Download Chapter 6 - Population and Community Ecology

Population and
Community Ecology
Chapter 6
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
Population ecology
 Study the factors that cause population to
increase and decrease
Input
Immigration
&
Births
Population size
Output
Emigration
&
Deaths
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
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
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
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
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
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
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
Survivorship Curves
 Patterns of survival over time:

Type I – high survival throughout most of their
lifespan


Type III – low survival early in life; few
individuals reach adulthood


K-selected species: humans, elephants
r-selected species: mosquitoes, dandelions
Type II – relatively constant decline in
survivorship throughout their lifespan

squirrels, coral
Survivorship Curves…
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
Community Interactions

Competition


Competitive exclusion principal


Individuals must ‘fight’ over the same limiting resource
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.
2.
3.
Temporal resource partitioning – use the same resource but at
different times (coyotes and wolves)
Spatial resource partitioning – use different locations (plants with
shallow roots vs. deep roots)
Morphological resource partitioning – evolution of different body plans
to use different parts of the resource (Darwin’s finches)
Resource partitioning
Community Interactions…
 Predation - the use of one species as a
resource by another

Four categories:
1.
2.
3.
True predators – kill and eat their prey
Herbivores – consume plants as prey; typically
only eat some of the plant; rarely kill the plant
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
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
Keystone Species

1.
2.
3.
The species on which the
ecosystem stability
depends – removing it
leads to instability:
Food supply species (figs)
Predator-mediated
competition – the predator
keeps the numbers of the
superior competitor in
check. Without the predator,
the competitor overpopulates the ecosystem
(sea stars)
Ecosystem engineers –
create habitat for other
species (beavers)
Keystone Species…
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
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