There are many factors that account for the changes to the vegetation over time within ecosystems in the British Isles, such as human activity, climate, soil, light availability and intensity and natural disasters. The characteristics of the vegetation that are influenced by these factors are height, distribution, variety of species, adaptations and density of the vegetation. Some of these factors have relatively little influence on the succession development, whereas others have a dramatic influence over a long period of time, such as human activity. These factors, over time, result in the progression of a succession until the climatic climax vegetation is reached. However, sometimes these factors can mean that a plagioclimax is reached, as they prevent the succession from progressing any further and the climatic climax vegetation is never achieved, which in the UK are Oak, Hawthorne and Birch trees.
This succession results in the development of an ecosystem. An ecosystem is a dynamic, stable environment which is composed of interacting and functioning biotic and abiotic components and can be of any size. There can be composed of variety of successions, such as lithoseres in grassland and woodland areas for example the Isle of Aaron, psamoseres along the coastline or small scale urban successions in urbanised areas. In an ecosystem, all the component are characterised as biotic, for example animals and plants, or abiotic for example climate, pH, soil characteristics or drainage. These biotic and abiotic factors determine the changes to the vegetation in the ecosystem overtime as they have a great effect on it, such as flooding and animal grazing. Within the ecosystem, there are inputs, outputs, stores and flows which transfer minerals, nutrients, water and light energy. The most important input is light energy from the sun, which is the source of any food chain.
Producers start a food chain, but they would not be able to do so without the 2% of energy provided by the sun to allow photosynthesis to occur. 10% of this energy is then transferred through the trophic levels in the food chain, from the autotrophs, through the consumers to the detritivores. Energy is lost at each stage as a result of movement, excretion and heat loss. As long as there is sufficient sunlight, consumers will continue to be able to use the vegetation in an ecosystem is a food source, which will have a dramatic affect on the characteristics of the vegetation. In this case, both human and physical characteristics are equally important for accounting to changes in the vegetation. Nutrients in an ecosystem are transferred through the nutrient cycle, a balanced cycle which ensures that there are enough nutrients for the ecosystem to survive.
This cycle is represented in a Gersmehl Diagram. An example of an ecosystem is a temperate deciduous woodland. The main human factor that affects the development of a temperate deciduous woodland is deforestation. Deforestation occurs for a number of reasons; to use the timber for commercial use, to use the land for agriculture or to use the land to build on. Once the land has been deforested, it will never be able to be used for the same environment again as the soil washes away in the rain. Therefore, human activity has an extremely large impact on the development of vegetation as it can result in the environment becoming inhospitable. One of the physical factors that affects the distribution of vegetation is the height and density of the exsisting vegetation.
This is because the more dense the canopy layer, te less light will filter through the leaf mosaic to the shrub layer. This means that only plants that require little sunlight will be able to survive, unless they have adaptations that enable them to absorb as much sunlight as possible such as having bread flat leaves so that they have a large surface area, or flowering earlier or later than other plants, such as Dogs Mercury and Bluebells. Another physical factor that affects the distribution of vegetation is the climate. If an area receives too little or too much rainfall, the plants will not be able to survive. On average, the temperate deciduous woodland should receive around 750-1500mm of rain per year. If this number is greatly exceeded, then the soil will become saturated and the area will flood, suffocating the plants as they do not have access to sufficient oxygen and carbon dioxide.
On the contrary, if there is not enough rainfall then the plants will also die because they do not have access to a plentiful supply of minerals that the water contains such as nitrogen and magnesium. One type of succession that can form an ecosystem is a lithosere. A lithosere is a plant succession that begins life on a newly exposed rock surface, such as one left bare as a result of glacial retreat, tectonic uplift as in the formation of a raised beach, or volcanic eruption. The lithosere succession is initiated by pioneer plants, such as blue and green algae with colonise the sock as there are into a lot of nutrients available and they have no root systems, so they can survive in the hostile conditions.
The succession then progresses due to the pioneer plants dissolving the rock, resulting in a release of nutrients. This then means that mosses, lichens and liverworts are able to thrive on that ground as the moss can absorb nutrients from rain water and they provide their own energy from photosynthesis. These plants chemically decay the rock by releasing acids, and physically decay the rock as a result of their root systems, leading to an amelioration of the plants so that the lithosere can progress to the next seral stage.
Herbs grasses and small flowering plants can now colonise in these conditions, which are followed by ferns, brackens and small shrubs as the soil is now thick enough to support their roots and their seeds become lodged in the cracks made by the moss, so colonisation continues. Once these plants die and decay, they leave humus which adds to the richness of the soil and enables more developed plants to grow in their place. At this stage, the soil is developed enough and contains enough nutrients that it can host small tress, such as Alder and Rowan. Once these trees die, the lithosere is able to reach its climatic climax vegetation of Oak, Birch and Pine.
There are many physical and human characteristics that affect the development of a lithosere. In the Isle of Aaron, there is no vegetation in areas of high relief. This is because the seeds for the plants slide down the slope, and do not colonise the steepest areas but colonise the top and bottom of the slope. Another physical factor which causes a lack of colonisation is the salt spray from the sea. The salt dehydrates the rocks that it lands on, so the area becomes inhospitable to the hardiest of plants. The prevailing winds from the south west result in a higher wind chill factor, therefore temperatures are lower, resulting in niche conditions. The winds also deliver seeds which can colonise the area, if the niche conditions that have been created are an appropriate for their survival.
The southerly facing beaches receive more light and heat energy, therefore decreasing the time in which it takes for the succession to reach its climatic climax vegetation. This is because the increased heat energy results in more photosynthesis taking place, as photosynthesis needs carbon dioxide, water and sunlight to occur. As a result, the plant has more energy and so the root systems develop at a fast rate, weathering the rock. In this particular example, the physical factors have a great effect on the development of the lithosere as its development is regulated by factors such as climate, relief of land and pH of the environment.
A plagioclimax is when human activity interferes with the succession of a sere and prevents it from reaching its climatic climax vegetation. Human activity which causes a plagioclimax to occur could be deforestation, animal grazing or fire clearance. Conversely, it is not just human activity that can result in a plagioclimax. Natural disasters such as floods and volcanic eruptions can also result in a plagioclimax being reached. However, if the plagioclimax is reached as a result of physical activity then it is generally not maintained and secondary succession can occur, which is when plants develop on land that has previously been vegetated. An example of a plagioclimax is the heather moorland in the North York Moors.
The North York Moors were once covered in woodland, and heather would have featured in very small quantities. However, as a result of sheep grazing and deforestation, the regrowth of the climatic climax vegetation has been prevented and now heather is the dominant species. To control the uplands and encourage new heather shoots, managed burning is employed every 15 years, a technique that eliminates the less fire resistant species, allowing heather to dominate and conserves as many nutrients as possible.
To conclude, I think that both physical and human factors play a significant role in accounting for changes in vegetation over time in the British Isles. However, I feel that physical factors have a greater impact in the long term that human factors, as physical factors are such that they generally change the environment or habitat on a permanent basis, such as climate change. Nevertheless, I believe that human factors play a significant role in changes to vegetation over time, but I feel that human intervention is not as long term as a change made as a result of physical factors because humans generally have the control and power to continuously change the environment and possibly restore it back to the way in which it was originally, whereas physical factors are not controlled.