Freshwater Reviews, Vol 1, No 2 (2008)

The ecology of riffle beetles (Coleoptera: Elmidae).

Malcolm Elliott
DOI: 10.1608/FRJ-1.2.4 | Pages: 189-203

Abstract

Riffle beetles in the family Elmidae are frequent members of the invertebrate community of running water.  Over 80 species have been recorded in North America and 46 in Europe; this number decreases in the western and northern fringes of Europe with only 12 species in Britain, many rare, four in Ireland and three in Norway.  The present review describes their habitat, food and predators, their life cycles, their dispersal, and human threats to their survival.  All elmid species have aquatic larvae with five to eight instars, depending on the genus.  Adults of a few species are terrestrial, but most are aquatic with plastron respiration.  Most elmid species occur in well-aerated streams and rivers, but some also occur on wave-washed lake shores.  A few species live in more unusual habitats, such as thermal pools and hot springs, subterranean habitats, and decaying wood.  Little is known about the food of adults or larvae, but they appear to be collector-gatherers and scrapers that feed chiefly on algae and detritus.  Adults and larvae are rarely taken by invertebrate predators, but are eaten by fish, especially salmonids.  However, their proportion in the diet is always lower than that in the benthos, indicating low availability to the fish.  To obtain a quantitative description of the life cycle, information is required on the number of larvae in each instar, the timing of oviposition, the number of eggs laid, the timing of pupation, and the number of adults.  Few studies meet these criteria.  However, three excellent studies from North America and a detailed study of the four commonest British species illustrate the variation in the life cycles of different species.  Life tables, identifying critical periods for survival, are provided for the British species, these being the only such tables currently available for riffle beetles.  The ability to disperse by flight varies among species and among individuals of the same species.  Adults and larvae disperse downstream in the invertebrate drift, especially at night.  In one study, drift densities of different life stages were related positively to their monthly losses in the benthos, but not to their benthic densities.  Less information is available on their upstream–downstream movements on the substratum.  There are ontogenetic shifts in diel drift periodicity and dispersal, and both relate to seasonal changes in drift density and critical periods for survival in the life cycle.  Human threats to the survival of elmids include reduced oxygen concentrations, elevation of water temperature, extremes of flow, especially spates, and pollution, especially by soaps and detergents.  Therefore, riffle beetles provide excellent indicators of water quality and perhaps also climate change.
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