Community Ecology

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Community Ecology

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Title: Community Ecology

1
Community Ecology

Reading Freeman, Chapter 50, 53

2
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.

3
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 pattern- herbivory,
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.

5
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.

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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.

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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
Simpsons diversity index D1-S (pi)2

14
Example Problem

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
community?

15
Answer

Total Individuals (104197153)202
PA104/202.51 PB19/202.09
PC71/202.35 PD5/202.03PE3/202.02
D1-(.51)2(.09)2(.35)2(.03)2(.02)2
D1-.40.60

16
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

17
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.

20
Some Agents of Disturbance

Fire
Floods
Drought
Large Herbivores
Storms
Volcanoes
Human Activity

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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 animal-dispersed 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.

24
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.

30
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.

32
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

33
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.
-Gleasons 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.

36
A Simple Food Web
Killer Whales
Sharks Harbor Seals
Yellowfin Tuna Mackerel Cod
Halibut

Zooplankton Unicellular Algae and Diatoms
37
Killer Whales Harbor Seals Mackerel Zooplankton
Phytoplankton
One path through a food web is a food chain.
38

The niche concept is very important in community
ecology.
A niche is an organisms habitat and its way of
making a living.
An organisms 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.

40
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

41
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.

42
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.

43
Badly scanned from Rose and Mueller (2006)
44
Types of Interspecific Interactions

Effect on
Effect on
Species 1
Species 2
Neutralism 0 0
Competition - -
Commensalism 0
Amensalism - 0
Mutualism
Predation, -
Parasitism, Herbivory

45
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

46
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

47
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.

48
Some specific types of competition

Consumptive competition
Preemptive competition
Overgrowth competition
Chemical composition
Territorial competition
Encounter competition

49
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.

50
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.

51
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.

52
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).

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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.

55
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
treesthe 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 others young).

58
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.

59
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.

60
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.)

61
Two organisms cannot occupy exactly the same
niche.

This is sometimes called Gausses 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.

62
Amensalism

Amensalism is when one species suffers and the
other interacting species experiences no effect.
Example Redwood trees falling into the ocean
become floating battering-rams during storms,
killing large numbers of mussels and other
inter-tidal 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 one-sided interference
competition-in fact it is both.

64
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.

65
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 anemones stings.
They gain protection by living in this way.

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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.

68
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.

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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.

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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.

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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)

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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.

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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.

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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.

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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.

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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.

84
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.

87
The time-lag effect may lead to predator-prey
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.

88
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.
89

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.