6. BIO1426 Population density and dispersion

What is the definition of a population?

A group of individuals of a single species living in the same general area.

They share resources, are influenced by the same environmental factors and are likely to interact with each other.

What is the definition of population density?

The number of organisms per unit area/volume.

E.g., the number of E.coli bacteria per millilitre in a test tube.

How are populations’ boundaries defined?

Population boundaries are a grey area that are not so clearly defined and can vary.

The population may be defined by what suits the ecologist/organisms and the questions being asked, or they can be natural such as an island or a lake.

How to measure population size and density simply

In some cases this can be determined by counting all individuals within the boundaries of the population. E.g. counting all the sea stars in a tide pool or all the elephants in a population (from a plane).

How to measure population size and density when it is impossible to count all individuals

Sampling techniques to estimate a smaller area/portion of the population, which is then extended to the population size of the entire area.

E.g., sample 5cm of a plant stem with ants on, then multiply that by the estimated number of 5cm stretches that the population occupies.

E.g., count the number of oak trees in several random 100m x 100m plots, calculate the average density in the plots, and then extend the estimate to the entire population size of the area.

What is the advantage of sampling a portion of the population?

By measuring individuals per unit area comparisons between populations of different sizes can be made.

How to make sampling more accurate?

The more sample lots used the more accurate the population estimated will be.

Why may organisms be too hard to find?

They may have good camouflage, be nocturnal, live underground.

How to measure population size and density when organisms are hard to find?

Use indicators such as the number of nests, burrows, tracks or feral droppings.

How is population size calculated?

The capture-mark-recapture method.

What is the capture-mark-recapture method?

Individuals in a population are captured and marked (ethically and in a way that will not impact their chances of survival).

They are then released back into their population and allowed to settle.

Then researchers return to the same population and recapture individuals in the same way.

The more individuals recaptured that are marked = smaller the population.

What are the problems with capture-mark-recapture?

Assumes that none have died.

That tags/marks haven’t come off.

That there’s been no migration.

That there’s been no births.

That they remixed with their original population.

That no one else has interfered with the population.

That markings haven’t impacted their survival.

Capture-mark-recapture worked example

N = ?

Capture and mark 10 snails. Release them.

Recapture 5.

3/5 of these are marked.

so 3/5 = 10/N.

3×3.333 = 10, so 5×3.333 = 16.66667.

What is the definition of population dispersion?

The spatial distribution of individuals within the population (on the basis of favourable conditions for their survival).

Can be clumped (aggregated in patches), uniform (evenly spaced) or random (unpredictable spacing).

What does dispersion indicate?

Differing dispersion/spacing indicates the biotic and abiotic factors that impact individuals in the population.

Why are most populations clumped?

As animals like to be in groups for better protection etc and as they will come together to breed/for mating reasons.

Plants and fungi favour environmental factors such as certain soil conditions for germination and growth.

Insects also favour certain environmental factors, such as high humidity under a log.

Why are some populations uniform?

May result from interactions between individuals in the population such as antagonistic social interactions like territoriality...

E.g., some plants secrete chemicals that inhibit the germination and growth of nearby individuals that could compete for resources

Why are some populations random?

The position of each individual is independent of other individuals.

Therefore there are no strong attractions or repulsions among individuals.

E.g., plants established by windblown seeds.

What does it mean if populations are dynamic?

Density/population size is not static but can change over time as individuals are added to/removed from a population.

How do populations change over time?

Births and immigration.

Deaths and emigration.

What is the definition of immigration?

The influx of new individuals from other areas.

What is the definition of emigration?

The movement of individuals out of a population and into other locations.