Náttúrufræðingurinn - 1975, Page 13

NÁTTÚRUFRÆÐINGURINN
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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.