Population Dynamics and Sampling

In short: Ecologists estimate population size by sampling, because counting every organism is rarely possible. Quadrats and transects sample plants and slow animals, and capture-mark-recapture estimates moving animals with $N=\frac{M\times C}{R}$. Populations grow until they reach the carrying capacity.

Starting point: what you already know

• A population is all the members of one species in an area (see the previous chapter).
• Calculating a mean and a percentage.
• Working scientifically: a larger sample gives a more reliable result.

Why sample?

Counting every organism is usually impossible. Instead, ecologists count a sample and use it to estimate the whole population. Samples must be random (to avoid bias) and large enough to be reliable.

Population density = number of individuals ÷ area.

Quadrats: for plants and slow animals

A quadrat is a square frame, often $1\text{ m}\times 1\text{ m}$. Place it randomly many times and count the organisms inside.

When counting is hard (grasses), use percentage cover: estimate how much of the quadrat each species covers.

Transects: for changes across an area

A transect is a line across a habitat, such as from the water's edge up a beach or across a rock platform. Organisms are recorded at regular intervals along it. A belt transect places quadrats along the line. Transects show how species change with an abiotic factor like height above the tide.

Capture-mark-recapture: for moving animals

1. Catch a sample of animals, mark them harmlessly and release them.
2. Later, catch a second sample and count how many are marked.

$$N=\frac{M\times C}{R}$$

where $N$ = population estimate, $M$ = number marked first, $C$ = total caught second time, $R$ = number recaptured with marks. This is the Lincoln index.

The method assumes marked animals mix back in evenly, marks do not fall off or make animals easier to catch, and no animals are born, die or move in between samples.

How populations change

A population grows by births and immigration (moving in). It shrinks by deaths and emigration (moving out).

When a few animals enter a new place with plenty of food, numbers grow slowly, then very fast (exponential growth). As food and space run short, growth slows and numbers level off at the carrying capacity: the largest population the environment can support. This makes an S-shaped curve.

Limiting factors stop growth: food, water, shelter, disease, predators.

Predator and prey

1. Lots of prey: predators have plenty to eat and their numbers rise.
2. More predators eat more prey, so prey numbers fall.
3. With less food, predator numbers fall.
4. With fewer predators, prey recover, and the cycle repeats.

Quick summary

• Samples must be random and large enough. Density = number ÷ area.
• Quadrats estimate plants and slow animals. Transects show change across a habitat.
• Capture-mark-recapture: $N=\frac{M\times C}{R}$.
• Populations grow toward the carrying capacity, limited by food, space, disease and predators.
• Predator numbers rise and fall after prey numbers.

Quiz