Population and Community Ecology

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. 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) Keystone Species

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