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Populations and Resources
Population
Is a group of organisms of one species
that lives in the same place, at the
same time, and can successfully
reproduce.
Example: Elephants.
Dynamic characteristics of populations
Size, number of
individuals (N)
Density (N/ area)
Dispersion,
Random, uniform,
clumped,
(appropriate scale)
Population Dispersion Patterns
Random
Uniform
Clumped
Dynamic characteristics of populations
Age distribution,
proportions of young, middle-aged, old
Differs in growing, stable, decreasing populations
Changes in populations
Growth
Decline
May affect size, density, dispersion, age
distribution.
May be affected by size, density, dispersion,
age distribution.
Changes in populations
Growth
Expansion of species’ populations may lead to
evolution of new species
Decline
Shrinking species’ populations may lead to
extinction
Small populations
Narrowly specialized species
02 June 2010
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Changes in populations
ΔN = +B +I –D –E
+B = births (birth rate)
+I = immigrants (immigration rate)
– D = deaths (death rate)
– E = emigrants (emigration rate)
(For many [most] natural populations I and E are
minimal.)
Exponential Growth
Accelerating growth
that produces a J-like
curve when the
population is
graphed.
Only occurs for short
periods in nature
Usually when the
organism has a lot of
resources.
Limiting Factors
Exponential growth cannot be
sustained in nature because no
ecosystem has an unlimited supply of
resources.
These restrictions are called limiting
factors.
Abiotic limiting factors: temperature,
pH, oxygen, light, hiding places
Biotic limiting factors: food, etc.
Factors which can limit the
growth of a population include:
Predation
Shortage of food
*Competition
Lack of water
Lack of space
Mating Opportunities
Disease
Disaster
Contamination of the environment
Carrying Capacity
Is the size of a population that can be
supported indefinitely by the resources and
services of an ecosystem.
When a population is maintained at its carrying
capacity, it is at an
equilibrium or balance.
There is an equal
amount of organisms
being born or moving
into the ecosystem
as there are dying
or leaving.
Carrying Capacity
“The largest number of individuals in a
species that an environment can
support.”
4 factors that determine carrying capacity:
1. Materials/energy
2. Food Chains
3. Competition
4. Density
Materials and Energy
All populations are limited by the
availability of resources:
Energy (from the sun)
Water
Oxygen
Carbon
Nitrogen
Other nutrients
Food Chains
The Pyramids of Biomass, Energy, and
Numbers demonstrate how the population
of a given organism is limited by:
its position in a food chain/web
the population size of those populations above and
below it
Relationships:
Producer – consumer - decomposer
Predator - prey
Competition
Ecosystems have limited resources.
Organisms with similar needs must compete to
get the resources they need.
Supply versus Demand
Competition occurs between:
Members of the same species = Intraspecific
Competition
Members of different species = Interspecific
Competition
Density
An organisms’ need for space depends on
their size, environment, and way of life.
Different species have different needs for
space.
This need for space determines how many
individuals of a given species can live in the
same area at the same time.
Factors affecting population size may be:
1. Density-Dependent Factors
2. Density-Independent Factors
Density Dependent Factors
If population density increases, it can
cause a decrease in the population.
Overcrowding leads to:
Increased stress = spread of disease and parasites
Increased aggression = neglect of offspring
Reduced access to resources, mates and habitat
Increased death rate/decreased birth rate
=decrease in population
Density Independent Factors
Events that can decrease population size
that are not related to how many
individuals are in the population to
begin with:
Forest fire
Flood
Volcano
Extreme weather (ex. Hurricane)
Pollution
Graphing Carrying Capacity
Humans and Carrying Capacity
Humans alter natural ecosystems often,
causing a change to carrying capacities.
Urban sprawl is the growth of relatively lowdensity development on the edges of urban
areas.
Human population in the Golden Horseshoe
is expected to increase by 3.7 million
between 2005 and 2031. How do we make
room for all these people without affecting
other ecosystems?
Easter Island
Discovered by Polynesians ~ A.D. 1000
Population grew to several thousand
Used trees for canoes to hunt dolphins
Used wood for cooking
Also ate birds, eggs, vegetables
Resources (trees) depleted
No canoes, no dolphins
Warfare over land, food resources
Population fell to ~ 100 when discovered by
Dutch, Easter Sunday 1722.
Ecological Niche
As species go about their daily
activities, they interact with other
species.
An ecological niche is the way an
organism occupies a position in an
ecosystem, including abiotic and biotic
factors.
Example: brown bat
Biotic niche: insects
that it eats, its
competitors like the
nighthawk, and its
predators.
Abiotic niche: place
for roosting and
hibernation, the time
of night it hunts,
airspace it flies in,
the temperature
range it can live at.
Population Regulation
Predators: organism that kills and consumes
other organisms
Prey: is eaten as food by a predator
Bottom-Up Population regulation: a shortage
of plant resource at the base of the food
chain causes declines in the animals in the
higher trophic levels.
Top-Down Population regulation: more
rabbits in a population, lead to more coyotes
to eat them, then more coyotes lead to fewer
rabbits.
Competition
Competition occurs when 2 or more
organisms compete for the same
resource.
Can limit size of population
Can influence the ecological niche of
an organism
Symbiosis
Mutualism: 2 species benefit from the
relationship
Example: algae lives in coral. The algae
gives the coral energy and the coral gives
the algae protection and carbon dioxide.
Commensalism: 1 species benefits
while the other is unaffected.
Parasitism: when an organism benefits
at the expense of a different organism.