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
Input
Immigration
&
Births
Output
Emigration
&
Deaths
Population size

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:
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)

18. Resource partitioning

19. Community Interactions…
Predation - the use of one species as a resource by another
Four categories:
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
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:
Food supply species (figs)
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)
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:
Primary succession – occurs only on surfaces without any soil (new volcanic area; abandoned parking lot)
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