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.     

Non-consumptive effects of predatory mites on thrips and its host plant

Recent reviews on trait-mediated interactions in food webs suggest that trait-mediated effects are as important in triggering top–down trophic cascades as are density-mediated effects. Trait-mediated interactions between predator and prey result from non-consumptive predator effects changing behavioural and/or life history traits of prey. However, in biological control the occurrence of trait-mediated interactions between predators, prey and plants has been largely ignored. Here, we show that non-consumptive predator effects on prey cascade down to the plant in an agro-ecological food chain. The study system consisted of the predatory mites P. persimilis and N. californicus , the herbivorous non-target prey western flower thrips F. occidentalis and the host plant bean. Irrespective of predator species and risk posed to prey, the presence of predator eggs led to increased ambulation, increased mortality and decreased oviposition of thrips. Furthermore, the presence of predator eggs reduced leaf damage caused by thrips. To our knowledge this is the first experimental evidence suggesting a positive trophic cascade triggered by non-consumptive predator effects on non-target prey in an augmentative biological control system.     

Cascading effects of predatory fish exclusion on the detritus-based food web of a lake littoral zone (Lake Vico, central Italy)

An exclosure experiment was carried out in the reed-dominated littoral zone of a volcanic lake (Lake Vico, central Italy) to test whether the impact of predatory fish on benthic invertebrates cascades on fungal colonisation and breakdown of leaf detritus. The abundance, biomass, and Shannon diversity index of the invertebrate assemblage colonising Phragmites australis leaf packs placed inside: (1) full-exclosure cages, (2) cages allowing access only to small-sized fish predators, and (3) cageless controls, were monitored over a 45-day period together with the mass loss and associated fungal biomass of leaf packs. The species composition of the fungal assemblage was further assessed at the end of the manipulation. In general, invertebrate predators did not show any significant response to fish exclusion, either on a trophic guild or on a single taxon level. In contrast, the exclusion of large predatory fish induced a diverse spectrum of changes in the abundance and population size-structure of dominant detritivore taxa, ultimately increasing the biomass and Shannon diversity index of the whole detritivorous guild. These changes corresponded with significant variations in leaf detritus decay rates as well as in the biomass and assemblage structure of associated fungal colonisers. Our experimental findings provide evidence that in Lake Vico effects of fish predators on invertebrate detritivores influence the fungal conditioning and breakdown of the detrital substrate. We conclude that in lacustrine littoral zones predator-driven constraints may structure lower trophic levels of detritus-based food webs and affect the decomposition of leaf detritus originated from the riparian vegetation.     

Indirect effects of predatory trout on organic matter processing in detritus-based stream food webs

Indirect effects of predators on basal resources in allochthonous-based food webs are poorly understood. We investigated indirect effects of predatory brown trout ( Salmo trutta ) on detritus dynamics in southern beech ( Nothofagus spp.) forest streams in New Zealand through predation on the obligate detritivore, Zelandopsyche ingens (Trichoptera, Oeconesidae). Trout presence/absence and Z. ingens density were manipulated in flow-through tanks to investigate the lethal and sub-lethal effects of trout on litter processing by Z. ingens . An experiment that allowed trout access to Z. ingens showed trout predation reduced densities of Z. ingens resulting in slower breakdown of coarse particulate organic matter (CPOM) and reduced production of fine particulate organic matter (FPOM). A second experiment that prevented trout access to Z. ingens , but allowed the transmission of trout cues, resulted in no change in litter processing rates in the presence of trout. Litter processing rates were higher in high Z. ingens density treatments compared to low density treatments. Thus, trout effects on litter processing were due to reduced Z. ingens densities, not trout-induced modifications to Z. ingens feeding behaviour. Field assays of litter processing rates using artificial leaf packs in natural streams showed significant reductions in CPOM loss in trout streams compared to fishless streams. Z. ingens dominated biomass in fishless stream leaf packs, but a facultative shredder, Olinga feredayi , dominated trout stream leaf packs. Thus, the absence of Z. ingens drove differences in processing rates between trout and fishless streams and the indirect effects of trout on litter processing observed in mesocosms were evident in complex, natural food webs. Overall our study provides evidence that predators can influence resource dynamics in donor-controlled food webs through their effects on consumers.     

Do non-native Platanus hybrida riparian plantations affect leaf litter decomposition in streams?

In forest headwater streams where the riparian canopy limits autochthonous primary production, leaf litter decomposition is a key process controlling nutrient and carbon cycling. Any alteration of the riparian vegetation may influence litter decomposition and detrital food webs. We evaluated the effect of non-native Platanus hybrida riparian plantations on leaf litter decomposition in Mediterranean streams. The experiment was conducted in six headwater streams; three lined by native riparian vegetation and three crossing P. hybrida plantations. We have characterized the processing rates of alder leaves and the assemblages of shredder macroinvertebrates and fungi. Litter decomposition was significantly faster in the P. hybrida than in the reference streams. Although the dissolved inorganic nitrogen concentration was higher in P. hybrida, no significant effect was observed in decomposition rates. Differences in decomposition rates reflected the macroinvertebrate and shredder colonization in alder litter, with higher abundance and richness in the P. hybrida streams. However, aquatic hyphomycete sporulation rate was higher in reference streams, suggesting that the variation in decomposition rates is a direct consequence of shredder abundance. Our findings support part of the substrate quality-matrix quality (SMI) hypothesis, which expects that high-quality litter will show increased decomposition rates in a low-quality litter matrix.     

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.     

Litter identity mediates predator impacts on the functioning of an aquatic detritus-based food web

During past decades, several mechanisms such as resource quality and habitat complexity have been proposed to explain variations in the strength of trophic cascades across ecosystems. In detritus-based headwater streams, litter accumulations constitute both a habitat and a resource for detritivorous macroinvertebrates. Because litter edibility (which promotes trophic cascades) is usually inversely correlated with its structural complexity (which weakens trophic cascades), there is a great scope for stronger trophic cascades in litter accumulations that are dominated by easily degradable litter species. However, it remains unclear how mixing contrasting litter species (conferring both habitat complexity and high quality resource) may influence top–down controls on communities and processes. In enclosures exposed in a second-order stream, we manipulated litter species composition by using two contrasting litter (alder and oak), and the presence–absence of a macroinvertebrate predator (Cordulegaster boltonii larvae), enabling it to effectively exert predation pressure, or not, on detritivores (consumptive versus non-consumptive predation effects). Leaf mass loss, detritivore biomass and community structure were mostly controlled independently by litter identity and mixing and by predator consumption. However, the strength of predator control was mediated by litter quality (stronger on alder), and to a lesser extent by litter mixing (weaker on mixed litter). Refractory litter such as oak leaves may contribute to the structural complexity of the habitat for stream macroinvertebrates, allowing the maintenance of detritivore communities even when strong predation pressure occurs. We suggest that considering the interaction between top–down and bottom–up factors is important when investigating their influence on natural communities and ecosystem processes in detritus-based ecosystems.     

Leaf breakdown, detrital resources, and food webs in streams affected by mine drainage

Breakdown of leaf litter is essential for providing detrital resources for food webs but can be impaired by anthropogenic activities, which may disrupt energy flow to consumers. We investigated the relationship between leaf breakdown and food web structure in 12 streams with or without mining impacts on South Island, New Zealand. Six streams received inputs of acid mine drainage (pH 2.5–4.9), three were naturally acidic (pH ~5.0), and three were circumneutral (pH ~6.8). Streams affected by mining either had highly acidic water (pH <3) or iron precipitates present on substrata. Breakdown rates of leaves were significantly lower in mining-affected streams than circumneutral (by almost 50%) but not naturally acidic streams and were driven primarily by microbial activity, as shredding invertebrates were often absent. Mining-affected stream webs were simplified structures with fewer species and links than those in other streams. With few species to process leaf litter and transfer detrital resources, inputs of AMD disrupted both the mechanisms responsible for breakdown and links for energy flow. While faster breakdown rates were associated with larger food webs, limited function maintained in mining-affected streams was sufficient to support primary consumers and small food webs.     

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.

     

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.     

Fish determine macroinvertebrate food webs and assemblage structure in Greenland subarctic streams

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Indirect effects of fish on foraging behaviour and leaf processing by the isopod Lirceus fontinalis

1. We examined whether the isopod, Lirceus fontinalis, an important facultative shredder in low-order streams in the eastern United States, responded to chemical cues of the skin mucus of five fish species varying in relatedness and feeding habits, and if fish-induced alterations in the foraging activity of isopods indirectly affected leaf processing. 2. In the laboratory, isopods generally responded to the presence of fish mucus by reducing their activity. Isopods were most responsive in water containing the skin mucus of predatory sunfish (Lepomis) and least responsive in water treated with mucus from algivorous stonerollers (Campostoma anomalum). 3. Rates of leaf disc breakdown by isopods in the laboratory were significantly reduced when isopods were exposed to chemical cues of confined green sunfish (L. cyanellus). 4. The rate of leaf processing in a fishless reach of a headwater stream was four to five times greater by isopods 7—10mmTL (total length) than those ≤ 5mm TL. Rates of leaf processing by isopods in low-order streams may be affected directly by the size structure of the isopod population and indirectly by the presence of fish.     

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.
     

Experimental Detritus Manipulations Unite Surface and Cave Stream Ecosystems Along a Common Energy Gradient

Subsidies of detritus from donor habitats are important energy sources for many ecosystems, but understanding their role in structuring recipient food webs requires comparative experimental studies along the full spectrum of detrital fluxes. Here we report results from an experimental addition of maize (Zea mays L.) litter to a detritus-poor cave stream ecosystem, which we then compare with analogous, past experiments using detritus-rich surface stream ecosystems that similarly have detritus-based food webs and extremely low in situ primary production. Bulk-tissue and compound-specific stable isotope analyses showed that maize litter carbon (C) was rapidly assimilated by microbes and transferred via successive trophic levels to the top of the cave stream food web (omnivorous crayfishes and predatory salamanders). All trophic levels increased in abundance and biomass, but only facultative cave taxa, that is those also found in surface streams, contributed to this numerical response. The lack of response by obligate cave species presumably occurred because evolutionary trade-offs associated with adaptations to low-C environments constrained their population-level responses during the one-year period of the litter addition. Comparison of the responses of the cave community with the analogous litter manipulation experiments in surface streams showed strong convergence in the functional relationship between invertebrate and detritus biomass (R ² = 0.72, P < 0.0001). Our results suggest that these seemingly disparate stream food webs lie along a single, common gradient of detritus supply, occupied at its extreme minimum by communities of obligate cave taxa adapted to low-energy environments.     

Assessing the functional importance of large‐bodied invertebrates in experimental headwater streams

Recent theoretical advances in food web ecology emphasize the importance of body size disparities among species for the structure, stability and functions of ecosystems. Experimental confirmations of the functional importance of large species, independent of their trophic position, are scarce. We specifically examine the multiple ecological roles of large invertebrates from two distinct trophic levels in headwater streams. We experimentally manipulated the presence of large predatory invertebrates (two Perlid stoneflies) or detritivores (a limnephilid caddisfly and a Pteronarcys stonefly) in a two‐by‐two design in stream channels open to immigration/emigration of smaller biota. We assessed treatment effects on the trophic structure of the benthic invertebrate community, dynamics of basal resources (benthic algae and leaf litter of cedar and alder), and stability of litter decomposition rates against an experimental pulse perturbation (fine sediment input). The presence of the large invertebrates was associated with a ten‐fold decrease in the biomass of invertebrate filterers whereas other trophic groups were unaffected by the large species. The biomass of benthic algae was lower and the rate of mass loss of alder litter was higher in channels lacking the large predators, thus revealing trophic cascades operating along both algal‐based and detritus‐based food chains. The large predators had no detectable effect on the decomposition of cedar whereas both cedar and alder disappeared faster in the presence of the large detritivores. Furthermore, the large predators and large detritivores interactively influenced the decomposition of the cedar–alder mixture through a litter diversity effect and the variability of the rate of alder decomposition after a pulse of fine sediment. Because the large invertebrates affected multiple ecosystem properties, and as their absence was not rapidly compensated for by small immigrant species, our findings support the notion that large species could be critically important in controlling ecosystem structure and functioning.     

Trophic roles of tadpoles in tropical Australian streams

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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 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.     

Food Web Structure and Basal Resource Utilization along a Tropical Island Stream Continuum, Puerto Rico

Tropical stream food webs are thought to be based primarily on terrestrial resources (leaf litter) in small forested headwater streams and algal resources in larger, wider streams. In tropical island streams, the dominant consumers are often omnivorous freshwater shrimps that consume algae, leaf litter, insects, and other shrimps. We used stable isotope analysis to examine (1) the relative importance of terrestrial and algal‐based food resources to shrimps and other consumers and determine (2) if the relative importance of these food resources changed along the stream continuum. We examined δ¹⁵N and δ¹³C signatures of leaves, algae, macrophytes, biofilm, insects, snails, fishes, and shrimps at three sites (300, 90, and 10 m elev.) along the Río Espíritu Santo, which drains the Caribbean National Forest, Puerto Rico. Isotope signatures of basal resources were distinct at all sites. Results of two‐source δ¹³C mixing models suggest that shrimps relied more on algal‐based carbon resources than terrestrially derived resources at all three sites along the continuum. This study supports other recent findings in tropical streams, demonstrating that algal‐based resources are very important to stream consumers, even in small forested headwater streams. This study also demonstrates the importance of doing assimilation‐based analysis (i.e., stable isotope or trophic basis of production) when studying food webs.     

Comparison of leaf processing rates under different temperature regimes in three headwater streams

1. We studied the effects of different temperature regimes on leaf litter processing in three forested Appalachian headwater streams of different pH (mean pH = 4.2, 6.5, 7.5).
2. We compared leaf breakdown rates, microbial biomass and macroinvertebrate shredder density and biomass between two 12-week processing periods (October–January and November–February) in each stream. Leaf processing rates were calculated both as k (day^–1) and k _d (degree day^–1).
3. There were no significant differences in processing rates ( k day^–1) between the two study periods for any leaf species in any stream. The average difference in temperature between the two study periods was 175 degree days. Shredder density was significantly higher during the earlier study period on 40% of the sample dates, but shredder biomass was not significantly different between the two study periods. ATP concentration was significantly higher during the early study period for 60% of the sample dates.
4. More significant differences in these variables (shredder density and biomass, ATP concentration) were seen among the three study streams than between the two study periods. This indicates that in this study other factors, particularly stream pH, contributed more to processing rate variation than did differences in thermal regime.