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 overshoot followed
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