Shredder Richness Research Articles

Drying and fragmentation drive the dynamics of resources, consumers and ecosystem functions across aquatic‐terrestrial habitats in a river network

Rivers form meta‐ecosystems, in which disturbance and connectivity control biodiversity, ecosystem functioning and their interactions across the river network, but also across connected instream and riparian ecosystems. This aquatic–terrestrial linkage is modified by drying, a disturbance that also naturally fragments river networks and thereby modifies organism dispersal and organic matter (OM) transfers across the river network. However, little evidence of the effects of drying on river network‐scale OM cycling exists. Here, we assessed the effects of fragmentation by drying at the river meta‐ecosystem scale by monitoring leaf resource stocks, invertebrate communities and decomposition rates, across three seasons and 20 sites, in the instream and riparian habitats of a river network naturally fragmented by drying. Although instream leaf resource quantity and quality increased, leaf‐shredder invertebrate richness and abundance decreased with flow intermittence. Decomposition was, however, mainly driven by network‐scale fragmentation and connectivity. Shredder richness and invertebrate‐driven decomposition both peaked at sites with intermediate amounts of intermittent reaches upstream, suggesting that upstream drying can promote the biodiversity and functioning of downstream ecosystems. Shredder richness, however, had a negative effect on decomposition in perennial sites, likely due to interspecific competition. Leaf quantity, invertebrate communities and invertebrate‐driven decomposition became more similar between instream and riparian habitats as drying frequency increased, likely due to homogenization of environmental conditions between both habitats as the river dried. Our study demonstrates the paramount effects of drying on the dynamics of resources, communities and ecosystem functioning in rivers and presents evidence of one of the first network‐scale examples of the co‐drivers of ecosystem functions across terrestrial–aquatic boundaries.

Leaf breakdown rates as a functional indicator were influenced by an invasive non-native invertebrate in urban ponds

Impacts on biodiversity and the functioning of ecosystems are often overlooked in small waterbodies as they are not regularly monitored as part of the Water Framework Directive (WFD). These small systems are, however, often essential for freshwater biodiversity and are frequently characterized by high beta-diversity. In this study we examined shredder diversity and ecosystem functioning (leaf litter breakdown) as indicators of environmental stress along a gradient of urbanisation. The native isopod Asellus aquaticus and the non-native shrimp Crangonyx pseudogracilis (Amphipoda) were dominant species of the shredder communities in 26 study ponds. Variation in shredder community composition among ponds was explained by pond surface area (P < 0.05) and the coverage of impermeable surface within 500 m (P < 0.05), indicative of urban impact, which was strongly, and negatively correlated with shredder richness. Site mean total leaf breakdown was 0.0020 k d−1 (range 0.0003 – 0.0053), whilst breakdown rates owing to microbial and fungal activity were lower and averaged 0.0010 k d−1 (range 6.72−5 − 0.0036). Both total and microbial activity driven breakdown rates showed relatively weak relationships with the environmental gradient reflecting degrees of urbanization. A model-averaging procedure suggested that total litter breakdown rates could be attributed to activity of shredders with A. aquaticus and C. pseudogracilis contributing most, explaining 65% of the variance (R2 = 0.65, F = 13.7, P < 0.001), which both significantly and similarly predicted total leaf breakdown rates (P < 0.05). Total leaf breakdown rates were significantly correlated with the densities of both C. pseudogracilis (P < 0.001) and A. aquaticus (P < 0.001), however partial correlations indicated that the former showed the strongest association.The process of urbanisation can have significant, and negative impacts upon freshwater ecosystems, including small water bodies such as ponds both regarding biodiversity and ecosystem functioning. The findings of the current study suggest that ecosystem functioning indicators such as leaf litter decomposition should be paired with structural measures. This is because function sustained by tolerant invading species would not otherwise be detected, and their presence is increasingly likely due to climate change and urbanization.

Recent land‐use changes affect stream ecosystem processes in a subtropical island in Brazil

AbstractLand‐use changes such as conversion of natural forest to rural and urban areas have been considered as main drivers of ecosystem functions decline, and a large variety of indicators has been used to investigate these effects. Here, we used a replicated litter‐bag experiment to investigate the effects of land‐use changes on the leaf‐litter breakdown process and leaf‐associated invertebrates along the forest–pasture–urban gradient located in a subtropical island (Florianópolis, SC, Brazil). We identified the invertebrates and measured the litter breakdown rates using the litter bags approach. Litter bags containing 3 g of dry leaf of Alchornea triplinervia were deployed on forest rural and urban streams. Principal component analysis, based on physico‐chemical variables which, confirmed a gradient of degradation from forest to urban streams with intermediate values in rural areas. In accordance, shredder richness and abundance were lower in rural and urban than in forest streams. The land‐use changes led also to the dominance of tolerant generalist taxa (Chironomidae and Oligochaeta) reducing the taxonomic and functional diversity in these sites. Leaf‐litter breakdown rates decreased from forest to rural and finally to urban areas and were associated with changes in pH, water velocity, dissolved oxygen and abundance of leaf‐shredding invertebrates, although global decomposition rates did not differ between rural and urban streams. Overall, this study showed that land‐use changes, namely to rural and urban areas, have a strong impact on tropical streams ecosystems, in both processes and communities composition and structure. Despite of being apparently a smaller transformation of landscape, rural land use is comparable to urbanisation in terms of impact in stream functioning. It is thus critical to carefully plan urban development and maintain forest areas in the island of Florianópolis in order to preserve its natural biodiversity and aquatic ecosystems functioning.

Species turnover and invasion of dominant freshwater invertebrates alter biodiversity–ecosystem‐function relationship

AbstractFreshwater ecosystems rely on allochthonous resources. Integration of these subsidies depends on diversity of both terrestrial resources and aquatic shredder and decomposer communities, but the diversity effects on leaf litter breakdown and decomposition are less clear in aquatic than terrestrial ecosystems. We need a better understanding of this relationship because aquatic communities are rapidly changing with species invasions and anthropogenic impacts. Here, we experimentally disentangled the effects of leaf and shredder richness on leaf litter breakdown by macroinvertebrates in mesocosm experiments using three species of amphipods, a dominant guild of crustaceans in European freshwater ecosystems. Increased leaf richness led to lower‐than‐predicted leaf consumption by native shredders, with mixed evidence of resource‐switching or prioritization of preferred food items within a leaf mix. Higher shredder species richness never promoted leaf consumption rates compared to predictions from relevant single‐species experiments, and instead sometimes substantially decreased leaf consumption. We then conducted a meta‐analysis of leaf litter consumption rates by seven widely distributed amphipod species (the three used in the experiments and four additional species). As expected based on our own experiments, nonnative amphipod species generally had lower biomass‐adjusted leaf litter consumption rates, although their larger body size led to higher per‐individual leaf consumption rates. Contamination of the water by metals, pesticides, and other chemicals additionally significantly decreased leaf litter consumption by multiple native and nonnative species compared to unpolluted systems. While the meta‐analysis suggested that litter consumption, and thus breakdown, would decline if native shredders are replaced by nonnative heterospecifics, complete species replacement is not the only outcome following immigration in a meta‐community context. Our experiments suggest that breakdown rates could remain reasonably high where native species coexist with nonnative arrivals. Experiments that neglect the ecological realism of species coexistence will necessarily mischaracterize effects on ecosystem functioning.

An efficient semi-quantitative macroinvertebrate multimetric index for the assessment of water and sediment contamination in streams

Environmental issues and their associated legislative pressures are demanding, especially regarding costs, limited budgets, and data collection efforts. The development of effective biotic indices for monitoring generated via comprehensive data collection within human-impacted landscapes can support district- to regional-level environmental management plans. We designed a macroinvertebrate multimetric index that fulfills the requirements of the EU Water Framework Directive and measures ecological health as a deviation from a reference state, taking into consideration water and sediment contamination in streams of Central Italy. Macroinvertebrate samples were obtained by a semi-quantitative technique. The Semi-quantitative Multimetric Index (SMI) we developed has a high discrimination efficiency (reference vs. impacted sites), high stability (i.e., low coefficient of variation) at reference sites, and low specificity regarding the environmental variables reflecting contamination. The SMI is composed of 4 metrics weighted by their discrimination efficiency against reference conditions: number of families, score of the biological monitoring waters pollution index (SBWMP), Margalef’s diversity index, and relative richness of shredders. These metrics that best distinguished reference from impacted sites were selected from 71 metrics divided into 4 categories (one metric for each category): richness/abundance, tolerance, diversity, bioecological traits. The SMI exemplifies a cost-effective method that could complement supranational techniques because it is robust in terms of biological, ecological, and socioeconomic contexts used to generate it; requires little effort for the data collection; and responds to a large number of environmental stressors. To ensure comparability, however, we recommend calibrating the new SMI index against the national index currently in use.

Effects of wood addition on stream benthic invertebrates differed among seasons at both habitat and reach scales

Addition of large wood (LW, wood pieces longer than 1m and wider than 10cm) into stream channels is a common restoration practice aimed at enhancing habitat diversity and fish populations, as well as the abundance and diversity of benthic invertebrates. Nevertheless, there is some controversy regarding the effects of LW restoration on invertebrate assemblages, and the effects could differ depending on the season of the year as well as on the scale of observation (habitat vs reach scale). In this study we analysed the effects of a LW restoration experiment performed in 4 mountain streams following a BACI (Before-After/Control-Impact) design. We sampled benthic invertebrates in 3 main habitats (fine inorganic sediments, called gravel; coarse inorganic sediments, called cobbles; and particulate organic matter, called POM) in winter and summer before and after the addition of LW into experimental reaches, and compared the results to those obtained from upstream control reaches. LW addition promoted the retention of gravel and organic matter, resulting in an overall decrease in the areal cover of cobbles and a significant increase in the cover of organic matter in summer. Invertebrate richness was highest in cobbles and lowest in POM, whereas density and biomass were highest in POM and lowest in gravel. At the habitat scale, LW addition promoted invertebrate and shredder richness, diversity and biomass in summer, but the opposite effect was found in winter. Community composition changed significantly with wood addition, most notably as a result of increased density of elmids, limnephilids and limoniids, and decreased density of baetids. Density of limnephilids increased 20-fold and that of limoniids 5-fold. At the reach scale, LW addition enhanced the biomass of invertebrates and shredders in summer but the effects were opposite in winter. LW addition did not affect invertebrate density. The results show the effects of LW restoration on invertebrates to differ among seasons. Positive effects on biomass occur in summer, when retention by LW enhances food availability compared to unrestored reaches, whereas effects are slightly negative in winter, a period of large food availability.

Does leaf litter from invasive plants contribute the same support of a stream ecosystem function as native vegetation?

AbstractRiparian habitats are highly susceptible to invasion and the spread of non‐native plants. Many freshwater organisms and processes are dependent on allochthonous subsidies, and thus, riparian invasions are often associated with changes in aquatic ecosystems. We studied the potential effects of riparian plant invasions on instream decomposition and detritivore communities. We compared decomposition rates (k) of leaf litter from species native and invasive to coastal British Columbia, and the macroinvertebrate assemblages associated with the local and novel resource subsidy. Five native and five invasive species of riparian plants representing various growth forms (i.e., herbs, shrubs, and trees) were used in a litter bag experiment. Litter bags were distributed in stream‐side experimental channels, ensuring similarity in background environmental conditions (substrate, flow, and temperature). Compared to native litter, the k‐values of invasive plants were 2.6 times higher for herbs and 1.3 times higher for trees, while shrubs did not differ. Shrubs also decomposed significantly slower than trees and herbs. These patterns could be partially explained by the chemical properties of the litter. Throughout the whole experiment, decomposition rates were negatively related to the content of structural carbohydrates which tended to be higher in native leaves of herbs and shrubs. The k‐values also had positive relationships with nitrogen content measured in the unconditioned and conditioned litter, and invasive plant species had some of the highest values of leaf nitrogen. We found no significant relationships between decomposition rates and the lignin content of the conditioned and unconditioned leaf litter. Taxonomic richness of all aquatic macroinvertebrates and densities of all aquatic macroinvertebrates and shredders were significantly higher on the invasive litter incubated for 21 but not for 53 d. Finally, richness of shredders explained the highest proportion of the variance in decomposition rates, when compared to other biological (all macroinvertebrate densities and richness) and litter chemistry variables. Our results indicate that riparian plant invasions are associated with changes in aquatic litter decomposition; however, the direction of the change is largely dependent on litter quality and plant identity rather than growth form or exotic status.

Assessing the long-term ecological effects of riparian management practices on headwater streams in a coastal temperate rainforest

Our understanding of how effective forestry Best Management Practices (BMPs) can mitigate logging impacts on the structure and functioning of temperate headwater streams tends to be based upon studies conducted within the first few years post-harvest. Little is known about how the affected biological parameters of streams change during riparian and upland forest regeneration, particularly the time taken to return to pre-harvest levels, or to within the range of reference conditions. By following up on an earlier study in southwestern British Columbia, Canada, we examined the post-logging recovery trajectories of stream attributes under multiple forest treatments. These treatments provided a gradient of riparian protection, including streamside clear-cutting with no reserves, and with fixed-width reserves (10m and 30m), and thinning with 50% basal area removal of riparian trees, which were compared with unharvested control sites. About 15years after clear-cutting and 9years after thinning, slower decomposition of red alder leaves (Alnus rubra) was reported in streams affected by forest harvesting, which was attributed to lower density of shredder invertebrates. Thinned reaches had lower litter decomposition rate than, but similar shredder density and richness to, unharvested reference sites. After a further 7years of forest regrowth (time elapsed between the previous and present study), we found that litter decomposition rate and shredder density in logging-affected reaches converged with values in reference reaches. Stream functional integrity, as reflected by litter decomposition, increased across all forest treatments. Some differences in rarefied shredder richness among forest treatments emerged in this study. The legacy effects of thinning on some biotic variables likely diminished more quickly (2–9years post-harvest) than those of clear-cutting with and without riparian reserves (8–15years). Stream recovery from thinning to reference conditions should continue to warrant attention, as wider, larger-scale partial harvesting practices are recommended within emerging paradigms of forest management. Overall, our findings indicate time scales towards recovery of an important ecological process of streams under common forestry practices, which could be longer than other stream properties, such as water quality. Such differences in recovery time frames should be considered for the planning of forestry operations and monitoring of harvesting legacies.

Shifts in leaf litter breakdown along a forest-pasture-urban gradient in Andean streams.

Tropical montane ecosystems of the Andes are critically threatened by a rapid land‐use change which can potentially affect stream variables, aquatic communities, and ecosystem processes such as leaf litter breakdown. However, these effects have not been sufficiently investigated in the Andean region and at high altitude locations in general. Here, we studied the influence of land use (forest–pasture–urban) on stream physico‐chemical variables (e.g., water temperature, nutrient concentration, and pH), aquatic communities (macroinvertebrates and aquatic fungi) and leaf litter breakdown rates in Andean streams (southern Ecuador), and how variation in those stream physico‐chemical variables affect macroinvertebrates and fungi related to leaf litter breakdown. We found that pH, water temperature, and nutrient concentration increased along the land‐use gradient. Macroinvertebrate communities were significantly different between land uses. Shredder richness and abundance were lower in pasture than forest sites and totally absent in urban sites, and fungal richness and biomass were higher in forest sites than in pasture and urban sites. Leaf litter breakdown rates became slower as riparian land use changed from natural to anthropogenically disturbed conditions and were largely determined by pH, water temperature, phosphate concentration, fungal activity, and single species of leaf‐shredding invertebrates. Our findings provide evidence that leaf litter breakdown in Andean streams is sensitive to riparian land‐use change, with urban streams being the most affected. In addition, this study highlights the role of fungal biomass and shredder species (Phylloicus; Trichoptera and Anchytarsus; Coleoptera) on leaf litter breakdown in Andean streams and the contribution of aquatic fungi in supporting this ecosystem process when shredders are absent or present low abundance in streams affected by urbanization. Finally, we summarize important implications in terms of managing of native vegetation and riparian buffers to promote ecological integrity and functioning of tropical Andean stream ecosystems.

Testing the Performance of Macroinvertebrate Metrics as Indicators of Changes in Biodiversity After Pasture Conversion in Patagonian Mountain Streams

Conversion of native forest to pastures is a common practice worldwide; it has complex effects on river biota that are related to activity type and intensity. This work was conducted in order to evaluate the effects of cattle grazing on environmental features and to select the most appropriate measures based on the macroinvertebrate community as indicators of ecological changes. Physicochemical features, riparian ecosystem quality, habitat condition, and benthic macroinvertebrates were investigated in streams draining pastures and were compared to reference streams located in nonimpacted native forested catchments. Strong evidence of sediment deposition was observed at pasture streams, which had higher percentage of sand in the streambed but also increased levels of suspended solids. Pasture sites had significantly higher water temperature and conductivity as well as lower dissolved oxygen concentrations than did forested sites. Both riparian quality and in-stream habitat condition were degraded at pasture sites. Among metrics, total richness, Ephemeroptera, Plecoptera, and Trichoptera (EPT) richness, Plecoptera richness, Shannon diversity, percentage of dominant taxon, percentage of Chironomidae species, shredder richness, and percentage of predator resulted to be the most consistent measures by displaying stronger responses to impairment. Our results suggest that forest conversion to pasture for livestock practices is diminishing macroinvertebrate biodiversity and potentially changing functioning and dynamics of Patagonian mountain streams as well. Damage prevention on riverine landscapes by restoring riparian ecosystems, replanting missing vegetation in buffer areas, and limiting livestock access to the riverbanks could contribute to enhance the ecological integrity at converted areas. A more extended and comprehensive use of macroinvertebrate metrics could contribute to better understand and evaluate impact effects of these extended land use activities and to promote conservation strategies.

Predicting richness effects on ecosystem function in natural communities: insights from high-elevation streams

. Despite the increased complexity of experimental and theoretical studies on the biodiversity-ecosystem functioning (B-EF) relationship, a major challenge is to demonstrate whether the observed importance of biodiversity in controlled experimental systems also persists in nature. Due to their structural simplicity and their low levels of human impacts, extreme species-poor ecosystems may provide new insights into B-EF relationships in natural systems. We address this issue using shredder invertebrate communities and organic matter decomposition rates in 24 high-altitude (3200-3900 m) Neotropical streams as a study model. We first assessed the effects of stream characteristics and shredder diversity and abundance on organic matter decomposition rates in coarse- and fine-mesh bags. We found the interaction term shredder richness x shredder abundance had the most significant impact on decomposition rates in the field, although water discharge may also play a role locally. We also examined the relative contribution of the three most abundant shredders on decomposition rates by manipulating shredder richness and community composition in a field experiment. Transgressive overyielding was detected among the three shredder species, indicating complementary resource use and/or facilitation. By integrating survey and experimental data in surface response analyses we found that observed B-EF patterns fit those predicted by a linear model that described litter decomposition rates as a function of increasing shredder richness and the relative abundance of the most efficient shredders. Finally, the validity of our approach was tested in a broader context by using two independent but comparable data sets from 49 French and Swedish streams showing more complex shredder community structure. Results revealed that richness and identity effects on decomposition rates were lost with increasing shredder community complexity. Our approach of combining experimental and empirical data with modeling in species-poor ecosystems may serve as an impetus for new B-EF studies. If theory can explain B-EF in low-diversity ecosystems, it may also have credibility in more complex ones.

Litter decomposition can detect effects of high and moderate levels of forest disturbance on stream condition

Considerable research efforts have been devoted to determining what forest management practices most affect stream ecosystems, and how those impacts might be mitigated. Recent studies have stressed the relevance of litter decomposition to assess the conditions of headwater streams affected by riparian and upland forest harvest. Here we specifically examined whether litter decomposition can detect ecological effects of clearcutting to stream edges on headwater streams eight years after logging and if large (30 m) and narrow (10 m) riparian reserves (8-year post-harvest), and selection logging at 50% removal of basal area of riparian trees (1-year post-harvest), are effective protection measures for streams. We measured decomposition rates of red alder ( Alnus rubra) leaf litter in sixteen stream reaches, including reference reaches in a 70-year-old forest. We further examined assemblages of two main litter consumer groups, shredder invertebrates in riffles and aquatic hyphomycete fungi developing on decaying alder leaves. Alder decay rate was significantly lower in clearcut reaches than in reference reaches, and we found no evidence that any alternative riparian management practices examined in this study were able to mitigate against such an effect of logging. In unlogged reaches, rapid litter decomposition (0.0050–0.0118 day −1) was associated with high density and diversity of shredders (up to ten taxa). Slower litter decomposition in wide and narrow reserve reaches (0.0019–0.0054 day −1) and clearcut reaches (0.0024–0.0054 day −1) was attributed to lower density and richness of shredders. By contrast, the low decay rate in recently established thinned reaches (0.0031–0.0049 day −1) was not associated with a numerical response of shredders. Smothering of submerged leaves by sediments may have caused the reduction in alder decay rate in thinned reaches. Across all forest treatments fungal biomass or diversity remained fairly similar. Our findings suggest that stream ecosystems are extremely sensitive to small changes in riparian and upland forest cover. We propose that litter decomposition as a key ecosystem function in streams could be incorporated into further efforts to evaluate and improve forestry best management practices.

Development of a local-scale urban stream assessment method using benthic macroinvertebrates: an example from the Santa Clara Basin, California

Research that explores the biological response to urbanization on a site-specific scale is necessary for management of urban basins. Recent studies have proposed a method to characterize the biological response of benthic macroinvertebrates along an urban gradient for several climatic regions in the USA. Our study demonstrates how this general framework can be refined and applied on a smaller scale to an urbanized basin, the Santa Clara Basin (surrounding San Jose, California, USA). Eighty-four sampling sites on 14 streams in the Santa Clara Basin were used for assessing local stream conditions. First, an urban index composed of human population density, road density, and % urban land cover was used to determine the extent of urbanization upstream from each sampling site. Second, a multimetric biological index was developed to characterize the response of macroinvertebrate assemblages along the urban gradient. The resulting biological index included metrics from 3 ecological categories: taxonomic composition (% Ephemeroptera, Plecoptera, and Trichoptera), functional feeding group (shredder richness), and habit (% clingers). The 90th-quantile regression line was used to define the best available biological conditions along the urban gradient, which we define as the predicted biological potential. This descriptor was then used to determine the relative condition of sites throughout the basin. Hierarchical partitioning of variance revealed that several site-specific variables (dissolved O2 and temperature) were significantly related to a site’s deviation from its predicted biological potential. Spatial analysis of each site’s deviation from its biological potential indicated geographic heterogeneity in the distribution of impaired sites. The presence and operation of local dams optimize water use, but modify natural flow regimes, which in turn influence stream habitat, dissolved O2, and temperature. Current dissolved O2 and temperature regimes deviate from natural conditions and appear to affect benthic macroinvertebrate assemblages. The assessment methods presented in our study provide finer-scale assessment tools for managers in urban basins.

The effects of land use on environmental features and functional organization of macroinvertebrate communities in Patagonian low order streams

Benthic invertebrates, water quality variables, chlorophyll a and particulate organic matter (POM) were studied in 18 sites of mountain streams in Patagonia (Argentina) subjected to six different land uses: native forest, pine plantation, pasture, harvest forest, urban and reference urban. Three streams of each land use were studied in March 2006. Macroinvertebrates were sampled in three riffles and three pools ( n = 108) and biomass of detrital fractions of POM were quantified. Overall benthic POM biomass was higher at native and harvest forest than pastures, whereas fine fraction (FPOM) was higher in harvest forest than in pastures. Regarding to autotrophic subsidies bryophytes were the only that changed consistently among uses. These differences in energy resources were correlated with changes in community attributes. Shredder richness was clearly higher at native and harvest forest than exotic pine plantations, collector gatherers density was consistently high at harvest sites, and total density was significantly higher at urban and harvest forest. Multidimensional scaling ordination based on macroinvertebrate density data showed a clear separation of forested (either native or exotic species) from riparian modified areas (pasture, urban and harvest sites). Environmental variables having explanation power on macroinvertebrate assemblages were mostly related with: detritus availability (wood and leaves biomass) and impairment (total phosphorous and % sand). These results confirm that macroinvertebrate assemblage structure in Patagonian low order streams can be altered by land use practices. Among guild structure measures, those indicators based on benthic community functional attributes, shredders richness and collectors density, resulted good candidates to assess land use impacts. On account of riparian corridor management may be critical to the distribution of benthic taxa, the maintenance of good conditions of vegetation adjacent to rivers will enhance water quality and the environment for highly endemic headwater communities of Patagonian streams.

Assessment of functional integrity of eutrophic streams using litter breakdown and benthic macroinvertebrates

Currently, interest is growing in evaluating stream functional integrity while river assessment schemes are still exclusively based on structural indicators determined from various aquatic communities. Although some approaches relying on macroinvertebrate functional groups and combinations of traits have been advocated as means to assess ecosystem function, there has been no attempt to test the reliability of these methods with any direct functional indicator even though litter breakdown has been recently proposed as a functional indicator of stream impairment. The purpose of this study was to compare nine benthic macroinvertebrate-based structural metrics with functional metrics based on leaf litter breakdown in coarse and fine mesh bags in nine streams distributed along a eutrophication gradient. In coarse mesh bags, a 10-fold drop in breakdown rate of alder litter indicated a high sensitivity of this functional indicator to the deleterious effects of ammonium and its associated products, ammonia and nitrite. In contrast, microbial breakdown measured in fine mesh bags did not vary substantially along the gradient. Taxonomic and functional structures of macroinvertebrate assemblages were altered in the most eutrophic streams, as shown by drops in IBGN (French biotic index), BMWP (Britain biotic index) and EPTC (Ephemeroptera, Plecoptera, Trichoptera and Coleoptera) richness. Shredder richness and an ecological diversity index calculated from functional groups defined from multiple trait combinations exhibited the highest correlation with litter breakdown rate. Our results indicate that indirect assessment of stream functional integrity by structural indicators is realistic but requires specific metrics not necessarily based on traits or functional groups. Leaf litter breakdown is confirmed as a reliable indicator for direct assessment of streams impacted by eutrophication.

Catchment land‐use, macroinvertebrates and detritus processing in headwater streams: taxonomic richness versus function

1. We used a litter bag technique to assess the effect of catchment land‐use (forest, wetland, agriculture, urban) on the processing of red maple (Acer rubrum L.) litter in 17 streams in Maine, U.S.A. Litter processing by fungi was predicted to increase with nutrient concentrations along a gradient of land use, from relatively unmodified to highly modified. Litter processing by litter‐shredding macroinvertebrates was predicted to decline along this gradient because of a decline in their taxonomic richness and biomass.2. Land use was associated with the anticipated gradient in nutrient and macroinvertebrate attributes, and a significant relationship was found between land use and nitrate concentration. There was, however, no significant relationship between land use and soluble reactive phosphorus (SRP) concentration. Similarly, shredder taxonomic richness was significantly related to land use type, whereas shredder biomass showed no significant relationship to land use.3. Attributes of the shredder assemblage structure and nutrient concentrations were both strong determinants of litter processing. Increasing biomass and taxonomic richness of shredders was significantly related to increasing rates of litter mass loss. Increasing concentrations of nitrate and SRP were significantly related to increasing rates of litter softening below threshold concentrations (approximately 0.20 mg NO3‐N L–1 and 5 μg SRP L–1).4. The potentially additive effects of nitrate and SRP concentrations or shredder richness and biomass on litter processing rates were confounded by the lack of significant correlation between these pairs of variables. Consequently, rates of litter processing (as rates of softening or mass loss) did not vary systematically among different land use regimes.

Spatial and temporal variation in the macroinvertebrate fauna of streams of the northern jarrah forest, Western Australia: functional organization

SUMMARY. 1. Spatial and temporal changes in functional organization of the invertebrate community of streams of the northern jarrah forest, Western Australia, were examined in the light of predictions of the river continuum concept (Vannote et al., 1980).2. The composition of the fauna was largely as predicted for forested headwater streams with detritivores, principally collectors, dominating the fauna. Although shredders are generally considered to be codominant in such streams, they were not abundant in the riffle areas sampled in this study.3. Some of the observed temporal changes in the composition of functional feeding groups were in agreement with those predicted by the generalized model. The abundance and richness of collectors were higher during the summer than winter, whereas the converse was true for filterers. This corresponded to the presumed availability of deposit and suspended fine‐particulate organic matter during summer and winter respectively. Scrapers and predators showed no consistent temporal pattern among sites. However, the density of the latter was highly correlated with the density of other invertebrates, particularly collectors.4. Temporal changes in the abundance and richness of shredders were not synchronized with the marked summer input of coarseparticulate organic matter. The possible reasons for this, and the role of shredders in Australian stream systems, are discussed.