Náttúrufræðingurinn

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Náttúrufræðingurinn - 1975, Síða 13

Náttúrufræðingurinn - 1975, Síða 13
NÁTTÚRUFRÆÐINGURINN 139 chemistry is a limiting lactor for the distribution of these two species. In addi- tion, the distribution does not seem to follow any climatic pattern. Lindroth (1931) and Fristrup (1942) brieíly explored the area where P. cingulatus occurs now, but they only found A. zonella larvae and adults. It thereíore seems likely that either P. cingulatus has been present in Iceland for a few decades (old people in NE-Iceland have known this caddis larva for many years) and is steadily spreading to the west, or it has been in Iceland even longer, but has had a local distribution until recently, when it started to expand to the west. A. zonella larvae leed primarily on diatoms (Nielsen 1943). P. cingulalus larvae in England feed to some extent on diatoms (Scott 1958, Elliott 1971), but mainly on detritus. Since very little allochtlionous material is brought into streams in Iceland, the only available food for the larvae is diatoms and in some cases aquatic mosses and green algae. Competition for food could explain why tliey so rarely occur together. It is suggested that A. zonella has become extinct from streams that P. cingulatus has occupied in Iceland. To my know- ledge there is no evidence of these species living together in the same stream in other countries. A. zonella is parthenogenetic; it might therefore be at a disadvantage when a new competitor is introduced into its environment. This may perhaps explain its disappearance from areas that it formerly occupied, where only P. cingulalus is now to be found.

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