71 THE MACROINVERTEBRATE COLONIZATION OF LEAF BAGS: IS THERE A PATTERN? R. M. V. Cortes', M. Abelho2 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA & S. B. Rebelo' ' Univer\idade de Tra+os-M«nte\ e Alto Douro, Apartado 202, Vild Red Codex, Portugal Departdmento de Zoologra, Univerdade de Coimbra, 3000 Coirnbr'i, Portugal Keywords: Colonization, stream macroinvertebrate conimtinities, functional feeding groups, leaf litter SUMMARY There have been many studies of litter decompocition and the macroinvertebrates associated with litter. Neverthelesc, the reasons why invertebrates colonize leaf bags are still a matter for diccussion. Although invertebrates use leaves as food, it has been shown that they may use the leaf bag a substrate. In this study, we investigate whcther macroiiivertebrates colonize leaf bags decpite aburidant natural leof litter in the streams aiid whether they discriminate between leaves occurring naturally in the streainc zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP (Alnus zyxwvutsrqponmlkjihgfedcbaZYXWV glutino.su) and alien spe- cies íin this case, the South American Nurci crepitum). We found no differences in the relative abundance of chredders between Alrzris and HiiIyi leaf baga. In relation to the relative abundan- ce of shredders in the leaf bags and iii the streambed, we fouiid al1 the possible conibinationc. In two streams there were no differences between the benthic camples and the leaf bags. in another stream. the relative abuiidance of shredders was higher in the benthic samples, and in a further stream, the relative abundance of shredders was higher in the leaf bags. It seeins therefore, that when tliere ia ari abtindant supply «f high qtiality food in the streains. there is no reason for the aggrcgation zyxwvutsrqponm oí shredders in the leaf bags. INTRODUCTION Coinparisori of invertebrate colonization and breakdown rates between different leaf species is an essential procedure to obtain information about three relevant aspects: a) the preferen- ce of invertebrate taxa for a specific type of litter, b) the encrge- tic cupply to the ayuatic community through the leaf-processing continuum, aiid e) the ntitritional quality of allochthonoua detri- tus. This approach has already bcen used in the lbrrian strearn systenis (e.g., [MBERT & POZO, 1989; ESCUDERO rt d., 1991). The saine methodology has also been applied in thic geographical region to assess the ccological implications of extensive afforestations with the exotic species Ei{cci/~pm.\ slo- hiiíir.~. Comparison of processing rates, coloniration aiid coii- sumption of eucalyptiis and alder (AIIIUS ,qíufirzn.sti), one of the iiiost coinmon riparian species in northern low-order streains, rnay allow predictions on the impact of eucalyptus afforesta- tioii.; on the energetic budgct of streams (POZO. 1993: BASA- CUREN zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA di POZO, 1994: CORTES et u/., 1994: CANHOTO 81 GRACA, 1995). The differcnce iii texturc uid litter quality betwecn alder and eucalypttie rnakes very predictable the distinction in decay rates, and also in the structure and composition of the inverte- emhlages colonizing these leaves. A subject pooriy invcstigated is the potential iinpact of an exotic species similar in leaf chemistry and decay mes to the natural occurring onec. The colonization of natural aiid artificial leaf packs by macroinvertebrates in streams has been a matter «f extensive discucsion (BU". 1988: DOBSON, 1991 : BASAGUREN & POZO, 1994: MALMQVIST & OBERLE, 1995). Somc atithors auggest that, by coloniLing artificial leaf packs. invertebrates seek an additional food source (e.g., ROWE ot d., 1996). Many of these xtudics focuc only on the proportion oí shredders pre- serit in the leaf packs. without reference to the strcarii inverte- brates (but ser MA1,MQVIST 81 OBERLE, 1995). The objectivcs 01 the prcscnt study werc: (1) to compare the ciyiiaiiiics of invertebrate coloiiizntion on Hurci crrpircmv. a South Aiiierican species not occurring in Europe, with the Europenn A/I~LIS ,ylutinrwu. and (2) to conipare invertebrates in leaf bags with invertebrates sainpled in the stream benthos. Since in headwaters leaf processing can be highly dependent o11 stream typology (HORTON &L BKOWN. 1991: POZO.