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Topic C

The effects of horse grazing on species diversity in permanent pasture on an organic farm in Worcestershire.

Contents:

1. What type of investigation will this be?

2. Background information

3. Aim and objectives

4. Population and Sampling

5. Controls

6. Variables

7. Replication

8. Statistics

9. Influencing outcomes

10. Assumptions and Bias

11. Repeatability

12. Back to the beginning

1. What type of investigation will this be?

1

Is this an observational investigation or an experiment?

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We are starting out with a question or hypothesis so this is an experiment.Check your answer

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2. Background information

The undergraduate reviewed the literature on this topic.

i. The justification

Grasslands are affected by their management, including the effects of grazing animals (e.g. Putman, 1986).

ii. Information about practicalities

In this investigation the seasonality of plant growth meant that the time of data collection could affect the species recorded. Reading indicated that most grassland species would be visible above ground during the summer.

Reading around on this subject indicated that two measures could be used. The first is species diversity i.e. the total number of species found in each quadrat. The second measure that could be used is abundance, indicated by using a Domin score. It was also clear that grassland surveys usually use 2m x 2m quadrats.

iii. The context

The two adjacent permanent pastures used in this study had been established for at least 15 years and grazed exclusively by horses for the past 10 years. The stocking rate had varied during that time but the pattern of management had been the same throughout. The two pastures had not been artificially fertilised. In both pastures during late autumn-early spring the horses were allowed to graze the whole field. On the 'grazed field' during the summer the grazing was restricted on a rotational basis and the horse manure removed by hand. The 'rested' field was not grazed during the summer but the grass was lightly mown (topped) in late autumn

References

PUTMAN, R.J. (1986). Grazing in temperate ecosystems - Large herbivores and the ecology of the New Forest. Croom Helm Ltd, London.

3. Aim and objectives

2

Draft an aim and objective(s).

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When this study was carried out the student had two objectives. We have included only one.

Draft Aim: To examine the effect of grazing on permanent pasture.

Draft Objective: Compare plant species (diversity and abundance) in a permanent pasture grazed all year with a pasture that is periodically rested from grazing by horses.

Check your answer

4. Population and Sampling

3

What is the statistical population? Will you need to sample? By what method(s)? Reflect on whether your sampling method will generate a representative sample. Indicate roughly the number of samples and/or observations you intend to collect and your rationale.

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In this investigation the statistical population is the abundance of particular plant species in the permanent pasture and the species diversity in permanent pasture.

It is not realistic for the student to record the presence and abundance of every plant species within the approximately 5 acres of each field. Therefore 2m x 2m quadrats were used. 20 samples were collected in each field by placing the quadrat at random locations. The random locations were determined using a random numbers table.

The number of samples was determined by time considerations balanced against the need for a representative sample.

Step 8 may lead to a revision in the number of samples chosen for the experiment and must always be carried out therefore as part of the planning process.

Check your answer

5. Controls

4

Do you need a control(s). If yes - what? If no, why not?

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No. This investigation is a comparison between two fields and there are no other base lines that need to be investigated at the same time.Check your answer

6. Variables

5

What is(are) your treatment(s)? What may cause non-treatment variation?

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What is(are) your treatment(s)?

There is one treatment variable - the management, in terms of horse grazing, of the two grasslands.

What may cause non-treatment variation?

Variations in the environment such as aspect, edge effects, wind direction, drainage and geology.

The quadrats could not all be examined on one day and the ability of the student to identify plants in August may not be the same as in July either because plants have died back or because some species may only produce above ground material later in the year.

Initial inexperience in identification of species.

Grazing of some areas by other species such as rabbits.

Check your answer

6

For each possible cause of non-treatment variation how might the effect be minimised?

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Potential source of non-treatment variation

How to minimise impact on hypothesis testing.

Variations in the environment such as aspect, edge effects, wind direction, drainage and geology.

The two fields are adjacent to one another which will minimise potential environmental differences.

The effect of any variation in the environment cannot be randomised or controlled. Therefore it is important for the student to be aware of any difference in the environment in the two fields and consider this in her evaluation of her results.

The quadrats could not all be examined on one day and the ability of the student to identify plants in August may not be the same as in July either because plants have died back or because some species may only produce above ground material later in the year.

To allow for the time it takes to record diversity and abundance in a total of 40 quadrats the student should alternate her recording. For example she might collect data from two quadrats in one field and then two quadrats in the next field.

Initial inexperience in identification of

species.

This is addressed in part by the alternation of sampling outlined in the row above. In addition the student should carry out a preliminary survey to ensure a level of competence in identifying the plant species found in the two fields.

Grazing by other species

This is problematic since other species may be either deterred or encouraged by the presence of horses. Ideally, two surveys should be completed in different years when different fields are being rested, but the time scale of the project did not allow for this.


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7. Replication

7

Will you use replicates? Why? Indicate how many and explain why this number.

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Each quadrat within a field is a replicate. The student decided to collect data from 20 replicates (quadrats) to provide a representative, analysable sample within the time available for data collection.

Step 8 may lead to a revision in the number of replicates chosen for the experiment and must always be carried out, therefore, as part of the planning process.

Check your answer

8. Statistics

8

Have a go at choosing what might be the correct test to analyse the data from this experiment. Explain your choice.

This is invariably the step that students find the hardest we therefore return to this in interactive exercises in chapters 4 - 8.

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The student recorded the number of species in each quadrat and Domin scores for each species in each quadrat.

Using the information in appendix b of the book:

B.1. What type of investigation am I designing?

In this investigation we are starting out with a question so this is an experiment.

B.2. Which type of hypotheses am I testing?

There are three types of hypotheses that you need to choose between. If you are not sure which type of hypotheses you will be testing read the information in B.2.1 - B.2.3 before deciding. For more information about hypotheses and hypothesis testing read Chapter 4.

  1. Does the data match an expected ratio?

or

  1. Is there an association between two or more variables?

or

  1. Do samples come from the same or different populations?

It is not always easy to decide which type of hypothesis you are testing. In this case the student first wanted to compare the number of species in each quadrat in the two fields. She had no expectation and did not want to examine the association between the snails and plants. She was therefore testing the third type of hypotheses. The same is true for the comparison for any one plant species of the Domin scores for that species in the 20 quadrats in each field.

B.2.3. Do samples come from the same or different populations?

To test this type of hypothesis you need to decide if the data are likely to be parametric. To tell if your data are likely to be parametric (Normally distributed) you should refer to Box 3.2 in the book and in the Statistical Software section of the Online Resource Centre. When designing an investigation you can only use criterion a to decide whether your data may be parametric and in this case a decision can be made based on this criterion.

Criterion a. Are the data measured on an interval scale which is therefore quantitative and continuous, such as mm and grams?

For the first measure (numbers of species in each quadrat) the answer is NO, the scale of measurement is numbers of species which is an ordinal scale since you cannot have one and a half species for example.

For the second group of measures for any one species the students recorded Domin scores which is also an ordinal scale.

B2.3.2. Non-parametric tests

Choosing a non-parametric test depends on how many treatment variables you are planning to examine, how many categories in each variable, and how many replicates in each category. In this example there is one treatment variable, (grazing regime). There are 20 quadrats (observations) in each field. From the table you can see that the statistical test that is most likely to be appropriate is the Mann Whitney U test.

Experimental design

Test

You have one treatment variable. You are going to compare two samples. The data is unmatched. You have 20 observations or less in each sample.

Mann Whitney U test (8.1.)

You have one treatment variable. You are going to compare two samples. The data is unmatched. The data is measured on a continuous scale and you have more than 40 observations in each sample.

z test for unmatched data (7.1.)

You have one treatment variable. You are going to compare two samples. The data is unmatched. You have more than 20 observations in each sample.

Sokal & Rohlf, 1981.

You have one treatment variable. You are going to compare two samples. The data is matched. You have less than 30 pairs of observations.

Wilcoxen's rank paired test (8.2.)

You have one treatment variable. You are going to compare two samples. The data is matched. You have more than 30 pairs of observations.

z test for matched data (Chapter 7 (7.2)).

You have one treatment variable. You are going to compare two or more samples. You wish to test general and specific hypotheses.

One-way ANOVA (Kruskal Wallis test)( 8.3. and 8.4)

You have more than one treatment variable. You are going to compare two or more samples. You wish to test general and specific hypotheses. You will be using a calculator.

Two-way non parametric ANOVA (8.5. and 8.6)

You have more than one treatment variable. You are going to compare two or more samples. You wish to test general hypotheses. You want to use a computer.

Scheirer - Ray - Hare test (8.7.).



The criteria for using the Mann Whitney U test (8.1.1.) are that you:

  1. Wish to test for differences in population medians.
  2. Have one treatment variable and two samples.
  3. Have data that is non-parametric and unmatched.
  4. Have data that can be ranked (3.1. and 3.8.2.).
  5. Have two samples which both have a similar shaped distribution. For example if one distribution is skewed to the left and the other to the right (3.4.4.) then you should not use this test. (If this does arise you could try transforming the data (3.9.))
  6. Should not use this test if one sample has only one observation or if both samples have less than 5 observations each.
  7. Need not have equal sample sizes.

With our current design we can see that most of these criteria are met. We cannot at the moment check criterion 5 and need to do this when the data are collected.

(In reality the student decided to use multivariate analysis using specialist ecological software called TWINSPAN and DECORANA. These two analyzes have additional advantages as they are able to 'extract' more information from the data than the tests we have outlined in our book and are, therefore, recommended for studies of this type if you have access to the software).

Check your answer

9

Finalise your aim and objectives.

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In this example there has been no revision to the draft aim and objective as a result of our planning process. Check your answer

10

Try drafting your hypotheses.

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If you did use the Mann Whitney U test to compare the numbers of species present per quadrat in the two fields, the hypotheses would be:

H0: There is no difference between the median numbers of plant species in the 'grazed' and 'rested' fields.

H1: There is a difference between the median numbers of plant species in the 'grazed' and 'rested' fields.

If you used the Mann Whitney U test to compare the Domin scores for each of the species recorded the hypotheses would be:

H0: There is no difference between the median Domin scores for species 1 in the 'grazed' and 'rested' fields.

H1: There is a difference between the median Domin scores for species 1 in the 'grazed' and 'rested' fields.

Check your answer

9. Influencing outcomes

11

Is this relevant to your experiment? In what way?

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It is unlikely that the investigator could influence the outcomes of this investigation.Check your answer

10. Assumptions and Bias

12

List all the assumptions and possible causes of bias in your design.

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The student has identified all the species correctly.

The quadrat samples do produce representative samples for the two fields.

The time taken to record data from all the quadrats has not influenced the results.

The effect of environmental factors other than grazing regimes is minimal. Check your answer

11. Repeatability

13

Could this experiment be repeated?

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a) Yes
b) No
Yes. Either the survey could be repeated the following year or if one or more pairs of fields with a similar history could be identified then the project could be repeated there.Incorrect. Either the survey could be repeated the following year or if one or more pairs of fields with a similar history could be identified then the project could be repeated there.
Check your answer

12. Back to the beginning

14

Provide a complete review of your experimental design written in such a way that another person could use this review to carry out your experiment.

Aim: To examine the effect of grazing on permanent pasture.

Objective: Compare plant species (diversity and abundance) in a permanent pasture grazed all year with a pasture that is periodically rested from grazing by horses.

Experimental design:

The following is a summary of our experimental design. Clearly there are some further details that need to be included before this is a finalised method.

Two adjacent fields were identified where a consistent management regime had existed for 10 years. These fields were believed to differ primarily in the grazing regime. In both fields the horses were allowed to graze from late autumn to early spring. In one field the horses were then removed and the pasture rested. A light grass cut was carried out in late autumn after which the horses were returned. This is the 'rested' field. In the grazed field after the grazing during the winter the horses continued to graze but only in rotation on the field. In autumn the rotation ceased and the horses were allowed full access to the field.

Plant species were examined in July and August in 20 2m x 2m quadrats in both the grazed and rested field. Each quadrat was placed at random by using random numbers table. In each quadrat the total number of species was recorded and the abundance of each species was measured using Domin scores. The data was analyzed using TWINSPAN and DECORANA.

Check your answer

Reflection

15

Describe how you felt at the various stages of planning this experiment. Can you identify one feature of your experience that you can seek to improve. How does our answer differ from yours? In what ways were your ideas better?

Well done! You have now completed this interactive exercise. You may now wish to print this page for your reference, and also the Word documents if you wish to keep a permanent record of your answers.Check your answer