An Investigation To Observe the Preferred Habitat of common rough woodlice.

AN INVESTIGATION TO OBSERVE THE PREFERRED HABITAT OF

COMMON ROUGH WOODLICE

Melanie Newberry

A2 Human Biology April 2003

Introduction

There are many species of Woodlice found in the British Isles and because of their abundance they provide good material for an ecological investigation into their preferred habitat. Woodlice are of the order Isopoda which means, “the legs are alike”1. Different species prefer different dwellings however the Common Rough Woodlouse (Porcellio scaber) which has been used for this investigation, is commonly found around residential properties. They can be found under rocks or around compost heaps where the soil is moist and a humid atmosphere is maintained. One abiotic factor that affects the habitat of Woodlice includes damp or moist soil. Compost heaps also provide a continuous source of dead plants, which is part of the diet of Woodlice2. This is another abiotic factor. Woodlice are believed to provide a vital role in the decomposition process as their eating habits return essential nutrients to the soil3.

Closer examination of the anatomy of the Porcellio scaber may give more understanding of why they may be found in damp conditions. The external structure (fig.1) shows a hard exoskeleton, which is shed at intervals throughout their lives in order that they can grow longer. The exoskeleton consists of segments and the first section has a fusion of the thorax and the head4. The uropod at the rear of the Woodlouse is used for the uptake of water and this is achieved when they are pressed against moist surfaces5. Woodlice cannot retain water for long periods of time and because they have a large surface area to volume ratio they loose water quickly by diffusion6. Their exoskeleton is not waterproof which means that they will also loose water through this.

Fig 1. Dorsal View of Porcellio scaber

The ventral view of the anatomy of Woodlice (fig. 2) shows that they have simple pseudo-lungs as their respiratory surface and this is also towards the rear4. There is a pore opening that allows the exchange of gases in and out of the lungs and this does not close allowing the continual diffusion of water into and out of the inner surfaces of the pseudo-lung.

Fig.2 Ventral view of Porcellio scaber

This investigation aims to analyse habitat preference of the Porcellio scaber and in particular ascertain ...

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Fig 1. Dorsal View of Porcellio scaber

Fig.2 Ventral view of Porcellio scaber

This investigation aims to analyse habitat preference of the Porcellio scaber and in particular ascertain whether a dry or humid atmosphere is preferred as this may play a essential role in the choice of habitat of this particular species.

Null Hypothesis

The Porcellio scaber will not show preference to either a humid or a dry atmosphere.

Alternative Hypothesis

The Porcellio scaber will prefer a humid atmosphere.

Implementation

The implementation of this investigation was carried out according to the plan specified previously.

Results

The controlled experiment showed an even distribution of Woodlice throughout the choice chamber.

The following results were obtained:

Chi-Squared Test

χ2=∑ (Ο−Ε)2

χ2 = 4.232 + 4.232 = 8.464

Degrees of freedom = 2 – 1= 1

Conclusion

The result of the chi-squared test was 8.464. When this figure was compared against a table of χ2 values with 1 degree of freedom it was shown that the figure fell below the level of significance of 0.05 therefore, the null hypothesis is rejected.

Evaluation

From this investigation I have discovered that the Porcellio scaber did prefer a humid environment. The control experiment also suggested this by the fact that there was an even distribution of Woodlice throughout the choice chamber. Likewise, the anatomy of this species of Woodlice would propose that a humid environment would be more beneficial. This is because they are unable to retain water for any length of time as it is easily diffused out of their bodies. They do not contain a waterproof outer layer, which is an added disadvantage in a dryer atmosphere as they are more prone to desiccation. Their respiratory organs are found underneath their torso and are situated close to the ground, which would suggest that in a humid environment there could be an efficient exchange of gases. The diet of the Porcellio scaber would similarly be more abundant in a humid environment as this is a suitable atmosphere for the decomposition of plant tissue.

Although the results of this investigation do favour the alternative hypothesis there are some limitations and sources of error that should be considered. The species of Woodlice were selected carefully however; the age or genetic composition of the individuals could not guaranteed. I did not choose an equal sample of male and female Woodlice and different results may have been achieved if the ratio had been equal. The sample of Woodlice used was also small and could not necessarily be representative of the overall population. Similarly there was no guarantee that all 10 individuals came from the same habitat. Another limitation may be that the Porcellio scaber could behave completely different in their normal habitat outside in the garden in comparison to how they behaved under controlled conditions. If I were to repeat this is investigation I would choose a larger sample of Woodlice and would increase the amount of repetitions of the individual trials. I would collect the sample on a previous day and keep the Woodlice under a controlled environment (i.e. similar humidity and temperature) for 24 hours to ensure that they have all received similar pre-treatment. I would also try to ensure that the sample had a similar ration of males to females.

Plan

Apparatus needed:

1 clean Choice Chamber

a piece of gauze wide enough to stretch over the chamber
Silica gel (self-indicating)
Cotton wool (soaked in water)
Cellulose tape
Stopwatch

10 Woodlice

I will use a clean choice chamber for this investigation as this will ensure that there are no odours or chemicals left by previous occupants as a variable may be created i.e. the behaviour of the Woodlice may be changed.

I will warm the Silica gel in an oven until it changes from a pink colour to a uniform blue colour. This will indicate that the conditions will be dry. The silica gel will then be placed into two adjacent sections in the choice chamber. The cotton wool will be soaked in water with any excess being squeezed out. I will then place it into the two other sections to provide a humid atmosphere (see below).

I will stretch a piece of gauze over the choice chamber to ensure that the Woodlice do not touch the contents of the chamber as this may also affect their behaviour.

The upper part of the choice chamber will then be pressed into position over the gauze to keep it taught. The holes in the top of the chamber will be sealed using cellulose tape. The chamber will then be left for 20 minutes to stabilise the internal environment.

I will collect a sample of 10 Woodlice from my garden and ensure that they are all of the same species. They will also be collected from the same area to eliminate any variations that may be present between habitats.

The Woodlice will all be stored in the same container and once the choice chamber is ready introduced into the holes on the top of the chamber randomly.

I will observe their behaviour for 5 minutes. After this time I will annotate the number of Woodlice on either the dry or humid side of the choice chamber on a results table (see results section).

The test will be carried out at least 5 times to obtain sufficient data and a Chi-squared test will be used to analyse the results. This is because the chi-squared test will investigate the significance of the results by comparing the expected results with the observed results.

The control experiment will be carried out by placing soaked cotton wool in all sections of the choice chamber to observe whether there is a significant difference in the distribution of Woodlice.

Control of Variables

Variables for this investigation will be controlled in the following ways;

I will ensure that the choice chamber is kept in a room, which maintains a constant temperature, and that the light intensity is not varied for each repetition of the investigation. This will ensure that the behaviour of the Woodlice should not be affected by changes in temperature or light.

The choice chamber will not be moved whilst the Woodlice are inside as this may change their behaviour and invalidate the results.

Because I will be selecting the same species of Woodlice and collecting them from the same area this should ensure that the genetic composition of the species will be similar and that they are from the same habitat.
I will not change the silica gel or cotton wool in between repetitions as this will mean that the Woodlice are not being exposed to the same conditions. In addition, I will use the same selection of Woodlice for each repetition to ensure continuity of behaviour.

References

1. 1

2. 2 3 4 5 6

3. Baker et.al. Further Studies in Human Biology. Hodder & Stoughton. 2001

4. Griffin Choice Chamber. Griffin & George Ltd. 1973