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AP Biology
Community Ecology
Part 1
Important concepts from previous
units:
• 1) Producers are responsible for providing
available energy, in the form of sugars, for the
food chain.
• 2) Consumers eat the producers to obtain the
energy found in their biomass.
• Community – The term refers to a
collection of interacting populations
within the same given area.
Community
Community
– Species Richness – Refers to the number of different species within a
given area.
– Relative Abundance – Refers to the population size for each species
within that given area.
• Rare – few exist ; common – many exist
Species richness is the same, however, relative abundance is not.
– Rivet model is the best way to explain the interaction among species
(A.K.A.food web model)
• It basically is stating that every species is important in the proper
functioning of that ecosystem. If you start removing species (extinction)
from the system (what we commonly call a food web), it is like removing a
rivet from an airplane. It damages the whole system. The more you
remove, the more damage done. At some point, you have removed so
many that the whole system collapses…the plane crashes killing all or
most.
Consider the ecosystem inhabited by the organisms
in the food web below. Suppose a disease caused
most of the rabbits in the ecosystem to die.
•
•
•
•
#1. Predict what effects this will have on
(a) the hawk population
(b) the amount of grass available
(c) the field mouse population
• Interspecific Interactions - These are between
two different species.
– Competition ( - );( - ) It is considered a negativenegative relationship.
• Competition exists because a resource is in small
supply.
• Active competition drains energy away from
reproduction. So populations are small.
Competition and also predation
• Competitive exclusion model - States each species has
it’s own niche in an environment.
– Two species cannot occupy the same niche… So one is
excluded from getting the resource.
– Resource partitioning (dividing) occurs due to displacement.
This can cause evolution to proceed faster – such as occurred
with Darwin’s finches of the Galapagos.
• The “excluded” species has three basic
options:
– 1) Leave the area to look elsewhere for the
resource .
– 2) Die off or
– 3) Find a replacement resource. (This promotes
evolution.)
Predation ( + ); ( - ) Considered a positive –
negative relationship.
1. Normal predation (Carnivore or Omnivore eats
an herbivore.)
2. Herbivory - Eating plants. (Also considered
predation…since they are a different species.)
Herbivory
(This is also predation.)
Skip to #4
Coming back to #3
Adaptations for being a predator – claws, teeth,
poisons, fast locomotion, muscular (All help
kill.)
Self defense adaptations against predators –
long legs, faster, flight, horns, coloration, very
good smell
i. Cryptic coloration – camouflage (like encryption)
ii. Aposematic (warning) coloration – bright colors like
reds or oranges
iii. Mimicry
• Batesian type – A harmless looks like a harmful
organism. This becomes an associative learning
exercise for the attacking species. They become very
scared to attack organisms that look similar to that
bad experience. This
increases survival rates for the mimickers.
• Műllerian type – A harmful looks like a another
harmful.
Cryptic Coloration
(Can you see the bird?)
Aposematic Coloration
Batesian Mimicry
(Catapillar and snake)
Mullerian mimicry
(Cuckoo bee and yellow jacket)
Back to #3
3. Parasitism – Death does not occur; but harm is done
to another species.
a. Three types occur
1) Ectoparasites – These attack from the outside.
(mosquito)
2) Endoparasite – These attack from the inside.
(tapeworm)
3) Parasitoidism – Insects laying eggs on a host that
will eventually become food for the larva. “oid”
means “like” It is like parasitism in that there is still
harm, but it is the larva (secondary) causing the
harm and not the insect (primary) that laid the eggs
Parasitism
Ectoparasite
Parasitism
Endoparasite (Tapeworm)
Mutualism ( + ); ( + ) Considered a positivepositive relationship.)
This relationship promotes co-evolution , But
remember that co-evolution can either be good or
bad, such as the predator/ prey relationship… it is
co-evolution too.
"A sea anemone makes an
ideal home for a clownfish. Its
poisonous tentacles provide
protection from predators and a
clownfish makes its meals from
the anemone’s leftovers."
"A clownfish can help an
anemone catch its prey by
luring other fish toward over so
that the anemone can catch
them. Clownfish also eat any
dead tentacles keeping the
anemone and the area around
it clean. "
D. Commensalism ( + ); ( 0 ) Considered a
positive- no effect relationship.
1. Few exist in nature and it is hard to see if
there is no reciprocal effect.
• Community Control Models – Essentially what
factor is controlling the species composition in
an area.
– Bottom Up Model (Nutrients in the soil determine
community composition.)(Most cases)
• Nutrients  Plants  Herbivores  Predators
– Top Down Model (Predators determine the
community composition.)
• Predators  Herbivores  plants  Nutrients
• Stability – A community at equilibrium. Very
little disturbance/change occurs over time.
• Non-equilibrium Model
– Periodic disturbances occur altering the
environments species composition over time.
– Examples of disturbances: Fire, flood, drought,
human actions, storms, hurricanes/typhoons.
– These help create new possibilities in an
environment by opening up new niches.
Number of species
present
Disturbance and
harmful effects
20
With Pisaster (control)
15
10
Without Pisaster (experimental)
5
0
1963 ’64 ’65 ’66 ’67 ’68 ’69 ’70 ’71 ’72 ’73
Disturbance
(Beaver Dams)
Fire and Disturbance
Before a controlled burn.
A prairie that has not burned for
several years has a high proportion of detritus (dead grass).
During the burn. The detritus
serves as fuel for fires.
After the burn. Approximately
one month after the controlled
burn, virtually all of the biomass
in this prairie is living.
• Ecological Succession – Change in community composition
due to time and disturbance.
– Two types can occur within environments
• Primary Succession – This is “starting from scratch”
using pioneer species – lichens and mosses.
– Hawaii going from barren volcanic rock to plush,
tropical island.
– Pioneers make the dirt needed for the plants &
birds bring seeds in their feces as they feed upon
lichens.
– Lichens  grasses  bushes  gymnosperms 
hardwood trees  Climax
• Climax Community – Hardwood forest exists all over.
Stages of Succession
Pioneer stage, with fireweed dominant
Dryas stage
60
Soil nitrogen (g/m2)
50
40
30
20
10
0
Pioneer
Dryas
Alder Spruce
Successional stage
Nitrogen fixation by Dryas and alder
increases the soil nitrogen content.
Spruce
stage
Primary Succession
(Remember Hawaii?)
Primary Succession
(Pioneer species)
Primary Succession
Primary succession
Primary Succession
Climax Community (Hawaii today)
Succession
(Mount St. Helens, WA)
https://www.youtube.com/watch?v=-H_HZVY1tT4
https://www.youtube.com/watch?v=BaRAGzjSkzo
Secondary Succession – This is “starting over
at the grasses level” not from scratch.
(Such as the farming of fields to grow crops.)
a. Dirt already exists
b. Grasses  bushes  Gymnosperms 
hardwood trees  Climax
Secondary succession
(Forest fire destroys the forest)
Secondary Succession
(Then over time…life returns)
• Biodiversity
– This measurement includes Species Richness and
Relative abundance in the calculation.
• Primarily looking at the degree of Heterogeneity.
(difference)
Richness and land area size
• Size and Location
of an environment
– Large Area –
Usually means
more species.
– More habitats and
niches present 
therefor more
species can exist.
Energy, water, and species richness
Rates of Evapotranspiration (determined by solar radiation,
temperature, and water availability) increases with abundant
rainfall…more rainfall results in greater species diversity
Equatorial  Polar gradient (going from equator
to north or south pole)
• Less biodiversity the farther north or south you get
from equator.
– Temperature and seasons, sunlight for photosynthesis,
water availability all change toward a more harsh
environment the closer you get to the poles.
– Equator  It is summer ALL year… so more plants and
animals.
– Poles  Winter ALL year… so less plants and animals.
– Above the tropics  Seasonal changes occur over the year.
– Tropics  desert (More plants and animals in the tropics
versus the desert.
Equitorial Gradients and Species
Islands Biodiversity (Defined by E. O. Wilson – 1960’s – He
is a native of Alabama.)
– Size of Island (Big – usually has much biodiversity;
Small usually has less.)
» Pygmism – Smaller sizes exist on some islands
due to smaller environment. (Smaller amounts
of resources tends to lead toward the evolution
of smaller organisms and populations.)
– Location of island in relation to mainland (Close –
less variation; far – more variation exists)
» Similarity of the environments
– Amount of disturbance can also affects the
biodiversity. If there is recurring, frequent
disturbances there will be less biodiversity. A calmer
environment promotes progression and evolution.
E.O Wilson
A Native Alabamian
Richness and Islands
As the number of species on the island increases,
immigration rate of new species decreases because any
individual reaching the island is less likely to represent
a species that is not already present.
As more species inhabit the island, extinction rates on
the island increase because of the greater likelihood of
competitive exclusion.
Pygmy Elephant of Indonesian
Islands