Dynamic Ecosystems

Part 3: Population Dynamics

• Population growth depends on several things. The four factors affecting population size are:

(natality) (mortality)

• What factors are involved in the reproduction and death of a species? (What kind of things can change the population?)

– Food supply

– Disease

– Birth control

– Climate

– Job availability

– War/predation

– Availability of space

– Natural disasters

* These are called limitingfactors. They can be characterized in two categories.

1. Density Dependent

– Factors that can change the population because of the amount of organisms that live in that population.

• Examples

– Competition

– Predation

– Parasitism

– Disease

Limiting Factors can be characterized in

two categories:

Competition

• Competition within an ecosystem occurs when organisms seek the same limited resource for survival (such as food, water or shelter).

• If the niches for two different species contain the same fooditem or the same living space, then members of those species will be in conflict with one another for those parts of their niches.

• The greater the population, the greater the competition.

Predation +/-• This is when one organism kills and eats

another organism.

• Predator (hunter) – hunts and kills other animals for food

• Prey (the hunted) – animals the predator hunts

2. Density Independent

–Factors that can change the population regardless of the amount of organisms that live in that population.

• Examples

–Unusual weather

–Natural disasters

–Human activity

Natural Occurrences

Human Activity

Parasites **Not in notes

Parasitism +/-

• One organism benefits, the other is harmed– Some live within the host

• Tapeworms

• Heartworms

• Bacteria

– Some feed on the external surface of the host• Ticks

• Fleas

• Mistletoe

– Most do not kill their host (at least not quickly)

Population Curves

• These show changes, growth, decline and limits of a population. They are useful to determine how healthy a population is in an ecosystem.

• Is it possible to have infinite population growth?

0

20000000

40000000

60000000

80000000

100000000

120000000

140000000

160000000

180000000

200000000

1 2 3 4 5 6

Houseflies Pop. Growth

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2

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4

• 1 – slow growth: only a few able to reproduce; the population adjusts to environmental conditions

• 2 – rapid growth: many are able to reproduce; the steepness depends on limiting factors

• 3 – zero growth: the population growth is stable and at its maximum. The population has reached its carrying capacity

• 4 – decline: the population decreases from lack of resources, predation, human activity, etc.

1

2

3

4

*Carrying capacity – the largest population of a species that a particular ecosystem can support

Predator-Prey Relationships

The density of each population is dependent on the density of the other.

Predator-Prey Relationships

• The prey population increases when there are few predators

• Now the predator has lots of food, so its population will increase

Predator-Prey Relationships

• As the predator population increases, there is more predation and competition, so the prey population decreases

• Now the predators have less food, so their population decreases

• Notice that:– The two population curves follow the same shape– The peaks and valleys of the predator curve trail the peaks and

valleys of the prey curve.

Predator-Prey Relationship