A Test of Resource Limitation Among Shredding Detritivores in Low Order Streams in Southern England

Abstract

1. The retention characteristics of stretches of four low order streams in south-east England were manipulated over a period of 2 years by the addition of small traps which accumulated leaf litter. Unmanipulated control stretches were retained in each stream. Our aim was to test the prediction that the local density of 'shredders', invertebrates which comminute autumn-shed leaves, would increase in response to leaf litter availability. One stream naturally retained a high leaf litter biomass, two had few natural leaf packs and one was intermediate. 2. After manipulation had begun, simultaneous samples of the benthos and leaf litter were taken from control stretches and the traps and intervening stream bed ('intertraps') in the manipulated stretches. 3. The effects of manipulation were slight in the highly retentive site, in terms of both litter standing stock and shredder abundance. The other three sites showed rapid responses, with increased leaf litter and shredder abundance in traps compared with controls. 4. In the least retentive site, manipulation significantly increased the biomass of coarse particulate organic matter in the intertrap samples. In one other site, there was also an increase in shredder density in the intertraps. 5. The changes associated with manipulation were consistent despite the differences in pH among streams, which determined the species assemblage of shredders at each site, but not the pattern of response to increased retention. 6. The results indicate that resource limitation may occur among mobile detritivorous invertebrates in low-order streams. The nature of the limiting resource is discussed; it is speculated that leaf litter may be limiting as a habitat for shredders as well as a source of food.