Community Ecology 10/27/06

Review of last time: Multiple ChoiceS Which of the following are true of the following equations: Circle ALL correct answers: The equations could describe competition between the two species. The equations could describe a predator-prey relationship. Interspecific competition dominates intraspecific competition for species F. d.When the population S is at zero, and population F is close to zero, the model predicts that population F will grow exponentially

Bottom-up vs. Top-Down control Predators can promote diversity by keeping competition in check Predator-Prey Relationships

Mutualism  Both species benefit

Mutualism  If it is a mutually beneficial relationship, then the two populations increase each other’s size  Population 1  N 1  ti  Population 2  N 2 = r 1 * N 1 = r 1 * N 1 dN 1 dt K 1 - N 1 + a 12 N 2 K 1 = r 2 * N 2 = r 2 * N 2 dN 2 dt K 2 - N 2 + a 21 N 1 K 2 Because this is a positive term, K is increased

Commensalism  One species benefits, the other is unaffected

Commensalism  If the relationship is commensalistic, one species benefits (the commensal) and the other is unaffected  Population 1  N 1  Population 2  N 2 = r 1 * N 1 = r 1 * N 1 dN 1 dt K 1 - N 1 + a 12 N 2 K 1 = r 2 * N 2 = r 2 * N 2 dN 2 dt K 2 - N 2 K 2 Because this is a positive term, K is increased Because there is no a 21 term, K is unchanged

Parasitism How would you model it???

Assumptions of Lotka-Volterra Models  All assumptions of logistic growth model for each species… plus:  Interaction coefficients, carrying capacities, and intrinsic growth rates are constant.

Summary of Interaction Equations: Competition: (-, -) Predator/Prey:(+, -) Mutualism:(+, +) Commensalism: (+, 0)

Test you knowledge! What type of relationship– what equation to use?  A coati eats tree fruit.  Your dog has a flea  You use a fast bicyclist to “draft” off of

Problems with Simple Logistic Growth 1. Births and deaths not separated -you might want to look at these processes separately -predation may have no effect on birth rate 2. Carrying capacity is an arbitrary, set value 3. No age structure

1. Separate Births and Deaths = Births - Deaths Births = b*N Deaths = d*N dN dt Births and deaths may be density dependent

1. Separate Births and Deaths = Births - Deaths Births = b*N Deaths = d*N dN dt Births rate may be density dependent Death rate may be dominated by predator effects Example: Births = b*N(1- N ) K Deaths = d b +a 21 N 2

2. Refine Carrying Capacity If the population is a herbivore, K may depend on the population of plants = r H * N H (1 – ) = r H * N H (1 – ) dN H dt NHNPNHNP K herbivore = N plant

Remaining Problems  Age Structure  Space: animals rely on different parts of landscape for different parts of their life cycle  Individuality: Populations are collections of individuals, not lumped pools

General Notes on Using Models  How complex should model be? K.I.S.S.  Identify research needs:  Build model structure  Test model to see what it is most sensitive to  Do research to find values of unknown parameters  If build a model that accurately predicts dynamics, it can be used as a management tool.  Look critically at assumptions!

Community Dynamics Community: a group of populations (both plants and animals) that live together in a defined region.

Trophic Cascade Eagles Foxes Mice Plants1 st trophic level 2 nd trophic level 3 rd trophic level 4 th trophic level autotroph/ primary producer herbivore/ primary consumer predator/ secondary consumer predator/ tertiary consumer

How would we Model the Fox Population? Why not include the effect of the plant population? What if foxes had a competitor?

Trophic Cascade Eagles Foxes Mice Plants1 st trophic level 2 nd trophic level 3 rd trophic level 4 th trophic level if eagles go extinct, what could happen to… foxes? mice? plants?

Trophic Cascade Eagles Foxes Mice Plants1 st trophic level 2 nd trophic level 3 rd trophic level 4 th trophic level If a new predator on mice is introduced, what could happen to… mice? plants? foxes? eagles?

Trophic Cascade Eagles Foxes Mice Plants1 st trophic level 2 nd trophic level 3 rd trophic level 4 th trophic level If drought caused a dip in plant production, what would happen to… mice? foxes? eagles?

Simplified Temperate Forest Food Web What happens to when it’s a WEB instead of a CHAIN? Oak seedling Deer Wolf Fox Rabbit GrassesHerbs Caterpillars Shrews Eagle In long term, balance is restored

Food Web doesn’t account for Keystone Species

Summary  Modeling Species Interactions  Competition  Predator-prey  Mutualism  Commensalism  Community Dynamics  Food Webs  Keystone Species GOOD LUCK ON MONDAY’S MIDTERM!!! NO BLUE BOOKS OR CALCULATORS NEEDED.