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

F

SFFFF

F

K

NNKNr

dt

dN 2.0

S

FSSSS

S

K

NNKNr

dt

dN 5

•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 N1

ti

Population 2 N2

= r= r11 * N * N11 dN

1 dt

K1 - N1 + a12 N2 K1

= r= r22 * N * N22 dN

2 dt

K2 - N2 + a21 N1 K2

Because this is a positive term, K is increased

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 N1

Population 2 N2

= r= r11 * N * N11 dN

1 dt

K1 - N1 + a12 N2 K1

= r= r22 * N * N22 dN

2 dt

K2 - N2

K2

Because this is a positive term, K is increased

Because there is no a21 term, K is unchanged

ParasitismHow 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)

1

2121111

1 ?

K

NaNKNr

dt

dN

2

1212222

2 ?

K

NaNKNr

dt

dN

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 dependentDeath rate may be dominated by predator effects

Example:Births = b*N(1- N )

KDeaths = db+a21N2

2. Refine Carrying Capacity

If the population is a herbivore, K may depend on the population of plants

= r= rHH * N * NHH (1 – ) (1 – )dNH

dtNH NP

Kherbivore= Nplant

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 structureTest model to see what it is most sensitive toDo 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

Plants 1st trophic level

2nd trophic level

3rd trophic level

4th 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?

F

EFEMFMFFFF

F

K

NaNaNKNr

dt

dN

Trophic Cascade

Eagles

Foxes

Mice

Plants 1st trophic level

2nd trophic level

3rd trophic level

4th trophic level

if eagles go extinct, what could happen to…

foxes?

mice?

plants?

Trophic Cascade

Eagles

Foxes

Mice

Plants 1st trophic level

2nd trophic level

3rd trophic level

4th trophic levelIf a new

predator on mice is introduced, what could happen to…

mice?

plants?

foxes?

eagles?

Trophic Cascade

Eagles

Foxes

Mice

Plants 1st trophic level

2nd trophic level

3rd trophic level

4th trophic levelIf 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

Grasses Herbs

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.