Zonation On Rocky Shore Essay Research Paper

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 a 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.