The impacts of human disturbance on stream benthic invertebrates and their drift in North Sulawesi, Indonesia

Abstract

1. Benthic invertebrates and their drift in eight streams in equatorial North Sulawesi were sampled to provide a first description of lotic communities on this large Indonesian island. Both undisturbed rainforest streams and sites affected by anthropogenic activities were investigated. 2. Fifty-four invertebrate families and 127 morphospecies, mostly insects, were collected. Benthic community composition reflected the attenuated Oriental fauna of Sulawesi and, at the family level, was intermediate between the reduced fauna of Papua New Guinea and the rich fauna of the rest of Southeast Asia. Net-spinning hydropsychids and philopotamids dominated the Trichoptera and there were few scrapers. Heptageniid and ephemerellid mayflies were poorly represented, while tricorythids and prosopistomatids were relatively abundant. Plecoptera were rare. Overall, predatory insects were scarce and shredders especially so. 3. Ordination by non-metric multi-dimensional scaling (MDS) revealed that benthic community structure was strongly affected by channelisation and conversion of forest to agriculture and human-dominated sites were well separated from other streams. Differences among the relatively pristine streams reflected stream size and degree of shading. MDS site groupings resulting from ordination of drift samples were the same as for benthic samples. 4. MDS site groupings were similar whether data were analysed at the morphospecies level or grouped into families, but family-level data better represented underlying patterns in the data set. Ordination of family-level data is a promising method for rapid bioassessment of stream condition in North Sulawesi and elsewhere in Southeast Asia, and has advantages when dealing with a fauna that has received limited taxonomic study. 5. Drift showed clear diel patterns, with more morphospecies and individuals drifting at night. This was unexpected, given the paucity of native fishes, as drift at night has been interpreted as an adaptation to avoid visually hunting predators. Insect responses to nocturnal predatory shrimps (Macrobrachium spp.: Palaemonidae) could have caused diel drift periodicity, but exotic fishes present at the human-dominated sites may have been influential also. 6. Drift and benthos samples yielded the same total number of species, but differed somewhat in composition. Atyid and palaemonid shrimps were not found in the benthos but were present in drift samples at night. Sampling drift and benthos provided more comprehensive information on community composition than drift or benthic samples alone. However, the extra data on shrimps obtained from sampling drift at night and the relative ease of processing drift samples, mandates their use in bioassessment wherever shrimps are important components of stream communities. 7. Human activities in parts of North Sulawesi that were formerly pristine rainforest have recently degraded the streams where this research was carried out. These impacts underscore the urgent need for freshwater biodiversity research in Indonesia and elsewhere in Southeast Asia, and the requirement for bioassessment methodologies to monitor stream conditions and the effectiveness of management or mitigation measures. © 2006 The Author.