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 litter breakdown rates in boreal streams: does shredder species richness matter?

1. Leaf litter breakdown rates were assessed in 23 boreal streams of varying size (first–seventh order) in central and northern Sweden. 2. Shredders were most abundant in small streams, while shredder species richness showed a hump-shaped relationship with stream order, with most species in fourth order streams. 3. In a partial least-squares regression analysis, year, water temperature, shredder species richness and shredder abundance were those factors correlating most strongly with leaf breakdown rates. Shredder species richness was more strongly correlated with leaf litter breakdown rates than shredder abundance, and shredder biomass showed no such correlation. 4. These data suggest that shredder species richness is an important variable in terms of leaf litter dynamics in streams.     

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.     

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.     

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

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Effects of litter patch area on macroinvertebrate assemblage structure and leaf breakdown in Central Amazonian streams

Leaf breakdown in streams is affected by several factors, such as leaf characteristics, water chemistry, microbial activity, and abundance of shredders. In turn, shredders may be resource-controlled. We hypothesized that the size of litter patches affects leaf breakdown, because large patches should be stable over time and therefore harbor high densities of shredders. We selected litter patches (area 0.25–10 m²) in 10 pools of three first-order streams (Manaus, Brazil). We installed 10 leaf packs of Mabea speciosa (Euphorbiaceae) in each patch, and sampled one after 1 day and three after 5, 19, and 28 days. The leaf packs were quickly colonized by the shredding caddisflies Triplectides and Phylloicus. The leaf breakdown rate (mean k = 0.026 ± 0.0015 SE) was high and similar to values reported for other tropical and temperate streams, although much higher than values reported for the adjacent Cerrado biome. Assemblage composition varied over time, but was not related to the size of litter patches. Contrary to our hypothesis, litter patch area did not affect breakdown rates (r ² = 0.012, P = 0.766) or abundance of shredders after 5, 19, and 28 days (r ² < 0.243, P > 0.147). We found, however, a positive relationship between the abundance of tropical shredders and leaf breakdown after 19 days (r ² = 0.572, P = 0.011), suggesting that shredders play an important role in leaf breakdown in these headwater streams. Our study indicates that leaf breakdown rates in tropical streams are variable and can be as high as those of temperate streams.     

Leaf Quality of Some Tropical and Temperate Tree Species as Food Resource for Stream Shredders

We tested the hypotheses that (1) plant defenses against consumers increase in the tropics, and that these differences in quality are perceived by detritivores; and (2) microbial conditioning of leaf litter is important for the feeding ecology of shredders from both geographical regions. We compared quality parameters of 8 tree species from Portugal and 8 from Venezuela. The tropical leaves were tougher, but did not differ from temperate leaves in terms of N, C: N, and polyphenols. In multiple‐choice experiments, shredders from Portugal (Sericostoma vittatum and Chaetopteryx lusitanica) and from Venezuela (Nectopsyche argentata and Phylloicus priapulus) discriminated among conditioned leaves, preferentially consuming softer leaves. In another set of experiments, all shredders preferentially fed on conditioned rather than unconditioned leaves, grew faster when fed conditioned than unconditioned leaves and fed more on temperate than tropical leaves. We conclude that leaf litter from the tropics is a low‐quality resource compared to leaves in temperate systems, because of differences in toughness, and that tropical shredders benefit from microbial colonization, as previously demonstrated for temperate systems. We suggest that leaf toughness could be one explanation for the reported paucity of shredders in some tropical streams. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Assessing the influence of wildfire on leaf decomposition and macroinvertebrate communities in boreal streams using mixed-species leaf packs

1. We investigated how compositional differences in riparian leaf litter derived from burned and undisturbed forests influenced leaf breakdown and macroinvertebrate communities using experimental mixed-species leaf packs in boreal headwater streams. Leaf pack mixtures simulating leaf litter from dominant riparian woody-stem species in burned and undisturbed riparian zones were incubated in two references and two fire-disturbed streams for 5 weeks prior to measuring temperature-corrected breakdown rates and macroinvertebrate community composition, richness, and functional metrics associated with decomposers such as shredder abundance and % shredders.
2. Leaf litter breakdown rates were higher and had greater variability in streams bordered by reference riparian forests than in streams where riparian forests had been burned during a wildfire. Streams bordered by fire disturbance showed significant effects of litter mixture on decomposition rates, observed as significantly higher decomposition rates of a fire-simulated leaf mixture compared to all other mixtures.
3. Variation among sites was higher than variation among litter mixtures, especially for macroinvertebrate community composition. In general, fire-simulated leaf mixtures had greater shredder abundances and proportions, but lower overall macroinvertebrate abundance; however, the shredder abundance trend was not consistent across all leaf mixtures at each stream.
4. These results show that disturbance-driven riparian forest condition and resulting composition of leaf subsidies to streams can influence aquatic invertebrate community composition and their function as decomposers. Therefore, if one of the primary goals of modern forest management is to emulate natural disturbance patterns, boreal forest managers should adapt silvicultural practices to promote leaf litter input that would arise post-fire, thereby supporting stream invertebrate communities and their function.

     

Temporal niches of shredders in lake littorals with possible implications on ecosystem functioning

Temporal growth separation of shredders is known in streams but has not been reported from lakes. In the present study, temporal niche/trait differentiations among shredders in lakes were investigated. We sampled quantitatively three lakes in SE Sweden over a period of 18 months. Dry weight and number of individuals of the collected shredders were measured monthly. Standing stock of detritus types was also monitored. The same 10 species of lentic shredders were found in each lake, one isopod and larvae of nine trichopterans. Functionally, the shredders could be categorized into two main groups; winter and summer growing species. However, also within these groups, temporal differences in growth pattern existed. The main input of detritus occurred during the autumnal leaf fall and a majority of winter shredders had the start of their lives tied to this period. A succession in loss of detritus types was evident with easily degraded matter disappearing first followed by more resistant matter. Shredder species richness, shredder biomass per m² and the ratio coarse/fine detritus all reached its annual low in late summer. We propose a temporal link between the shredder groups and the organic matter subject to decomposition; the successive palatability of coarse detritus is likely to make a temporally separated community of shredders efficient in terms of decomposition. We believe that a temporal differentiation per se is sufficient to conclude that different impact on ecosystem function exists among shredders. Additionally we discuss impacts of differences in abundance and shredding capacity among the species.     

Understanding of colonization and breakdown of leaves by invertebrates in a tropical stream is enhanced by using biomass as well as count data

We hypothesized that (i) the importance of shredders for leaf breakdown is more evident in terms of their biomass than their abundance, due to the large bodies and high-feeding efficiencies of some typical shredders; (ii) non-shredder invertebrates select more refractory leaves because these are a more stable substrate for colonization or to obtain other forms of food. To test these hypotheses, we performed a decomposition experiment with leaves of contrasting chemical composition in a tropical stream, and determined the changes in the ash-free dry mass (AFDM) of the litter, and the invertebrate abundance and biomass during a 44-day period. The biomass of shredders showed a positive relationship with AFDM remaining, whereas their abundance was unrelated to AFDM. While shredder abundance represented only 4–12% of total invertebrate abundance, shredder biomass constituted 19–36% of total invertebrate biomass. We conclude that (i) shredder biomass expresses better than abundance the role of this guild in the decomposition of leaf detritus, demonstrating that they are important for the functioning of tropical streams; (ii) incubation time rather than stability of leaf litter as a substrate influences colonization by non-shredder invertebrates.     

Breakdown and detritivore colonisation of leaves in three New Zealand streams

Breakdown of leaves from three native riparian tree species, and their colonisation by shredding and collecting insect larvae, were investigated in three streams on Banks Peninsula, New Zealand. Leaves were introduced in baskets at the time of leaf fall. Breakdown rates of leaves were faster than previously recorded in New Zealand streams and were comparable to those of many northern hemisphere deciduous species. Shredder and total detritivore densities and biomass in leaf baskets were also greater than previously found in New Zealand streams. Peaks of shredder biomass on red beech and mahoe leaves were found when only about 20% of leaf biomass remained. No shredder peak was recorded on fuchsia leaves, and no collector peaks occurred in any of the streams. Relative shredder and collector biomass (per g DW leaf) in leaf baskets did not exceed or was smaller than in leaf litter accumulations of mixed origin and conditioning throughout the streams during leaf breakdown although absolute shredder and collector biomass (per m² stream bottom) was occasionally larger in baskets than in the rest of the stream. These findings support contentions that spatial and temporal relationships between detrital inputs and detritivore biomass and life histories are weak in New Zealand streams.     

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.     

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)     

Effects of diversity loss on ecosystem function across trophic levels and ecosystems: A test in a detritus‐based tropical food web

Abstract We investigated the effects of biodiversity loss across trophic levels and across ecosystems (terrestrial to aquatic) on ecosystem function, in a detritus‐based tropical food web. Diversities of consumers (stream shredders) and resources (leaf litter) were experimentally manipulated by varying the number of species from 3 to 1, using different species combinations, and the effects on leaf breakdown rates were examined. In single‐species shredder treatments, leaf diversity loss affected breakdown rates, but the effect depended on the identity of the leaves remaining in the system: they increased when the most preferred leaf species remained, but decreased when this species was lost (leaf preferences were the same for all shredders). In multi‐species shredder assemblages, breakdown rates were lower than expected from single‐species treatments, suggesting an important role of interspecific competition. This pattern was also evident when oneleaf species was available but not with higher leaf diversity, suggesting that lowered leaf diversity promotes competitive interactions among shredders. The influence of diversity and identity of species across trophic levels and ecosystems on stream functioning points to complex interactions that may well be reflected in other types of ecosystem.     

Leaf litter quality drives the feeding by invertebrate shredders in tropical streams

Amazon and Cerrado‐forested streams show natural fluctuations in leaf litter quantity along the time and space, suggesting a change on litter quality input. These natural fluctuations of leaf litter have repercussion on the organic matter cycling and consequently effects on leaf decomposition in forested streams. The effects of the quantity of leaf litter with contrasting traits on consumption by larvae of shredder insects from biomes with different organic matter dynamics have still been an understudied question. The Trichoptera Phylloicus spp. is a typical shredder in tropical headwater streams and keep an important role in leaf litter decomposition. Here, we assessed the consumption by shredder Phylloicus spp., from Amazonia and Cerrado biomes, on higher (Maprounea guianensis) and lower quality leaves (Inga laurina) in different proportions and quantities. Experiments were performed concomitantly in microcosms approaches, simulating Cerrado and Amazonian streams. Higher leaf consumption occurred in Cerrado microcosms. Litter quantity influenced negatively leaf consumption by shredders in Cerrado, in opposition to Amazonia, where consumption was not affected by leaf quantity. In both sites, we observed higher consumption by shredders in treatment with only M. guianensis and no difference between other treatments with mixture of leaves. In treatment with litter of I. laurina, we noted the use of substrate for case building (due to the higher leaf toughness), affecting the fragmentation process. Therefore, our results indicate that leaf litter quality drives the preference of consumption by Phylloicus larvae in Cerrado and Amazonia streams.     

Food quality, feeding preferences, survival and growth of shredders from temperate and tropical streams

1. The importance of leaf quality to the nutritional ecology of lotic shredders is well established for temperate species but virtually unknown for tropical taxa. In the present study, we compared the feeding behaviour and performance of two tropical and two temperate shredders in a series of pair‐wise experiments. 2. Specifically, we tested whether leaf conditioning status (stream‐conditioned versus unconditioned leaves) and geographical origin (temperate Alnus glutinosa versus tropical Hura crepitans leaves) affect the food preference, survivorship, and growth of selected shredders from low and high latitudes in a consistent manner. The animals used in experiments were the caddis‐flies Nectopsyche argentata and Phylloicus priapulus from Venezuela, Sericostoma vittatum from Central Portugal, and the amphipod Gammarus pulex from Northern Germany. 3. In general, all shredders exhibited the same high preference for conditioned over unconditioned leaves, irrespective of the geographical origin of the leaf or shredder species. 4. A corresponding tendency for higher growth was found for sets of animals offered conditioned leaves, with the differences in growth being clearer in the two tropical shredders. Survivorship of the two temperate species was consistently high (> 83%) regardless of the diet offered, whereas the tropical shredders survived better on conditioned (77–90%) as compared with unconditioned (54–87%) leaves, although not significantly so. 5. With the exception of the temperate S. vittatum, shredders did not select or perform better on leaves to which they had previously been exposed, indicating a potential adaptation to native leaf species is over‐ridden by intrinsic leaf properties. 6. Taken together, the results of this study suggest that tropical shredders may exhibit the same basic patterns of food exploitation as their temperate counterparts. Consequently, current concepts relating to the role of shredders in stream detritus dynamics may well be applicable to tropical streams, although essentially derived from temperate systems.     

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.     

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.     

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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大型底栖无脊椎动物对溪流中三角枫落叶分解的影响

采用复合网袋法,研究了冬季南京紫金山三角枫落叶在无干扰溪流和生态恢复溪流中的分解过程.结果表明：112 d后,三角枫落叶无灰干质量剩余率为31％～62％,分解速率符合指数衰减模型(P<0.05).在生态恢复溪流和无干扰溪流的流水生境中, 三角枫落叶的分解速率分别为0.0030 d^-1和0.0064 d^-1,静水生境中分别为0.0018 d^-1和0.0016 d^-1.流水生境中,无干扰溪流网袋内的大型底栖动物多度和生物量显著高于恢复溪流(P<0.05),而静水中无显著差异(P>0.05).无干扰溪流中的撕食者多度比例最高(70.4%),以栉水虱为主;生态恢复溪流中滤食者的多度比例最高(37.8%),以长跗摇蚊属为主.流水生境中三角枫落叶的分解速率与撕食者物种丰富度和多度相关性显著(P<0.01),而与生物量的相关性不显著(P>0.05).说明冬季溪流中撕食者的物种丰富度和多度决定三角枫落叶的分解速率.