Leaf Litter Breakdown Rates Research Articles

Sedimentary context controls the influence of ecosystem engineering by bioturbators on microbial processes in river sediments

By modifying the physical environment, ecosystem engineers can have inordinately large effects on surrounding communities and ecosystem functioning. However, the significance of engineering in ecosystems greatly depends on the physical characteristics of the engineered habitats. Mechanisms underlying such context‐dependent impact of engineers remain poorly understood even though they are crucial to establish general predictions concerning the contribution of engineers to ecosystem structure and function.The present study aimed to decrypt such mechanisms by determining how the environmental context modulates the effects of ecosystem engineers (bioturbators) on microorganisms in river sediments. To test the effects of environmental context on the role of bioturbators in sediments, we used mesocosms and recreated two sedimentary contexts in the laboratory by adding a layer of either fine or coarse sand at the top of a gravel‐sand matrix. For each sediment context, we examined how the sediment reworking activity of a bioturbating tubificid worm (Tubifex tubifex) generated changes in the physical (sediment structure and permeability) and abiotic environments (hydraulic discharge, water chemistry) of microorganisms. Microbial characteristics (abundances, activities) and leaf litter decomposition – a major microbially‐mediated ecological process – were measured to evaluate the impact of bioturbation on biotic compartment.Our results showed that the permeability, the availability of oxygen and the activities of microorganisms were reduced in sediments covered with fine sand, in comparison with sediments covered with coarse sand. Tubifex tubifex significantly increased permeability (by about six‐fold), restored aerobic conditions and ultimately stimulated microbial communities (resulting in a 30% increase in leaf litter breakdown rate) in sediments covered with fine sand. In contrast T. tubifex had low effects in sediments topped by coarse sand, where O2 was already available for hyporheic microorganisms. Our study supports the idea that context dependency mainly modulates the effects of engineering by controlling the ability of engineers to create changes on abiotic (O2 in the present study) factors that are limiting for surrounding communities.

Elevated chloride and consumer presence independently influence processing of stream detritus

Headwater streams are in close contact with the landscape and known to mediate disturbances to downstream waterways through important ecological interactions. We studied how elevated chloride from road deicer, a pollutant of rising concern in urban ecosystems, influenced the leaf-microbial matrix in streams, and subsequent processing of C and N. In a multi-factorial laboratory experiment, we determined if elevated chloride interacts with nitrogen loading and invertebrate consumer feeding to alter rates of leaf litter breakdown and N immobilization. Naturally colonized leaf litter, the dominant C source in small streams, was collected from five Piedmont streams (Maryland, USA) and subjected to a gradient of NaCl (0, 1,000, 5,000 mg Cl l−1) and dissolved nitrogen (ambient, elevated), and an invertebrate treatment (presence, absence) in a total of 60 microcosms. Loss rate and C:N content were determined from remaining leaf litter after 16 d of incubation. Chloride loading significantly (P < 0.05) reduced loss rate regardless of N loading, and C:N content significantly (P < 0.05) increased with Cl concentration, interacting marginally (P < 0.10) with N loading. Invertebrate feeding had a marginally-significant (P < 0.10), negative effect on loss rate, but not C:N content. Overall, elevated chloride significantly influenced organic matter loss rate and N immobilization, despite N loading and the presence of invertebrates. We conclude that there is the potential for chloride loading as road deicer runoff to negatively influence microbial processing of C and N by stream-dwelling microbial communities.

Leaf litter recycling in benthic and hyporheic layers in agricultural streams with different types of land use

Changes in land use and intensification of agricultural pressure have greatly accelerated the alteration of the landscape in most developed countries. These changes may greatly disturb the adjacent ecosystems, particularly streams, where the effects of pollution are amplified. In this study, we used the leaf litter breakdown rate to assess the functional integrity of stream ecosystems and river sediments along a gradient of either traditional extensive farming or a gradient of vineyard area. In the benthic layer, the total litter breakdown process integrates the temporal variability of the anthropogenic disturbances and is strongly influenced by land use changes in the catchment even though a low concentration of toxics was measured during the study period. This study also confirmed the essential role played by amphipods in the litter breakdown process. In contrast, microbial processes may have integrated the variations in available nutrients and dissolved oxygen concentrations, but failed to respond to the disturbances induced by vineyard production (the increase in pesticides and metal concentrations) during the study period. The response of microbes may not be sensitive enough for assessing the global effect of seasonal agricultural practices. Finally, the leaf litter breakdown measured in the hyporheic zone seemed mainly driven by microbial activities and was hence more affected by vertical exchanges with surface water than by land use practices. However, the breakdown rate of leaf litter in the hyporheic zone may constitute a relevant way to evaluate the impact on river functioning of any human activities that induce massive soil erosion and sediment clogging.

Flow intermittence controls leaf litter breakdown in a French temporary alluvial river: the “drying memory”

The inputs and breakdown of terrestrial leaf litter in streams is a fundamental ecological process that sustains in-stream foodwebs and secondary production. In temporary rivers, litter breakdown is reduced during dry phases, but the long-term effect of alternating drying and wetting cycles on litter breakdown is still poorly understood. We tested the hypothesis that leaf litter breakdown (LLB) in temporary rivers is primarily controlled by flow permanence (the number of flowing days over a given period expressed in %), and that drying events affect LLB during leaf fall periods through reduction of microbial activity and the modification of aquatic invertebrate assemblages. LLB rates (k), microbial activity and invertebrate assemblages were determined in winter at ten cross-sections scattered along a flow permanence gradient on the temporary Albarine River, France. Results demonstrated that summer drying events affected the breakdown process for up to 6 months after flow has resumed in the river. LLB rates decreased exponentially with decreasing flow permanence, and with increasing drying event duration and frequency. These exponential relationships were observed for flow permanence variables calculated for the river for both 24-years and 1-year time periods prior to the experiment. A decrease in flow permanence from 100 to 85% led to a four-fold decrease in leaf litter breakdown rate. Microbial activity, which typically did not differ between cross-sections, failed to explain the between-cross-section differences in k. By contrast, invertebrate assemblages and, shredders, in particular, decreased exponentially with decreasing flow permanence and with increasing drying event duration and frequency.

Effects of upland clearcutting and riparian partial harvesting on leaf pack breakdown and aquatic invertebrates in boreal forest streams

Summary1. Leaf litter breakdown and associated invertebrates were compared among three logged and three reference stream reaches 2–3 years before and 3–4 years after logging to assess the environmental impacts of partial‐harvest logging as a novel riparian management strategy for boreal forest streams.2. Partial‐harvest logging at three sites resulted in 10, 21 and 28% average basal area removal from riparian buffers adjacent to upland clear‐cut areas.3. Leaf litter breakdown rates were not significantly different between reference and logged sites after logging, but litter breakdown was significantly different from year to year at all sites.4. Significant post‐logging differences in aquatic invertebrate communities were detected at only one of the three logged sites. These differences were largely the result of increases in some leaf‐shredding stoneflies and a detritivorous mayfly and a decrease in a chironomid group 2–4 years after logging. This site where significant change was detected had the lowest intensity of riparian logging (average 10% removal) but the highest proportion of the catchment area that was clear cut (85%).5.The post‐logging differences in invertebrate communities at this site were more related to catchment‐wide influences (e.g. weather patterns, water yield, possibly upland clearcutting) than to reach‐level disturbances from riparian logging.6.The study indicates that partial‐harvest logging in riparian buffers at up to 50% removal should pose little risk of harm to leaf litter breakdown processes or aquatic invertebrate communities beyond any impacts that might arise from upland logging disturbance or catchment‐wide influences. However, the results should be viewed in the context of the natural disturbance (summer drought conditions) through the post‐logging assessment period of this study. Post‐logging summer drought conditions may have masked or confounded logging impacts on streams.

Present Effects of Past Wildfires on Leaf Litter Breakdown in Stream Ecosystems

We investigated the present effects from a 10-year-old wildfire on leaf litter breakdown rates in 3 head-water streams in central Idaho. These systems experienced a massive debris flow one year after the fire. Based on soil instability and burn patterns, we identified 3 stream conditions; unburned, burned only, and burned/scoured. We placed leaf bags containing willow leaves (Salix sp.) in each stream type and removed bags at various time intervals until all bags were collected 100 days after their introduction. Leaf material was dried and weighed, and decay rate coefficients were calculated. Macroinvertebrates colonizing the bags were enumerated and identified, and selected taxa were placed into trophic groups. We found that the unburned stream had the fastest leaf litter breakdown rate, the lowest level of incident light reaching the stream, and the largest amount of benthic organic matter. The burned/scoured stream was nearly opposite in all respects. Numbers of 2 detritivore invertebrate taxa, Serratella tibialis and Zapada oregonensis, were highest in the unburned stream but lowest in the burned/scoured stream. A third taxon, Baetis sp., showed the opposite relationship. Presence of predatory invertebrates did not affect detritivore abundance or leaf decay rate in the bags, Our research suggests that recovery response variables of some stream systems may not have returned to prefire levels even a decade after the initial wildfire. In this study, the recovery of our streams appears to be connected to the return of the riparian zone, though fire-induced debris flows may slow or alter final recovery of the stream system.

Interactions between fauna and sediment control the breakdown of plant matter in river sediments

Summary1. A substantial portion of particulate organic matter (POM) is stored in the sediment of rivers and streams. Leaf litter breakdown as an ecosystem process mediated by microorganisms and invertebrates is well documented in surface waters. In contrast, this process and especially the implication for invertebrates in subsurface environments remain poorly studied.2. In the hyporheic zone, sediment grain size distribution exerts a strong influence on hydrodynamics and habitability for invertebrates. We expected that the influence of shredders on organic matter breakdown in river sediments would be influenced strongly by the physical structure of the interstitial habitat.3. To test this hypothesis, the influence of gammarids (shredders commonly encountered in the hyporheos) on degradation of buried leaf litter was measured in experimental systems (slow filtration columns). We manipulated the structure of the sedimentary habitat by addition of sand to a gravel‐based sediment column to reproduce three conditions of accessible pore volume. Ten gammarids were introduced in columns together with litter bags containing alder leaves at a depth of 8 cm in sediment. Leaves were collected after 28 days to determine leaf mass loss and associated microbial activity (fungal biomass, bacterial abundance and glucosidase, xylosidase and aminopeptidase activities).4. As predicted, the consumption of buried leaf litter by shredders was strongly influenced by the sediment structure. Effective porosity of 35% and 25% allowed the access to buried leaf litter for gammarids, whereas a lower porosity (12%) did not. As a consequence, leaf litter breakdown rates in columns with 35% and 25% effective porosity were twice as high as in the 12% condition. Microbial activity was poorly stimulated by gammarids, suggesting a low microbial contribution to leaf mass loss and a direct effect of gammarids through feeding activity.5. Our results show that breakdown of POM in subsurface waters depends on the accessibility of food patches to shredders.

Biodiversity and ecosystem function in species-poor communities: community structure and leaf litter breakdown in a Pacific island stream

Pacific island stream communities are species-poor because of the effects of extreme geographic isolation on colonization rates of taxa common to continental regions. The effects of such low species richness on stream ecosystem function are not well understood. Here, we provide data on community structure and leaf litter breakdown rate in a virtually pristine stream on a remote island in the eastern Caroline Islands of Micronesia. The Yela River catchment on Kosrae, Federated States of Micronesia, is uninhabited, completely forested, and not traversed by any road. At each of 5 sampling stations along the Yela River we measured physical and chemical variables, characterized the benthic invertebrate community, and estimated the relative abundance of macroconsumers (fishes, shrimps, and snails) using snorkeling surveys and trapping. Benthic invertebrate biomass decreased with stream size and was dominated by macroconsumers (i.e., decapods and gastropods). Benthic insect species richness and density were extremely low, characterized by the complete absence of Ephemeroptera, Trichoptera, and Plecoptera, with nonshredding larval Chironomidae making up ∼85% of the exceptionally low insect biomass (∼0.6 mg dry mass/m2). Six species each of fishes and shrimps and 4 snail species were found. We examined the functional role of these noninsect macroconsumers in organic matter processing by comparing litter breakdown rates inside and outside electrified macroconsumer-excluding quadrats at a single site in the lower Yela River. Breakdown rates were slow relative to those reported in most tropical litter decay studies (k = 0.004–0.018 among quadrats). Despite differences in macroconsumer density (p = 0.01), the leaf breakdown rate was not significantly different between reference and electrified treatments (p = 0.77). Thus, macroconsumers at this site had neither a direct role in shredding nor an indirect effect on leaf breakdown through their interactions with any nonexcluded taxa. In the absence of a significant role played by insect or macroconsumer shredders, leaf litter breakdown in streams of remote Pacific oceanic islands might be driven purely by physical and microbial processes. Our results suggest that biogeographic processes have placed some Pacific island streams at the extreme low end of a continuum of shredder biodiversity and biomass, restricting the ecosystem function of their species-poor communities.

Effects of intense agricultural practices on heterotrophic processes in streams

In developed countries, changes in agriculture practices have greatly accelerated the degradation of the landscape and the functioning of adjacent aquatic ecosystems. Such alteration can in turn impair the services provided by aquatic ecosystems, namely the decomposition of organic matter, a key process in most small streams. To study this alteration, we recorded three measures of heterotrophic activity corresponding to microbial hydrolasic activity (FDA hydrolysis) and leaf litter breakdown rates with ( k c) and without invertebrates ( k f) along a gradient of contrasted agricultural pressures. Hydrolasic activity and k f reflect local/microhabitat conditions (i.e. nutrient concentrations and organic matter content of the sediment) but not land use while k c reflects land-use conditions. k c, which is positively correlated with the biomass of Gammaridae, significantly decreased with increasing agricultural pressure, contrary to the taxonomic richness and biomass of Trichoptera and Plecoptera. Gammaridae may thus be considered a key species for organic matter recycling in agriculture-impacted streams.

Seasonal variation in leaf-litter breakdown in nine boreal streams: implications for assessing functional integrity

Although leaf-litter breakdown rates have been suggested as a complementary bioassessment tool to measure stream functional integrity, surprisingly few studies have assessed how seasonal variability in decomposition might affect detection of impact. In this study, leaf-litter breakdown (Alnus glutinosa (L) Gaertner) was assessed during two seasons (spring and autumn) and over three years to determine the efficacy of this method for detecting human-induced change. The leaf-litter breakdown studies, performed in nine boreal streams in south-central Sweden along a gradient of nutrient enrichment, gave little support to the conjecture that rates of decomposition were related to nutrient enrichment. Only fine mesh bags in one season (autumn-winter 2005) showed a significant effect of impact. Seasonal differences in leaf mass loss in coarse mesh bags after 34 and 55 days of incubation and in fine mesh bags after 34 days were attributed to changes in temperature. In addition, other factors such as high water levels resulted in the loss of many litter bags confounding data interpretation.

Stream ecosystems respond to riparian invasion by Japanese knotweed (Fallopia japonica)

There are growing concerns about the rapid spread of exotic plants into riparian zones, yet little information is currently available on their influence on stream ecosystems. This study assessed the impact of riparian invasion by Japanese knotweed (Fallopia japonica), an aggressive invader, on leaf litter breakdown and its associated biota (aquatic hyphomycete fungi and benthic invertebrates) in heterotrophic, low-order streams in The Pennines (England) and the Pyrenees (France). Our results suggest that leaf consumers (aquatic hyphomycetes and invertebrate shredders) can readily use knotweed leaf litter even in stream sites where it was not previously present. However, aquatic hyphomycete and invertebrate assemblages differed between stream sites with and without knotweed. Leaf litter breakdown rate and relative abundance of large invertebrate shredders (mainly Trichoptera) were enhanced in the Pyrenean invaded site, whose channel contained a high proportion of knotweed leaf litter, whereas no such effects were observed in The Pennines, possibly because of the less extensive knotweed invasion. Alteration of riparian vegetation by plant invaders could therefore increasingly influence instream community and ecological functions as the severity of invasion rises.

Effects of pesticides on community structure and ecosystem functions in agricultural streams of three biogeographical regions in Europe

There is a paucity of large-scale field investigations on the effects of organic toxicants on stream macroinvertebrate community structure and ecosystem functions. We investigated a total of 29 streams in two study areas of France and Finland for pesticide exposure, invertebrates and leaf-litter breakdown. To link pesticide exposure and community composition we applied the trait-based Species At Risk (SPEAR) indicator system. In the French region, pesticide stress was associated with a decrease in the relative abundance and number of sensitive species in the communities. The presence of undisturbed upstream reaches partly compensated the effects of pesticide contamination. Functional effects of pesticides were identified by a 2.5-fold reduction of the leaf-litter breakdown rate that was closely correlated with the structural changes in the contaminated streams. No effects of pesticides were observed in Finnish streams since contamination with pesticides was very low. In a follow-up analysis, the SPEAR approach successfully discriminated between reference and contaminated sites across different biogeographical regions, also including results of a previous field study in North Germany. Furthermore, change of the community structure was detectable at a concentration range as low as 1/100 to 1/1000 the acute 48 h-LC50 of Daphnia magna. Our findings demonstrate that pesticides may influence the structure and function of lotic ecosystems and that the SPEAR approach can be used as a powerful tool in biomonitoring over large spatial scales.

Assessing the Impact of Effluents from a Modern Wastewater Treatment Plant on Breakdown of Coarse Particulate Organic Matter and Benthic Macroinvertebrates in a Lowland River

Wastewater treatment plants (WWTP) with insufficient technologies for wastewater purification often cause a distinct nutrient pollution in the receiving streams. The increased concentrations of dissolved nutrients can severely disturb the ecological integrity of streams, which has been recently shown for basic ecosystem processes like mineralization of coarse particulate organic matter (CPOM). The present study investigated the impact of a modern WWTP (Zentralklaranlage Jena) on breakdown rates of CPOM exposed in net bags (1 mm mesh size) to the effluent of a large municipal WWTP and an upstream control site in the Saale River (Thuringia, Germany) from April to October 2005. Control and effluent site differed significantly in water chemistry with increased concentrations of dissolved organic carbon (DOC), ammonium, sulfate, and chloride at the effluent site, while the control site displayed higher concentrations of nitrate. However, breakdown rates of toothpickers and small twigs were not significantly different between the sites, whereas breakdown rate of leaf litter was significantly higher at the effluent site (k = 0.0124 day−1) than at the control site (k = 0.0095 day−1). Benthic invertebrate assemblages inhabiting the sandy stream bed at both sites were dominated by Chironomidae and Oligochaeta, typical inhabitants of fine sediments. Although the Shannon diversity of the benthic invertebrates was slightly higher at the effluent site (0.85) than at the control site (0.63), no significant difference could be detected. Bacterial numbers in water samples and surface biofilms on glass slides also displayed no significant differences between the two sites. This study showed that the effluents of a WWTP with modern technologies for wastewater purification did not directly affect breakdown rates of CPOM, bacteria numbers in epibenthic biofilms and the water column, and the community composition of sediment inhabiting aquatic macroinvertebrates in an effluent-receiving river with already increased concentrations of dissolved nutrients.

Evaluation of macrofaunal effects on leaf litter breakdown rates in aquatic and terrestrial habitats

Abstract Decomposition of the organic matter is a key process in the functioning of aquatic and terrestrial ecosystems, although different factors influence processing rates between and within these habitats. Most patterns were described for temperate regions, with fewer studies in tropical, warmer sites. In this study, we carried out a factorial experiment to compare processing rates of mixed species of leaf litter between terrestrial and aquatic habitats at a tropical site, using fine and coarse mesh cages to allow or prevent colonization by macroinvertebrates. The experiment was followed for 10 weeks, and loss of leaf litter mass through time was evaluated using exponential models. We found no interaction between habitat and mesh size and leaf litter breakdown rates did not differ between fine and coarse mesh cages, suggesting that macroinvertebrates do not influence leaf litter decomposition in either habitat at our studied site. Leaf breakdown rates were faster in aquatic than in terrestrial habitats and the magnitude of these differences were comparable to studies in temperate regions, suggesting that equivalent factors can influence between‐habitat differences detected in our study.

Leaf-litter breakdown in 3 streams in temperate, Mediterranean, and tropical Cerrado climates

The objectives of our study were to assess leaf-litter breakdown in 3 streams in 3 climates and to determine the contributions of associated microbial and invertebrate communities to the process. We incubated leaves of Alnus glutinosa in 1 stream in each of 3 climate zones: temperate (mountains of Central Portugal), Mediterranean (South Portugal), and tropical Cerrado (Minas Gerais, Brazil). Leaf-litter breakdown rates (/d) were faster in temperate (k = 0.023–0.017) than in tropical (k = 0.014) or Mediterranean (k = 0.014–0.009) streams. Leaf-litter breakdown rates (/degree day) also were higher in the temperate stream (k = 0.0018–0.0032) and similar between the other 2 streams (k = 0.008–0.0012). Colonization of leaves by aquatic hyphomycetes was faster in the temperate stream (maximum = 421 μg ergosterol/g of leaf by day 24) than in the tropical Cerrado or Mediterranean streams. However, peak ergosterol content was highest in the tropical Cerrado stream (573 μg/g on day 75). Ergosterol content was lowest in the Mediterranean stream (maximum = 341 μg/g on day 7). Total microbial biomass (as ATP) was higher in the tropical Cerrado stream (maximum = 531 nmoles/g on day 75) than in the Mediterranean (maximum = 108 nmoles/g on day 92) and temperate (maximum = 93 nmoles/g on day 7) streams. These results suggest either that not all microorganisms associated with leaves were involved in leaf-litter breakdown or that other less efficient microorganisms than fungi were involved in leaf-litter breakdown in the tropical stream. Leaves exposed to invertebrates (coarse-mesh bags) decomposed significantly faster than leaves protected from invertebrate feeding (fine-mesh bags) only in the temperate stream. This result suggests that invertebrates were important mediators of leaf-litter breakdown only in the temperate stream. A larger proportion of invertebrates recovered from decomposing leaves were shredders in the temperate stream (nearly 5%) than in the Mediterranean (1%) and tropical Cerrado (0%) streams. Leaf-litter processing rates increased with discharge and NO3 concentration in the water. Our results suggest that the positive effect of temperature on breakdown rates of allochthonous organic matter in streams can be overridden by nutrient content in the water and the presence of invertebrate shredders.

Flow‐substrate interactions create and mediate leaf litter resource patches in streams

Summary1. The roles that streambed geometry, channel morphology, and water velocity play in the retention and subsequent breakdown of leaf litter in small streams were examined by conducting a series of field and laboratory experiments.2. In the first experiment, conditioned red alder (Alnus rubra Bongard) leaves were released individually in three riffles and three pools in a second‐order stream. The transport distance of each leaf was measured. Several channel and streambed variables were measured at each leaf settlement location and compared with a similar number of measurements taken at regular intervals along streambed transects (‘reference locations’). Channel features (such as water depth) and substrate variables (including stone height, stone height‐to‐width ratio, and relative protrusion) were the most important factors in leaf retention.3. In the second experiment, the role of settlement location and reach type in determining the rate of leaf litter breakdown was examined by placing individual conditioned red alder leaves in exposed and sheltered locations (on the upper and lower edges of the upstream face of streambed stones, respectively) in riffle and pool habitats. After 10 days, percent mass remaining of each leaf was measured. Generally, leaves broke down faster in pools than in riffles. However, the role of exposure in breakdown rate differed between reach types (exposed pool &gt; sheltered pool &gt; sheltered riffle &gt; exposed riffle).4. In the third experiment, the importance of substrate geometry on leaf litter retention was examined by individually releasing artificial leaves upstream of a series of substrate models of varying shape. Substrates with high‐angle upstream faces (were vertical or close to vertical), and that had high aspect ratios (were tall relative to their width), retained leaves more effectively.5. These results show that streambed morphology is an important factor in leaf litter retention and breakdown. Interactions between substrate and flow characteristics lead to the creation of detrital resource patchiness, and may partition leaf litter inputs between riffles and pools in streams at baseflow conditions.

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.

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

Summary1. 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 Macrobrachium hainanense predation on benthic community functioning in tropical Asian streams

Summary1. Macrobrachium hainanense is a large predatory palaemonid shrimp, present at high densities in pools of low‐order forested streams in Hong Kong. The present study investigated the impacts of M. hainanense on benthic community structure and functions in pools of two streams: Tai Po Kau Forest Stream and Tai Shing Stream.2. Repeated whole‐pool experiments involving shrimp density manipulations (removal, control and addition) were conducted in both streams between October 2000 and April 2002, and included a wet (May to September) and two dry (October to April) seasons. The three objectives of the study were to determine the effects of M. hainanense predation on benthic macroinvertebrate abundance and species richness, rates of leaf litter breakdown because of effects on detritivores, and periphyton standing stocks by way of an effect on herbivores.3. Wet season results showed consistent reductions in benthos densities and species richness following heavy rainfall, irrespective of shrimp manipulation. These results suggested that spate‐induced disturbances might override biotic effects and play a dominant role in structuring benthic communities in stream pools in Hong Kong.4. No significant, reproducible effects on any of the response variables measured in either stream were found during the dry season. Litter breakdown was reduced in the absence of shrimps during one experiment only, suggesting it might be a type I error. These results signified no effect of shrimp removal on benthic communities, or on the functional processes of litter breakdown, or on periphyton accumulation. The large scale of the experimental units (8–40 m2), refuge availability, and the presence of benthic predatory fishes that cropped excess prey made available by removal of M. hainanense, may have contributed to the lack of any effect, despite the abundance of the predatory shrimps.

Leaf diversity alters litter breakdown in a Piedmont stream

AbstractWork in terrestrial ecosystems has suggested that the breakdown rate of leaf litter may not change predictably with increasing plant species richness. Indeed, it may be that certain combinations of leaf species demonstrate significant non-additive effects on breakdown rates, mediated by the presence of a single key species. Such effects have not been explored in running-water ecosystems despite the strong interest in the conservation and restoration of riparian systems. We documented the magnitude and species composition of leaf litter inputs, the species richness and composition of leaf litter on the streambed, and estimated the breakdown rate of mixed litter in both the summer and autumn in a warmwater stream. We found that leaf species richness of litter packs on the streambed varied from 1 to 11 species, and leaf species composition reflected the composition of litter inputs from May through November. We did not find a general relationship between breakdown rate and leaf litter species richnes...