IB HL Biology Internal Assessment- October 2017
GLQ523
Title
An investigation into the impact of natural and human disturbance on the dispersion of Achillea
millefolium on the shingle ridge of Slapton Ley coastline in Devon, United Kingdom
Research Q
To what extent does variance in soil compaction along transects of the shingle ridge in Slapton Ley
affect the abundance of Achillea millefolium?
Introduction
I am fascinated by the medicinal properties of plants and therefore found it interesting to learn that the
genus name of Achillea millefolium is allegedly derived from the Greek hero Achilles, who used extracts
of the plant to stop his soldiers’ wounds from bleeding during the Trojan wars. The plant is still employed
for medicinal uses today, as an indigestion relief for example, due to the anti-inflammatory flavonoids it
contains.
Slapton Ley in Devon, UK is home to a rare shingle ridge habitat, on which Achillea millefolium is
common. Shingle is composed of large pieces of sediment, which therefore affects its ability to retain
moisture and remain secure during severe weather and high tides, presenting it as a harsh environment
for vegetation to establish. Natural cyclic disturbance from tidal movements inundates the established
vegetation, causing soil erosion and hence leading to less compact and stabilised soil. However, further
from the shore, where disturbance is less frequent, vegetation begins to arise following the colonization
of pioneer species. These hardy species begin the process of ecological succession by providing organic
matter for soil development. This in turn facilitates the colonization of grasses and herbaceous plants,
which further develop and stabilise the soil. The change in number and type of organism that is
encountered further from the sea shows the pattern of succession on the shingle ridge. The shingle ridge
is also a popular tourist destination, and human activity causes disturbance to the ecology. Trampling
and other compressive processes disturb the topsoil and cause it to become more compact, in contrast
to natural disturbance.
Achillea millefolium, shown in the adjacent picture1, is a
herbaceous plant that typically arises in the early stages of
successional development due to the improving soil conditions
and relative lack of competition. The plant is able to adapt to
many environments and, as a mesophyte, actually favours the
shingle ridge due to the well-drained soil. It is a ruderal plant,
meaning it can grow well in disturbed habitats and thrives in
poorly developed soil. It has rosette leaf structures that lie low to
the ground, and far-creeping rhizomes, which keep it deeply
rooted and serve as protection from sources of external
disturbance. However, as the plant is low-lying, it is fairly intolerant of competition with other plant
species for light, which is essential for photosynthesis.
I have decided to measure how soil compaction, as a measure of disturbance in the shingle ridge
community, affects the dispersion and abundance of Achillea millefolium. As all the plant species on the
shingle ridge are adapted to the poor soil conditions typical of an early successional stage, they are likely
to all respond to natural disturbance from high tides in the same way. Therefore soil compaction as a
measure of human disturbance rather than natural disturbance will better explain the dispersion of plant
species. Achillea millefolium is very tolerant of surface disturbance unlike many other species typical of
the shingle ridge, and therefore, in areas more disturbed by human activity there will be less competitor
species, and Achillea millefolium can thrive.
Hypothesis- in areas with greater soil compaction, there will be a greater abundance of Achillea
millefolium
1
Rutgers New Jersey Agricultural Experiment Station (2018) Large Image of Yarrow, Available
at: https://njaes.rutgers.edu/weeds/full.asp?yarrow (Accessed: 29.10.17).
1
, Variables
Independent variable- Soil compaction (± 0.1 c m3)
The soil compaction was measured using a penetrometer, which measures the depth at which a metal
rod is driven into the ground when dropped from a quantified height.
Dependent variable- Abundance of Achillea millefolium (± 1)
This was measured using a 50 X 50 cm gridded quadrat
Monitored variables
Although these abiotic variables that occur naturally in the field cannot be controlled, the measurements
can be monitored. Therefore, if a change in the abundance of Achillea millefolium correlates well with
one of these monitored variables, it may be concluded that the variable is having an effect on the
abundance.
Light intensity (lux)- Light levels may affect the abundance of Achillea millefolium, as it is a shade
intolerant plant and therefore requires a high light intensity in order to photosynthesise. Light intensity
was measured from the position of the plant on the ground, so that the effect of taller vegetation is
accounted for. All data was taken and recorded on the same day to ensure that variation in light levels
remained relatively consistent, however the data is instantaneous and varied throughout time.
Wind speed (m/s)- Wind speed may affect the abundance of Achillea millefolium as a higher wind speed
leads to a greater rate of transpiration and therefore increased water loss, which the plant may not be
able to tolerate. This may not present as much of a problem for Achillea millefolium in particular as it is a
low-lying, waxy plant, which prevents rapid loss of water, however it was monitored at regular intervals at
different locations along the shingle ridge transect to observe any major changes. As with light intensity,
the data is instantaneous and varied considerably, however all samples were taken on the same day so
variation was consistent.
Relative air humidity (%)- This is a measure of the water content of the air relative to the maximum water
content for that temperature. If air humidity is too low, the rate of transpiration will become too high
leading to water loss. If it is too high, water will not evaporate from the plant and nutrients from the soil
cannot be taken in, so the plant will rot. The relative air humidity will stay fairly constant throughout the
day but may fluctuate depending on wind speed, so was monitored at frequent intervals.
Soil moisture content (%)- Vegetation requires water to photosynthesize, and therefore soil moisture
content may have an impact on abundance of plant species. This was therefore monitored by taking soil
samples and analysing the water content in the laboratory. The shingle ridge at Slapton Ley is unusual in
that the shingle pieces are smaller than in other areas, meaning that the soil can retain more water due
to less drainage. As the data was only collected over a small area, the soil moisture was not expected to
change significantly due to distance from the sea, however if the soil is more compacted it may retain
more water. If a correlation is found between compaction of the soil and soil moisture content, then it
may be increased water availability that is affecting the abundance of Achillea millefolium.
Air Temperature (°C)- Temperature was monitored using a thermometer. Seasonal variations in
temperature may have an effect on the distribution of plant species, however daily variations will not. It
was measured every three metres along the transects to see whether it varied with factors such as wind
speed and light intensity.
Controlled variables
Similarity of the surrounding ecosystem for each transect- The areas where transects were set up were
ecologically similar, with similar colonizing plant species. They were within a small distance of each
other, so that the level of regular disturbance was more likely to be similar.
Apparatus uncertainty- The same pieces of equipment were used throughout the fieldwork, so that if
there were any calibration errors in the equipment, the error would be consistent
Oven temperature to calculate moisture content- The temperature of the oven used to evaporate the
moisture from the soil was kept at 105°C
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