Ecology

YOU ARE LEARNING:

The Amount of Organisms

Understanding how many organisms occupy an area is important for conservation and management techniques. Different methods can be used to determine this, based on the organism's biology.

What is a population?

Measuring the size and distribution of a population in an ecosystem is an important part of practical ecology. Without this information, it is not possible to understand how an ecosystem is functioning or changing.

Sometimes, counting the number of individuals in a population is easy. Sometimes, it can be really difficult and time-consuming or even impossible! For which of these populations would it be easy to count the actual total number of individuals?

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In practice, the majority of populations can only be measured indirectly to give an estimated population **** size. We have several different techniques that we can use to give those estimates.

Imagine you want to sample the number of small ground plants in an area, for example grass on a playing field.

We can use a simple device called a quadrat to help us. It is a metal frame, normally no larger than $1 m^2$ and often divided into smaller squares, to help with the counting.

In this example, there are 2 species: Large flowers and small flowers. How many small flowers are in this quadrat?

When we give the size of the quadrat, it is normal to do that in $m^2$ . Now, our quadrat is $0.5m$ along each side. How big is the quadrat in $m^2$ ?

Species density

So we give species density by stating how **** many individuals of a species is in a given area, often $1m^2$ .

So what is the species density of small flowers in $1m^2$ , based on this sample?

The whole field is measured at $15m^2$ . What is the estimated population of small flowers in this field?

Do you think this estimate is reliable? Answer yes or no.

The estimate of 480 small flowers in the field is not entirely reliable

We only checked the number of small flowers in a single quadrat; maybe we happened to look in a place where there was a lower density of small flowers.

So it's not very reliable to base estimates on just a single quadrat. How could this estimate be made more reliable?

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The quadrat is good for estimating the overall population of plants in an area.

However, we can't use it to study the distribution of the individuals in an area, for example a field or a shore line. For that, we can use a belt transect.

The transect line is right in the middle of the sampling area.

We count the number of individuals of a species that we find within a certain distance from the transect line (the width of the belt). We do that for set intervals, say every $1m$ , along the transect line.

Can we use a quadrat to estimate the population along a belt transect? Yes or no?

Quadrats and belt transects work so long as the individuals are sedentary, such as plants or animals like limpets on the sea shore. If they move around (like most animals do) and maybe cover large areas, we can't use quadrats and belt transects for anything, really. You couldn't estimate the population of dolphins in the Atlantic with quadrats!

The capture-recapture technique can be used to estimate the population size of a mobile species. There are some limitations, but we will look at those later.

First, you capture a number of individuals

They have to be from the same location.

Then you place a marker on them, like a tag or a blob of paint. You record the number of individuals you captured and marked. Now, the marker should not...

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Now you wait.

You want the marked individuals to mix with the rest of the population.

After a while, you capture another set of individuals in the same location.

You count the number you caught this time, and you count how many of them are marked already - so how many of them you have captured for the second time.

Finally, you do the maths! If you capture 10 individuals in the first round and 15 individuals in the second round of which 5 individuals were already marked, your estimated population size would be 30. Now, which of these formula do you think is the correct one for calculating the estimated population size?

What is the estimated population if 15 snails were captured and marked, then 12 were captured, the second time of which 3 snails were already marked?

This estimate is based on a number of assumptions about the technique and about the nature of the population. Can you work out what these assumptions might be?

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