Download A Field Atlas of the Seashore

A Field Atlas of the Seashore
ZONATION ON THE ROCKY SHORE
General features to look for:
• There is a marked banding of the communities on the seashore (zonation)
• Productivity (biomass) is low on the upper shore but increases towards the lower shore
• Descending the shore there is an increase in species diversity and community complexity
• Competition increases towards the lower shore
Zonation is well marked on rocky shores. This is due to the environmental gradient which occurs across the shore. No
one reason accounts for the distribution but is the result of interactions between a number of physical, chemical and
biotic factors. To help understand zonation we must consider these limiting factors and the adaptations of the organisms
to them. Remember the “n” dimensional fried egg (page ) where the dimensions are the various factors creating the
parameters of the niche.
Summary of Dominant Species
_______________ Zonation of brown algae from upper shore (left) to lower shore (right) ___________________
Pelvetia
Cell wall
thickness/ µm
% metabolism
recovery after
reimmersion
Wavelength of
light for full
photosynthesis
Reproduction
F. spiralis
F. vesiculosus
Ascophyllum
F. serratus
Laminaria
Pelvetia
1.2
F. spiralis
1.47
Ascophyllum
1.02
F. vesiculosus
0.69
F. serratus
0.42
Laminaria
-
95%
49%
35%
20%
0
0
-
-
-
600
500
150
Hermaphrodite
Hermaphrodite
Separate sexes
Separate
sexes
Separate
sexes
Separate
sexes
________Zonation of periwinkles and topshells from upper shore (left) to lower shore (right) ___________
Melaraphe
Nitrogenous
excretory product
Main diet
Lethal Temp
Limit °C
water loss/day as
% of body wt
L. saxatilis
L. littorea
Melaraphe
neritoides
uric acid
L. saxatilis
lichens
L. littoralis
G. umbilicalis
Calliostoma
L. littoralis
Calliostoma
uric acid
L. littorea
G. umbilicalis
ammonia
ammonia
ammonia
micro/macro
algae
L=46 G=42
Fucus spp.
red/green algae
46.3
lichens/micro
algae
45
44
34.5
3.7%
5.6%
5.35%
8.35%
-
A Field Atlas of the Seashore
The seashore is a transition from marine to a terrestrial ecosystem and produces an environmental gradient of limiting
factors. Thus the conditions of the shore will change across the habitat.
SUMMARY of LIMITING FACTORS AFFECTING ZONATION
1. DESICCATION as a result of exposure at low tide;
influences the upper and middle shore.
2. WAVE ACTION increases humidity; see separate
section page .
3. LIGHT is needed for photosynthesis. Plants need to
be in water for this to occur. The water will filter off
some of the wavelengths of light and reduce the
intensity. Smaller algae for example, the red algae, will
photosynthesise with 10% of the light required by the
browns. The compensation point for Laminaria is only
30ft. cad. A typical land plant is nearer 200.
4. TEMPERATURE; immersion in water buffers
against temperature change and so upper shore species
will have to tolerate the greatest variation. It will affect
the rate of metabolism. Bladdderwrack can just respire
at -17°C, Enteromorpha at -22°C. 17°C is optimum.
High temperatures will increase drying out and
increase salinity in pools
5. ASPECT is the direction in which the shore faces:
south and it has more illumination and warmth, but
dries faster; north is cooler and darker. Thus on a
north facing slope community bands will be wider and
higher up.
6. SLOPE. A flatter shore will expose a greater area of
substrate for colonising and will not drain as fast as a
steeper one.
7. TURBIDITY is the cloudiness of the water. Large
amounts of plankton can increase the turbidity, as will
detritus and sewage pollution. This restricts the light
reaching the algae on the rocks.
8. SUBSTRATE. The hardness and particle size of
rock will influence an organisms ability to attach itself.
Soft rocks will be suitable for burrowers, e.g.
piddocks. Large boulders and rocks give greater shelter
for animals and the angle of dip of the rock strata may
produce more crevices and pools.
9. FRESHWATER. Seepage of water from the cliff
can dilute the seawater. Few of the organisms on the
shore can tolerate salinity changes. Enteromorpha is so
tolerant it is a good indicator of freshwater on rocky
shores. Upper
shore rockpools are particularly vulnerable to salinity
variation.
10. BIOTIC. These are the biological factors
influencing the community. Grazing is perhaps the
most important to consider, where herbivores will
often determine the presence or absence of the
dominant seaweeds. Feeding and competition is dealt
with below. Algal turf, like Laurencia and Chondrus,
will slow down the drainage on the shore and reduce
desiccation. The fucoids have a "whiplash" affect,
which is where water movement causes a sweeping
action of the alga across the substrate and prevents the
attachment of spores and larvae.
VERTICAL RANGE, ORGANISM ADAPTATION AND COMPETITION FOR RESOURCES
Organisms are rarely able to live in a variety of environmental conditions, having adapted to a narrow range of
conditions by natural selection so that each has its own specific niche. Spores and larvae may be deposited on any part
of the seashore but they will only develop if the environmental conditions are favourable enough to let them. For
example, Laminarian spores do settle on the upper shore whilst Pelvetia spores settle on the lower. It would be unlikely
if they grew because Laminaria cannot survive drying and Pelvetia could not cope with the competition; where one
perform well another will find it lethal. In this way organisms become partitioned into zones where they do best. There
will be some overlap and where this occurs each organism is at the edge of its niche, that is at the limit of the
conditions which it can tolerate. Consequently, they may not be typical specimens but poorly developed variations, e.g.
mussels on the upper shore are small as they do no obtain enough food.
The photo, left, of the splash zone and upper shore is
on a vertical rock face. The arrows show the
Lichen
distribution of the different species and so represents
their vertical range. The spores of the Channel
Wrack will be spread over much of the rock and yet
they only grow in this limited range. The upper limit
Tar Lichen
is probably due to abiota such as desiccation – it is at
the limit of the spring tide and above this point it is
just too dry. The lower limit could be abiota but is
more likely to be the biota – competition with the
larger Spiral Wrack. In this way they spores outside
Channel
of its vertical range may settle but as fast as they
Wrack
become establish die of these abiotic or biotic
factors.
Remember that this vertical range should be related
to chart datum for a particular shore. It will be highly
unlikely that you will be working on a rocky shore
that is vertical but is actually quite undulating with
Spiral Wrack
outcrops and dips. To determine species vertical
range needs a measurement of the shore profile from
chart datum.
A Field Atlas of the Seashore
SEDENTARY ORGANISMS tend to be more highly specialised than mobile ones. Barnacles can maintain life at
50°C. Mobile animals can move away from stressful situations; fish move with the tide and crabs will shelter under
rocks. Zonation in animals is best seen in the static species, which, like the algae, are anchored to the rock and must
tolerate the micro-climate. Animals that do move have the problem of possibly straying from their niche.
MAINTENANCE OF ZONATION is crucial. Animal movements are controlled by taxes (responses to light, gravity
and humidity) which change, often with the state of the tide. This helps to keep the organism in the conditions which
suit it best. Edible Periwinkles constantly stray from their niche. Resources in an ecosystem, e.g. space, light and food,
are limited. Organisms compete for these resources. Specialisations like faster growth rate will aid the competitor to
win and dominate over its rival in a constant environment. See Thongweed and Toothed Wrack page .
INTERSPECIFIC COMPETITION
This is the competition between difference species.
Better feeding efficiency and reproductive rate will
help the winner. But if conditions were to vary, such
specializations become a hindrance. F. spiralis will
shade Pelvetia on the upper shore, Ascophyllum shades
out F. vesiculosus in the middle and Laminarians with
a flexible stipe survive over the rigid variety. The
fucoids with the whiplash prevent barnacles from
settling and as barnacles grow their plates move across
the rock cutting off any attached algae. The two
species of barnacle, Chthamalus and Semibalanus,
compete for space in the upper and middle shores. The
former tolerates drying better as it has a non-porous
shell, an operculum, high temperature tolerance and
larvae. settle out of the plankton on just a film off
water. Hence, it is found on the highest part of the
beach. As it merges with the Semibalanus population it
begins to loose the battle because all those
specialisations are at the expense of growing slowly.
The shell of Semibalanus is porous but quick to grow
and soon cuts out the competitor. Semibalanus is
restricted by dogwhelk predation. Competition in
overlapping niches has led to zonation of the barnacles.
INTRASPECIFIC COMPETITION
Competition is greatest with members of the same
species. Older plants will shade out younger ones
which try to develop below their fronds. Only on the
death of the plant will spores have a chance to grow.
Animals usually have the ability to move away from a
competitor and in this instance we see territorial
behaviour. Beadlet Sea Anemones have been shown to
butt each other over a period of time with the weaker
individual moving away. Anemones on an overhang of
rock may be seen to have a regular distribution pattern
- all a similar distance apart.
Extensive splash zone of lichens
Dark band of Tar Lichen Verrucaria spp. Also
with Rough Winkles, Littorina saxatilis
Lichina pygmaea replacing
upper/mid shore algae
White barnacle
zone with
dogwhelks and
limpets
Kelp Zone: a mixture of Laminaria digitata,
L. hyperborean and Alaria esculenta
Red algae: Corallina and Lithothamnion.
Both calcareous and so able to survive wave
action
Zonation on a rocky shore which is exposed to a high levels of wave action
A Field Atlas of the Seashore
•
•
•
•
•
FEATURES OF THE WAVE EXPOSED ROCKY SHORE:
the Verrucaria/Littorina zone is very extensive.
note the poor growth of seaweed and therefore
low productivity
there is an absence of fucoid algae, these being
replaced by a barnacle zone.
the lower limit of the barnacles will be
determined by the dogwhelks
in crevices are mussels and occasionally beadlet
sea anemones.
•
•
•
•
the limpet Patella aspersa has a thick shell; may
replace P. vulgata especially in the lower shore.
the attached Laminaria, with their large holdfasts,
and encroaching Alaria esculenta. The latter has a
very strong midrib. Typical of extreme exposure
Corallina and Lithophyllum are calcareous and
form the dominant algae of the shore
On gradual slopes Himanthalia can form a zone
near the edge o the red algal zone.
THE AFFECT OF WAVE ACTION on ZONATION
The communities described so far are the most typical.
The increased or decreased action of waves will have a
modifying affect on the communities changing the
species such that it maybe unrecognizable. (See facing
page). Examples of adaptations include:
1. A change in the dominant community by removal
of competitor species.
• large surface areas are a disadvantage with wave
action: F. serratus will be removed, favouring
strap-like Himanthalia; Ascophylum cannot
survive wave action and F. vesiculosus takes
over; Laminaria is replaced by Alaria esculenta ,
the Dabberlocks, (photo below) in extremes.
•
Patella vulgata is replaced by P. aspersa which is
able to exert a greater suction to the rock for
attachment. The firmly attached barnacles and
mussels increase
2. There is a change in the shape and form of growth,
e.g. dogwhelks
• bladderwrack varies in the number of air
bladders; production reduces with wave action so
in extreme cases it becomes bladderless
• limpet shells tend to be flatter in shelter, domed
with wave action. (page )
3. Productivity decreases with increased wave action.
•
Waves create erosion of the softer, large
seaweeds and they may be unable to remain
attached; mainly calcareous, small slow growing
seaweeds survive
4. Zonation is displaced up the shore with an
extension of the splash zone.
• waves will increase the humidity on the shore so
that desiccation is less of a problem: algae and
animals survive further up the beach; Laminaria
may remain uncovered at low tide as the spray
keeps them wet.
• the spray up the cliff will extend the Verrucaria
and Littorina zone (splash zone)
5. Difficult conditions reduce diversity
A Field Atlas of the Seashore
Ballantine’s Exposure Scale
For any scientific exercise it is
important to compare and use a
standard against which you can
measure. For example, chart
datum is a good standard on
which to relate the position of a
species when comparing shores.
When looking at wave action it
is very difficult to find a
measurable point of reference.
The force of waves is extremely
variable from day to day so
measuring the force one day.
The organisms have to exist for
long periods of time. In this way
we can use species as indicators
of the abiota. High densities of
barnacles suggest strong wave
action whilst high cover of
wracks suggest, on average,
sheltered conditions.
Ballantine’s Exposure Scale where BES 1 is most exposed to wave action and BES
The ecologist Bill Ballantine
8 is the most sheltered (After Ballantine 1961)
uses these indicators to
construct a scale of eight where
one is extreme wave action that
one might expect on a headland facing the open Atlantic.
Eight is extreme shelter which you would see in the back
end of a sea loch or fjord where the only disturbance of the
water is the gentle rise and fall of the tide. A simple
representation of the scale is shown above. Using this
compare the two photos, right, to see where they might fit
on the scale. Both were taken a mid-tide level but still it is
possible to judge their position on the scale. The one to the
right is around BES 7 and the left one is BES 3.
FEEDING RELATIONSHIPS on the ROCKY SHORE
By photosynthesis, the algae absorb sunlight and convert
solar energy into chemical energy which they use or store
within the cell. They are the primary producers within this
ecosystem. However, the algae exist in many forms.
Seaweeds produce thousands of cells on the frond surface
which, like human skin cells, erode away from the plant.
The tip of Laminaria blades erode continually; 40% of the
productivity of Ascophyllum escapes into the sea. These
cells become suspended in the water and subject to
bacterial action. Along with other particles of organic
matter, like dead animal remains, this forms microscopic
detritus. This is filtered and consumed by detritivore
animals. The surface of the rock may appear to be devoid
of algae but will have a fine layer of microscopic
cyanobacteria as well as young developing spores
(germlings) of the macro algae.
BES1 Note the extensive
splash zone, dominant and
barnacles with some red algae
and F.serratus
The primary food sources available to animals in this
therefore
ƒ macro algae or seaweeds, e.g. Fucus, Laminaria
ƒ micro algae, e.g. Calothrix and cyanobacteria
ƒ diatoms in the phytoplankton
ƒ cells in suspension and converted to detritus by bacteria
ƒ lichens, e.g. Verrucaria
BES7 Note the small splash
zone, Pelvetia, F.spiralis and
extensive Ascophylum
and F. vesiculosus weed
ecosystem are
A Field Atlas of the Seashore
The sea is one of the most productive of ecosystems; phytoplankton has a very fast reproduction; Saccorhiza grows to
over five metres in a year and weighs several kilograms. There are actually very few animals that are dedicated
herbivores. Most feed on microscopic material. The Flat Perwinkles are somewhat of an exception.
FACTORS AFFECTING THE FOOD WEB:
SEASONS: Algae, with some exceptions, grow best in summer; some animals only appear on the seashore when
spawning occurs, e.g. sea slugs. Land birds often come on to the shore to feed in winter when the fields are covered by
snow. ABUNDANCE of PREY: Animal density varies and a predator will usually take prey items which are those
easiest to come by i.e. those most abundant. This may mean an alternative to the usual diet. POLLUTION: Crude oil
adheres to the rock and by covering any lichens will prevent photosynthesis; trampling by man will prevent spores
developing and reduce productivity of the shore. Reduction in top carnivores (seabirds) by oil will give an expansion in
the prey items below them in the web. INVASION by OTHER SPECIES: Foreign species will not fit in to any
particular zone and may take over dominance, shading (killing off) indiscriminately species. For example, Sargassum
muticum, arrived on the south coast in 1970 and some seashores around the Isle of Wight have become swamped, with
the weed replacing natural communities. It has now colonized shores many hundreds of miles from this original shore.
FEEDING MECHANISMS USED BY ANIMALS
ON THE ROCKY SHORE
1. Plankton and detritus feeders
• gills e.g. mussels
• appendages e.g. barnacles use limbs
• pores e.g. sponges
• tentacles e.g. sedentary polychaete worms
• perforated pharynx e.g. sea squirts
• cilia e.g. brittle stars
2. Debris feeders (scavengers)
• mouthparts and pincers e.g. Common Shore Crab (photo,
right)
3. Seaweed feeders
• grazing gastropod molluscs using a radula e.g. topshells
and periwinkles
4. Predators
• shell borers e.g. dogwhelk, with radula and chemicals.
• shell openers e.g. starfish with their tube feet and inverted
gut. Photo right is the underside of the common starfish,
Asterias, with the central mouth and the tube feet.
• paralysis of prey e.g. sea anemones, with nematocysts.
• capture of prey e.g. cuttlefish, with tentacles; e.g. blenny,
with teeth on powerful jaw; e.g. cormorant and gulls with
beak
A Field Atlas of the Seashore
PREDATION AND ITS AFFECT ON DIVERSITY
The richness of the species within a habitat will depend on a)
food preferences and b) the intensity of grazing/predation.
Fig. 8 demonstrates the affect that increasing predation has upon
the richness of species in a community such as that found on the
lower shore. Edible periwinkles are selective feeders here on
seaweed germlings. With a minimum of grazing competition
between seaweed is strong and will favour only a few species
often with one becoming dominant. As grazing increases so all
algae will be affected and will be present. If over grazing occurs
then algae may never reach maturity, spores are eaten before
they can grow and only a few, grazing-tolerant species survive.
Compare these facts with fig. 9. When populations of the
predator, dogwhelks, declined in the 1970’s and 80’s so did the
variety of species on the shore often becoming dominated by
mussels.
PARASITISM
Feeding relationships are inevitably one sided; the dogwhelk
kills the barnacle and consumes it in one go, moving then on to
another one. Therefore it lives on “capital”. Parasitism is a
nutritional method whereby the organism does not kill the prey
but lives upon it deriving a steady income without killing it.
Parasites are very specialised for this existence and invariably
cause a modification to the host. The crab, Carcinus, is affected
by a barnacle-type parasite, Sacculina. The name is derived from
a yellow sac attached to the underside of the crabs abdomen. It
looks nothing like a barnacle. Feeding on the crab it causes
many physiological and behavioural changes, e.g. stimulating
them to move in to deeper water. Periwinkles are susceptible to flukes. Adult flatworms live in the gut of gulls and
when the bird defaecates eggs pass out too. As the gulls roost on the upper shore the eggs may then be eaten by L.
saxatilis. Larval flukes develop and if the periwinkle is eaten by gulls the cycle is completed. The periwinkle acts as a
second host enabling the parasite to get back to the primary one.
Underside view of the
Common Shore Crab,
Carcinus maenas.
Left, this crab has been
parasitized by the
crustacean, Sacculina.
By comparison the
photo right shows a
normal female crab
holding the bunch of
eggs (up to 800,000
typically) in the same
place under the
abdomen.
ROCKPOOLS
When the tide goes out it will leave pockets of water trapped in hollows and depressions to produce distinct rockpool
communities. For some species this will enable them to survive further up the shore; in spring it is possible to find
Laminaria growing in the middle and upper shore, but high temperatures in summer will kill it. Fucus serratus can
invariably be found in rockpools of the middle shore. Many organisms will use the pool for shelter as well as
combating desiccation. However, not all the species discussed previously will be found in the pools, these pockets of
still water excluding them. It may be that they are already specialised for their niche and life in the rockpool will
require additional adaptations for survival. The limiting factors operating in a zone will not necessarily apply to the
rockpools there and the difference will be represented by a different community. Deep rockpools will have their own
vertical zonation. Over a period of several hours it is the condition of the static water which change. Therefore, most of
the limiting factors relate to the length of time it is standing before the tide returns.
A Field Atlas of the Seashore
Upper Shore Rockpool
The green alga, Enteromorpha, is common as
freshwater collects here and the weed is very
tolerant of brackish water. The white covering to
the rock is an encrusting red seaweed like
Lithothamnion that has been bleached by the high
light levels here. Diversity of species is low due to
the abiotic conditions varying so much during the
day, e.g. high temperatures in mid-summer,
freezing temperatures in winter. Enteromorpha can
be frozen but still survive. Pelvetia is on the dry
rocks at the top of the picture.
Middle Shore Rockpool
Shown in close-up, the red encrusting
algae are dominant on the rocks as
well on the limpets. The latter have
grazed the pool preventing too much
other algae from growing. Other red
algae are also calcareous like the
Coralina and these form fringes (top
right and bottom). Calcareous weeds
and very tolerant of grazing. The
orange patches are sponges, keeping
within the wet crevices.
This type of pool is typical of exposed
shores. Sheltered shores would have a
higher diversity of species.
Lower Shore Rockpool
The deeper pools like this one
can have a very high
biodiversity as the abiota
remains fairly stable from tide
to tide. Water is changed
regularly and rarely has
chance to significantly heat up
or cool. Red algae fringing the
pool are abundant as are the
Laminarians which cannot
tolerate desiccation. Algae
like the Sea Oak, Halidrys,
are typical rockpool species
found only here. Zonation
with a pool of this size is
quite marked. The green
seaweed, mainly Sea Lettuce,
Ulva lactuca, is a transient
species abundant in the
summer when the light can
reach the pool especially
when the tide receeds.
A Field Atlas of the Seashore
THE LIMITING FACTORS operating in ROCKPOOLS
Temperature: High air temperatures in summer heat
the water affecting the dissolving of gases in water as
well as increasing evaporation. In winter, upper shore
rock pools may freeze.
Salinity: Increased evaporation will cause the salinity
to rise. Rain will dilute the seawater. Either will cause
an osmotic problem with the organisms because few
have the ability to regulate the body concentration.
Carcinus is one of the exceptions.
Oxygen and pH: The amount of dissolved oxygen
varies with temperature and the degree of
photosynthesis. In sunlight it is easy to see bubbles of
oxygen on the fringe weeds. By late afternoon this
slows stopping completely at night. The balance
between respiration and photosynthesis also affects the
carbon dioxide content. This latter gas produces an
acid in water and its addition and subtraction from the
water changes the pH during 24 hours. Too much
dissolved oxygen will slow down photosynthesis
(called the Warburg effect) and seaweeds high up will
reach their peak by mid morning. A decline in
photosynthesis then occurs.
Organic Matter: Rockpools will trap dead and
decaying organisms; seaweeds washed in from the
lower shore often become caught. This attracts the
scavenging animals. Shrimps are typically found here
as will most of the detritus eating Crustacea.
ORGANISMS OF THE ROCK POOLS
Upper Shore: These pools will be dominated by
Enteromorpha species as they are most tolerant of
temperature extremes as well as ionic changes due to
the lower salinity. Pelvetia is notably absent. Carcinus
and Palaemon both can osmoregulate and may survive
here.
Middle Shore: F.serratus may grow well here but
few other lower shore species do. Lithophyllum and
Corallina both grow, particularly if shaded. They may
dominate and give their name to the type of pool.
Many small, delicate species perform well in these
pools, e.g. Ceramium, Nemalion, Scytosiphon. The
sides of the pools will have the most abundant growth called fringe weed because grazing limpets cannot
climb the side. The floor will be grazed clear except
for Lithophyllum. Small weeds will grow on the shells
of the limpets - the only place they will not be
consumed. Shallow pools will be colonised by the
Snakeslock Sea Anemone. The green form a symbiotic
alga (zooxanthella) living in the cells. In bright
sunshine the plant will be able to produce foods for
both. Barnacles are absent from rockpools, producing
a clear dividing line near the edge. This may be due to
problems with larvae or competition with the sheltered
rock pool species.
Lower Shore: With the displacement of organisms up
the shore in these desiccation-free pools sub-littoral
species will colonise the lower shore. "Copses" of kelp
live in the deeper ones with the much branched
Halidrys siliquosa. This is aptly named sea oak as it
has an abundance of animals live amongst it. These
pools may have echinoderms and octopus more typical
of offshore habitats.
Studying Rockpools
As noted below in the next section on investigations rockpools from different parts of the shore or of different sizes and
volumes can be compared. This invariably will look at biodiversity with the calculation of a biodiversity index to create
a value for each pool. These values can then be analyzed and comparisons attempted. However you should measure the
changes in abiotic factors with time
and state of the tide, such as
temperature, oxygen, carbon dioxide,
salinity. Temperature is
straightforward but the others would
need samples taken and analyses
later using titrations. The easier way
of carrying out the later is to use a
datalogger. Set to record the data
over whatever time you choose the
logger can have four sensors able to
measure values at a faster rate than a
human can! At the end the data is
saved and taken back to download on
to a computer. Salinity can be
measured using a conductivity sensor
whilst carbon dioxide can be
estimated by measuring pH. When
the gas is dissolved in water it forms
a mild acid. As plants carry out
photosynthesis in the pools the level
of carbon dioxide reduces and the
water goes more alkaline.
Screen shot of data collected with a datalogger and downloaded to a computer
A Field Atlas of the Seashore
PERSONAL INVESTIGATIONS and PROJECT WORK
A personal investigation is found in most biology courses and rocky shores are ideal habitats for this due to the wide
range of species and their comparatively large size. The latter means that specimens are easily found and measured.
The criteria categories are:
• Planning
• Implementing
• Analysing evidence and drawing conclusions
• Evaluating evidence and procedures
• A synthesis of principles and concepts
All investigations need to
• Produce quantitative data
• Contain statistical analysis
• Have a brief literature review
• Have a Risk Assessment of the habitat and
procedure
Data - What makes a good subject for investigation?
Comparative investigations are good as they generate a
good deal of data and that is what is needed for graphs and
statistics. To this end, population studies are often easier
than community ones. The data can be either as counts
(density) or measurements (size of individuals).
Percentages do not make good data for analysis and
abundance scales should be avoided. Start by choosing the
type of organism you think you might like to work with,
either animal or plant. Here are some further pointers to
help your selection.
1. Use species which you know are common and easily found
2. Once you have an aim set a clearly testable hypothesis; this means one that is not ambiguous and at the end
you can accept or reject, e.g. Rough Periwinkles are smaller on exposed shores than on sheltered shores. Data
can be collected to easily prove or disprove this. This hypothesis is the H1. When using stats you create a null
hypothesis (H0), in this case, there is no difference in the size of Rough Periwinkles found on exposed and
sheltered shores
3. Minimise the number of variables used
4. Keep it simple, succinct and manageable
5. Plan your method carefully including the data analysis as this affects the type of data collection (see below)
6. Counting things is generally non-parametric and works with most data including abundance scales
7. Measuring things is generally parametric and works by comparing the means and standard deviations of two
sets of data, typically, 25 samples in each set
8. Consider carefully what equipment to use, e.g. if it is a quadrat you need to state clearly the size and type,
justifying why the selection.
9. Are the samples random or taken systematically? Probably the former. If you are comparing the size of rough
periwinkles on two shores they should be at the same height above chart datum. At these two points random
samples would be taken to find the individuals needed. Even if you compare specimens in the upper and
middle shore these would be from specific heights above chart datum, say 4 metres and 6 metres, and random
samples taken at each height.
10. Take repeat readings or samples, but how many?
How many samples to take? We tend to think that the more
we take the more accurate the result. This is not so and
secondly will be influenced by the stats test we employ.
Statistical tests will be considered below but first the principle
of the Running or Accumulated Mean.
As an example, how many samples should we take to find the
mean density of Purple Topshells in the middle shore? Laying
down a one metre square quadrat at the mid-tide level on a
rocky shore, the number of individuals could be counted. Lets
say the figure is 46, i.e. 46/metre square. Do we need to do this
again? Is that enough samples to get an accurate, representative
figure? No, because it is almost certain that if we take another
quadrat sample at random it will not have 46. What needs to be
done is to keep taking samples until adding more data to create
Sample
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Topshell
Density
46
23
31
65
26
17
29
32
20
34
28
19
31
40
Running Mean
calculation
46/1
46+23/2
46+23+31/3/3
46+23+31+65/4
46+23+31+65+26/5
46+23+31+65+26+17/6
Complete this yourself!
Running
Mean
46.0
34.5
33.3
41.3
38.2
34.7
33.9
33.6
32.1
32.3
31.9
30.8
30.8
31.5
A Field Atlas of the Seashore
Running Mean
a mean produces negligible difference to the mean value. Look at the table on the right to see how this works.
It is possible to keep this going but already there
A plot of the running means of Purple Topshells
are two values at the twelfth and thirteenth sample
50.0
which are the same. Adding more data is not
making a significant change to the value and so
45.0
15 samples may well be enough in this case. It is
40.0
best to plot this as a graph so that the changes can
35.0
be seen more easily (right). Plan to do something
30.0
like this in the field to justify the number of
25.0
samples you take.
20.0
15.0
Statistics: this section merely touches the subject
10.0
of stats and is too lengthy to show methods here.
5.0
The main statistics used at A Level are mentioned
0.0
here. The principle of stats is to run the data
1
2
through a test so that it is possible to reject or
accept the Null Hypothesis. It gives weight to
your argument that there is or there is no difference between the data.
3
4
5
6
7
8
9
10
11
12
13
14
Sample Number
Chi square of association
Classical method for analysing frequencies of categorical data. This would be used in only a very few investigations
such as niche or micro-habitat selection. For example, the numbers of flat periwinkles associated with different types of
wrack.
Spearman Rank Correlation Coefficient
This test determines whether two variables, which need not be normally distributed, are related. For example, a
correlation exists between the density of beadlet anemones at different heights on a rocky shore. 5 – 10 values required.
Student t-test
This compares two sets of data that are normally distributed. To be used with measurements such as the length of
periwinkles on two different shores. Only two sets of data can be compared, if there are three then each set needs to be
checked against the other, i.e. 3 separate test combinations. Each set requires between 15 – 30 values or measurements.
Ratios or two sizes can be used, e.g. height to base ratio of limpets (see below).
Mann-Whitney U-test
This compares two sets of data that are not normally distributed. To be used with counts such as the density of
periwinkles on two different shores. Only two sets of data can be compared, if there are three then each set needs to be
checked against the other, i.e. 3 separate test combinations. Each set requires between 8 -15 values or counts.
The idea of ratio values was mentioned above. A measurement of the length of a periwinkle is a useful value of size but
wave action might influence the shell morphology (shape). Size is a possible indication of age and as molluscs grow so
they get larger. Consider the shapes of the limpets below. A and B are from the same shore but the larger one (B) is
much older. C and D are from the same but different shore to A and B. D is the older individual. A single measurement,
such as the base, will be an indication of age but if you want to see changes in shell shape between different specimens
two dimensions are needed to get a ratio between the two. Dividing the base, x, by the height, y, will give a ratio that
then excludes the affect of age and looks just at the shape. The ratios would then be compared using the t-test. For
example, in A and B the ratio is close to one as x and y are similar and it does not matter how old or large the
specimens as the ratio will tell you how the two dimensions relate to each other. C and D will have a ratio closer to
four.
Measuring Limpets
B
A
y
x
C
D
A Field Atlas of the Seashore
Topics and Suggestions
These suggestions are simple ideas for expansion. Keep it simple. In many cases the hypothesis can be written for a
wide range of species and comparative projects always work well.
1.
2.
3.
4.
5.
6.
7.
8.
9.
The affect of exposure to wave action on animal populations using either rough periwinkles, purple topshells,
dogwhelks, beadlet anemones or limpets. This could be either density or shape/size.
The affect of tides (height above chart datum) on animal populations. This could be either density or
shape/size.
The affect of exposure to wave action on algal populations. This could be length of fronds, biomass or number
of bladders in the case of bladderwrack
The affect of exposure to wave action on lichens populations in the splash zone. This could be density,
diversity location as well as size of individuals.
Microhabitat selection by animal populations in an area of shore, e.g. density of purple topshells in crevices,
pools, on weed, bare rock etc. this could be done with most species including sedentary ones like sponges as to
whether they are in crevices which fill with water or under stones, etc.
Competition between two species, e.g. knotted wrack and bladderwrack
Prey Predator relationships, e.g. a correlation between the density of dogwhelks and live (or dead) barnacles
Comparison of species diversity found in rockpools of different volumes or between different areas of the
shore
Measuring the weight loss of different seaweeds with time and relate this to their position on the shore.
The following are more general, Mini-Projects, and are not so suitable for assessment.
1. Animal communities (epifauna) on the dominant seaweeds. Select several fresh samples of different seaweeds such
as the fucoids, kelps and Halidrys. Vigorously, wash each species separately in a bucket of seawater and identify the
organisms which are dislodged. Compile a list for the different seaweeds and record the part of the plant to which they
were attached. Halidrys siliquosa has a surprisingly large macrofauna living upon its fronds whilst Laminaria digitata
has an extensive micro-community within its domed holdfast. Some algae, e.g. Ascophyllum have a poor epifauna.
Construct foodwebs for the communities. If a record is made of the numbers of each animal species comprising the
epifauna a pyramid of numbers could be constructed.
2. Epifauna of fringe weeds. Collect small samples of fringe weeds, e.g. Ceramium, Halopitys, Corallina, and keep
them fresh in seawater until you are back in the lab. Put them in a petri dish of seawater and examine under a binocular
microscope. The vast numbers of micro-organisms crawling and swimming around the fronds are quite amazing. For a
detailed analysis, the fresh plant can be soaked in the dye, Rose of Bengal, made up in seawater. This helps remove the
organisms and stain them pink. They can then be identified and counted.
3. Marking Animals. Much can be learnt about animal movement by marking animals on the seashore. One of the
commonest experiments is to mark a number of limpets with enamel paint and watch their progress over a period of
days. By carefully removing them and placing them at different distances from their original sites, up to several metres
away, the homing behaviour can be studied. Numbering crabs on the carapace can be used to measure their distribution
patterns.
5. Rockpools. An interesting comparison can be made between rockpools selected in the different zones on the
seashore. They can be mapped by placing a tape measure along on edge (the standard) and a second one at right angles.
At intervals along the standard tape measurements are recorded to the edges of the pool. Using a suitable scale it can
then be drawn on to graph paper. On the pool outline can be drawn a map of the vegetation and relative positions of
animal grazers. Temperature and oxygen readings should be taken at different times of the day.
6. Plankton. Samples taken with suitable nets at different times of the day at a fixed depth or at different depths or
locations.
Risk Assessment and Environmental Issues
It is essential to keep the welfare of the organisms in mind whilst carrying out the project. Avoid removing animals
from the shore, aim to measure them in situ. Snails like periwinkles, topshells and dogwhelks can be easily picked up to
be measured and released back where they were found. Do not move species between zones. Assessed projects
invariably carry marks for this care as well as a risk assessment of the investigation. For this make sure you have
assessed all the issues regarding the fieldwork such as potential problems on the shore, for example:
1. States of the tides, i.e. not getting cut off by a returning tide
2. Slippery seaweed, especially fine micro-algae which may cause you to over balance
3. Sharp barnacles, being carefully not to put out your hand on to a rock covered in barnacle shells
4. Tell someone where you are going and never work alone!