Chapter 53: Community Ecology

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Chapter 53CommunityEcology
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Community Ecology

• A community consists of all of the organisms
  living within a certain geographical area
• These organisms include conspecifics as well as
  members of other species
• These organisms interact with each other both
  directly and indirectly
• Numerous (pessimists might say "endless")
  parameters affect what species are present and in
  what abundance
• "Simple generalizations can rarely explain why
  certain species commonly occur together in
  communities.
• "The distributions of most populations in
  communities are probably affected to some extent
  by both abiotic gradients and interactions with
  other species."

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Interspecific Interactions
A key distinction between intraspecific and
interspecific interactions is that the former but
not the latter share a gene pool
  Intraspecific interactions do not generally
lead to the extinction of a species in
interspecific interactions, losers can go extinct
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Commensalism
The Knifes Edge!
For the host species to be truly not affected
by the commensal, either negatively or
positively, is probably somewhat rare
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Interspecific Interactions
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Mutualism
The ants feed on sugar produced by nectaries on
the tree and on protein-rich swellings (orange in
the photograph) at the tips of the leaflet. The
acacia benefits because the pugnacious ants,
which attack anything that touches the tree,
remove fungal spores and other debris and clip
vegetation that grows to close to the acacia. p.
1164, Campbell Reece (2005)
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Mutualism
the fish is able to produce a special mucus
that causes the anemone not to release its
stings In return for the anemone's protection,
the fish brings scraps to it, and lures larger
fish into the anemone's tentacles.
http//mangrove.nus.edu.sg/pub/seashore/text/265.h
tm
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Interspecific Interactions
One individual partly consuming another
(especially a plant)
One individual killing another
A symbiont replicating at the expense of a hosts
health
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/ - more than just predation
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Defense against / or /

• Secondary compounds (plants)
• Nutritional deficiencies (plants)
• Mechanical defenses (plants)
• Production of poisons (animals)
• Mechanical defenses (animals)
• Running away hiding (animals)
• Fighting back (mostly animals)
• Cryptic coloration (mostly animals)
• Batesian mimicry (animals)
• Müllerian mimicry (animals)
• Immune systems (animals)

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Plant Defenses
It is important to keep in mind that herbivores
can be big (cows) as well as small (insects,
fungi, bacteria), so more than one defense is
typically necessary to defeat all possible
predators
Plants tend to be eaten in pieces rather than as
a whole organism, so anything a plant can do to
spare part of the plant from being eaten can also
be advantageous
Typically a plant will not manage to achieve
complete avoidance of predation, but instead will
limit their own predation to those organisms that
possess appropriate morphological or biochemical
adaptations
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Secondary Compounds

• Secondary compounds are chemicals that plants
  produce that are distinct from the primary
  metabolism that to some extent common to all
  plants
• One role of secondary compounds are as defenses
  against predation, e.g., toxins
• What is toxic to one herbivore may be useful to
  another particularly humans take great advantage
  of plant secondary chemicals using them as drugs
  (both recreational and medicinal), spices, etc.
• Some animals (e.g., monarch butterflies) can
  actually incorporate these toxins into themselves
  to make themselves unpalatable to some of their
  own predators

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Nutrient Deficiencies (plants)
Such nutritional deficiencies force predators to
diversify what plants they consume, thus
preventing herbivores from getting too good
(specialized) at exploiting a particular plant
species
Plants tend to lack certain nutrients (e.g.,
essential amino acids)
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Mechanical Defenses

• Anything a plant can do to keep a herbivore from
  reaching, biting, or deriving benefit from it
  once a piece that has been removed can serve to
  protect the plant from being consumed
• Thorns, for example, prevent larger things from
  comfortably eating a plant, while hairs and other
  small appendages can keep small things from
  reaching the plant
• Plants can interfere with chewing by,
  essentially, being less than succulent, e.g.,.
  the shell of a nut or silica deposited in the
  leaves of grass

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Production of Poisons
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Mechanical Defenses (animals)
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Running Away and Hiding
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Fighting Back
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Cryptic Coloration
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More Cryptic Coloration
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Coevolution

• Coevolution represents the evolutionary
  modification of organisms in response to other
  organisms, particularly when two organisms are
  mutually modified in response to modifications
  displayed by the other.
• "Despite the problems in assessing cause and
  effect in the evolution of complex ecological
  relationships, biologists agree that the
  adaptation of organisms to other species in a
  community is a fundamental characteristic of
  life. Put another way, interactions of species in
  ecological time often translate into adaptations
  over evolutionary time.
• Strictly, coevolutionary relations may be limited
  to interactions between two species rather than
  modifications that affect a suite of species for
  example, an ability to run faster in order to
  escape predators is not quite the same thing as
  an ability to run faster in order to escape one
  predator species (which, if it wants a meal,
  would then be exposed to selection to run even
  faster).
• This narrowing limits the applicability of the
  idea of coevolution since it creates a criteria
  that is stricter then simply more effectively
  interacting with other species in terms of
  survival and reproduction.

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Parasite-Host Coevolution
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Aposematic (warning) Coloration
Aposematic coloration is how organisms advertise
unpalatableness, at least visually (humans, or
course, display a distinct bias in terms of
visual input)
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Batesian Mimicry
Batesian mimicry is the tendency of palatable and
otherwise succulent prey species to pretend to be
unpalatable by looking like unpalatable species
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Müllerian Mimicry
Müllerian mimicry is when two unpallatable
species both display the same aposematic
coloration
This increases the rate at which predators learn
to recognize unpallatableness
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Interspecific Interactions
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Interspecific Competition

• Interspecific competition represents a lose-lose
  interaction (-/-), that is, both species are less
  able to convert resources into progeny because
  the other species is laying claim to the same
  resources
• Note that this is an unstable situation that will
  tend to select for either better means of
  acquiring the contested resources, or a switching
  to a different resource
• Additionally, note that while a competing species
  may be more effective in exploiting any given
  resource, conspecifics will always be competing
  with any given individual for a larger variety of
  resources than will interspecifics
• Thus, growth of a given species may be limited by
  both conspecifics (intraspecific
  competition/density-dependent factors) and
  interspecific competition (a density-independent
  factor)

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Defense against /

• Interference Competition
• Fighting back
• Running away
• Avoidance
• Losing
• Exploitative Competition
• Competitive exclusion
• Resource partitioning
• Character displacement
• Fundamental vs. Realized Niche

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Competitive Exclusion
"Two species with similar requirements (cannot)
coexist in the same community one species would
inevitably harvest resources and reproduce more
efficiently, driving the other to local
extinction. Even a slight reproductive advantage
would eventually lead to the elimination of the
inferior competitor."
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Resource Partitioning
Extinction of one of two sympatric populations
competing over resources is not the only possible
outcome of interspecific competition
An alternative outcome is the evolution of a
divergence of resource needs, called resource
partitioning
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Character Displacement
"The tendency for characters to be more divergent
in sympatric populations of two species than
allopatric population of the same two species is
called character displacement."
That is, characters diverge presumably in
response to interspecific competition, and thus
do not diverge in populations not subject to the
same interspecific competition
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Ecological Niche

• What is being fought over in interspecific
  competition is various aspects of the ecological
  niche
• A niche is the sum total of what an organism does
  in its environment, including all of the
  resources consumed
• All of the resources a population could exploit
  under ideal conditions, where there exists no
  interspecific competition, is termed the
  fundamental niche of an organism
• The fundamental niche basically represents as
  good as things can get for an organism
• A population able to exploit its fundamental
  niche would be able to achieve its maximal
  population size
• A realized niche is those resources a population
  can exploit in a real environment, particularly
  one in which interspecific competition occurs

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Fundamental Realized Niches
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Interspecific Interactions
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Trophic Structure

• Trophic structures are the feeding relationships
  within communities and therefore within
  ecosystems, that is, who's eating whom
• Trophic levels refer to how far removed from the
  original source of energy an organism is within a
  trophic structure
• The first trophic level is made up of the primary
  producers, the organisms that obtain from
  inorganic sources the energy that powers
  ecosystems
• Primary producers typically are photosynthetic
  organisms more generally, primary producers are
  autotrophs (i.e., they fix CO2)
• Consumers are the heterotrophs, i.e., organisms
  that obtain their carbon from other organisms
• The typical consumer is a chemoheterotroph that
  consumes other organisms or parts of other
  organisms to obtain their carbon and energy

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Food Chains
Note that it is the words in this figure rather
than the images that are important
All other consumers are carnivores, detrivores,
or decomposers
Primary consumers are herbivores
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Food Chains and Webs

• A simplification of the trophic structure of an
  ecosystem is the food chain
• Food chains refer to the passage of nutrients and
  energy from a primary producer to a primary
  consumer to a secondary consumer, and so on
• Food chains are usually a simplistic
  representation because they assume that a given
  organism consumes only one kind species and that
  the predators of any given consumer also consume
  only one kind of species
• Far more realistic is the concept of food webs
• Food webs are like food chains but more
  realistic, i.e., allowing for species to consume
  more than one other kind of species and allowing
  individual species to consume at more than one
  trophic level

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Aquatic Food Web
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Marsh Food Web
Most food chains have no more than about five
trophic levels
Reasons for this limitation include energy losses
while moving up trophic levels and that higher
trophic levels are more prone to extinction
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Trophic Levels vs. Energy Available
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Field Food Web
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Soil Food Web
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Species Richness vs. Abundance
Same species richness, different species abundance
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Predation Species Diversity

• A way that two directly competing species can
  achieve coexistence results from both species
  sharing a predator
• Both competing species (i.e., the prey) can
  coexist especially if the weaker competing
  species happens to be better at escaping
  predation
• Additionally, optimal foraging can result in prey
  caught and consumed as a function of their
  population densities such that predation
  maintains prey diversity by frequency-dependent
  effects in the same manner that
  frequency-dependent selection can maintain a
  balanced polymorphism
• One can describe a predator whose presence has a
  profound impact on the species diversity of a
  given community as a keystone species

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Keystone Species
Predator controls species abundance top-down
control
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Keystone Species
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Bottom-Up Control
Resources (e.g., food, or here rain) control
species abundance bottom-up control
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More Bottom-Up Control
Evotranspiration is a function of both water
availability and temperature
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Foundation Species
Foundation species facilitate ecosystems by their
presence
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Ecosystem Engineers
Ecosystem engineers serve as foundation species
as a consequence of their actions
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Interspecific Interactions
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Ecological Succession

• Limiting the carrying capacity for many organisms
  is that the presence of these organisms
  essentially spoils the environment for their
  continued presence
• Such organisms typically are good at finding
  environments they can exploit, exploiting those
  environments, then giving way to organisms which
  are better at hanging on in those environments
• The exploitation of an environment by one
  population, followed by the exploitation by a
  second (third, etc.) population is termed
  ecological succession
• Ecological succession continues in a habitat
  until species whose young are good at maturing
  within the same environment (as well as good at
  excluding other species) comes to dominate the
  environment, or until catastrophic change
  essentially wipes the slate clean, making an
  environment once again exploitable to the
  r-selected populations

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Ecological Succession
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Primary Succession
When the environment being exploited is
essentially lifelesslacking in both living
organisms and in their remainsthen the first
round of exploitation is termed primary
succession
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Primary Succession
Primary succession occurs, for example, following
volcanic or glacial destruction of an environment
The first organisms that exploit an otherwise
lifeless terrain are termed primary successors
Primary succession is a fairly rare occurrence
especially relative to the much-more familiar
secondary succession that we observe in disturbed
habitats all around us
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Secondary Succession
Secondary succession is succession that follows
primary succession, i.e., of an environment that
already contains life (or, at least, soil)
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Dryas
  "Because resource availability changes over the
course of succession, different species compete
better at different stages. Early stages are
typically characterized by r-selected species
that are good colonizers because of their high
fecundity and excellent dispersal mechanisms.
Many of these may be described as fugitive or
weedy species that do not compete well in
established communities, but maintain themselves
by constantly colonizing newly disturbed areas
before better competitors can become established
in the same places."
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Secondary Succession

• Alders Cottonwoods are better competitors when
  N concentrations are lower

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Big Sitka Spruces
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Ecological Succession

• The community that exists following ecological
  succession is termed the climax community
• A climax community is made up of organisms that
  are good at reproducing in the face of
  interspecific competition
• "At the climax stage, environmental conditions
  are such that the same species can continue to
  maintain themselves. For example, the
  maple-beech forest that is the climax stage of
  old-field succession in much of Ohio maintains
  the moist, shaded environment that allows
  offspring of these species to grow, while
  inhibiting most of the species typical of earlier
  stages of succession."
• Climax communities will remain in place until
  either the climate changes, a better competitor
  arrives, or the community is catastrophically
  disrupted, e.g., by fire or, more recently, by
  extensive logging

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Canadian Hemlock
Intermediate levels of disturbance, e.g.,
low-level fire or large storms, that disrupt
climax communities can result in greater levels
of community species diversity
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Secondary Succession
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Island Biogeography

• In order for an ecosystem to go through
  succession, the organisms in each wave of
  succession must be available in the local
  environment
• The farther an ecosystem is from a source of
  these organisms, the less likely these organisms
  will be present and therefore that succession
  will occur
• The smaller an ecosystem is, the less likely that
  species will find their way to there and the more
  likely that species present will go extinct (due
  to smaller size and due to resultantly smaller
  populations, respectively)
• This can be seen most obviously on islands
• the farther an island is from a source of
  organisms, the less likely the given organisms
  will find their way to the island
• and the smaller an ecosystem is, the less able
  it is to hold on to the species that it has

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Island Biogeography
Application of these ideas to our environment is
somewhat profound because they tell us that we
can't go on destroying ecosystems forever without
risking their very existence
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Island Species Richness
If we convert every last forest into farmland,
housing tract, or parking lot, the remnants of
ecosystems will be so small that they will be
unable to sustain what species they start with
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Island Species Richness
and ecosystems will be so far apart that they
will be unable to reacquire species from similar
ecosystems
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Island Species Richness
In this and the previous 2 graphs, just
understand the trends rather than memorizing
numbes or scales
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Ecological Footprint
The lesson from Island Biogeography is that
ultimately we humans are genetic bottlenecking of
the entire world, and if our goal is to survive
past this environmental disaster of our own
making, then the big losers will most definitely
be ourselves
Or, to paraphrase George Carlin, If we're so
smart, why are we peeing in our water bowl?
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The End