Ch 6 Population and Community Ecology.ppt

Individual
Population – same species, same time, same area
Community – all the different populations in an area
Ecosystem – all the different communities plus the abiotic factors in an area
Biosphere – all areas on Earth where life exists

1. Population and
Community Ecology
Chapter 6

2. Levels of complexity
Individual
Population –same species, same time, same
area
Community –all the different populations in
an area
Ecosystem –all the different communities
plus the abiotic factors in an area
Biosphere –all areas on Earth where life
exists

3. Population ecology
Study the factors that cause population to
increase and decrease
Population size
Input
Immigration
&
Births
Output
Emigration
&
Deaths

4. Basic population characteristics
Population size= total number of individuals (N)
Population density= number of individuals per unit of area
Helps us understand if the species is rare or abundant
Population distribution= how individuals are spaced relative to
others in the population
Random–no pattern of location (trees in a forest)
Uniform–fairly even spacing (nesting birds)
Clumped–individuals gather around each other (schooling fish)
Population sex ratio= the ratio of males to females
Usually 50:50
Population increase is related to the number of females
Population age structure= the number of individuals in each age
category
Populations with large numbers of young increasing
Populations with large numbers of old decreasing

7. Factors that influence population size
Density-dependent factors
Influence an individual’s odds of survival in a
manner that depends on the size of the
population
Example: available food
These factors are also called limiting resources
The population limit in an ecosystem is its
carrying capacity

8. Factors that influence population
size…
Density-independent factors
Have the same effect on an individual’s odds
of survival regardless of the size of the
population
Example: a tornado

9. Population growth models
Exponential growth model
Growth rate= number of offspring –deaths
Under ideal conditions (with unlimited
resources) each species has a particular
intrinsic growth rate–the max for that
species
This model calculates this maximum rate and
displays it as a J-shaped curve(because
there are no limits)
Only beginning populations can actually show
this type of growth

10. Population growth models…
Logistic growth model
Includes environmental limits on the
population growth
As the population reaches the carrying
capacity, the growth slows and then stops
This produces an S-shaped curve
Some populations cycle above and below the
carrying capacity –this is overshootfollowed
by die-off

12. Reproductive strategies
K-selected species
Low intrinsic growth rate
Slowly reach the carrying capacity and then
stay there
Characteristics:
Large
Later maturing
Few offspring
Substantial parental care

13. Population growth models…
r-Selected Species
High intrinsic growth rate
Rapid population growth followed by
overshoots and die-offs
Characteristics:
Small
Early maturity
Small offspring
Little or no parental care

14. Survivorship Curves
Patterns of survival over time:
Type I –high survival throughout most of their
lifespan
K-selected species: humans, elephants
Type III –low survival early in life; few
individuals reach adulthood
r-selected species: mosquitoes, dandelions
Type II –relatively constant decline in
survivorship throughout their lifespan
squirrels, coral

15. Survivorship Curves…

16. Metapopulations
Smaller, fragmented parts of a larger overall
population
Occasionally members of one metapopulation
move from one to the other
This can reduce the risk of extinction:
Moving individuals increase genetic diversity
as well as the size of a population
Human development is causing more and
more metapopulations to form

17. Community Interactions
Competition
Individuals must ‘fight’ over the same limiting resource
Competitive exclusion principal
Two species competing for the same limiting resource cannot coexist
Resource partitioning
Two species divide the resource based on differences in behavior or
morphology
This can lead to natural selection which over time will increase the
differences between the 2 species
Three possibilities:
1.Temporal resource partitioning –use the same resource but at
different times (coyotes and wolves)
2.Spatial resource partitioning –use different locations (plants with
shallow roots vs. deep roots)
3.Morphological resource partitioning –evolution of different body plans
to use different parts of the resource (Darwin’s finches)

18. Resource partitioning

19. Community Interactions…
Predation -the use of one species as a
resource by another
Four categories:
1.True predators –kill and eat their prey
2.Herbivores –consume plants as prey; typically
only eat some of the plant; rarely kill the plant
3.Parasites –live on or in a host organism; rarely
causes the death of their host
Pathogen –disease-causing parasite
4.Parasitoids –lay eggs inside another organism

20. Community Interactions…
Mutualism –two species interacting in a way
that increases the survivability of both
Plants and the insects that pollinate them
Acacia trees and ants
Commensalism –one species benefits from
an association with another but the other is
not helped nor harmed
Birds nesting in trees

21. Keystone Species
The species on which the
ecosystem stability
depends –removing it
leads to instability:
1.Food supply species (figs)
2.Predator-mediated
competition –the predator
keeps the numbers of the
superior competitor in
check. Without the predator,
the competitor over-
populates the ecosystem
(sea stars)
3.Ecosystem engineers –
create habitat for other
species (beavers)

22. Keystone Species…

23. Changes in communities over time
Ecological succession –predictable replacement of one group
of species by another
Two types:
1.Primary succession –occurs only on surfaces without any
soil (new volcanic area; abandoned parking lot)
2.Secondary succession –occurs in disturbed areas that
have not lost their soil –the original vegetation has been
removed as in a forest fire or even abandoned farmland
Pioneer species –plants that are able to colonize new areas at
the early stages of succession. They grow rapidly and need
lots of sunlight
Climax community –the later stages of succession. Generally
considered to be the ‘typical’ type of community for that biome

24. Factors affecting species richness
Latitude:
equator to poles number of species
declines
Time:
longer areas have been aroundmore
species
Habitat size:
larger habitat area more species
Distance from other habitats:
increase distance fewer species