In streams what is invertebrate drift and how may upstream populations compensate for loss of individuals downstream.
Bio3122 id:12922668
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) and Denham found that there was very little drift during periods of low discharge and the highest amount of drift during the snow melting. Less turbulent streams produced lower amounts of drift and that riffles produce more than pools. Humphries (1938) and Ruxton (2002) noted that it has been argued that drift is simply a production in excess of carrying capacity, and that the loss of numbers will decrease in response to the carrying capacity being attained. So, the suggestion then seems to be that perhaps drift is density dependant. Although drift occurs regardless of disturbances, it does however exhibit a diel tendencies or periodicities (Waters 1961, Muller 1963). The diel tendencies reaching maxima during the night, with the peak being reached just after sunset. Perhaps this is in response to the depletion of oxygen levels during the night where it reaches a minimum in streams. Although Elliott (1965) found that the diel tendencies could be influenced by the amount of light, he showed that by illuminating the stream at night, the amount of drift was reduced and comparable to daytime levels, and Anderson, N.H (1966)., found that drift did not increase on nights where the full moon was out. These responses are perhaps due the fact that these behavioural patterns reduce the changes of the animal being entrained by the current. In lotic environments can provide an immediate escape rate and mode of dispersal, at little cost to the insect. The aquatic insects have also shown that they will voluntarily enter the water column when they are subjected to stress. Drift therefore can occur as a response to a stress stimuli. Mayfly nymphs have also shown that they will enter the drift when exposed to stress. 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.
