Why do populations vary in size?

•Population growth•Limiting factors •Interspecific competition• Intraspecific competition• Predation

Slow growth Rapid Stable state – no growth

Population growth curves

Small fluctuations due to changes in factors such as food supply.

The growth of a population is slowed by limiting factors.

Imagine a single celled Algae living in a new pond in summer, the algae divides rapidly at first as there are no limiting factors. What will slow the algal growth?

Lack of mineral ions Algae at the surface prevent light reaching

algae further down. Other species – competition or predators Winter – lower temperatures or light intensity

Limiting factors

Abiotic Temperature: each species has an optimum

temperature.Below, the metabolic rate drops.Above, enzymes denature.Even animals that can maintain a relatively

constant temperature will be affected by temperature as they will expend more energy trying to maintain their normal body temperature. So they mature and reproduce slower.

Limiting factors

Abiotic LightThe rate of photosynthesis increases as the

light intensity does. Therefore more plants grow and more animals can be fed.

Water and humidityLack of water results in a small population of

animals adapted to dry conditions. Humidity affects plant transpiration rates and

evaporation of water from animals bodies.

Limiting factors

Why do populations never grow indefinitely?

What is the difference between biotic and abiotic factors?

Suggest the level and type of abiotic factor that is most likely to limit the population size of the organisms:

a. Ground plants on a forest floorb. Hares in a sandy desert.c. Bacteria on a high mountain summit.

Questions

Where would only the blue species be found?

Where would moisture and temperature allow both species to live?

The temperature too high for the blue’s?

There is competition between the 2?

Application

1 2

3

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Competition between members of the same species.

Competing for food, water, breeding sites etc.

Availability of resources determines the size of the population.

Examples?

Intraspecific competition

Competition between members of different species.

Where 2 species occupy the same niche one may have a competitive advantage and will increase in size while the other diminishes. This can lead to the complete removal of a species = the competitive exclusion principle. No two species can occupy the same niche indefinitely when resources are limiting.

Interspecific competition

When cultured in isolation, the population density of each species of Paramecium increases rapidly and then levels off

In mixed culture, competition for food, space and other minerals results in the elimination of Paramecium caudatum after about 20 days; the more rapidly

reproducing Paramecium aurelia outcompetes its rival

Predator and Prey

An organism’s niche includes how it avoids being eaten and how it finds or captures its food. Predators are

consumers that capture and eat other consumers. The prey is the organism that is captured by the

predator. http://www.bbc.co.uk/nature/adaptations/Predation#p00g9r9k

Niches and barnacles

Balanus balanoides

chthamalus stellatus

Predator and Prey

The deer is being eaten by the python. It is prey.

The python has captured and is eating the deer.

Predator and Prey

Predator

Prey

Predator and Prey

PredatorPrey

Predator and Prey

Prey Predator

Predator and Prey

This alligator is the predator. It eats the python.

This python is the prey. It is eaten by the alligator.

Lynx vs. Hare

Predator/Prey Relationship

Predator/Prey Relationship

The presence of predators usually increases the number of different species that can live in an ecosystem. Predators limit

the size of prey populations. As a result, food and other

resources are less likely to become scarce, and competition

between species is reduced.

Predator/Prey Relationship