The River continuum concept (RCC) (Vannote et al. 1980) attempted to provide a unifying theory of river function. The Functional Feeding Group (FFG) concept (Cummins 1973, 1974) became a major component of the RCC. The FFG concept provides testable hypotheses about the changes in proportions of FFGs along a downstream gradient in a river, in response to the changing nature of food resources. The following short-comings of the FFG concept have been identified: 1) the variability of macroinvertebrate feeding, 2) problems with gut analysis as a method for assigning taxa to FFGs, and 3) inconsistent criteria defining FFGs. The objective of this study was to investigate the macroinvertebrate fauna of the Buffalo River in order to assess the applicability of aspects of the RCC and the FFG concept. The specific aims of the study were: 1) to describe the distribution of macroinvertebrate assemblages in the Buffalo River; 2) to clarify aspects of the FFG concept listed above; 3) to establish whether selected taxa could be assigned to FFGs; 4) to assess whether the proportions of different FFGs in successive reaches of a southern African river conformed to the predictions of the RCC; and 5) to test whether a functional classification is a useful alternative to a taxonomic classification. Macroinvertebrates were collected seasonally from a variety of biotopes at three sites, one each in the upper, middle and lower reaches. Riffles were sampled in summer at 16 sites. Over 100 taxa were identified and an hierarchical classification was prepared using two-way indicator species analysis. Invertebrate assemblages in the narrow headwater stream were taxonomically distinct from those of the middle/lower reaches and were not positively associated with subjectively identified biotopes. Biotopes were characterised by distinct assemblages in the wider middle/lower reaches. Sixteen abundant taxa whose feeding had not been previously investigated were selected for feeding studies, four from the headwaters and 12 from the middle/lower reaches. Methods used included gut content analysis, behavioural observations, food choice experiments and morphological studies using scanning electron microscopy. Three aspects of the functional feeding group concept were clarified. 1) Dietary variability was assessed using gut contents as an index of diet. The gut contents of both early (small) and late (large) instar larvae of all 16 taxa collected from different sites and biotopes, and in different seasons were compared using a multifactor analysis of variance. For all taxa the most consistently significant differences in gut contents were between large and small larvae. These were due to differences in the amount of material in the gut and in varying amounts of rarer dietary items. Dietary variability did not prevent taxa from being assigned to FFGs. 2) Gut content analysis satisfactorily provided basic information about the feeding biology of taxa but proved to be an inadequate single method for positively assigning taxa to FFGs. 3) Before taxa could be assigned to FFGs the definitions for some FFG categories had to be described clearly. It is suggested that the term shredder be based on the observation of shredding and a predominance of leaf fragments in the foregut. The presence of algae was not diagnostic of scrapers and a morphological basis is suggested. A morphological basis for the brusher FFG is described for the first time. All 16 taxa were assigned to FFGs. Three headwater taxa were shredders ((Goerodes caffrariae (Lepidostomatidae), Dyschimus ensifer (Pisulidae), Afronemoura spp. (Notonemouridae)) and one was a collector:brusher (Adenophlebia auriculata (Leptophlebiidae). These results were consistent with RCC predictions. All 12 of the taxa from the middle/lower reaches were filterers or collectors and this result was also consistent with RCC predictions. The Hydropsychidae, Cheumtopsyche afra and Macrostemum capense, were passive net filterers; Neurocaenis reticulatus (Tricorythidae) was a passive setal filterer; Caenidae sp. Band Pseudocloeon maculosum (Baetidae) were active filterers; caenidae sp. A, and the Baetidae, Baetis harrisoni, Centroptilum excisum and Cloeon africanum, were collector: gatherers; the Leptophlebiidae, Choroterpes elegans and Choroterpes nigrescens, were collector:brushers; and Afronurus harrisoni (Heptageniidae) was a scraper. Gut content analyses alone were insufficient to assign taxa to FFGs, but when augmented by morphological and/or behavioural data, taxa could be assigned to FFGs with confidence. In all cases the FFG designation referred to the most frequent style of feeding. N. reticulatus, A. harrisoni and A. auriculata were particularly flexible in their feeding behaviour. A functional classification of macro invertebrates in the Buffalo River was compared with a taxonomic classification. In both cases similar groups were identified, but their taxonomic and functional descriptions yielded different information. It is suggested that functional and taxonomic classifications should be viewed as complementary rather than alternative options.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:rhodes/vital:5646 |
| Date | January 1992 |
| Creators | Palmer, C G (Carolyn Gay) |
| Publisher | Rhodes University, Faculty of Science, Zoology and Entomology |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis, Doctoral, PhD |
| Format | 264 leaves, pdf |
| Rights | Palmer, C.G. (Carolyn Gay) |
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