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In streams what is invertebrate drift and how may upstream populations compensate for loss of individuals downstream.

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IN STREAMS WHAT IS INVERTEBRATE DRIFT AND HOW MAY UPSTREAM POPULATIONS COMPENSATE FOR LOSS OF INDIVIDUALS DOWNSTREAM. Invertebrates, in all types of running water exhibit a widespread tendency to drift downstream with current in varying degrees according to species. This behaviour is seen in all kinds of benthic invertebrates. Although, more common in some species, such as Ephemeroptera, Chironomidae and Plecopters and not so common in Oligochaete, Coleoptera and Hydracarina (Hynes 1970). Waters (1961) suggest that this phenomenon must be contributed to ecological relationships that are unique to flowing water. Denuded areas downstream are colonised by the occurrence of drift. However, even under these conditions, the upper reaches or headwaters still manage to remain populated. Dendy (1944) notes that the concept of the drifting of stream invertebrates is a normal process in streams and that drift occurs even in the absence of floods or abnormal current velocities. However, studies that Hurton (1961) carried out showed that high water levels increased the amount of drift; also in support of this Logan (1963) ...read more.


Huges (1966) showed that the mayfly nymphs in response to respiratory stress would enter the drift. The need to escape aggressive competitors or predators can also be the impetus that causes insects to exhibit drifting behaviour. Willey and Kohler (1981) support this with their studies on the foraging of the predaceous caddis fly on black fly larvae. The black flies consequently release their hold on the substrate and drift away, thereby avoiding predation. The distances that are travelled by the drifting organism varies; Waters(1965) suggests that organisms will usually drift approcximately 50-60 metres downstream, however others have suggested that they can drift for several hundred metres; Elliott (1967) on the other hand suggest that the distances travelled are much less. So, how then can the upstream reaches prevent their population being depleted? Muller (1954) noticed several behavioural patterns that could perhaps partly answer this question. Muller noticed that the adult forms of several insects would migrate upstream in order to lay their eggs. Muller suggests that by this behavioural pattern, the cycle is completed, which sees the immature forms of the insects drifting downstream, and the depletion of the population density upstream is prevented by the mature adults laying their eggs. ...read more.


So when the area becomes overcrowded as a result of the animals breeding, some of the animals will be displaced as a result of drift. These animals in the drift will probably go on to replace, animals further down stream that have either died or have been eaten as a result of predation. This then supports the theory of mature flying insects migrating upstream to lay their eggs and drift then compensates for overstocking. This it seems is how the upstream populations compensate for loss of individuals through drift (Waters 1961). Hynes (1970) suggest that a large amount of this upstream movement or migration occurs deep down in the substratum. Waters supports this by suggesting that the function of drift is density related, and therefore it would not be expected that drift would reduce the population levels of the upstream reaches to less than the carrying capacity. Humphries and Ruxton (2002) in their findings argue that although drift is an important determinant of benthic population dynamics, it is more a question of individual fitness. In that the individuals that are left upstream have a higher degree of fitness, which has enabled them to remain in the upstream reaches. ...read more.

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