Transcript Community Ecology

Community Ecology
Communities

A community is a
group of organisms
of different species
that live in a
particular area
Individualistic Hypothesis vs.
Interactive Hypothesis

Individualistic Hypothesis:


A community is a chance group of species
found in the same area because they have
similar abiotic requirements
Integrated (Interactive) Hypothesis:

A community is a group of closely linked
species locked together in mandatory biotic
interactions that cause the community to
function as an integrated unit
Interspecific Interactions


Interspecific interactions are interactions
that occur between populations of different
species living together in a community
There are 4 major interspecific interactions:




Predation (and parasitism)
Competition
Commensalism
Mutualism
PredationParasitism
Predation (and Parasitism)

(+ -)


The interaction is
beneficial to one
species and
detrimental to the
other
Predation:


When a predator
eats its prey
Example in
picture:
Predation (and Parasitism)

Parasitism:

Predators that live
on or in their
hosts, usually
feeding off their
body tissues or
fluids
• Usually do not kill
their hosts

Examples in
picture (tick,
leech)
Parasitism


One organism (the
parasite) gets its
nourishment from another
organism (the host), which
is harmed in the process
Endoparasites:


Ectoparasites:


Live within host tissues
(tapeworms)
Feed on external
surfaces (mosquitoes)
Parasitoidism:

Insect lays eggs on or in
a host. The eggs feed on
the host . . . eventually
killing it
Disease

Pathogens are
similar to
parasites
(typically
bacteria, viruses
or fungi)
Plant Defenses Against
Herbivores


“Plants Fight Back!”
Plants have 2 major
mechanisms by which
they defend
themselves against
being eaten

Mechanical
Defenses
• Thorns, hooks, etc.

Chemical Defenses
• Poisons
Plant Defenses Against
Herbivores

Chemical Defenses



Produce
chemicals that are
distasteful or
harmful to an
herbivore
Morphine (opium
poppy)
Nicotine (tobacco)
Animal Defenses Against
Predators



Animals defend
themselves against
predators passively
(hiding) or actively
(fleeing)
Cryptic coloration
(camouflage) makes
prey difficult to spot
Aposematic
coloration (warning
coloration)warns
predators not to eat
animals that may be
toxic or may sting.
Animal Defenses Against
Predation

Mimicry


When one species “imitates” or “mimics” another
Batesian mimicry
• When one edible or harmless species mimics an badtasting (unpalatable) or harmful species
• Example: hawkmoth mimics a snake
Animal Defenses Against
Predation

Mimicry

Mullerian mimicry
• Two species, both of which are unpalatable (taste
bad) or harmful, resemble each other
• Example: monarch butterfly (unpalatable) and queen
butterfly (unpalatable) resemble each other
Competition
Interspecific Competition



(-/-)
Competition between organisms of different species
for a particular limited resource
The Competitive Exclusion Principle:
 Two species with similar needs for the same
limiting resources cannot coexist in the same
place
 Niches may overlap but they may not be identical.
• Niche: sum total of a species’ use of the biotic
and abiotic resources in an environment
Ecological Niches

An organism’s niche is the
specific role it plays in its
environment


All of its uses of biotic and
abiotic resources in its
environment
Example: oak tree in a deciduous
forest
• Provides oxygen to plants,
animals, etc.
• Home for squirrels
• Nesting ground for blue jays
• Takes water out of the soil
• Etc., etc.
Fundamental v. Realized
Niche


Fundamental Niche
includes resources an
organism could
theoretically use (if no
competition)
Realized Niche
includes resources it
actually does use
given competition
from other species.
Resource Partitioning

Similar species develop ways to
partition/divide resources in order to
coexist.
Character Displacement


Evidence for competition can sometimes be
determined by looking at closely related species
 Allopatric (geographically separate): are
morphologically similar and use similar
resources
 Sympatric (overlapping geography): show
different morphology and use different
resources
Character displacement: the trend of
sympatric species to be more divergent than
allopatric species
Symbiosis
A close relationship between two
organisms.
Symbiosis

Parasitism (+,-)

Already discussed
Commensalism (+,neutral)
 Mutualism (+,+)

Commensalism



(+0) relationship
One partner benefits, the other
is not affected
Examples:
 Sea anemone and
clownfish
• Clownfish gets a place to
live, sea anemone is not
affected
Mutualism
(++) relationship
Both partners benefit from the
relationship
“You scratch my back, I’ll
scratch yours”
Examples:
Ants & acacia tree
tree provides high
protein food in beltian
bodies & habitat for nests
inside thorns; ant protects
against predators
Mycorrhizae-fungal
extentions on plant roots
Plant gets increased
water/nutrition, fungi
gets food
Hummingbirds & flowers
Hummingbirds get
food, flowers can
reproduce
SUMMARY

Relationship
Organism #1
Organism #2

Commensalism
+
0

Mutualism
+
+

Parasitism
+
-

Predator Prey
+
-

Competition
-
-
Evolutionary component

Many of the relationships discussed
could be a result of coevolution

Each species influences the heritable
traits of another,closely associated,
species
Community
Structure
Community structure
Community structure describes the
make up and interactions of the
species in a community.
 Many times this is a result of 2 factors:

Species diversity
 Feeding relationships

Species diversity

Species diversity if made of two
components:
Species richness is the total number
of different species in a community
 Relative abundance is the proportion
of each species that makes up the
community

Who eats who?

A trophic structure describes the feeding
relationship between organisms in a
community
 Feeding relationships always start with
some sort of primary producer
(generally a photosynthetic organism)
 Then you will have primary consumers
(herbivores) and various secondary and
tertiary consumers (carnivores)
 Eventually, the cycle ends with
decomposers
Food chains

Food Chains- A single pathway of
energy relationships among
organisms in an ecosystem
Energy transfer

The arrows DO NOT merely show what gets
eaten
 The purpose of the arrows is to show where
the energy is going
 Scientists refer to eating as an energy
transfer, because when one organism eats
another, the main goal is to get energy from
the organism.
 SO, the arrow points at the organism that
GETS the ENERGY (the organism doing the
eating)
Limits of food chain length


A food chain is usually only a few links long
 Can be as few as 2 to as many as 5 OR more
Why?
 One hypothesis is the energetic hypothesis:
the length of a food chain is limited by the
inefficient transfer of energy from one
organism to the next (only about 10%)
• Food chains with more photosynthetic
organisms should be longer because you
have MORE starting energy
Limits of food chain length

Second hypothesis is the dynamic
stability hypothesis: long food chains
are less stable than short food chains
 The more organisms involved in a
food chain the more potential for
variation
• Extinction, migration, climatic
changes
• With more species, you have more
chances to disrupt the food chain
Food web
Food chains are a very inaccurate
depiction of feeding relationships in
an ecosystem…Food webs are
more accurate
 Food webs are interrelated food
chains of an ecosystem

Large impact

In communities, certain species may
have a larger impact on the
community structure than other
They may be highly abundant
 Play a pivotal role in maintaining the
balance in a community

Dominant Species

Dominant Species:
 Species in a community that have the highest
abundance or highest biomass
 These species have a powerful effect on the
distribution and eating patterns of all other
species in a community
 Possible reasons for a dominant species
• Dominant species is most competitive in
acquiring limited resources
• Dominant species is most successful at
avoiding predators OR disease
• This may be the reason invasive species can take
over a community that lack their natural predators
and pathogens
Keystone Species

Keystone Species:
 Important to a community because of
their ecological roles (niches), not by
numbers
 When these species become extinct,
or scarce, the entire community
changes and usually many other
species are affected
Sea otters and sea urchins
Keystone Species- Pisaster

When a species of starfish (pisaster) that feeds
on mussels was removed from an intertidal
zone, the mussel began to dominate and eat
other species (decreasing biodiversity)
Foundation Species

Some organisms exert their influence
by altering the environment

This changes the landscape and
alters the structural dynamics of the
environment
• They may act as facilitators that have a
positive affect on the community (protect
from salt variations, maintain soil
cohesion)
• They may also be harmful . . . What
species do you think has been altering
the environment the most?
Species interactions

Two simple ways to explain the effects of
organisms in a community
Bottom-up model – lower levels of the
food web influence the levels above them
(producers are the most influential and
the higher levels, top consumers, have
the least influence)
 Top-down model – the opposite of above
 There are also many intermediates of
these two

Ecological Disturbances
Disturbance


Classic view of communities:
 Communities are in a state of equilibrium
unless seriously disturbed by outside
influences (they are stable)
• Constant composition of species
Newer model is the nonequilibrium model
 Communities are constantly changing in
response to disturbances
• Disturbance: anything that changes the
community, removes organisms, or alters
the natural resources (shocker: humans
have the highest impact)
Ecological Succession

Ecological succession is a change in the
species that live in a given area over a
period of time



One community replaces another
Primary succession = occurs in places
where soil is not yet formed (bare bedrock)
Secondary succession = occurs in places
where there is soil, but where some
disturbance has eliminated the previous
community (fire, tidal wave, natural disaster)
Ecological Succession
Ecological Succession

The first organisms to inhabit an area
undergoing succession are known as
pioneer organisms


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These are usually small organisms (bacteria,
lichens, algae, etc.)
The ecosystem goes through a number of
stages, with each new stage usually
consisting of larger organisms than the last
one
Once a community has become stable and
is not changing much, it is known as a
climax community
Causes of Ecological Succession

There are 3 major causes of ecological succession:
1.
Human Activities
- logging, mining, development, etc.
2.
Natural Disasters/Disturbances
- fires, volcanic eruptions, etc.
3. Natural Competition Among Species
- Fictitious example:
- turtles and frogs both eat crickets
- frogs are faster, turtles are slower
- frogs eat more crickets, turtles starve
- turtle population dies out, frog population
gets bigger
Biodiversity
Biodiversity

Two factors are usually associated
with species diversity:
Geographic location
 Geographic size

• In general, fewer organisms on islands,
than on main continent
Tropical diversity
In general, there is a larger diversity of
species in tropical regions than in
temperate or polar regions
 Possible reasons:


Evolutionary history – in general,
tropical regions are generally older
than other biomes (more consistent
climate means fewer major
disturbances)
Tropical diversity

Climate is another reason for more
diversity in the tropics
Fairly consistent solar input
 Abundant rainfall

• Evapotranspiration measures how much
water remains in a biome as opposed to
the loss due to transpiration
• In the tropics, much of the water remains
in the biome