Detritivores in Kenyan highland streams: more evidence for the paucity of shredders in the tropics?

1. The relationship between coarse particulate organic matter (CPOM) standing stock and benthic macroinvertebrate assemblages in Kenyan highland streams was determined by sampling seven sites on three rivers (2000-2700 m a.s.l.). Taxa recorded were allocated to functional feeding groups using published literature, mouthpart analysis and examination of gut contents. Patterns were compared with five structurally similar streams in three areas of Europe (south-west France, south-east England, north-east England). 2. Number of individuals and proportion of detritivores in Kenyan streams were equivalent to, or greater than, those in European sites. Shredders were, however, almost completely absent from Kenyan sites, despite high standing stocks of CPOM. Shredders were abundant in all European sites. 3. The phenomenon of low shredder abundance has been observed in other tropical streams in south-east Asia and Central and South America but, in contrast to these regions, the African rivers studied were devoid of shrimps or fish which may occupy the shredding niche elsewhere. 4. These preliminary data suggest that shredder-mediated detritus processing, which is a key functional component of streams in the North Temperate Zone, does not operate in East African streams. There are three possible reasons for this. The first is that tropical African rivers are functionally different to those in temperate regions. This could be because of enhanced microbial activity replacing shredder activity at high temperatures. Alternatively, it could be a result of low palatability of detrital inputs from dominant riparian trees in the region. The second and third are methodological: that our allocation to functional feeding groups is incorrect, and that our sampling methods missed a potentially key shredding taxon - the freshwater crab Potamonautes sp.