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Estimating Population Size: Definition
Before describing how to estimate the size of a population, let's define what a population is.
A population is a group of individuals of the same species living in the same habitat.
Population ecology is the study of populations in relation to their natural habitat. Populations naturally fluctuate, depending on biotic (living) and abiotic (nonliving) factors.
In the natural world, multiple populations live and interact with each other in a community. Biodiversity is measured at the community level.
Biodiversity is the biological richness of a habitat. It encompasses the number of species, their abundance, distribution, and genetic diversity.
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
Without data, ecologists cannot predict how a community will change over time. A change in one species' population may have a knockon effect on another population.
Population dynamics looks at the effect that various factors have on the size of a population over time.
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.
Change  Potential Causes 
Increase 

Decrease 

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 markrecapture method is used for mobile organisms.
The Quadrat Method
A quadrat is a square frame of a fixed size used to outline a sample area.
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.
Grid references indicate a location on a map using grid lines. These lines are identified using letters and numbers. For example, on a 4x4 grid, the reference could be B2, with this representing: two cells in →→ and one down↓.
The quadrat is placed on the ground using random, predetermined coordinates. The individuals within the quadrat are identified and counted.
A quadrat of 1 m^{2} was placed in a rock pool. Three barnacles and four limpets were inside the area of the quadrat.
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.
A gridded quadrat of 1 m^{2} was placed in a meadow. It contained 25 smaller squares, each equivalent to 4% of the quadrat's area (100/25 = 4%). Thirteen squares were primarily grass, eight were mainly moss, and the remaining four were bare soil. This translates to an estimated percentage cover of:
 13 squares x 4% = 52% grass
 8 squares x 4% = 32% moss
 4 squares x 4% = 16% bare soil
Multiple quadrat samples are taken to build up a representation of the entire habitat.
The Transect Method
A transect is a line placed through a habitat. It used to measure vegetation frequency systematically.
Continuous transects record vegetation along the entire length of the transect line. As a result, they are best suited for shorter distances.
Interrupted transects are used to record vegetation at regular intervals along the line.
Transects often measure vegetation changes in successional habitats.
Successional habitats gradually change from one end to the other. The soils, plants, and animals present will vary as you move through the habitat.
Dunes are an example of successional habitat. They change from exposed sandy slopes to established grasslands and forests.
The species present in this habitat depend on abiotic (nonliving) factors:
 Proximity to the sea
 Salinity
 Wind exposure
 Soil composition
And biotic (living) factors:
 The existing species that are present
Kite diagrams are used to show how vegetation frequency changes along a transect. The area of the graph shows frequency  the wider the chart, the higher the frequency. These diagrams give an overall impression of how a successional habitat changes.
The MarkRecapture Method
Ecologists capture animals and mark them with tags, paint, markings, or bands. The marked animals are then released back into their environment, where they naturally mix with the rest of the population.
Later, a second animal sample is collected. This will likely include some marked individuals (known as recaptures) and some unmarked individuals.
How are animals safely captured?
 Longworth traps capture small mammals with bait.
 Pitfall traps catch arthropods that live on the ground
 Beating trays are used to collect arthropods living on plants
 Pooters are used to suck insects into a small chamber (don't worry, you can't suck them into your mouth accidentally!)
 Nets are used to sample freshwater organisms of varying sizes.
The ratio of marked to unmarked individuals allows ecologists to estimate the population size. The markrecapture 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.
Method  Pros  Cons 
Quadrats and Transects 


MarkRecapture 


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 m^{2} quadrat. The total habitat size was 12,000 m^{2}.
$\frac{12000}{1}x3=36000$
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 MarkRecapture Formula
The formula for the markrecapture 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).
You may need to decide the appropriate estimation method for some activities. Pay attention to the chosen species and the nature of the habitat.
Equipment:
Quadrat
Two 10 m tape measures
Clipboard
Pen
Paper
Method:
Measure the size of your chosen habitat. Select your quadrat locations using random coordinates within the habitat.
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 m^{2}.
Use a random number generator to obtain two numbers between 0 and 100. You will use these as coordinates within your 100 m^{2 }area.
Place the quadrat at the random coordinates (the quadrat size can be 1 x 1 meters, 1m^{2}). Count and record the number of individuals of your chosen species present within the quadrat.
Take nine more random samples within the 100 m^{2} area.
Repeat this process at the predetermined quadrat locations within the habitat.
Estimate the total population size.
Control Variables:
Quadrat size
Number of quadrat samples taken
Counting method
Risks:
Cuts or stings
Allergies
Adverse weather conditions
Estimating Population Size  Key takeaways
Estimating population size is a method used to determine the abundance of different species living in a community.
Estimating population size is important because significant changes can lead to extinction and affect the entire community.
The quadrat method is suitable for plants and sessile animals. Within each quadrat, individuals are counted, or percentage cover is calculated. This data is then used to calculate the size of the population.
The markrecapture method is suitable for mobile organisms. Individuals are captured and marked. A second sample of the population is captured later. The ratio of recaptured to newly captured organisms is used to estimate the size of the population.
It's easy to estimate a population size yourself. It doesn't involve much equipment and carries little risk.
^{1. Field Studies Council, Fieldwork Techniques, 2022}
^{2. John R. Platt, Island Fox May Have Lowest Genetic Variability of Any Animal, Scientific American, 2016}
^{3. Meredyth Glass, Advantages and Disadvantages of Quadrat Use, Sciencing, 2018}
^{4. Michael Pidwirny, Plant Succession, Physical Geography.Net, 2018}
^{5. Michael Reiss, Environmental Biology, 2000}
^{6. National Science Foundation, General Biology: Boundless, 2022}
^{7. Neil Campbell, Biology: A Global Approach Eleventh Edition, 2018}
^{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}
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Frequently Asked Questions about Estimating Population Size
What are the methods used to estimate population size?
The quadrat, transect, and markrecapture methods are used to estimate population size.
What is the formula for estimating population size?
The formula for estimating population size is:
 For quadrats: population = mean number of individuals per quadrat x (area of habitat / area of quadrat)
 For markrecapture: population = total number of animals in 1st sample x (total number of animals in 2nd sample) / number of marked animals in 2nd sample)
How to you estimate population size from a sample?
To estimate the population size (n) from a sample, you can extrapolate your data from the area you sampled (a quadrat, plot, or any sample unit) with the formula n = (total area of habitat / area of sample unit) x mean number of individuals per sample unit. For the markrecapture method, you use the Lincoln Index: n = number of animals 1^{st} sampling x (number of animals 2^{nd} sampling / number of marked animals 2^{nd} sampling).
Why are population estimates important?
Population estimates are important because ecologists need to predict how a community will change over time. Population estimations can detect if a population is experiencing a significant decline, which could present an issue for the species because small populations experience a greater risk of extinction. Moreover, change in one population may have a knockon effect on other populations in the community.
How do ecologists determine the size of a population?
Ecologists determine the size of a population by using population ecology techniques and formulas that include estimating the number of individuals or the percentage cover of a species within a specified area (quadrats or transects) with different methods (like counting the individuals or the markrecapture method).
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