Different colonization patterns of two closely related stream insects (Austrosimulium spp.) following disturbance

1 In many streams, blackfly larvae rapidly colonize scoured surfaces, then decline in numbers as other species are still increasing. Such a colonization pattern may be generated by species that seek disturbed substrata because they prefer scoured surfaces having low epilithon cover (i.e. they are opportunists) or because of lowered faunal densities (i.e. they are fugitives), but this has rarely been tested. 2 In the Acheron River, south‐eastern Australia, the larvae of the blackfly Austrosimulium torrentium show the typical pattern of increase and decline whereas A. victoriae shows a different pattern of more gradual increase. 3 An experiment showed that A. torrentium, consistent with its colonization pattern, responded strongly to treatments that mimicked the effects of disturbance; it colonized bricks with few other animals present regardless of epilithon cover and therefore is potentially a fugitive species. A. victoriae did not respond convincingly to treatments, which is also consistent with its colonization pattern. 4 Blackfly larvae of both species preferentially colonized bricks that were in fast‐moving water. Hence, A. torrentium only responded to the densities of other fauna when bricks occurred in currents greater than approximately 30 cm s−1. 5 Whilst epilithon cover did not affect abundance of blackfly larvae, it did affect attachment positions. In one experimental treatment, epilithon was reduced on one half of the top surfaces of bricks. More than 50% of larvae on these bricks were attached to the epilithon‐reduced halves. 6 Our results show that functional feeding or taxonomic groups do not necessarily contain ecologically equivalent taxa; colonization patterns may provide a more useful way of categorizing species for the purposes of modelling these assemblages.