Riparian plant species loss alters trophic dynamics in detritus-based stream ecosystems

Riparian vegetation is closely connected to stream food webs through input of leaf detritus as a primary energy supply, and therefore, any alteration of plant diversity may influence aquatic ecosystem functioning. We measured leaf litter breakdown rate and associated biological parameters in mesh bags in eight headwater streams bordered either with mixed deciduous forest or with beech forest. The variety of leaf litter types in mixed forest results in higher food quality for large-particle invertebrate detritivores (‘shredders’) than in beech forest, which is dominated by a single leaf species of low quality. Breakdown rate of low quality (oak) leaf litter in coarse mesh bags was lower in beech forest streams than in mixed forest streams, a consequence of lower shredder biomass. In contrast, high quality (alder) leaf litter broke down at similar rates in both stream categories as a result of similar shredder biomass in coarse mesh bags. Microbial breakdown rate of oak and alder leaves, determined in fine mesh bags, did not differ between the stream categories. We found however aquatic hyphomycete species richness on leaf litter to positively co-vary with riparian plant species richness. Fungal species richness may enhance leaf litter breakdown rate through positive effects on resource quality for shredders. A feeding experiment established a positive relationship between fungal species richness per se and leaf litter consumption rate by an amphipod shredder (Gammarus fossarum). Our results show therefore that plant species richness may indirectly govern ecosystem functioning through complex trophic interactions. Integrating microbial diversity and trophic dynamics would considerably improve the prediction of the consequences of species loss.     

Identifying the Relative Importance of Leaf versus Shredder Species Loss on Litter Decomposition in Streams

Understanding how species loss influences ecosystem function is a contemporary issue in ecology. However, most research has focused on species loss at one trophic‐level. We explored the relationship between functional diversity (FD) and species richness separately for trees and aquatic leaf‐shredding detritivores. For trees, we collected information on species‐specific leaf tissue chemistry and species co‐occurrences in the mid‐Atlantic region (USA). For shredders, we used a published trait database with information on communities from 38 streams in the same region. We used a clustering algorithm to estimate FD for each community and for randomly assembled communities. If FD was high, we concluded that species loss was important to change in function; if low, species were functionally redundant and insensitive to species loss. We found tree FD to be significantly different than expected, but shredders exhibited FD levels similar to patterns based on random assembly. Furthermore, there were more leaf species exclusively associated with very high or very low levels of functional diversity compared to shredders. This approach revealed greater implications for leaf than shredder species loss for litter breakdown. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

How biological diversity influences ecosystem function: a test with a tropical stream detritivore guild

We investigated the relationship between diversity and ecosystem function, which is controversial and has rarely been examined for consumer assemblages, for the process of leaf breakdown by the shredder guild in a tropical stream. We manipulated species richness, evenness and identity of four macroinvertebrate shredder species (three caddisflies and one mayfly) in microcosms and tested their effect on leaf breakdown rates measured as leaf mass loss per capita and per milligram of animal. Species richness, evenness and species identity all affected leaf breakdown rates. Breakdown rates tended to increase with higher richness, but only for the three caddisflies, probably through a release of intraspecific interference, although other mechanisms such as niche complementarity or facilitation cannot be discarded. Leaf breakdown by the caddisflies was reduced in the presence of the mayfly, possibly because of its mode of movement by swimming instead of crawling and its similarity to some predators that are common in leaf litter. Species identity was more important than species richness in determining leaf breakdown rates, indicating that some species within the shredder guild are not redundant, and suggesting important consequences of particular species loss for the functioning of the ecosystem.     

An invasive species may be better than none: invasive signal and native noble crayfish have similar community effects

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Simulating species loss following perturbation: assessing the effects on process rates

We removed stream-living macroinvertebrate shredder species in the sequences in which they are predicted to disappear, in response to two common types of anthropogenic disturbances: acidification and organic pollution, and analysed the effects on leaf breakdown rates. The experiment was performed in field microcosms using three shredder species. Species identity significantly affected leaf breakdown rates, while species richness per se was non-significant. The simulated sequential species loss showed large effects on leaf breakdown rates, with observed rates being significantly higher than expected from single-species treatments in two, out of four, two-species, and in all four three-species treatments. The invertebrates used in this study were taxonomically distinct (Insecta: Plecoptera and Trichoptera; Crustacea: Amphipoda), and of different sizes, hence a high degree of complementarity was probably present. A method to study the effects of species loss, characteristic of perturbation type, could be more useful than a random approach when investigating the impact of perturbation. Our results may have general applicability for investigations on the effects of diversity loss on ecosystem functioning in any ecosystem exposed to human perturbations, given that the order of extinction is known or can easily be assessed.     

Species richness–decomposition relationships depend on species dominance

Human disturbances both decrease the number of species in ecosystems and change their relative abundances. Here we present field evidence demonstrating that shifts in species abundances can have effects on ecosystem functioning that are as great as those from shifts in species richness. We investigated spatial and temporal variability of leaf decomposition rates and community metrics of leaf‐eating invertebrates (shredders) in streams. The shredder community composition dramatically influenced the diversity–function relationship; decomposition was much higher for a given species richness at sites with high species dominance than at sites where dominance was low. Decomposition rates also markedly depended on the identity of the dominant species. Further, dominance effects on decomposition varied seasonally and the number of species required for maintaining decomposition increased with increasing evenness. These findings reveal important but less obvious aspects of the biodiversity–ecosystem functioning relationship.     

Litter decomposition in a temperate and a tropical stream: the effects of species mixing,litter quality and shredders

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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.     

Shredder–tadpole facilitation of leaf litter decomposition in a tropical stream

1. Leaf litter decomposition is one of the most important ecosystem processes in streams. Recent studies suggest that facilitation, in which litter is processed by a succession of species with differing abilities and requirements, may be important in making the nutrients bound in litter available to the stream assemblage.
2. We predicted that stream invertebrates that feed on terrestrial leaf litter (shredders) and tadpoles would facilitate leaf litter decomposition by changing the quality of leaf material directly via physical contact or indirectly via nutrient release. We experimentally examined the ability of shredders and tadpoles to break down leaves, independently and together, in artificial streams beside a natural forest stream.
3. The decomposition rate was greater when shredders and tadpoles were together than was expected from rates in single-species treatments, indicating that facilitation occurred. This facilitation operated in one direction only: the rate of leaf breakdown by tadpoles was higher when leaves had been partly processed by shredders, but there was no similar effect when leaves previously occupied by tadpoles were processed by shredders. We did not detect facilitation caused by indirect nutrient release.
4. Shredders may have benefited tadpoles by roughening leaf surfaces, making them easier for the tadpoles to consume and enhancing leaf breakdown in the presence of both taxa. This indicates that the loss of a single species can have impacts on ecosystem functioning that go beyond the loss of its direct contribution.     

Effects of a predatory fish on a tropical detritus-based food web

In contrast to that for grazing systems, relatively little information exists for trophic cascades in detritus-based stream food webs, which are predominant in forested headwater streams. Predator–prey interactions are thought to be weak in these systems, but studies are very scarce, their results are equivocal, and they do not separate the effect of direct consumption from a behavioural response of shredders. We examined the effect of predatory fish on leaf litter breakdown in headwater tropical Australian streams at three levels: (1) the behavioural response of shredder species to predator presence as indicated by chemical cues; (2) the rates of leaf breakdown resulting from shredder activity; and (3) the relationship between shredder species richness and leaf breakdown rates. Our results suggest that predatory fish can have a trait-mediated effect on detritus-based food webs in streams, by reducing consumer activity. We identified reductions in short-term overall activity in response to the presence of predatory fish cues, comparable to those found for grazers. We also observed a visible, albeit statistically non-significant, reduction in consumption rates. Shredder species richness did not affect leaf breakdown rates, and fish presence did not modify this relationship or the differences in breakdown rates among species, suggesting that the overall reduction in leaf breakdown caused by fish presence is due to a reduction in activity in every species. Thus, our laboratory studies have shown that there can be a behavioural basis for trait-mediated trophic cascades linked to fish presence in detrital food webs in streams. However, the strength of fish effects depends on environmental circumstances, and field studies of litter breakdown in streams with and without predatory fish are required if we are to elucidate the ecological significance of our observations.     

Leaf diversity influences in-stream litter decomposition through effects on shredders

1. The functioning of many aquatic ecosystems is controlled by surrounding terrestrial ecosystems. In a view of growing interest in linking biodiversity to ecosystem‐level processes, we examined whether and how leaf diversity influences litter decomposition and consumers in streams. 2. We tested experimentally the hypothesis that the effects of leaf diversity on decomposition are determined by the responses of leaf consumers to resource–habitat heterogeneity. Leaves from three common riparian trees, beech (Fagus sylvatica), hazel (Corylus avellana) and ash (Fraxinus excelsior), were exposed alone and in all possible mixtures of two and three species in a stream. We analysed individual leaf species for decomposition rate, microbial respiration and mycelial biomass, and we determined the species composition, abundance and biomass of shredders in leaf bags. 3. We found that the decomposition of the fastest decomposing leaves (hazel and ash) was substantially stimulated (up to twofold higher than single species leaf packs) in mixtures containing beech leaves, which are refractory. In contrast, the decomposition of beech leaves was not affected by leaf mixing. Such species‐specific behaviour of leaves in species mixtures has been overlooked in previous studies that examined the overall decomposition of litter mixtures. 4. The effects of leaf diversity on decomposition varied with the abundance and biomass of shredders but not with microbial parameters. Beech leaves alone were less attractive to shredders than leaf packs made of hazel, ash or any mixture of species. Moreover, the presence of beech leaves in mixtures led to higher shredder abundance and biomass than we had expected from data from single species exposed alone. Lastly, we found that early instars of the caddisfly Potamophylax (the dominant shredder in terms of biomass) almost exclusively used the toughest material (i.e. beech leaves) to construct their cases. 5. Leaf pack heterogeneity may have altered shredder‐mediated decomposition. Shredders colonising diverse leaf packs benefited from the stable substratum provided by beech leaves, whereas ash and hazel leaves were primarily used as food. Thus, our findings provide strong evidence for an intimate linkage between the diversity of riparian vegetation and aquatic communities.     

The importance of crayfish in the breakdown of rhododendron leaf litter

1. Rhododendron (Rhododendron maximum) is a common evergreen shrub in riparian areas of the southern Appalachians, where its leaves can comprise a large proportion of leaf litter in streams. However, they are relatively refractory and generally considered a low quality food resource for detritivores. 2. Our objective was to assess whether macroconsumers [primarily crayfish (Cambarus bartonii)] influence rhododendron leaf breakdown in a forested southern Appalachian stream in both summer (when leaves other than rhododendron are relatively scarce) and autumn (when other leaves are relatively abundant). We conducted two leaf decay experiments, one in summer and one in autumn, using pre‐conditioned leaves. Macroconsumers were excluded from the benthos of a fourth‐order stream using electric ‘fences’; we predicted that excluding macroconsumers would reduce the decay rate of rhododendron leaves in both summer and autumn. 3. In both experiments, breakdown rate was lower in exclusion treatments. Macroconsumers accounted for approximately 33 and 54% of rhododendron decay in summer and autumn, respectively. We attribute this effect to direct shredding of rhododendron by crayfish. Biomass of insect shredders, insect predators and fungi did not differ between control and exclusion treatments, indicating that insectivorous sculpins (Cottus bairdi) had no effect on rhododendron decay and that omnivorous crayfish did not exert an indirect effect via alteration of insect or fungal biomass. 4. The influence of shredding insects varied between summer and autumn. In summer, when other, more palatable leaf types were not available, rhododendron leaf packs appeared to provide ‘resource islands’ for insect shredders. There was a significant inverse relationship between insect shredders and leaf pack mass in the summer exclusion treatment: insects were the only organisms eating leaves in this treatment and, as shredder biomass increased, remaining leaf pack mass decreased. In the control treatment, however, we did not see this relationship; here, the effect of insect shredders was presumably swamped by the impact of crayfish. In autumn, when other leaves were abundant, insect shredder biomass in rhododendron leaf packs was less than one‐third of summer values. 5. Even at low density (approximately 2 m^–2) crayfish were able to influence an ecosystem process such as leaf decay in both summer and autumn. Given the threatened status of many crayfish species in the United States, this finding is especially relevant. Even small alterations in crayfish assemblages, whether via loss of native species and/or introduction of exotic species, may have significant repercussions for ecosystem function.     

Experimental shading alters leaf litter breakdown in streams of contrasting riparian canopy cover

1. Headwater stream ecosystems are primarily heterotrophic, with allochthonous organic matter being the dominant energy. However, sunlight indirectly influences ecosystem structure and functioning, affecting microbial and invertebrate consumers and, ultimately, leaf litter breakdown. We tested the effects of artificial shading on litter breakdown rates in an open‐canopy stream (high ambient light) and a closed‐canopy stream (low ambient light). We further examined the responses of invertebrate shredders and aquatic hyphomycetes to shading to disentangle the underlying effects of light availability on litter breakdown. 2. Litter breakdown was substantially slower for both fast‐decomposing (alder, Alnus glutinosa) and slow‐decomposing (beech, Fagus sylvatica) leaf litters in artificially shaded stream reaches relative to control (no artificial shading) reaches, regardless of stream type (open or closed canopy). 3. Shredder densities were higher on A. glutinosa than on F. sylvatica litter, and shading had a greater effect on reducing shredder densities associated with A. glutinosa than those associated with F. sylvatica litter in both stream types. Fungal biomass was also negatively affected by shading. Results suggest that the effects of light availability on litter breakdown rates are mediated by resource quality and consumer density. 4. Results from feeding experiments, where A. glutinosa litter incubated under ambient light or artificial shade was offered to the shredder Gammarus fossarum, suggest that experimental shading and riparian canopy openness influenced litter palatability interactively. Rates of litter consumption by G. fossarum were decreased by experimental shading in the open‐canopy stream only. 5. The results suggest that even small variations in light availability in streams can mediate substantial within‐stream heterogeneity in litter breakdown. This study provides further evidence that changes in riparian vegetation, and thus light availability, influence organic matter processing in heterotrophic stream ecosystems through multiple trophic levels.     

Global patterns of stream detritivore distribution: implications for biodiversity loss in changing climates

Aim We tested the hypothesis that shredder detritivores, a key trophic guild in stream ecosystems, are more diverse at higher latitudes, which has important ecological implications in the face of potential biodiversity losses that are expected as a result of climate change. We also explored the dependence of local shredder diversity on the regional species pool across latitudes, and examined the influence of environmental factors on shredder diversity. Location World‐wide (156 sites from 17 regions located in all inhabited continents at latitudes ranging from 67° N to 41° S). Methods We used linear regression to examine the latitudinal variation in shredder diversity at different spatial scales: alpha (α), gamma (γ) and beta (β) diversity. We also explored the effect of γ‐diversity on α‐diversity across latitudes with regression analysis, and the possible influence of local environmental factors on shredder diversity with simple correlations. Results Alpha diversity increased with latitude, while γ‐ and β‐diversity showed no clear latitudinal pattern. Temperate sites showed a linear relationship between γ‐ and α‐diversity; in contrast, tropical sites showed evidence of local species saturation, which may explain why the latitudinal gradient in α‐diversity is not accompanied by a gradient in γ‐diversity. Alpha diversity was related to several local habitat characteristics, but γ‐ and β‐diversity were not related to any of the environmental factors measured. Main conclusions Our results indicate that global patterns of shredder diversity are complex and depend on spatial scale. However, we can draw several conclusions that have important ecological implications. Alpha diversity is limited at tropical sites by local factors, implying a higher risk of loss of key species or the whole shredder guild (the latter implying the loss of trophic diversity). Even if regional species pools are not particularly species poor in the tropics, colonization from adjacent sites may be limited. Moreover, many shredder species belong to cool‐adapted taxa that may be close to their thermal maxima in the tropics, which makes them more vulnerable to climate warming. Our results suggest that tropical streams require specific scientific attention and conservation efforts to prevent loss of shredder biodiversity and serious alteration of ecosystem processes.     

Drought impact on stream detritivores: experimental effects on leaf litter breakdown and life cycles

Predictions of effects of global climate change include decreased runoff for many parts of the world, which will result in drying of streams. Information of the effects of drought on aquatic ecosystems is limited and little is known of the effects on ecosystem functions. Our main objective was to measure the direct effects of drought on leaf litter breakdown by invertebrate shredders in a controlled laboratory experiment. We hypothesized a decreased breakdown at high drought level. Single-species and multi-species treatments with three shredder species (Asellus aquaticus, Limnephilus bipunctatus, and L. flavicornis) were set up in an experiment with three drought level treatments, control, medium, and high drought (6 cm water level, 1 cm water level, and water level below sediment surface, respectively). Breakdown measured as leaf litter loss was significantly lower in both medium and high drought treatments compared to the control. Previously, decreased breakdown due to drying has been reported, but attributed to low densities of invertebrate shredders. We show that even when shredders are present, drought decreases the breakdown. Drought treatments also induced earlier pupation for the caddisfly L. flavicornis. Shifts in species phenology due to drought, e.g., earlier emergence, may affect species ability to adult survival and reproduction. Shifts in timing of emergence may also affect terrestrial food webs, where emerging aquatic insects may constitute an important food subsidy. Our knowledge of the complex effects of droughts in aquatic systems is limited with an urgent need of extended knowledge of the ecological effects of droughts on freshwater ecosystem functioning.     

Leaf litter breakdown, microbial respiration and shredder production in metal-polluted streams

1. If species disproportionately influence ecosystem functioning and also differ in their sensitivities to environmental conditions, the selective removal of species by anthropogenic stressors may lead to strong effects on ecosystem processes. We evaluated whether these circumstances held for several Colorado, U.S.A. streams stressed by Zn. 2. Benthic invertebrates and chemistry were sampled in five second–third order streams for 1 year. Study streams differed in dissolved metal concentrations, but were otherwise similar in chemical and physical characteristics. Secondary production of leaf‐shredding insects was estimated using the increment summation and size‐frequency methods. Leaf litter breakdown rates were estimated by retrieving litter‐bags over a 171 day period. Microbial activity on leaf litter was measured in the laboratory using changes in oxygen concentration over a 48 h incubation period. 3. Dissolved Zn concentrations varied eightfold among two reference and three polluted streams. Total secondary production of shredders was negatively associated with metal contamination. Secondary production in reference streams was dominated by Taenionema pallidum. Results of previous studies and the current investigation demonstrate that this shredder is highly sensitive to metals in Colorado headwater streams. Leaf litter breakdown rates were similar between reference streams and declined significantly in the polluted streams. Microbial respiration at the most contaminated site was significantly lower than at reference sites. 4. Our results supported the hypothesis that some shredder species contribute disproportionately to leaf litter breakdown. Furthermore, the functionally dominant taxon was also the most sensitive to metal contamination. We conclude that leaf litter breakdown in our study streams lacked functional redundancy and was therefore highly sensitive to contaminant‐induced alterations in community structure. We argue for the necessity of simultaneously measuring community structure and ecosystem function in anthropogenically stressed ecosystems.     

Effects of shredding amphipod density on watercress Nasturtium officinale breakdown

Shredding stream invertebrates should have a positive influence on the breakdown rates of leaf litter via direct consumption and particle fragmentation. To determine the effects of shredder density on litter breakdown, breakdown of the emergent stream macrophyte, Nasturtium officinale , was investigated using three litter bag mesh sizes [fine (0.2 mm), medium (1 mm) and coarse (3 mm) mesh] and four stocking densities of the shredder, Gammarus pseudolimnaeus , (0, 4, 8 and 16 per bag). Watercress decayed very rapidly, with breakdown rates ( k values) ranging from 0.075 d^-1 for fine mesh with no shredders to 0.24 d^-1 for coarse mesh. Stocked Gammarus increased breakdown rates significantly in fine mesh bags (p < 0.001), but only marginally in medium mesh bags (p < 0.1). Breakdown rates also increased significantly with mesh size. A regression model showed a significant relation of breakdown rate to Gammarus density and mesh size. These results clearly show that shredders can significantly influence breakdown rates and can account for up to 30% of breakdown, but that mesh size effects such as particle size reduction and loss are also very important.     

Fish invasion alters ecosystem function in a small heterotrophic stream

The strength of trophic cascade effects in aquatic ecosystems depend, in part, on the identity of the top predator involved. We examined whether an invasive benthic fish (round goby, Neogobius melanostomus) altered the strength of cascade effects in a heterotrophic stream and in a controlled mesocosm experiment relative to the effects of a functionally similar, native fish. In the stream, the introduced fish had a direct effect on grazer and shredder abundance which led to a significant increase in periphyton chlorophyll a, a significant reduction in leaf breakdown rate, an increase in leaf biomass remaining, but no change in periphyton ash-free dry mass. In mesocosms, native and introduced fish similarly reduced shredder abundance, but this did not lead to an indirect effect on leaf breakdown rates or biomass remaining at the end of the experiment. Indirect effects of introduced fish on periphyton biomass and chlorophyll a in mesocosms were both significant and were stronger than in the field, but were the result of grazer behavioral modification and not reduced grazer abundance. Collectively, these results suggest non-native fish have the ability to initiate trophic cascades in heterotrophic streams, and that both fish identity and environmental context are important in determining the strength of cascades.     

Stronger effects of litter origin on the processing of conifer than broadleaf leaves: A test of home‐field advantage of stream litter breakdown

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Local Variation in Shredder Distribution can Explain their Oversight in Tropical Streams

Stream shredders play an important role in the breakdown of allochthonous leaf litter—a well-known, key process in temperate headwater streams. In contrast, it has been suggested that litter breakdown in tropical streams is driven by microorganisms, shredders being scarce or absent. We propose that shredders have been overlooked in some tropical streams for two reasons: (1) assuming that tropical shredders belong to the same taxa as temperate ones, without determining the diet of tropical litter fauna; and (2) the small spatial scale of most tropical stream studies, which do not account for intra- and inter-site comparisons. We explored shredder abundance and species richness in six streams in each of two tropical regions, the Australian wet tropics (AWT) and Panama (PAN), finding 734 individuals of 12 shredder species in AWT and 391 individuals of 16 species in PAN. Shredder species richness was positively related to altitude in AWT, but not in PAN. Shredder contribution to total leaf breakdown in the field was 24±3 SE percent in AWT and negligible in PAN, but this was probably due to the unsuccessful colonization of experimental cages by PAN shredders. In the laboratory, shredder contribution to total leaf breakdown was higher than in the field (35%±2 SE in AWT and 64%±3 SE in PAN) and varied with leaf decomposability. Our results support earlier indications that shredders are not scarce or functionally unimportant in the tropics, and suggest that their contribution to litter processing should be determined along altitudinal gradients.