Population Characteristics

By Joelle VanderVelde

Population Ecology

Population ecology is looking into the size, density, distribution, and changes of a population. By studying the changes in population, we are able to predict growth rates and trends. The health and location of species can also be determined, which makes it easier to manage the size of populations.

Distribution of Populations

The distribution of a population is how far apart or close together the inhabitants live compared to each other. It's determined by their geographic range, population size and density, and their dispersion throughout the area.

Geographic range

Geographic range is the area that a population of beings or plants inhabits. The Antarctic midge has a small range as they only live in one place: Antarctica. A Blue whale is an example of an animal that has a large range, as they're found in every ocean on the planet.

A population's habitat is more specific than their range, as it's the place where they tend to live.

Population Size and Density

Population size (N) is the number of individuals in a species that take up a given area at a specific time.

Population density (D) is the amount of individuals per unit area or per unit volume.

  • In order to take into account the fact that not all the space is habitable, they include the crude and ecological density

        • Crude density: the total number of individuals in the entire habitat

        • Ecological density: the number of individuals per unit area/volume

In order to find the density, you divide the number of individuals by the space occupied (S).

  • D = N / S

Dispersion

Clumped dispersion


Species clump together when resources are distributed in patches, when the animals are social, and when seeds aren't dispersed enough.



Random dispersion


Species are randomly dispersed when the resources are equally spaced, or the seeds are blown about before they are planted in the ground



Uniform dispersion

Uniform dispersion occurs when species compete for resources, breeding, or nesting grounds. Some plant species such as the Creosote bushes take up all the nutrients and secrete toxins, making it impossible for plants to grow near them.


Studying Populations

In order to understand population dynamics, we need to measure population size and density. That way the populations of both endangered and important species can be watched, and agricultural pests can be controlled. There are a couple different ways to record population size and density: Quadrat sampling, the mark-recapture method, and tracking.

Quadrats

Quadrats are squared structures, and they vary in the materials used to make them. The size of the quadrat reflects the size of area or species you're sampling. In order for the experiment to work properly, the need to be placed randomly inside the area. After placing them, you count the amount inside all the squares and find the average.

Estimated population density =

Total number of individuals ➗ sampling area

Estimated population size =

(estimated population density) (total size of study area)


Tracking

Through tracking, biologists are able to look further into animal's behavior. By using either GPS systems or DNA sampling, different species can be followed as they travel. This has allowed us to learn about the species' range, distribution, and population density.


Mark-recapture

The mark-recapture method is where a particular number of animals are captured and marked. After they've been successfully marked, they're released back into the population where it's hoped they'll spread out normally. After a period of time, some animals are captured again, and biologists are able to see how many are marked and how many aren't. Using the two numbers, they estimate the size of the population.


total number marked(M) / total population(N) = number of recaptures(m) / size of second sample(n)



OR



N = Mn / m




Five things are assumed during this method:


  1. The chances to catch each individual must be equal, one isn’t harder or easier to catch than the others

  2. The proportion of marked to unmarked animals stays the same between capture and recapture, no immigration or migration/birth or death

  3. There’s enough time between capture and recapture for the animals to spread out randomly

  4. The animals ability to live and survive isn’t affected by their mark

  5. The mark doesn’t come off




Ethics of Studying Populations

Some speculate that capturing the animals in order to mark them and such may change their behaviour in the wild. The effects of the animals being tranquilized multiple times are still unknown, so we should keep the their health in mind whenever biologists attempt to capture them. The effects of collars and tags are also unknown, but there's a chance they may be keeping the subject from being able to live freely.

When it comes to tracking, we need to make sure we handle the animals in a humane way. Their safety should be number one priority.