Community Ecology
Community is the assemblage of
populations of different organisms living in
an area and potentially interacting.
E.g. pond community, community of
decomposers in a rotten log, forest
community.
Interspecific interactions
Interactions between organisms of different
species are referred to as interspecific
interactions.
These include competition, predation,
parasitism, herbivory, mutualism and
disease.
Some interactions benefit only one
participant (e.g. predation, herbivory) but
some benefit both participants (mutualism).
In other cases neither participant benefits
(competition). Competition has been
extensively studied by ecologists.
Competition
Interspecific competition occurs when different
species compete for a resource that is in limited
supply. Some resources such as air are usually not
limiting so there is no competition for them.
Plants compete for water, nutrients and light.
Mussels and barnacles compete for space to settle
on rocks in the intertidal zone.
Owls, foxes and weasels compete for small
mammal prey.
Competitive Exclusion
If competition between two species is very
strong, one species may outcompete the
other and competitively exclude it.
Competitive exclusion first well
documented by Gause in 1934.
Competitive Exclusion
Gause studied two species of Paramecium:
P. aurelia and P. caudatum.
When cultured in separate containers each
species thrived and population leveled off at
carrying capacity of test tube.
Competitive Exclusion
However, when P. aurelia and P. caudatum
were grown together P. caudatum became
extinct.
Gause concluded P. aurelia had a
competitive advantage at competing for
food and outcompeted P. caudatum.
Competitive Exclusion
Gause developed from this and other
experiments his Competitive Exclusion
Principle: Two species competing for the
same limiting resources cannot coexist in
the same place.
If the inferior competitor cannot escape the
competition it will be driven to extinction.
Ecological Niches
The sum total uses that a species makes of
the biotic and abiotic resources in its
environment is referred to as its niche.
(Pronounced to rhyme with “itch” in the
U.S., but elsewhere to rhyme with
“sheesh.”).
Ecological Niches
An organism’s ecological niche is analgous
to its “profession,” what it does for a living.
Niche includes many components: the food
the organism eats, the places it occupies, the
time of day it is active, the temperature
range it can tolerate, etc.
Ecological Niches
The concept of niches can be used to restate
the competitive exclusion principle: Two
species cannot coexist if their niches are
identical.
Ecologically similar species, however, can
coexist if there are significant differences in
their niches.
Ecological Niches
As a result of competition a species
fundamental niche, the niche potentially
occupied by that species may be different
from its realized niche the niche it actually
occupies in a particular environment.
Connell’s work on barnacles
Joseph Connell demonstrated the effects of
competition on niche occupation in
barnacles.
He studied Balanus balanoides and
Chthamalus stellatus in the Scottish
intertidal zone.
These species have a stratified distribution
on intertidal rocks.
Connell’s work on barnacles
Chthamalus occurs higher than Balanus in
the intertidal zone.
Connell carried out experiments in which
he excluded one or other species from rocks
and observed what happened.
Connell’s work on barnacles
When Chthamalus was excluded Balanus
did not spread higher up the rocks because
it apparently cannot tolerate the stress of
drying out for long periods.
Balanus’s realized niche is thus similar to its
fundamental niche.
Connell’s work on barnacles
In contrast when Balanus was excluded,
Chthamalus spread down the rock. The
realized niche of Chthamalus in the
presence of Balanus is much smaller than
its fundamental niche.
Connell’s work on barnacles
Connell’s observations showed that
although Chthamalus settled in the lower
zone that Balanus smothered or crushed
Chthamalus and that the most mortality
occurred during the period of most rapid
Balanus growth.
Resource partitioning
Competition between species can result in
natural selection causing niches to
differentiate to escape the effects of
competition.
Anolis lizards in the Caribbean all feed on
similar prey and are similar in size, but they
make use of different foraging perches.
Character displacement
Natural selection can also result in
morphological changes in species that
reduce competitive effects.
Many competitors have populations that
occur sympatrically (overlap their
competitor) and allopatrically
(geographically separate).
Character displacement
Often in sympatric populations the two species
diverge physically.
For example, among Geopsiza finches on the
Galapagos Islands sympatric G. fortis and G.
fuliginosa populations differ in beak depth, but
measurements of allopatric populations overlap
greatly.
Character displacement
Because beak dimensions affect the
efficiency with which birds can consume
different size seeds, the differentiation of
bill depths in sympatry appears to reduce
the intensity of competition between the
two species.
Other interactions
Besides competition organisms engage in a
wide variety of other interactions:
Predation
Herbivory
Parasitism
Disease
Mutualism
Predation
Predation is another major interaction
between organisms that shapes
communities.
Predators and prey both have extensive
suites of adaptations designed to enable
them to catch prey or avoid being caught.
Adaptations
Predators possess weaponry: teeth, claws
poison, etc. They are also usually quick and
stealthy.
Prey generally flee or hide to avoid
predation, but may also possess defensive
structures (horns, armor, spines) or toxins
(pufferfish, plants).
Cryptic coloration
Warning colors
Many organisms have effective chemical
defenses and signal them by using warning
or aposematic colors.
Examples include Monarch Butterfly, Coral
Snake, and Poison arrow frog
Coral Snake
Poison arrow frog
Mimicry
Some organisms mimic the warning coloration of
toxic organisms to gain protection.
E.g. Viceroy Butterfly mimics pattern of Monarch
Butterfly
The mimicry of toxic organisms by non-toxic ones
is called Batesian mimicry.
Batesian Mimicry
Monarch Butterfly
Viceroy butterfly.
Coral Snake and mimics. Which is the coral
snake?
Wasp,
Hornet moth,
Wasp beetle,
Hoverfly
Mullerian mimicry
In Mullerian mimicry several toxic or
dangerous species all display the same
or similar warning colors. Result of
convergent evolution.
Mullerian mimics on left of red line
Batesian on right of line
Herbivory
Plants are subject to grazing by many
organisms and defend themselves with
mechanical defenses (thorns, silica, hard
shells) and by producing toxins (e.g.
capascin [substance that makes chilies hot],
strychnine, nicotine and tannins).
Parasitism
In parasitism the parasite derives its
nourishment from its host.
Parasites may be internal (endoparasites e.g.
tapeworm, fluke) or external (ectoparasites
e.g. tick, flea).
In addition parasitoids lay eggs in or on
prey and when the larvae hatch they
consume the prey.
Parasitism
Many parasites have complex life cycles that
include several host species (e.g. for malaria
humans and mosquito are hosts).
Parasites frequently modify behavior of host
species so they are more vulnerable to predation
by the next host species (e.g. acanthocephalan
worms enter cockroach’s brain and cause it to
wander about in the light where it can be caught
and eaten by a rat the next host species).
Disease
Just like parasites pathogens which cause
disease are harmful to their hosts.
Pathogens include bacteria, viruses, fungi and
protists (single-celled organisms).
Generally microscopic.
Disease
Pathogens that gain access to populations that
have not been previously exposed to the
disease can have devastating effects.
E.g. smallpox introduced to New World,
Dutch Elm Disease.
Mutualism
Mutualistic interactions are interactions that
benefit both species involved.
Acacias and Pseudomyrmex ants. Tree produces
hollow spines to house ants and food in form of
sugar and protein –rich nodules. Ants deter
herbivores, remove fungal spores and cut back
competing vegetation.
Acacias and Pseudomyrmex ants
Mutualism
Other mutualistic interactions include
nitrogen fixing bacteria and legume roots,
bacteria that live in the guts of grazing
mammals, and mycorrhizae (association
between fungi and plant roots).
Trophic structure
Community structure and dynamics
strongly influenced by feeding relationships
between organisms: the Trophic Structure.
Transfer of energy up through trophic levels
called the food chain.
Energy travels from the primary producers
(plants and other photosynthesizers)
through herbivores and various carnivores.
Food webs
Charles Elton recognized in the 1920’s that food
chains are linked together into food webs.
Food webs summarize the feeding relationships in
a whole community. Because many organisms
feed at multiple trophic levels, food webs can be
quite complex.
Fig 53.13
Food webs
However, webs can be simplified by
grouping species with similar feeding habits
into trophic groups (e.g. carnivorous
plankton in the previous web).
Food chain length
Most food chains are relatively short,
usually having no more than 5 links.
There are two major hypotheses for why
food chains are generally short.
Food chain length
Energetic hypothesis: because energy is
inefficiently transferred from one trophic level to
the next (only about 10% passes from one level to
the next) there is too little energy left to support
“superpredators.”
Explains why there are no tiger-eating birds for
example. Very large organisms generally feed at
the bottom of the food chain (e.g. whales)
Food chain length
Dynamic stability hypothesis: Suggests
that long food chains are less stable than
short food chains and that population
fluctuations at the bottom of such chains
would result in extinction of top predators.
Food chain length
Most available data support the energetic
hypothesis.
In studies of tree hole communities
manipulating the amount of litter deposited
in the tree holes affected the length of food
chains as predicted by the energetic
hypothesis
Food chain length
Energetic hypothesis predicts longer chains when
the supply of food at the base of the chain is
increased and shorter chains when food supply is
decreased.
Australian researchers found such a pattern in a
series of manipulative experiments where they
adjusted leaf litter levels (the food supply) for tree
hole communities.
53.15
Effects of single species on
community structure
Certain species can have a large impact on
community structure because of their
abundance or because they alter conditions
for other species.
Dominant species
In some communities one species
predominates and exerts a strong influence
on the other species in the community (e.g.
sugar maples shade out other plants and
their shallow roots consume most available
water).
Dominant species
Dominant species may be dominant because
they are better competitors or because they
have few predators or pathogens (often the
case with invasive species).
Removing the dominant species may or
may not have a large impact on community
structure.
Dominant species
For example, the near extinction of the
American Chestnut (once the dominant tree
in eastern forests before 1910 forming 40%
of the large trees) had little effect on
community structure. Other trees became
more common, but birds and mammals
were apparently almost unaffected.
Keystone species
Some species may not necessarily be
abundant but can strongly affect community
structure. These species are referred to as
keystone species.
Removal experiments can show the effect
of keystone species.
Keystone species
Robert Paine removed the sea star Pisaster
from a section of rocky shoreline in
Washington.
Pisaster preys on mussels and when
Pisaster was removed the mussels increased
in abundance and crowded out other
invertebrates and algae.
Keystone species
When Pisaster was present 15 to 20 species
of red algae occurred, but when it was
removed the number of red algae species
fell to fewer than 5.
Foundation species (“ecosystem
engineers”
Some organisms have a profound effect on the
environment and influence community structure in
that way.
Classic example is the beaver which through tree
felling and dam building substantially modifies the
environment producing ponds and wetlands used
by many species
Effects of disturbance on species
diversity and composition
Historically ecologists thought that
communities were usually in equilibrium
and that stability was maintained by
interspecific competition.
However, disturbance is now recognized as
playing a major role in shaping
communities.
Disturbance
Disturbances are events that change
communities by removing species or
altering resource availability.
Common disturbances include fires, storms,
floods, droughts and human disturbance.
Disturbance
Disturbance disrupts the competitive
advantage of strong competitors and
provides opportunities for other species by
creating environmental patchiness.
Disturbance
Greatest diversity in communities is generally
produced by intermediate levels of disturbance.
Low levels allow superior competitors to
predominate. High levels of disturbance exclude
many species that cannot tolerate the associated
stresses and also species that only colonize
disturbed habitats slowly.
Ecological Succession
After a major disturbance a site is
recolonized in a predictable sequence.
Early colonizers that disperse quickly into
disturbed sites are replaced by later-arriving
but superior competitors.
Ecological Succession
McBride Glacier in Glacier Bay, Alaska has
retreated steadily since 1760.
Pattern of vegetation development in wake of
retreating glacier follows clear pattern.
First, in pioneer stage exposed ground colonized
by mosses, liverworts, fireweed and Dryas (a mat
forming shrub), willows and cottonwoods.
After about 30 years Dryas predominates
Next alder trees invade and form dense thickets.
The alder stands are next invaded by Sitka Spruce
which overgrow them forming a thick forest after
about 100 years.
Finally hemlocks invade the spruce forests and
after another 100 years spruce-hemlock forest
forms.
Biogeographic effects on
community diversity
Communities differ greatly in the number of
species they contain.
A community’s species diversity is strongly
influenced by its geographic location and
its size.
Biogeographic effects on
community diversity
Plant and animal life is more abundant near
the topics and small or remote islands have
fewer species than large island or those near
continents.
These observations suggest that community
diversity may be strongly affected by
biogeographic processes.
Equatorial-polar gradient
Tropical habitats support many more
species than temperate ones.
6.6 ha plot in Malaysia contained 711
species of tree. A similar plot in Michigan
contains only 10-15 species.
Equatorial-polar gradient
Two key factors in equatorial-polar gradient are
probably evolutionary history and climate.
Over evolutionary time species diversity may
increase as speciation occurs. Tropical habitats
are older (having been less disturbed by e.g.
glaciations) and so there has been more time for
speciation to occur.
Equatorial-polar gradient
Main reason for higher diversity at tropics,
however, is probably climate.
There is a very strong correlation between solar
energy input and water availability (combined into
one measure as evapotranspiration) and species
richness.
Evapotranspiration highest in areas of high
temperatures and rainfall.
Area effects
Species-area curve. Everything else being
equal, the larger the area a community
occupies the greater the number of species.
Most likely this is because larger areas
include a greater diversity of habitats and
microhabitats than smaller areas.
Area effects
Species–area curves differ among taxa, but
basic pattern applies across all groups.
In conservation biology knowledge of
species area curves for key taxa in
communities allows ecologists to predict
how loss of habitat may alter a community’s
biodiversity.
Island Equilibrium Model
Islands have been popular model systems
for studying the biogeographic factors that
affect community diversity.
“Island” refers not only to true islands but
to any patch surrounded by unsuitable
habitat (e.g. ponds, forest fragments,
mountain peaks).
Island Equilibrium Model
MacArthur and Wilson in the 1960’s
developed a general model to explain
diversity on islands.
Simple model used extinction and
immigration rates to predict the number of
species an island could support.
Island Equilibrium Model
MacArthur and Wilson assumed that small islands
would have higher extinction rates than large
islands because small islands have smaller
populations.
Also assumed that large islands and islands close
to landmasses would have higher rates of
immigration than smaller or more distant islands.
Island Equilibrium Model
Because immigration and extinction are
opposing forces, by graphing the two curves
(immigration and extinction) MacArthur
and Wilson were able to predict an
equilibrium number of species that an island
should contain.
Island Equilibrium Model
The MacArthur and Wilson model is simple
and applies only when colonization is major
factor affecting species composition, but
had a major effect on stimulating research
into the effects of habitat size on species
diversity (a topic of vital importance to
conservation biology).