Estimating Population Size

Measuring a population size presents a few problems. Habitats can be huge and brimming with millions of tiny organisms. Imagine how long it would take to count every woodlouse living in a forest! Plus, how would you know that you aren't counting the same woodlice again and again? So, ecologists prefer to estimate population sizes instead of measuring them. Estimation saves time, money, resources and… patience!

Estimating Population Size: Definition

Before describing how to estimate the size of a population, let's define what a population is.

Population ecology is the study of populations in relation to their natural habitat. Populations naturally fluctuate, depending on biotic (living) and abiotic (non-living) factors.

In the natural world, multiple populations live and interact with each other in a community. Biodiversity is measured at the community level.

Estimating population size is a method used to determine the abundance of one or more species living in a habitat. It's impossible to count every individual living thing in a habitat, so ecologists use special techniques and formulas to make an educated guess.

The Importance of Population Estimation

Populations experience constant change. The number of individuals can fluctuate either up or down, depending on several potential causes. Population dynamics can have a major influence on the stability, development, and growth of a population.

Table 1: Potential causes for the change in a population size.

Population estimations can detect if a population is experiencing a significant decline. Such a decline could present an issue for the species because small populations experience a greater risk of extinction. Why? They tend to have lower genetic diversity, making them more vulnerable to environmental changes and inheriting harmful genes.

A change in population dynamics can have a domino effect on the entire community. All species in a community rely on each other for food, resources, pollination, shelter, and oxygen. A sudden decline of one species could lead to the fall of another, which causes the reduction of another, and another…

Methods of Estimating Population Size

Ecologists estimate population size using three different sampling methods. Quadrats and transects are used for plants or sessile (unmoving) animals, whilst the mark-recapture method is used for mobile organisms.

The Quadrat Method

Before quadrat sampling, quadrat locations are determined using random coordinates within the study site. This prevents bias, ensuring an even spread of quadrats and species. Additionally, it is essential to understand the area of the study site. For square or rectangular habitats, multiply the length and width. However, many habitats will require the use of grid references.

The quadrat is placed on the ground using random, predetermined coordinates. The individuals within the quadrat are identified and counted.

Sometimes, estimating percentage cover is a more suitable method than counting individuals. A gridded quadrat is used to calculate percentage cover. This is a quadrat which is split into smaller squares. The dominant species is determined by seeing which is more present within each square, and from this, the estimated percentage cover can be calculated.

Later, a second animal sample is collected. This will likely include some marked individuals (known as recaptures) and some unmarked individuals.

The ratio of marked to unmarked individuals allows ecologists to estimate the population size. The mark-recapture method assumes that the larger the population, the lower the percentage of individuals recaptured.

Estimation Methods: Pros and Cons

All methods used for estimating population size have specific pros and cons.

Table 2: Main pros and cons of estimation methods for measuring population size.

Estimating Population Size: Formulas

Once data has been collected, it needs to be extrapolated to estimate the population size for the entire habitat.

The Quadrat Formulae

The formula required for extrapolating quadrat data depends on how the data was collected.

Counting Individuals

To estimate the population size, the habitat area is divided by the quadrat area and multiplied by the mean. The following formula can be used:

$\frac{areaofhabitat}{areaofquadrat}xmeannumberofindividualsperquadrat=population$

Suppose an ecologist found a mean of 3 thistles within each 1 m² quadrat. The total habitat size was 12,000 m².

$\frac{12000}{1}x3=36000$

From this calculation, it can be concluded that there are 36,000 thistles in the population.

Percentage Cover

To estimate the population size, calculate the mean percentage cover and multiply this by 100 to obtain a percentage.

$\frac{sumofpercentages}{numberofsamplestaken}x100=percentagecover$

Suppose an ecologist measured the percentage of moss cover for ten quadrats in a moorland habitat.

$\frac{14+78+36+67+54+98+91+86+90+75}{10}x100=68.9\%$

From this calculation, it can be concluded that the moorland habitat has 68.9% moss coverage.

The Mark-Recapture Formula

The formula for the mark-recapture method is known as the Lincoln Index.

$Population=totalnumberofanimalsin1stsamplex\frac{totalnumberofanimalsin2ndsample}{numberofmarkedanimalsin2ndsample}$

Suppose an ecologist captured and marked 15 snails during the first sample. They then caught 21 in the second sample; 3 were already marked as they had been captured in the first sample.

$105=15x\frac{21}{3}$

Using the Lincoln Index formula, the ecologist can conclude that there are 105 snails in the population.

Estimating Population Size: Activity

Estimating population size isn't as difficult as it sounds. All you need is a large open space (such as a school playing field or local park) and some measuring apparatus.

Aim: Estimate the population size of a chosen plant species in a habitat using the quadrat method (counting individuals).

Equipment:

• Quadrat

• Two 10 m tape measures

• Clipboard

• Pen

• Paper

Method:

1. Measure the size of your chosen habitat. Select your quadrat locations using random coordinates within the habitat.

2. At your first coordinates, extend the tape measures to their maximum length. Lay them on the ground, perpendicular to each other, to form a right angle. This will create an area of 100 m².

3. Use a random number generator to obtain two numbers between 0 and 100. You will use these as coordinates within your 100 m² area.

4. Place the quadrat at the random coordinates (the quadrat size can be 1 x 1 meters, 1m²). Count and record the number of individuals of your chosen species present within the quadrat.

5. Take nine more random samples within the 100 m² area.

6. Repeat this process at the predetermined quadrat locations within the habitat.

7. Estimate the total population size.

Control Variables:

• Quadrat size

• Number of quadrat samples taken

• Counting method

Risks:

• Cuts or stings

• Allergies

• Adverse weather conditions

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^{8. PMT Education, Edexcel Biology iGCSE: 4.2 Estimating Population Size, 2022}

^{9. StudyMind, Organisation & Trophic Levels - Quadrat and Transect Sampling (GCSE Biology), 2022}

^{10. W. Chris Funk, Monitoring Population Trends of Eleutherodactylus Frogs, Journal of Herpetology, 2003}