Zonation On Rocky Shore
The seashore is a habitat that contains a wide
range of microhabitats and ecological niches for
different creatures. This is
mainly due to the effects of the tides, that rise
and fall twice each day.
Tides are the vertical movement of water in a
periodical oscillation of the
sea, due to the gravitational pull of the sun and
moon. The tides are on a
semi-diurnal cycle, so there are two high tides and two
low tides each day.
Due to the orbit of the moon, the tides also have a monthly
cycle. This
creates neap (very low) and spring (very high) tides. The seashore
can be
divided into several zones, which are illustrated on the diagram
below:
Key: EHWS = Extreme High Water Spring (MHWS = Mean High Water
Spring) MHWN =
Mean High Water Neap (MTL = Mid Tide Level) MLWN = Mean
Low Water Neap ELWS =
Extreme Low Water Spring (MLWS = Mean Low Water
Spring) CD = Chart datum The
Supralittoral Zone: This is the highest zone
on the shore, and lies above the
EHWS mark, and therefore is never
covered by seawater. However, it may be
occasionally be spray wetted. Because
of this, it is mainly inhabited by
terrestrial species, such as lichen, that
can live in areas of very high
salinity. The Littoral (Intertidal) Zone: This
zone is the area that is covered
and uncovered by the tides, and therefore
organisms that live here must be able
to tolerate a large range of
conditions. It can be further divided into the
Littoral Fringe and the
Eulittoral zone. The Littoral Fringe (Splash Zone): This
part of the Littoral
zone lies above the area that is completely submerged by
the sea in normal
conditions. However, it is frequently covered by splash from
waves, and so is
far more marine in character that the Supralittoral Zone.
Lichens still
dominate this zone, but some species of periwinkles and topshells
may graze
them. The Eulittoral Zone: This zone is the area of the beach that
is
regularly submerged by the tides, and can be divided into three more
zones, the
upper, middle and lower shores. It shows the greatest species
diversity of any
of the zones. The Upper Shore: This region of the shore lies
between the EHWS
and MHWN marks, and so is only immersed during spring tides.
Because of this,
organisms that live here must be adapted to survive long
periods of desiccation.
The two seaweeds that are the most common here,
Fucus spiralis and Pelvetia
canaliculata have adaptations to survive in this
area. The Middle Shore: This
region of the shore lies between the MHWN and
MLWN marks, and will be submerged
for half of every day, even during neap
tides. The most common seaweed in this
zone Fucus vesiculosus. Mussel beds
will form and both limpets and periwinkles
will graze the rocks. Sea anemones
and crabs are residents of this zone. The
Lower Shore: This region of the
shore lies between the MLWN and ELWS marks, and
will be submerged for most of
each day, even during neap tides. The most
important seaweed in this area is
Fucus serratus, which will form large zones
wherever suitable. It shows the
greatest species diversity of any zone on the
seashore. The Sublittoral Zone:
This part of the shore lies below the ELWS mark,
and is therefore never
uncovered by the sea. There are many types of organism
found on the rocky
shore. The two main photosynthetic organisms are the lichens
and the
macroalgae or seaweeds. Lichen are the main organisms found in the
splash
zone and come in three distinct types; crustose, foliose and
fruiticose.
Crustose lichens form a thin crust on the rock surface, and
are impossible to
remove without damage. Foliose lichens are leafy lichens
that are not as firmly
attached to the rocks. Fruiticose lichens extent
vertically from the rock
surface, and can sometimes be confused with mosses
and small grasses. The leafy
part of a lichen is known as the thallus.
Seaweeds are primarily divided by
colour, into brown, red and green groups.
Most marine seaweeds are brown
seaweeds, with fewer red species, and even
fewer green species. The three main
parts of a seaweed are: 1. Frond (lamina,
thallus, blade) (often broad and flat)
2. Stipe region (often long and
cylindrical) 3. Basal attachment (holdfast) The
frond or thallus is the site
of most of the photosynthetic activity in the
organism, and also contains the
reproductive organs. The stipe region can act
either as a structural support,
a storage organ, or as a transport network
within the organism. The role of
the holdfast is to anchor the seaweed securely
to the substrate it lives on.
The holdfast must be strong enough to resist the
strong pull of the waves and
tides on the seaweed. The size and strength of the
holdfast varies between
species. The main heterotrophic organisms of the
seashore are the molluscs.
The most common molluscs are the gastropods
(periwinkles, limpets and
topshells), and the mussels. Periwinkles have coiled
shells and a circular
operculum (a small, retractable piece of shell used to
cover the opening of
the shell when the snail is inside.). They average about
15mm in length
and are the most common group of gastropods on the seashore.
Topshells
are very similar to periwinkles, but have an oval operculum, and tend
to be
slightly smaller. There are fewer species of topshells than periwinkles on
a
rocky shore. Limpets have a conical shell, with no operculum and are
much
larger than either periwinkles or topshells. Mussels have two shells,
and are
fixed to a single location in adult life. They can form large groups
on the
rocky shore. Describe LOWER SHORE There was only one species of
seaweed found in
the lower shore, Fucus serratus, and it was very abundant.
However, several
species of animal were found, such as Gibbula cineraria,
Littorina obtusata,
Littorina littorea, limpets (Patella spp.) and
mussels (Myttilus edulis). Of
those, Gibbula cineraria was the most abundant.
Fucus serratus: This species of
brown seaweed (Phaoephyta) was found only
below the MLWN mark in stations 10, 11
and 12. It was most common in station
11 (40% cover), but there was not a lot of
difference in the distributions
between these three stations. Fucus serratus is
a medium sized marine seaweed
with a flattened, branched thallus with a small
stipe for support and a small
holdfast. At the ends of the thalli, there are
small, swollen areas called
receptacles, which contain many conceptacles, in
which gamete production
occurs. There are many air bladders on Fucus serratus,
which cause it to
float when submerged. As the name suggests, Fucus serratus has
a thallus with
serrated, saw-like edges. Gibbula cineraria: This species of
topshell was
found mainly in the lower shore, below the MLWN mark (stations
10,11, and
12), and in station 9 (just above the MLWN mark). It was evenly
distributed
across stations 9, 10, and 11, with similar numbers in each quadrat
(between
40 and 50 individuals per quadrat). It was far less common in station
12,
where only two individuals were found. Gibbula cineraria is a
relatively
large snail, at just over 15-mm. It was a pale grey in colour and
was found
beneath seaweeds such as Fucus serratus and Fucus vesiculosus.
MIDDLE SHORE
Several species of seaweed were recorded in the middle
shore. Fucus vesiculosus,
Ascophyllum nodosum and Polysiphona lanosa were
all found, and Fucus vesiculosus
was the most abundant. Many animal species
were recorded, such as Gibbula
umbilicalis, G. cineraria, Littorina
saxatalis, L. obtusata, L. littorea,
limpets (Patella spp.) and mussels
(Myttilus edulis). Of these Gibbula cineraria
was the most abundant. Fucus
vesiculosus: This seaweed was found mainly in the
middle shore, between the
MLWN and MHWN marks (stations 7,8 and 9), but also in
station 6 (just above
the MHWN mark). There was a much lower density in stations
6,7 and 8
(between 3 and 12%), than in station 9, where the percentage cover
was
30%. Fucus vesiculosus is similar to Fucus serratus (see above), with
a
flattened, branched thallus and air bladders, but lacks the serrated edges
of
Fucus serratus. Littorina obtusata agg.: This species of periwinkle
was found in
the middle shore (stations 7,8 and 9) and the lower upper shore
(station 6). It
was also recorded in station 12, at the lower end of the
lower shore. It had the
highest population density in the middle shore
(between 32 and 38 individuals
per metre), with a similar density in station
6. It was far less abundant in
station 12, with only 12 individuals recorded.
Littorina obtusata agg. is a
small, flat periwinkle, mainly found on the
underside of seaweeds such as Fucus
vesiculosus, Fucus spiralis and
Ascophyllum nodosum, where it mimics air
bladders. It comes in a wide range
of colour, but most individuals are a dark
olive green to match the seaweeds
they live on. UPPER SHORE Again, several
species of seaweed were recorded in
this zone, such as Fucus vesiculosus, F.
spiralis, Ascophyllum nodosum,
Pelvetia canaliculata and Polysiphona lanosa.
Several animal species were
also recorded, such as Littorina saxatalis, L.
obtusata and limpets (Patella
spp.) Pelvetia canaliculata: This seaweed was
found in station 4 only (at the
very upper limit of the littoral zone, just
below the EHWS mark), but was
very abundant, covering 70% of the quadrat.
Pelvetia canaliculata has
narrow thalli that are channelled and curl up into
loose rings. It is browny
red in colour and has no air bladders for support.
Littorina saxatalis:
This species of periwinkle was found across the whole upper
shore (stations
4,5 and 6) and at the top of the middle shore (station 7). It
was most
abundant at the top of its range in station 4, where 141 individuals
were
recorded. It became less and less abundant down the beach, at the bottom
of
its range, in station 7, where only 20 individuals were recorded.
Littorina
saxatalis is a medium-sized periwinkle, about 16-mm long. It has a
ridged shell
that is orange-brown in colour, and is commonly found in
crevices and cracks on
the upper shore. SPLASH ZONE The only plants found in
the splash zone where
lichens such as Verrucaria maura, Xanthoria parientina,
Ramalina siliquosa,
Lecanora atra and Ochrolechia parella. No animal
species were recorded in this
zone. Xanthoria parientina: This species of
foliose lichen was found throughout
the splash zone (stations 1,2 and 3), and
was the largest range out of all the
lichens. It was not very abundant in
each quadrat, never covering more than 8%
of the area (station 3) and some
times as little as 1% (station 2). Xanthoria
parientina is a foliose lichen,
which means it is only loosely attached to the
rock, and has large thalli. It
was orangey yellow in colour. Explain The
environmental gradient on the
seashore is constantly changing. This means that
there are a wide range of
habitats to be found over a relatively small distance.
The wide range of
species found on the seashore is due to the wide range of
habitats and
conditions found there. Species can only be adapted to a small
range of
conditions, so as the conditions on the seashore change, so do the
species
found there. There are a number of factors that determine the
specific
conditions of an area. These factors can be either biotic or
abiotic. Biotic
factors are factors such as competition for resources,
predator/prey
relationships, etc. Abiotic factors are factors like
temperature, relief,
climate, etc. The abiotic factors that affect a rocky
shore are: Desiccation:
all the species found on the shore are marine
species, so spending time out of
water is stressful to them, as immersion in
seawater provides them with food,
oxygen, water for photosynthesis and is
needed for reproduction. Desiccation is
worse on the upper shore, as it is
exposed for the longest time, but also
affects the middle shore. Temperature:
Seawater remains at a far more constant
temperature that the land, (seawater
varies between 5° and 15° Celsius,
whereas the land temperature varies
between below freezing in winter and 30° C
plus in summer) so species that
are immersed in seawater for long periods of
time are buffered against large
temperature changes. The temperature of the
surroundings also affects the
rate of metabolism; very cold conditions will slow
it down, whereas very high
temperatures may denature vital enzymes. Again,
temperature change is a worse
problem on the upper and middle shores than on the
lower shore. Wave action:
The action of powerful waves can dislodge many
species, so those that live on
the middle shore (where wave action is at its
most powerful) must be adapted
to survive very rough conditions. Wave action
also increases the humidity of
an area, and so can help to reduce desiccation.
Light: Light is needed
for photosynthesis, and all seaweeds must be immersed in
water for this to
occur. Water filters off some of the wavelengths of light and
reduces the
intensity that reaches the seaweeds. To maximise the light that does
reach
them red and brown seaweeds have accessory pigments that help to
absorb
different wavelengths of light. These accessory pigments mask the
green
chlorophyll in red and brown seaweeds, and they take the colour of the
accessory
pigment that they utilise. Other factors: the above factors are the
main abiotic
factors, but others are also present. The aspect of a slope
affects the
temperature and rate at which water evaporates, so south facing
slopes are
warmer, but dry faster, while north facing slopes are cooler and
damper. The
steepness of a slope also affects the rate at which it drains, as
a steeper
slope drains faster than a shallower one, so desiccation is more
of problem.
The turbidity or cloudiness of seawater (due to
plankton, sewage and other
detritus) can affect the intensity of light
reaching submerged seaweeds. Another
factor is the seepage of freshwater onto
the shore. Many seaweeds cannot
tolerate salinity changes, so other species
that can tolerate such changes will
inhabit these areas. The biotic factors
that affect the rocky shore tend to
affect the lower limits at which a
species may live. The biotic factors that
affect the distribution of
organisms on the rocky shore are: Food supply: All
organisms need food to
survive and so can only flourish in areas in which they
can find food. Many
species that are found on the seashore left the sea in
search of food
supplies. For organisms, such as barnacles, which depend on food
carried by
the waves, far more food will be found in the intertidal zone that at
the
bottom of the sea. Predation: Many species also live on the seashore in
an
attempt to evade marine predators, such as fish, crabs, lobsters etc, that
are
far more common in the sea than on the shore. Organisms will also try to
live as
far up the shore as possible in order to avoid their less well
adapted
predators. Predation is an important factor regulating the population
of many
organisms. Reproduction: Most marine organisms still rely on the sea
for
reproduction, so animal species, such as crabs, may migrate lower down
the shore
in order to release their gametes. Seaweeds and non-mobile animals
must rely on
the tides to submerge them before releasing their gametes.
Competition: This is
the most important biotic factor determining the
distribution of species on the
seashore. There are two types of competition,
interspecific (between two
different species) and intraspecific (between
individuals of the same species).
Organisms compete for all the resources
that are in short supply. On the
seashore, most resources are in short
supply, so organisms compete for space,
food, and light. Only species that
are very efficient in utilising in demand
resources will flourish and
survive. Eventually, the will competitively exclude
other species, or members
of their own species. Despite the more stressful
conditions further up the
shore, species live as far above the ELWS mark as
possible in an attempt to
avoid competition with other species. For example,
Fucus spiralis is very
well adapted to surviving long periods out of water, so
it is found in the
upper shore. It is not found in the middle and lower shores
because
competition with other species of seaweeds such as Fucus vesiculosus
and
Fucus serratus prevents them from surviving, so no specimens are
found. Species
can adapt to these different factors in three ways. They can
adapt in physical,
physiological or behavioural ways. Physical adaptations
are those that modify
the external appearance of an organism, physiological
adaptations are those that
modify the internal organisation of an organism
and behavioural adaptations are
those that modify the behavioural of an
organism. Those species that are best
adapted to take advantage of a set of
conditions will do far better than those
that are not adapted will. This
survival of the fittest leads to wide diversity
of species found on the
seashore. The main factor affecting the species found in
the splash zone is
that although it lies above the EHWS mark, and is therefore
never covered by
the sea, it is regularly covered in salt spray from waves and
the wind. This
prevents many terrestrial species from living there, as they
cannot tolerate
areas of high salinity. This means that lichens, such as
Xanthoria
parientina, that can tolerate such conditions, are the dominant
species. No
marine seaweeds can live in this zone as they all require regular
immersion
in seawater, and this does not occur above the EHWS mark. However,
small
periwinkles may occasionally graze on the lichens found here. The
main
factors affecting the upper shore are the highly variable temperature,
and the
amount of desiccation that organisms have to endure as a result of
their
infrequent immersion in the sea. However, wave action and the light
that reaches
seaweeds are not major factors are waves do not cover this area
regularly, and
even when it is submerged, it is not submerged deeply, so the
light is not
affected. Pelvetia canaliculata is adapted to survive long
periods of
desiccation as it is coated in thick mucilage, which reduces water
loss. The
thick mucilage layer also helps to regulate the temperature of the
seaweed. It
has channelled fronds, which helps reduce the surface area of the
fronds that
are exposed to the air. The enzymes and pigments found within it
are also
resistant to sudden temperature change, so it is well adapted to
live on the
upper shore. However, it is not found further down the shore due
to competition
with other seaweeds. Littorina saxatalis can cope with low
temperatures far
better than it can with high temperatures, so it has a
ridged shell surface to
increase its surface are and therefore the amount of
heat that it radiates. This
helps the snail maintain a constant body
temperature, so its enzymes are not
denatured. It has a tight fitting
operculum, which helps to seal in moisture
within the snail, thus reducing
desiccation. All of the main abiotic factors
affect the Middle Shore. Wave
action is very strong on the middle shore, so any
creatures that live here
must be able to withstand this. Desiccation and
temperature change are also
important factors as the middle shore is regularly
exposed to the air. The
main seaweed found in the middle shore is Fucus
vesiculosus, which has thick
mucilage to conserve water. The enzymes and
pigments within are also able to
withstand a certain amount of temperature
shock, though not as much as those
found in Pelvetia canaliculata. It is very
firmly attached to the substrate
material, and so is able to withstand the wave
action. Grazing by limpets and
periwinkles is not a major problem on this shore,
so the seaweed cover is
very abundant. It is not found in the upper shore, as it
cannot cope with the
extremes of temperature and the lack of water in that zone.
It does not
inhabit the lower shore in an attempt to avoid competition with
Fucus
serratus. Littorina obtusata can withstand the moderate amounts
of
desiccation and temperature change on the middle shore by closing its
operculum
to seal in moisture and by resting under seaweeds to insulate it.
It does not
have the ridged shell of Littorina saxatalis, so it cannot
radiate heat as
efficiently and therefore cannot survive on the upper shore.
By remaining on the
middle shore, Littorina obtusata can avoid predators such
as dog whelks that
live further down the shore. However, 12 Littorina
obtusata were recorded in
12th station, just above the ELWS mark, which
is very unusual, as they are
normally out competed by lower shore snails such
as Gibbula cineraria in that
region. The conditions on the lower shore are
most like those in the sea. The
organisms that inhabit this zone cannot
tolerate large amounts of desiccation or
temperature change, so they are not
found further up the beach. As they are
submerged for long periods, the
amount of light reaching the seaweeds is an
important factor and only those
with the appropriate accessory pigments can
survive here. Predation is far
more of a problem for the animals that live here.
Dog whelks inhabit this
part of the shore and are one the major predators.
Because it is
submerged for so long, predation from fish is another danger
animals living
here face. Fucus serratus is very efficient at using the
resources that are
in short supply, so it out competes other species, such as
Fucus
vesiculosus and Pelvetia canaliculata. However, rapid temperature
changes
destroy the photosynthetic pigments in its cells, so it is not found
further up
the shore. It is brown in colour and so is very well adapted for
taking
advantage of all the available wavelengths of light that reach it.
Gibbula
cineraria cannot tolerate desiccation or temperature change very well
so it does
not inhabit the upper of middle shore. However, it is very good at
maximising
the resources around it, so it out competes other species of
snails, such as
Littorina saxatalis. It has a thicker shell than many
other snails, and so is
more difficult for predators to eat. Limitations The
method that was followed
had a number of limitations that lead to anomalous
results (such as finding
Littorina obtusata in the twelfth station). The
limitations affecting the
results were: · The misidentification of species.
Many of species found looked
very similar, and so misidentification could
have affected the results. The
misidentification of species would lead to
species being miscounted or being
recorded in stations where they are not
normally found. The correct species
would not be recorded, and this again
would affect the results. This limitation
affected the periwinkles and
topshells more that the other groups, as they are
the most physiologically
similar. · Species or specimens being miscounted or
missed altogether. Due to
the thick seaweed cover on the shore, it is possible
that many of the
periwinkles and topshells where either miscounted (as
individuals were
covered up) or missed altogether. Quadrats containing many
cracks or
crevices, or large rocks, which organisms could hide under, also made
it more
difficult to be confident that every specimen had been recorded, leading
to
inaccurate results. · Quadrats being placed in the wrong location. It
would
have been easy for errors to have been made while cross-staffing new
locations
for quadrats, which would lead to species being recorded at the
wrong heights
and in the wrong zones. This would make it harder to draw
meaningful conclusions
from the results. · Quadrats placed on uneven ground.
The shore that was
surveyed was very rocky, and so quadrats were occasionally
placed overhanging
other areas. This lead to larger areas being surveyed, as
the slopes were
surveyed as well as the flat ground. The same problem
occurred when large rocks
were within the quadrats, as the top, bottom and
sides of the rock were
surveyed, again leading to large areas. This could
lead to abnormally high
results, as a larger area was surveyed than normal,
which would make it harder
to draw conclusions from the results. · Animals
moving around. The majority of
the animal species recorded are mobile, and so
could move around while being
counted, leading to inaccurate results, or
could have been found far from their
niche, distorting the results. The
animals could move into a quadrat, leading to
higher results, or move out of
a quadrat, leading to lower results than would be
expected. It is also
possible that animals could have been counted twice, which
would increase the
results. All of these limitations would affect the accuracy
of the results,
making it harder to draw meaningful conclusions. Biological
Significance
An organism can only survive in a particular habitat if it is well
adapted to
that habitat. If a organism arrives in a habitat to which it is not
adapted,
then it will be either killed outright by the conditions there (e.g.
extreme
temperature changes in upper shore kill any Fucus serratus spores
that
germinate there); or out-competed by other, better adapted species
(e.g.
Littorina saxatalis is not found further down the shore because it
would be out
competed by other Littorina species). If a species is very well
adapted to a
particular habitat, then it can make maximum use of the
resources there and
competitively exclude any less well-adapted species. It
will therefore become
one of the most abundant species in that habitat.
Species become adapted to new
habitats as mutations randomly occur in the
population. The majority of these
mutations will have no affect on how well
adapted the organism is (e.g. a human
being born with webbed toes), some will
make it less well adapted (e.g. a bright
white lion is born and is unable to
be camouflaged against its prey and so
starves), and others may make an
organism better adapted to its habitat (e.g. a
giraffe is born with a longer
neck and so can reach more food). Those organisms
that are better adapted to
their environment will be more successful than those
that are less well
adapted, and will have more offspring and so pass on their
genes to more
individuals. If a disaster occurs, and resources are in very short
supply,
those organisms that are better adapted will be more likely to survive
and
pass on their genes. Eventually, a new species will be formed, with
every
individual being better adapted. When this occurs, the original species
may
become extinct (e.g. all the giraffes with short necks), or continue
surviving
if the new species is adapted to take advantage of a different
habitat (e.g. a
new seaweed evolves that can survive higher up the shore).
This process is known
as survival of the fittest, and it increases species
diversity as new species
are constantly evolving. This can be seen on a
miniature scale on the rocky
shore, where many different species have evolved
to take advantage of the many
different ecological niches available. My
results show that each species is only
found on a small area of the shore, an
area that it ha evolved to be adapted to,
and one where it is the most
successful species. This process of evolution is
constantly occurring,
producing better and better-adapted species, for many
different ecological
niches. It occurs all over the globe in many different
habitats, forming many
new species.