of a paradox, because so many
species exist in nature using the same resources.
-The more complex environments found in nature
may enable more resource partitioning.
Amensalism
• Amensalism is when one species
suffers and the other interacting species
experiences no effect.
• Example: Redwood trees falling into
the ocean become floating batteringrams during storms, killing large
numbers of mussels and other intertidal organisms.
• Allelopathy involves the production
and release of chemical substances by
one species that inhibit the growth of
another. These secondary
substances are chemicals produced by
plants that seen to have no direct use in
metabolism.
• This same interaction can be seen as
both amensalism, and extremely onesided interference competition-in fact it
Example: Allelopathy in the California Chaparral
• Black Walnut (Juglans nigra) trees excrete an
antibiotic called juglone. Juglone is known to inhibit
the growth of trees, shrubs, grasses, and herbs
found growing near black walnut trees.
• Certain species of shrubs, notably Salvia leucophylla
(mint) and Artemisia californica (sagebrush) are
known to produce allelopathic substances that
accumulate in the soil during the dry season. These
substances inhibit the germination and growth of
grasses and herbs in an area up to 1 to 2 meters
from the secreting plants.
Commensalism
• Commensalism is an interspecific interaction where one
species benefits and the other is unaffected.
• Commensalisms are ubiquitous in nature: birds nesting in
trees are commensal.
• Commensal organisms frequently live in the nests, or on the
bodies, of the other species.
• Examples of Commensalism:
• Ant colonies harbor rove beetles as commensals. These
beetles mimic the ants behavior, and pass as ants. They eat
detritus and dead ants.
• Anemonefish live within the tentacles of anemones. They
have specialized mucus membranes that render them
immune to the anemone’s stings. They gain protection by
living in this way.
Mutualism
• Mutualism in an interspecific interaction
between two species that benefits both
members.
• Populations of each species grow, survive
and/or reproduce at a higher rate in the
presence of the other species.
• Mutualisms are widespread in nature, and
occur among many different types of
organisms.
Examples of Mutualism
• Most rooting plants have mutualistic associations
with fungal mychorrhizae. Mychorrhizae increase
the capability of plant roots to absorb nutrients. In
return, the host provides support and a supply of
carbohydrates.
• Many corals have endosymbiotic organisms called
zooxanthellae (usually a dinoflagellate). These
mutualists provide the corals with carbohydrates
via photosynthesis. In return, they receive a
relatively protected habitat from the body of the
coral.
Mutualistic Symbiosis
• Mutualistic Symbiosis is a type of mutualism
in which individuals interact physically, or even
live within the body of the other mutualist.
Frequently, the relationship is essential for the
survival of at least one member.
• Example: Lichens are a fungal-algal symbiosis (that
frequently includes a third member, a
cyanobacterium.) The mass of fungal hyphae provides
a protected habitat for the algae, and takes up water
and nutrients for the algae. In return, the algae (and
cynaobacteria) provide carbohydrates as a source of
energy for the fungus.
Facultative vs. Obligate Mutualisms
• Facultative Mutualisms are not
essential for the survival of either
species. Individuals of each species
engage in mutualism when the other
species is present.
• Obligate mutualisms are essential for
the survival of one or both species.
Other Examples of Mutualisms
• Flowering plants and pollinators. (both
facultative and obligate)
• Parasitoid wasps and polydna viruses.
(obligate)
• Ants and aphids. (facultative)
• Termites and endosymbiotic protozoa.
(obligate)
• Humans and domestic animals. (mostly
facultative, some obligate)
Predation, Parasitism,
Herbivory
• Predators, parasites, parasitoids, and
herbivores obtain food at the expense of their
hosts or prey.
• Predators tend to be larger than
their prey, and consume many prey
during their lifetimes.
• Parasites and pathogens are
smaller than their host. Parasites
may have one or many hosts
during their lifetime. Pathogens
are parasitic microbes-many
generations may live within the
same host. Parasites consume
their host either from the inside
(endoparasites) or from the
outside (ectoparasites).
• Parasitoids hunt their prey like
predators, but lay their eggs within the
body of a host, where they develop like
parasites.
• Herbibores are animals that eat
plants. This interaction may resemble
predation, or parasitism.
Predator-Prey and Parasite-Host
Coevolution
• The relationships between predator
and prey, and parasites and hosts,
have coevolved over long periods of
time.
• About 50 years ago, an evolutionary biologist named
J.B.S. Haldane suggested that the interaction
between parasite and host (or predator and prey)
should resemble an evolutionary arms race:
• First a parasite (or predator) evolves a trait that allows it to
attack its host (or prey).
• Next, natural selection favors host individuals that are able to
defend themselves against the new trait.
• As the frequency of resistant host individuals increases, there
is natural selection for parasites with novel traits to subvert
the host defenses.
• This process continues as long as both species survive.
• Recent data on Plasmodium, the cause of malaria, support
this model.
Example of Parasite-Host
Coevolution
• The common milkweed, Asclepias syriaca has leaves
that contain cardiac glycosides: they are very
poisonous to most herbivores. This renders them
virtually immune to herbivory by most species.
• Monarch butterfly larvae have evolved the ability to
tolerate these toxins, and sequester them within their
bodies. They are important specialist hervivores of
milkweeds.
• These sequestered compounds serve the additional
purpose of making monarch larvae virtually inedible to
vertebrate predators.
Predator-Prey Population Dynamics
• Predation may be a density-dependent mortality
factor to the host population-and prey may represent
a limiting resource to predators.
• The degree of prey mortality is a function of the
density of the predator population.
• The density of the prey population, in turn, affects
the birth and death rates of the predator population.
• i.e, when prey become particularly common,
predators increase in numbers until prey die back
due to increased predation, this, in turn, inhibits the
growth of prey.
• Typically, there is a time lag effect.
• There is often a dynamic balance
between predators and prey that is
necessary for the stability of both
populations.
• Feedback mechanisms may control
the densities of both species.
Example of Regulation of Host Population by a
Herbivore
• In the 19th century, prickly pear cactus, Opuntia sp. was
introduced into Australia from South America. Because no
Australian predator species existed to control the population
size of this cactus, it quickly expanded throughout millions of
acres of grazing land.
• The presence of the prickly pear cactus excluded cattle and
sheep from grazing vegetation and caused a substantial
economic hardship to farmers.
• A method of control of the prickly pear cactus was initiated
with the introduction of Cactoblastis cactorum, a cactus
eating moth from Argentina, in 1925. By 1930, densities of
the prickly pear cactus were significantly reduced.
• Sometimes predator species can drive their
prey to localized extinction.
• If there are no alternate prey, the predator then
goes extinct.
• If the environment is coarse grained, this makes
the habitat available for recolonization by the prey
species.
• Example: The parasitic wasp Dieratiella rapae is a
very efficient parasitoid. One female can oviposit
into several hundred aphids during its lifetime.
Frequently, aphids are driven locally extinct and the
adults must search for new patches when they
emerge. Once the aphid and the host are gone, the
host plants may become re-infested with aphids.
• In other cases, there are alternate
prey to support the predator and
the prey is permanently excluded.
• Example: Freshwater fish such as
bluegills and yellow perch
frequently exclude small
invertebrates such as Daphnia
pulex from ponds. The fish then
switch to other prey such as insects
larvae.
The time-lag effect may lead to predatorprey oscillations.
• Most predators do not respond instantaneously to the
availability of prey and adjust their reproduction
accordingly.
• If predator populations grow faster than prey
populations, they may overshoot the number of prey
that are able to support them
• This leads to a rapid decline in the prey, followed by a
rapid decline in the predator.
• Once the predator becomes rare, the prey population
may begin growing again.
• This pattern is called a predator-prey oscillation.
Cycles in the population dynamics of the snowshoe hare
and its predator the Canadian lynx (redrawn from
MacLulich 1937). Note that percent mortality is an elusive
measure, it may, or may not, be useful since mortality
varies with environment and time.
•In the 1920s, A. J. Lotka (1925) and V. Volterra (1926)
devised mathematical models representing host/prey
interaction.
•The Lotka-Volterra curve assumes that prey destruction is a
function not only of natural enemy numbers, but also of prey
density, i.e., related to the chance of encounter.
•This model predicts the predator-prey oscillations sometimes
seen in nature. Populations of prey and predator were
predicted to flucuate in a regular manner (Volterra termed
this "the law of periodic cycle").
•Lotka-Volterra model is an oversimplification of reality. In
nature, many different factors affect the densities of
predators and their prey.