Mechanisms and Consequences of Interspecific Competition Between Two Stream Insects

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Abstract
(1) Grazing larvae of the net-veined midge, Blepharicera micheneri, and filter-feeding larvae of blackflies, primarily Simulium virgatum, occupy rock surfaces in fast-flowing water in Rattlesnake Creek, California, U.S.A. Based on observations of aggressive behaviour by Simulium toward Blepharicera, we conducted a series of field experiments to determine the presence, mechanisms and consequences of competition for attachment space between these two dipteran insects. (2) There was an inverse relationship between abundances of the two taxa, and when Simulium was removed from natural substrates, blepharicerid densities increased. Despite blackfly aggression, co-occurrence was common. To estimate the cost of co-occurrence, we measured behavioural, feeding and fitness responses of blepharicerid larvae to manipulated simuliid abundances. (3) Simulium caused Blepharicera to spend 5 .times. more time in avoidance responses than when alone, resulting in a 20% reduction in time spent feeding. Distance travelled was also increased by simuliid interference. The interactions were stongly asymmetrical in favour of Simulium, being reversed only when Simulium was much smaller than Blepharicera. (4) Diatom ingestion by Blepharicera was reduced 60% by the presence of Simulium. In mesocosms adjacent to the stream, blackflies inhibited the growth of blepharicerids and increased mortality and time to pupation, resulting in decreased blepharicerid production. (5) Baetis mayflies and higher Blepharicera density also tended to inhibit blepharicerid growth, probably via exploitative competition, but these effects were secondary to interference competition with Simulium. (6) The importance of interspecific competition varies within and between years, because Blepharicera and Simulium co-occur for a longer period in years of high rainfall, but may not overlap in dry years. Both taxa depend upon flood disturbance to open space and reduce competition from other taxa (macroalgae and the caddis Hydropsyche oslari Banks). Contrary to prevalent conceptions, competition in screams may be a common and important factor structuring populations and communities, while disturbance determines the form of interspecific competition rather than eliminating it.