Community structure of Trichoptera in a mountain stream: spatial patterns of production and functional organization

Abstract

SUMMARY. 1. Annual production was estimated for Trichoptera occurring in each of three distinct habitats of a mountain stream: bedrock‐outcrops, riffles and pools. Production was greatest on bedrock‐outcrops (2608 mg ash‐free dry weight m−2), followed by riffles (1038) and pools (950).2. Annual production in bedrock‐outcrops and pools was dominated by single functional groups, with collector‐filterers and shredders contributing 73% and 75% of the annual production, respectively. Production in riffles was due primarily to shredders (46%), followed by collector‐filterers (27%).3. Taking account of the amount of stream area occupied by each habitat type, total annual production was estimated at 1336 mg AFDW m−2. 53% of this production was attributable to four taxa: Parapsyche cardis Ross (25%), Pycnopsyche gentilis (MacLachlan) (10%), Neophylax mitchelli Carpenter (9%) and Rhyacophila nigrita Banks (9%).4. Habitat‐weighted production was distributed among functional groups as follows: collector‐filterers (41%), shredders (29%), engulfing‐predators (15%), scrapers (13%) and collector‐gatherers (2%).5. The distinct taxonomic and functional structures of trichopteran sub‐communities were shaped by the distinct physical characteristics of their principal habitats. Bedrock‐outcrops were characterized by low roughness and high current and were sites of low deposition or organic matter; thus the predominance of collector‐filterers. In contrast, the other habitats of greater roughness (riffles) and/or lower current (pools) were sites of deposition of food (e.g. leaf litter) and greatest shredder production.6. By distinguishing discrete mesoscale habitats, each with a functionally distinct caddisfly sub‐community, we speculate that small mountain streams provided the diverse physical templates essential for the evolution of the major feeding tactics (e.g. scraping, shredding, filter‐feeding) of the. Trichoptera.