Investigation to study the abundance of clover in mown and un mown grassland

The investigation of population sizes and distributions is a particular interest of ecologists.  The abundance of one species or the decline of a species is a key indicator of the features of an ecosystem

Population data is vital to document the effect of humans on ecosystems, for impact assessments and restoration ecology.  For example, an increase in the population of a prey species might indicate problems within the population of carnivores in the ecosystem or a proliferation in the plant food of the prey animal.

Population measurements are useful for comparing between species or between communities of the same species.  An investigation of a plant population can give information on the condition of soil preferable to the plant.  For example if two populations of plants growing in the same climate shoe different population growths it can be assumed that another factor, perhaps soil type, is having an effect. Regular observations of populations therefore are essential in ecology.

Ecologists usually find it is impossible to count every individual in a population so they rely on using a sampling technique to estimate population size.  A large number of samples should be taken to increase the proportion of the total population sampled and samples should be taken randomly to eliminate bias.  When the sampling technique is planned carefully the estimate of population can be very accurate.

White clover is an important legume in the UK.  It is a nitrogen converter - bacteria on its roots can convert atmospheric nitrogen into a form useful for plants.  It improves the soil structure and reduces the need to use a nitrogen fertiliser on soil. It provides grazing animals with high quality proteins.  It can improve animal health and performance. 

Clover is a very useful plant for both dairy and crop farmers.  Ecological investigations of clover populations are important to determine the ideal conditions for its growth and to try to optimize clover populations.  The factors that might affect white clover growth are whether the grassland it is growing in is mown or unmown, the humus content of the soil, the light level at the soil and the soil pH.
Soil pH is an indication of the acidity or alkalinity of soil.  The pH scale goes from 0 to 14 with 7 being a neutral pH.  A low pH of between 0 and 6 is acidic.  A pH of between 8 and 14 is basic.  Soil pH provides clues to properties of the soil.  White clover prefers to grow in moderately acidic soil with a pH of 5.8 or higher.

The humus content of soil is a good indication of its condition. Humus is decomposable organic matter such as fallen leaves and other dead organisms in the soil.  It contributes to plant growth through its effect on the physical, chemical and biological properties of the soil. Humus promotes good soil structure by improving aeration and permeability and enabling the soil to hold more water.  The organic residues in the soil lead to the formation of compounds that bind the soil into clumps.  These clumps help to maintain a loose, open granular condition of soil.  Water is better able to infiltrate and move through soil in this clumpy condition.  Oxygen can get to where it is needed at the roots of plants.  Humus is a source of nutrients such as nitrogen and phosphorus and serves as a source of energy for microorganisms and earthworms.  It is a black or brown carbon-containing matter that can be burnt.

The aim of this investigation is to determine if the clover population differs significantly in mown and unmown grassland.  The research hypothesis is that there will be more clover in mown grassland.  

Method

This experiment was carried out in a sloping meadow using a transect and quadrant method. 

To avoid any bias in the investigation a number of precautions were taken.  The same experimenter conducted every part of the investigation to eliminate variables due to experimenter competency and accuracy.  All parts of the investigation were carried out on the same day in the same climate to eliminate as many environmental variables as possible.  Enough samples were taken for the experimenters to be confident that any significant difference seen between populations was valid.

Apparatus

The apparatus used were a 1m squared quadrant divided into 100 squares, a tape measure, raw data sheets, a pH meter, a Bunsen burner, beakers, a weighing scale, light level reader.

Procedure

Investigation of the clover population

  • A 20m line was marked in the meadow in the unmown and mown areas using a tape measure.  The lines were parallel to each other.
  • The 1m squared quadrant was placed randomly 20 times along each line.
  • At each placement the number of squares in the quadrant taken up with white clover was noted.
  • As there are 100 squares in the quadrant each square occupied with clover is one percent of clover in that area.

Light reading

  • A light reading at the soil level was taken for the mown and unmown grassland
  • The light reader was calibrated at the mid-point of the 20m line in the unmown grassland.
  • The sensor was placed at the soil level and the reading in lux was noted.
  • The procedure was repeated at the mid-point of the 20m line in the mown grassland.

pH reading

A pH meter was calibrated using solutions of known pH, in this case pH 4 and pH 7.  The sensor was rinsed with distilled water, placed in the pH 4 solution and the readout adjusted to read 4.  The sensor was then rinsed in distilled water again, placed in the pH 7 solution and the readout adjusted to read 7.  The pH meter is now calibrated.

The sensor was placed in the soil at the midpoint of the 20m line in unmown grassland. The pH reading was noted from the screen. The sensor was rinsed in distilled water and the procedure repeated for mown grassland.

Water content of the soil

The water content of the soil in the mown and unmown grassland was measured at three points along the 20m lines.
The 20m line was divided into three imaginary sections, top, middle and bottom moving down the slope.  A point in each of these sections was chosen for a water content reading.

Site A in the unmown grassland was a point chosen at random along the top section of the 20m line.  A soil auger was inserted into the soil and a sample was withdrawn.  The soil was placed in a plastic bag and the bag was sealed and clearly labeled and taken bask to the lab.

This procedure was repeated for two other sites in the unmown grassland and for three sites in the mown grassland.
In the laboratory a beaker was weighed on a weighing scales and its weight noted.   Soil sample ‘Unmown A’ was emptied into the beaker.  The beaker and soil were weighed and the weight noted.  The weight of the soil sample was calculated by deducting the weight of the beaker from the weight of the beaker and soil.  The beaker was placed on a frame above a lit Bunsen burner and allowed to dry out.  When the soil was totally dry the beaker and soil was weighed again.  The weight of the dry soil was calculated.  The difference between the weight of the soil before and after drying was the weight of its water content.  The percentage of water in the soil was calculated by (weight of water/weight of wet soil)*100.

The procedure was repeated for each soil sample.

Humus content of the soil

The humus content of the soil in the mown and unmown grassland was measured at three points along the 20m lines.

The 20m line was divided into three imaginary sections, top, middle and bottom moving down the slope.  A point in each of these sections was chosen for a humus content reading.
Site A in the unmown grassland was a point chosen at random along the top section of the 20m line.  A soil auger was inserted into the soil and a sample was withdrawn.  The soil was placed in a plastic bag and the bag was sealed and clearly labeled and taken back to the laboratory.  This procedure was repeated for two other sites in the unmown grassland and for three sites in the mown grassland.
In the laboratory a glass dish was weighed on a weighing scales and its weight noted.    Soil sample ‘Unmown A’ was emptied onto the glass dish.  The glass dish and soil were weighed and the weight noted.  The weight of the soil sample was calculated by deducting the weight of the glass plate from the weight of the glass plate and soil.  The plate was placed over a lit Bunsen burner and dried out.    The dry soil was spread out on the glass plate and the flame of a lit Bunsen burner was applied directly to it.  The humus in the soil was burnt and the weight of the soil was calculated again.

The difference between the weight of the soil before and after drying and burning was the weight of its humus content.  The percentage of humus in the soil was calculated by  (weight of humus/weight of wet soil)*100.
The procedure was repeated for each soil sample.

Statistical analysis

Statistical analysis was conducted to test whether the data supports the research hypothesis. 

The mean, median and range of all the raw data were calculated.

A Mann-Whitney U test with a 5% significance level was chosen to test for a significant difference in the clover population in unmown and mown grass.  This test suits our data as we could not assume a normal distribution of clover population.  It will be a one-tailed Mann Whitney U test as our research hypothesis states there will be a difference in the populations.  A t-test with a significance level of 5% was used to investigate differences in water content and humus content of the soil in unmown and mown grassland.
Results

Investigation of the clover population

The raw data from the quadrants placed in the unmown and mown grasslands are in Appendix I.
The median percentage of clover in unmown grassland is 6.50 and the range is from 2% to 14%.  The mean percentage of clover in unmown grassland is 6.75%, and the standard deviation is 3.49%.
The median percentage of clover in mown grassland is 16.5, and the range is 8% to 55%.  The mean percentage of clover in mown grassland is 21.50%, and the standard deviation is 13.01%.
The mean percentages of clover in unmown and mown grassland were graphed (Figure 1).  The error bars are the standard error, calculated as the standard deviation/ Square root (N).

Figure 1 The mean percentage of clover in mown and unmown grassland ± standard error.

A Mann Whitney U test was conducted on the raw data of clover population.  A significance level of 5% was chosen.  The Na value was 20 and the Nb value was 20.  The U value obtained from the ranked data was 200.  This is higher than the critical table value of 138 so the null hypothesis was rejected and the research hypothesis was accepted.  There is a significant difference in the population of white clover in unmown and mown grassland.

Light reading

The light reading of unmown grassland was 1291.68 lux and the light reading of mown grassland was 90525.24 lux.  Soil in mown grassland therefore receives more light.

pH reading

The pH of soil in the unmown grassland was 7.5.  The pH of soil in the mown grassland was 5.52.  The soil in mown grassland is therefore more acidic.

Water content of the soil

The raw data for water content in unmown and mown grassland is in Appendix I.  The mean percentage water content in unmown grassland was 3.41% with a standard deviation of 0.69%.  The mean percentage water content in mown grassland was 6.47% with a standard deviation of 1.35%.
The mean percentage water content in unmown and mown grassland was graphed (Figure 2).  The error bars are the standard error, calculated as the standard deviation/ Square root (N).

Figure 2 The mean percentage water content in unmown and mown grassland ± SE
A two-tailed t-test was carried out to investigate if the water content of soil in unmown and mown grassland is significantly different.  A significance level of 5% was chosen.  N = 3 and df = 2.  The t value obtained was -2.69.  The critical value for a two-tailed t-test with df=2 is 4.30.  This exceeds the value for our test so there is no significant difference between the water content of soil in unmown and mown grassland.

Humus content of soil

The raw data for the humus content of soil in unmown and mown grassland is in Appendix I.
The mean percentage humus content in unmown grassland is 27.34% with a standard deviation of 2.59%.  The mean percentage humus content in mown grassland is 19.84% with a standard deviation of 1.61%.
The mean percentages humus content in unmown and mown grassland was graphed (Figure 3).  The error bars are the standard error, calculated as the standard deviation/ Square root (N).

Figure 3 The mean percentage humus content in unmown and mown grassland ± SE

A two-tailed t-test was carried out to investigate if the humus content of soil in unmown and mown grassland is significantly different.  A significance level of 5% was chosen.  N = 3 and df = 2.  The t value obtained was 6.04.  The critical value for a two-tailed t-test with df=2 is 4.30.  Our result exceeds the critical value test so there is a significant difference between the humus content of soil in unmown and mown grassland.

 

Evaluation

The sampling technique used in this investigation was carefully planned so as to give an accurate estimate of population size and distribution.  20 samples were taken in each habitat.  The quadrant was placed randomly on the ground.  The same experimenter conducted all the counts and measurements.  As many variables and sources of bias as possible were eliminated.

The measurement of soil water content, humus content, soil pH and light levels were conducted according to standard procedures.

Conclusion

This investigation into the abundance of white clover in unmown and mown grassland reveals a significant difference in populations.  There is significantly more clover in mown grass than in unmown grass (Figure 1).

There was significantly greater humus content of soil in unmown than mown grassland (Figure 3).  Humus content is influenced by the amount of organic matter falling onto the soil.  In am unmown grassland the grass and other plants are longer and so provide more organic matter when they fall onto the soil. 

High humus content means that soil is well aerated, has plenty of fuel for microorganisms and is generally soil in good condition.  It is a well-accepted fact that soil with plenty of humus is ideal growing conditions for many plants.  The humus content of soil does not seem to be an important factor for the growth of white clover however as there is significantly more clover in the less humus containing mown grassland.

There is no significant difference in the water content of soil from the unmown and mown grassland.  Water content therefore does not seem to have an important effect on the proliferation of white clover.
Probably the most important difference between the soil in the unmown and mown grassland is the light level.  The availability of light is a limiting factor for photosynthesis.  The clover in unmown grassland do not get enough light to thrive like their mown grass neighbours.

White clover is an important legume.  It has a role in improving soil condition and in improving grazing animal health.  If farmers want to increase the population of white clover in their grassland they should mow the grass regularly to ensure as much light as possible reaches the clover plants.

Source: Essay UK - http://www.essay.uk.com/free-essays/science/abundance-of-clover.php



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