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The Relationship between Species Diversity and Water Velocity Introduction The experiment will be looking to find out if there is a relationship between the species diversity of freshwater invertebrates and the velocity of a river. The experiment will take place in a glacial melt water stream within the French Alps. Aim - To discover whether water velocity has any effect on the species diversity of the river. Characteristically and regardless of precise habitat type any aquatic system can be divided into three spatial compartments within which different processes are located; The pelagic community of the water masses; the benthic community living in and on the underlying sediments or rock; and in shallow regions; the fringing communities dominated by emergent or submerged plants. Invertebrate animals of all kinds are found in lakes and rivers. The lakes of the world exhibit a horizontal and vertical zonations but their small size, shallower depth and freshwater content make them ecologically different in many ways from the sea. The river being studied is the Torrent De Corvaria. This is a river situated on the ubac side of a glacial valley
in the French Alps, and is fed mainly by glacial meltwater. Its situation on the ubac side means that it does not
receive as much sun as the adret side of the valley, making it cooler with longer lasting snow and Velocity is the factor being studied in relation to invertebrate species diversity within the river. Velocity
is a measurement of the speed of water flowing past a certain point within a specific amount of time. Velocity
is influenced by the slope gradient, but also the depth and width of the river as friction with the beds and banks
decreases the rivers velocity. Velocity is part of determining the amount of sediment carried by the river as well
as its ability to erode.
The river chosen is the Torrent De Corvaria. Grid reference 9293304. This river was chosen due to its change in gradient and stepped route which will change the velocity down stream so several sites can be tested, and measurements taken. By using the same river we are standardising the pH, temperature, aspect, and rock type. The width of the river is taken and recorded. At the midpoint the velocity is taken using a flow meter. Once the velocity has been recorded the next step is to test the stream by means of kick sampling. This involves standing in the river at the midpoint where the velocity was taken. The bottom of the net is placed at the bottom of the river bed, one meter downstream from where the sampler is standing. Kicking the river bed will dislodge organisms which will then be caught in the net. This method allows for sample time to be standardised for comparison between sites of species number and species diversity. Kicking of the riverbed will be taken four times. The contents of the net are placed into a shallow tray of water. Any species can then be identified by using
the AIDGAP (Aid to identification in difficult groups of animals and plants). This allows a form of identification
by using the key. Each key follows the traditional dichotomous form, consisting of pairs of conflicting questions
(couplets), one of which will be a characteristic of the invertebrate in question. This will either be a name,
a number (referring to another couplet) or a name together with a reference to a later key. It is essential to
read both parts of the couplet carefully to avoid error. These results are then recorded for later interpretation.
Why we chose the Shannon-Weiner:- The Shannon-Weiner function is a good measurement for species diversity, because it is large when the species
diversity is large and/or apportioned evenly amongst the species and is small when the species diversity is small
and/or apportioned unevenly amongst the species (Vandermeer, 1981). This means that species diversity and velocity
can be plotted on a bar graph to see the relationship between the two variables. Working out the Shannon-Weiner Index:
The table below compares The Shannon-Weiner Index to velocity:
The Shannon-Weiner Index is directly proportional to species diversity i.e. as the index increases so does the species diversity. Our results need to be squared because they are negative. They will then become directly proportionate to species diversity and can be compared to velocity on a bar chart.
Summary of Results
Species diversity tends to increase during periods of environmental stability, if environmental conditions change the communities are relatively small, but when conditions are stable numbers of organisms increase. Due to the river being an open system unlike lakes and ponds it was unable to stabilise due to constant changes in meltwater throughout different seasons. according to the stability-time hypothesis of Sanders (1980), long periods of stable conditions enable more and more species to co-exist, this however did not occur in the river we studied due to changing velocities throughout the river (Barnes et al, 1980). Velocity is the factor which affects invertebrates of a river the most. It also affects vegetation which plays a large part in species diversity. The presence of vegetation in a river provides a substrate on which animals can live and also a habitat for certain animals which might be unable to colonise the area because of the velocity. An absence of prolonged stagnation means that the rivers' plants manufacture little basic food materials themselves. The river that we studied was high in velocity, therefore had low vegetation cover. Without vegetation cover the invertebrates are unable to obtain the food they require, and are therefore low in numbers. In the areas of lower velocity there is likely to be more vegetation, allowing more invertebrates to survive. Under such intense competition for food some species will be more successful, because they can maintain maximum biomass while using the minimum of resources, and tend to eliminate others, by the process of competitive exclusion. This would lead to a decrease in species diversity, which is what was observed, as velocities increase (Barnes et al, 1980). The readings of the higher velocity were taken from the upper and mountain areas of the river, where the water was fast flowing, and therefore more erosive. The middle course of the river had lower velocities and flowed more slowly, but still fast enough to carry sand, silt and mud in suspension. The species diversity in the faster-flowing parts of the river was low although there were some species, such as Gammarus, which have adapted to these areas by relying on their swimming ability. They live in the crevices between stones and show a number of behavioural response to velocity. Another reason why there was a lower species diversity in the faster-flowing rivers is the deposition of sand, caused by friction of the boulders slowing down the velocities enough for the lighter sediment to be dropped. These areas also have increased erosion due to attrition and erosion of the river sides and bed because of the increased load in suspension. Sand and silt is more likely to support the smallest number of species (Mills, 1972). Species diversity is higher in the slower-flowing rivers because water movement is less, so they are able to
stabilise themselves on boulders. The number of species found was less than expected due to recent snow melt which
increased velocities even in the parts of the river which normally experience low velocities.
The investigation proved that the null hypothesis can be rejected, therefore the velocity does effect species diversity. As velocity increases species diversity decreases. Our result indicate this as in site number three the velocity is 293 counts/min. which is the lowest and has the biggest diversity if three species. The results supported our hypothesis, but there were limitations while carrying out the investigation. If doing the experiment again and had more time the results could have been collected over a longer period of time, therefore the limitation of climate i.e. snow melt and sunshine would have been omitted. The velocity of the river could have been standardise by taking the results from several equal section across the river, as the river bed and bank cause friction causing the velocity to change over the cross-section. Another factor that was standardised was one person doing the kick sampling due to different strength of individuals, however kick sampling was made difficult in some areas, due to the size of the bed rock, and velocity of the water. There could be slight changes of the level of pH through the course of the river, as it would depend on the stage of weathering on the parent material. Mary-Louise Biddlecombe Barnes, K. and Mann, K.H. (eds.) (1980) Fundamentals of Aquatic Ecosystems. Blackwell Humphries, G., Luck, S. and Adlington, F. (eds.) (1999) Phillips Illustrated Macan, T.T. and Worthington,. E.B (1962) Life in Lakes and Rivers. Collins, London. Mills, D.H. (1972) An Introduction To Freshwater Ecology. Oliver & Boyd, Edinburgh. Vandermeer, J. (1981) Elementary Mathematical Ecology. John Wiley & Sons, USA. |
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