Food quality, feeding preferences, survival and growth of shredders from temperate and tropical streams

1. The importance of leaf quality to the nutritional ecology of lotic shred ders is well established for temperate species but virtually unknown for tr opical taxa. In the present study, we compared the feeding behaviour and pe rformance of two tropical and two temperate shredders in a series of pair-w ise experiments. 2. Specifically, we tested whether leaf conditioning status (stream-conditi oned versus unconditioned leaves) and geographical origin (temperate Alnus glutinosa versus tropical Hura crepitans leaves) affect the food preference , survivorship, and growth of selected shredders from low and high latitude s in a consistent manner. The animals used in experiments were the caddis-f lies Nectopsyche argentata and Phylloicus priapulus from Venezuela, Sericos toma vittatum from Central Portugal, and the amphipod Gammarus pulex from N orthern Germany. 3. In general, all shredders exhibited the same high preference for conditi oned over unconditioned leaves, irrespective of the geographical origin of the leaf or shredder species. 4. A corresponding tendency for higher growth was found for sets of animals offered conditioned leaves, with the differences in growth being clearer i n the two tropical shredders. Survivorship of the two temperate species was consistently high (> 83%) regardless of the diet offered, whereas the trop ical shredders survived better on conditioned (77-90%) as compared with unc onditioned (54-87%) leaves, although not significantly so. 5. With the exception of the temperate S. vittatum, shredders did not selec t or perform better on leaves to which they had previously been exposed, in dicating a potential adaptation to native leaf species is over-ridden by in trinsic leaf properties. 6. Taken together, the results of this study suggest that tropical shredder s may exhibit the same basic patterns of food exploitation as their tempera te counterparts. Consequently, current concepts relating to the role of shr edders in stream detritus dynamics may wen be applicable to tropical stream s, although essentially derived from temperate systems.