Distribution and abundance of organisms

In a nutshell

The abundance and distribution of organisms can be investigated using pooters, pitfall traps, nets and the capture-recapture method. These sampling methods can then be used to estimate the entire population size. It is also important to investigate factors such as temperature that impact abundance and diversity. 

Abundance

Definition

Abundance is how many individuals of a particular organism you find in an area. Abundance can be estimated by counting the number of individuals in a small area and scaling up for the total area. This is because it would be too time-consuming to count all of the individuals in a large area.

Distribution

Definition

Distribution is where an organism is found in a habitat.

Example 

There may be more daisies in sunny areas of a field rather than shaded areas.

Like abundance, investigating distribution can be time-consuming so sampling is used. You can use different sampling methods to study the distribution of an organism such as comparing how common an organism is in two areas or studying how the distribution changes across one area. 

Pooters

Pooters are jars that are sealed with rubber bungs (2.) that have two tubes coming out of them. They are used to collect ground insects.

The longer tube (1.) is placed over the insect and then you suck on the shorter tube (3.) which forces the insect inside the jar. The jar is sealed with a bung (2.). You collect as many insects as you can in a defined area and count them. Then in a second area, you repeat and take a second sample. You can then compare the two organisms. There is also a fine mesh (4.) that stops the insect from being breathed in.

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Note: You should spend the same amount of time sampling each area and each area should be of a similar size. 

Pitfall traps

Pitfall traps are another way to collect ground insects. They are steep-sided containers that are placed in a hole in the ground. The top is partly open so insects will fall into the trap. The trap is left overnight and then the number of insects can be counted the next day. The trap can be placed in two different sampling areas and the results can be compared. 

Nets

Nets are used to collect animals from long grass and water. There are two types of net: a sweep net and a pond net. 

Sweep nets

Sweep nets are used to collect insects from long grass. They are lined with a strong cloth. The sampler stands still in the sample area and sweeps the net from left to right through the grass. The collected insects are then put into a container and counted. This is repeated in a second sampling area and the numbers can be compared. 

Pond nets

Pond nets are used to collect animals from ponds and rivers. The method is similar to using a sweep net, however, the collected organisms are placed in a white tray that contains water when they are counted. 

Note: All of these experiments should be repeated several times (at least three​) and the average should be used.

Scaling up from a small sample area

All of the experiments use small sample areas, these can be used to calculate the entire population size by scaling up. 

Example

Estimate the population size of woodlice in a forest of $$1000 m^2$$. The mean number of woodlice per $$1m^2$$ is $$47$$.

Multiply the mean number per $$m^2$$ by the total area:

​$$47 \times 1000 = 47,000$$​​

Woodlouse population size:

  $$=\underline{47,000}$$ ​

​$$\underline{Therefore,\ there\ would\ be\ approximately\ 47,000\ woodlice\ in\ a\ forest\ of\ 1000m^2.}$$​​

Capture-recapture method

The capture-recapture method involves capturing a sample of a population and marking them in a harmless way. They are then released. A sample of the population is then recaptured and you can count how many have the marking. This can then be used alongside the equation to estimate the population size.

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​$$Population\space size ={number\space in \space first \space sample \times number\space in\space second\space sample \over number\space in\space second\ sample\space previously\space marked}$$​​

Example

A pooter was used to capture beetles in a field. After one hour, $$40$$ beetles were captured and marked with harmless paint. They were then released. At the same time the next day, $$49$$ beetles were caught and $$8$$​ had the marking. Estimate the population size.

$${40 \times 49 \over 8} = {245}$$​

Beetle population size:

​$$=\underline {245}$$​​

​$$\underline{Therefore,\ the\ population\ size\ is\ approximately\ 245\ beetles.}$$​​

Assumptions

There are some assumptions about using this method that you must learn. This includes the assumption that there has been no change in the population size between sampling times (including births and deaths) and the marking method hasn't affected the organism's chance of survival, which is could do if it makes it more obvious to predators. 

Using a key

Keys can be used to identify unknown organisms. They are a series of questions or statements based on the physical features of the unknown creature. You start at question one and work your way down as you narrow your options of what this organism could possibly be. By the time you reach the bottom you should have just one possible species that your organism could be. 

Abiotic and biotic factors

The distribution of organisms is affected by abiotic factors like temperature, moisture level, light intensity and soil pH, as well as biotic factors such as competition. 

Measuring abiotic factors

It is important to investigate the reasons why there might be a difference in the distribution of organisms. This is why abiotic factors are measured.