EFFECTS OF DISTURBANCE ON STREAM MOSS AND INVERTEBRATE COMMUNITY STRUCTURE

Natural occurrence of overturned stones and the distribution of moss with respect to stone size and stability were examined in two North Swedish woodland streams. In total, 16.7% of moss-covered stones had obviously been overturned in the last few years. Moss was rare on small stones except for those that were embedded in the surrounding substrate. On large stones (> 12 cm), moss was abundant and embedment did not affect the moss distribution here. It is concluded that the stability of substrates markedly affected the distribution of stream moss. In a field experiment, moss-covered stones were overturned to simulate mild spate effects, and the recovery of moss and invertebrates was followed for 14 mo. The treatment reduced ash free dry weight of moss as well as diversity (H') and total abundance of invertebrates. Three out of 16 invertebrate taxa increased their density as a result of disturbance, showing peak densities on the moss-covered underside of overturned stones. All other taxa decreased their density upon overturning, and in these taxa peak densities were found on the moss-covered upper side of control stones. Recovery of moss and invertebrates was still weak 14 mo after the stone overturning. I conclude that overturning of stones has severe and prolonged effects on stream moss and, indirectly, on the benthic invertebrate community.