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Population and
Community Ecology
Chapter 35
Population Density
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Ecologists often
describe a population
in terms of its
density.
Population density is
the number of
individuals of a
particular species per
unit area or volume.
exponential growth
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the population multiplies by a constant
factor at constant time intervals.
Carrying Capacity
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In nature, a population may start growing
exponentially, but eventually one or more
environmental factors will limit its growth.
The population then stops growing or may
even begin to decrease. Space is one example
of a limiting factor, a condition that can restrict
a population's growth.
Other limiting factors include disease and
availability of food.
Carrying Capacity
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When such environmental factors limit a
population's growth rate, the population is
said to have reached its carrying capacity.
The carrying capacity is the number of
organisms in a population that the
environment can maintain
As a growing population approaches carrying
capacity, the birth rate may decrease or the
death rate may increase (or both), until they
are about equal.
Competition
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This competition within a single species limits
the growth of the population.
Within a community, interspecific competition
(competition between species) takes place when
two or more species rely on the same limited
resource.
Niches
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Within a community, each species has a
unique living arrangement called its niche.
A niche includes an organism's living place
(habitat), its food sources, the time of day
it is most active, and many other factors
specific to that organism's way of life.
The local loss of a species due to
competitive exclusion is most likely to
occur if two species have niches that are
very similar.
predation
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an interaction in which one
organism eats another.
Predators – animals that
hunt and eat other animals.
Prey- animals that get eaten
by the predator.
Symbiotic Relationships
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A symbiotic relationship is a close
interaction between species in which one
of the species lives in or on the other.
There are three main types of symbiotic
relationships: parasitism, mutualism, and
commensalism.
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Parasitism is a relationship in which one
organism, the parasite, obtains its food at the
expense of another organism, the host.
Examples include blood-sucking mosquitoes,
tapeworms that live and feed in the
intestines, wasps laying eggs on caterpillar
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In mutualism, both organisms benefit
from the symbiotic relationship.
Clownfish and sea anemones present an example of
facultative mutualism. The clownfish benefits by
having a protected home territory.
Commensalism
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is a relationship in which one organism benefits,
while the other organism is neither harmed nor
helped significantly.
This shows the symbiotic relationship
commensalism, the bird gets food from the back
of the cow but the cow is not affected
Ecological Succession
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Communities may change drastically as
a result of a disturbance.
A variety of species may colonize the
disturbed area.
These species may be replaced as yet
other species move into the area later.
This process of community change is
called ecological succession.
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When a community arises
in a lifeless area that has
no soil, the change is
called primary
succession.
Examples of such areas
are new islands created
by erupting volcanoes, or
the bare rock left behind
a retreating glacier.
Primary
Succession
Primary Succession
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Autotrophic microorganisms are first organisms to
appear.
Then lichens and mosses.
Soil develops gradually from the action of these early
colonizers and from their decomposed remains.
Once soil is present, grasses, shrubs, and trees grow.
Primary succession from barren ground to a community
such as a deciduous forest can take hundreds or even
thousands of years.
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Secondary
Succession
When a disturbance damages an existing
community but leaves the soil intact, the
change that follows is called secondary
succession.
An example is a forested area that has been
cleared for farming.
Grasses may grow from seeds carried by
animals or the wind.
They may be replaced by shrubs, and
eventually by trees similar to those in the
original forest.