Mechanisms of Population Equilibrium

Chapter 4.2

1a. Predator-Prey DynamicsExample:• Isle Royale – 45 mile-long island – Lake Superior• Eary 1900s – group of moose crossed the ice• Moose pop. grew• 1949 – pack of wolves crossed the ice• 1958 – Biologists began tracking• Moose pop. increase followed by wolf pop.

increase• Moose pop. decrease followed by wolf pop.

decrease• 1980 – new cycle begins but wolves are affected

by a canine virus

1a. Predator-Prey DynamicsExample: Isle Royale

Their effects are density dependent

• High prey pop. = high predation

• Low pop. = harder for effective predation

Often incapable of killing mature, healthy individuals. This prevents predators from eliminating prey.

1a. Predator-Prey Dynamics

1b. Parasite-Host Dynamics

Similarities with predator-prey dynamics

• High prey pop. = high parasitism

• Low pop. = harder for parasites to spread

Parasites and Predators

• Parasites and predators work together - prey weakened by parasites are easy targets for predators

• Population density of a species – consequences of relationships with all natural enemies.

• Many natural enemies contributes to more stable (s curve ) prey populations.

• Many prey species contribute to more stable predator populations - Biodiversity keeps things in balance

2. Introduced species

Example: Australian Rabbit• 1859 – introduction of rabbits for game• No natural enemies• Rabbit pop. explosion – overgrazing devastated

rangeland• Rabbit virus controlled population for a time• Rabbits developed resistance to virus• 2nd rabbit virus introduced – decreased

population by 95%• Kangaroos and rare plants thrive again• Foxes were introduced but found other species

easier to catch.

2. Introduced speciesExample: Australian Rabbit

2. Introduced species

Example 2: American Chestnut• Prior to 1900 – American Chestnut was dominate

tree of Eastern US deciduous forests• High quality wood and food• 1904 – Chestnut blight (fungal disease) introduced

through planting Chinese Chestnut in NY• 1950 – Nearly every A. Chestnut tree had died out• Oaks filled the gap but still great ecological and

commercial loss• Recently – American/Chinese hybrid – 94% native –

crossbred – resistant to blight

2. Introduced species

Activity: Read p. 88-91 and complete the table and list the following for each species:

• Species

• Negative/Positive/Neutral effects on env.

• Description of effect

• Year (if stated)

2. Introduced speciesPossible outcomes:

1. Species won’t be able to thrive in new conditions – dies off

2. Species is able to survive finding a vacant spot in the ecosystem – stable pop.

3. Species thrives with no natural enemies – pop. explosion

Species adapt over time to their ecosystem, independent of other ecosystems.

2. Introduced species

Introduced species can include the following organism categories:

PlantHerbivoreCarnivoreParasite

http://youtube.com/watch?v=yS7zkTnQVaMhttp://youtube.com/watch?v

=2ChwJiKKBdA&feature=related

2. Introduced species

Ecological Lessons:

1. The regulation of populations is a matter of complex interactions among the biotic community.

2. The relationships are specific to the organisms in each ecosystem.

3. Territoriality

Territory – area of adequate resources need to rear or brood successfully.

Territoriality – defence of territory against species interested in the same resources.

Examples of territoriality: bird songs, spotting with urine, fighting/intimidation and death (rare)

3. Territoriality

Territoriality restricts breeding to those capable of claiming and defending territory.

Absence of territories – even rationing of inadequate resources causing entire population to be malnourished (risky)

4. Plant-Herbivore Dynamics

Example: Rein-deer

• St. Matthew Island (128 mi2)

• 1944 – 29 deer introduced (5m 24f) - no predators

• 1963 – 6000 individuals

• Winter 63-64 – pop. crashed - lichens replaced by unpalatable sedges and grasses

• 1966 – 42 individuals

4. Plant-Herbivore DynamicsExample: Rein-deer

4. Plant-Herbivore Dynamics

Overgrazing – feeding on plants faster than they can grow back.

Uncontrolled populations of herbivores results in:

1. replacement of food plants with other plant species

2. desertification

4. Plant-Herbivore Dynamics

Factors influencing plant-herbivore balance:

1. Predators - killing predators can result in overgrazing of herbivores.

2. Migration – animals migrate when forage is reduced. Migration often prohibited by sectioned land (fencing)

3rd Principle of Ecosystem Sustainability

The size of consumer populations are controlled so that overgrazing or other

overuse does not occur.

5. Competition between plant species

1. Adaptation to specific conditions. Landscapes are not uniform.

2. Sharing resources – different adaptations to access different resources in the same area

Examples - root systems, light requirements, growth timing

5. Competition between plant species

3. Balanced herbivory – plant populations controlled by herbivory

Monoculture – vulnerable to pests and disease; support population explosions.

Diverse ecosystems – more balanced for everybody.

5. Competition between plant species

Example: Amazon Basin

• Single acre (0.4 hectares) >100 tree species

• Individuals of a species can be 0.5 miles apart

• Rubber tree plantations failed due to pests

Class Starter

1. What species caused the demise of the American Chestnut?

2. How has the Chestnut started to make a comeback?

3. What is the first principle of ecosystem sustainability?

4. What is one factor that affects the plant-herbivore dynamics?