Biodivers Conserv (2007) 16:2695–2713 DOI 10.1007/s10531-006-9081-2 ORIGINAL PAPER
Assessing riparian quality using two complementary sets of bioindicators Jenny Smith Æ Michael J. Samways Æ Stuart Taylor
Received: 18 February 2005 / Accepted: 29 May 2006 / Published online: 12 July 2006 Springer Science+Business Media B.V. 2006
Abstract Biological indicators are being increasingly used to rapidly monitor changing river quality. Among these bioindicators are macroinvertebrates. A shortcoming of macroinvertebrate rapid assessments is that they use higher taxa, and therefore lack taxonomic resolution and species-specific responses. One subset of invertebrate taxa is the Odonata, which as adults, are sensitive indicators of both riparian and river conditions. Yet adult Odonata are not necessarily an umbrella taxon for all other taxa. Therefore, we investigated whether the two metrics of aquatic macroinvertebrate higher taxa and adult odonate species might complement each other, and whether together they provide better clarity on river health and integrity than one subset alone. Results indicated that both metrics provide a similar portrait of large-scale, overall river conditions. At the smaller spatial scale of parts of rivers, Odonata were highly sensitive to riparian vegetation, and much more so than macroinvertebrate higher taxa. Odonate species were more sensitive to vegetation structure than they were to vegetation composition. Landscape context is also important, with the odonate assemblages at point localities being affected by the neighbouring dominant habitat type. Overall, benthic macroinvertebrates and adult Odonata species provide a highly complementary pair of metrics which together provide large spatial scale (river system) and small spatial scale (point localities) information on the impact of stressors such as riparian invasive alien trees. As adult Odonata are easy to sample and are sensitive to disturbance at both small and large spatial scales, they are valuable indicators for rapid assessment of river condition and riparian quality.
J. Smith School of Botany and Zoology, University of KwaZulu-Natal, P/Bag X01, Pietermaritzburg, South Africa M. J. Samways (&) Æ S. Taylor Department of Conservation Ecology and Entomology and Centre for Agricultural Biodiversity, University of Stellenbosch, P/Bag X1, Matieland 7602, South Africa e-mail:
[email protected]
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Keywords Riparian ecosystems Æ Bioindicators Æ Benthic macroinvertebrates Æ Adult Odonata Æ Complementarity
Introduction River quality throughout the world is deteriorating, with stream biotas being altered in numerous ways (Davies and Day 1998). Yet aquatic ecosystems are highly complex, and incorporate interactions between physical, chemical and biological entities. This makes them difficult and expensive to monitor by traditional physico-chemical means. In contrast, organisms, being biological endpoints, are good indicators of river quality, and reflect the overall ecological integrity of their environments (Wright et al. 1984; Rosenberg and Resh 1993; Metcalf-Smith 1994; Smith et al. 1999) and enable management decisions to be made (Karr 1991; Norris and Norris 1995; Karr and Chu 1999; Mancini et al. 2005). However, not all organisms are equally sensitive to disturbance (Lammert and Allen 1999; Dovciak and Perry 2002; Heino et al. 2005). Among the biological indicators that have been used for assessing river quality in southern Africa, as elsewhere, are benthic macroinvertebrates (Dallas 1997; Brown 2001; Dickens and Graham 2002). Macroinvertebrate assemblages have been used for comparing river systems, as well as for monitoring long-term trends. However, these macroinvertebrate metrics are a coarse method, as they are intended to be a rapid bioassessment method that is field based to reduce the time needed to process samples. Furthermore, the taxonomic resolution of samples is limited to taxonomic levels above species (Dickens and Graham 2002). They generally do not provide a measure of ecological integrity at the species level and therefore would not be able to give information relating to conservation issues of concern at this level (Brown 2001). In particular, as named species are not used, macroinvertebrate higher taxa do not generate assessments of the ways in which endemic species are being affected relative to more geographically widespread species. Monitoring abundant resident species may be important for detecting the early decline of a habitat (Hawking and New 2002). However, monitoring rare species is also important as they can be indicative of relict or undisturbed conditions and used to rate the importance of any site within its habitat groups (Eyre et al. 1986). It is also important to identify species that are restricted to a narrow range of conditions as they may be good indicators of change. One subset of aquatic macroinvertebrates is the Odonata, an insect order which occupies a spectrum of aquatic habitats (Corbet 1999). Odonates are sensitive to changes in water quality and to landscape disturbance and thus they reflect to some extent the ecological conditions of their habitats (Samways and Steytler 1996; Chovanec and Waringer 2001). Odonata are relatively well studied, and there is a sufficient number of species in most localities to give manageable number of species for assessments (Samways 1993). Adults are large and conspicuous, and most species are easily identified in the field, even by relative non-experts. These odonate characteristics, coupled with their long ontogenetic development, suggest they could be valuable for medium- or long-term monitoring. However, using only Odonata as indicators of ecological integrity does not necessarily offer a wide enough taxonomic umbrella on which to base sound biological conservation decisions. As a single taxon, they may not expose what is happening at higher community levels of
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organization. Conversely, not all benthic macroinvertebrate taxa can be monitored at the species level as this would be too time consuming, labour intensive, expensive and often the individual species cannot be identified. Determination of the habitat requirements of Odonata species allows development of characteristic assemblages of Odonata species, which can be used to monitor changes in the environment (Clark and Samways 1996; Osborn and Samways 1996). Environmental disturbances alter odonate assemblages in terms of both species composition and abundance. If the habitat preferences of certain species are known, a change in species composition is likely to be an indication of a type of disturbance. Species with more specific habitat preferences are more susceptible to certain types of disturbance (Clark and Samways 1996). In this regard, rare and threatened sunloving species are likely to be very indicative of invasion by alien woody plants with a dense canopy. As no single indicator can give a full picture of the particular environmental state of a river, it is necessary to look for complementarities among indicator metrics, and to identify indicators of change in structural, functional and compositional diversity at a range of scales and levels of organization (Rogers and Biggs 1999). One solution would be to combine a metric of higher benthic macroinvertebrate taxa, as a measure of ecosystem health, with the Odonata as a measure of ecological integrity at the species level. This is done here in an attempt to provide a more comprehensive picture of river health and conservation status. Special emphasis is given to the structure and composition of the riparian canopy, an ecosystem that is being severely modified in many parts of the world. The results are then used to assess the merits of using both or either metric for determining the effects of alien vegetation change upon the stream fauna. Sites and methods Sites and vegetation sampling units This study was done on three permanent rivers, the Msunduzi, Dorpspruit and Townbush in the Pietermaritzburg basin, South Africa 3020¢E, 2936¢S (660–690 m a.s.l.). A sampling unit (SU) was 10 m of river together with the 1 m wide strip of vegetation on either bank. Within each 10 m stretch, was a glide and a riffle to ensure that all habitats were included, minimizing variation (Dickens and Graham 2002). River depth was < 0.75 m. SUs along each river included extremes of structural diversity (shaded versus sunlit) and compositional diversity (alien versus indigenous plants). It was hypothesized that using such a range of environmental conditions would place extreme demands on the assemblages at point localities, while the three different river systems provided variation on a broader scale. Our aim was to ‘stretch’ the two metrics as far as possible, and to see whether they both responded in the same or in different ways to a spectrum of environmental conditions. To this end, 14 SUs were along the Msunduzi (M), 28 SUs along the Townbush (T) and 28 SUs along the Dorpspruit (D) giving 70 SUs in all. Each SU was divided into four categories according to its combination of vegetation structure and composition characteristics. Vegetation structure was defined as open (70% of river bank with tree canopy), and riparian vegetation composition as either principally indigenous, or principally alien vegetation along the rivers bank.
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Sampling of benthic macroinvertebrates and Odonata adults Sampling of benthic macroinvertebrates was standardized according to the South African Scoring System (SASS5) protocol, and included as much microhabitat variation as possible within each SU (Dickens and Graham 2002). Both emergent marginal and submerged vegetation were sampled. Samples were obtained using the kick-method, where rocks and other benthic material were disturbed by foot to flow downstream into a soft, 1 mm mesh net, 30 cm square. The content of each sample was washed to the bottom of the net and then tipped into a water-filled, white tray. Macroinvertebrates were identified to family level or above, while for the sensitive families Baetidae and Hydropsychidae, the number of species within each family was also recorded. Identification of the macroinvertebrates in white, plastic trays was for
