Effects of invasive clam (Corbicula fluminea) die‐offs on the structure and functioning of freshwater ecosystems

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A global experiment suggests climate warming will not accelerate litter decomposition in streams but might reduce carbon sequestration

The decomposition of plant litter is one of the most important ecosystem processes in the biosphere and is particularly sensitive to climate warming. Aquatic ecosystems are well suited to studying warming effects on decomposition because the otherwise confounding influence of moisture is constant. By using a latitudinal temperature gradient in an unprecedented global experiment in streams, we found that climate warming will likely hasten microbial litter decomposition and produce an equivalent decline in detritivore-mediated decomposition rates. As a result, overall decomposition rates should remain unchanged. Nevertheless, the process would be profoundly altered, because the shift in importance from detritivores to microbes in warm climates would likely increase CO(2) production and decrease the generation and sequestration of recalcitrant organic particles. In view of recent estimates showing that inland waters are a significant component of the global carbon cycle, this implies consequences for global biogeochemistry and a possible positive climate feedback.     

Using Stable Isotopes to Differentiate Trophic Feeding Channels within Soil Food Webs

The soil is probably the most diverse habitat there is, with organisms ranging in sizes from less than 1 μm to several metres in length. However, it is increasingly evident that we know little about the interactions occurring between these organisms, the functions that they perform as individual species, or together within their different feeding guilds. These interactions between groups of organisms and physical and chemical processes shape the soil as a habitat and influence the nature of the soil food web with consequences for the above‐ground vegetation and food web. Protists are known as one of the most abundant groups of bacterivores within the soil; however, they are also consumers of a number of other food sources. Even though they are responsible for a large proportion of the mineralisation of bacterial biomass and have a large impact on the C and N cycles within the soil they are regularly overlooked when investigating the complete soil food web. Recently, stable isotopes have been used to determine trophic interactions and here we describe how this technique has been used to highlight linkages between protists and the soil food web.     

Factors related to diversity of decomposer fungi in tropical forests

Recent studies suggest that host-preferences are common among certain groups of tropical fungal decomposers but rare in others, and sometimes occur where we least expect them. Host preferences among microfungi and ascomycetes that decompose leaf litter are common but usually involve differences in relative frequencies more than presence/absence, so their diversity may be loosely correlated with species richness of host trees. Strong host-specificity appears to be rare among wood decomposer fungi, whereas characteristics of their substrata and habitat are very important for this group. Anthropogenic disturbance predisposed a tropical forest to subsequent hurricane damage, and the resulting direct and indirect effects on host diversity and habitat heterogeneity were reflected in the decomposer fungal community more than sixty years after the original disturbance. While species richness of dictyostelid slime molds and functional diversity of their bacterial prey increased with disturbance, the more diverse microfungi and ascomycetes were apparently negatively affected by disturbance.     

Elemental composition and degree of homeostasis of fungi: are aquatic hyphomycetes more like metazoans,bacteria or plants?

Ecological stoichiometry generally assumes that heterotrophs have a higher degree of elemental homeostasis than autotrophs. Differences between fixed consumer nutrient requirements and nutrients available in resources allow prediction of the intensity of nutrient recycling ensured by heterotrophs. Despite their fundamental role in detritus decomposition, extremely few data are currently available on fungal elemental composition. In this study, we quantified the degree of elemental homeostasis of aquatic hyphomycetes used as model organisms. Contrary to metazoans, but similar to plants, aquatic hyphomycetes exhibited highly plastic elemental compositions. Mycelium also reached far higher C/nutrient ratios than reported for bacteria. Our results suggest that non-homeostasis of fungi should be explicitly included in stoichiometric models dealing with nutrient recycling, and that the discrepancy in homeostasis between some bacterial strains and fungi should certainly be considered when investigating interactions between both groups of decomposers.     

Role of first-order consumers in ecosystem carbon flow

Herbivores and detritus consumers (i.e. microbial decomposers and invertebrate and vertebrate detritivores) are pivotal components of trophic food webs and thus play a paramount role in the trophic transference and turnover of producer‐fixed carbon. Hence, elucidating patterns in carbon flux through these first‐order consumers is important to understand the nature and controls of carbon flow in ecosystems. Here, using the largest literature compilation to date, I show that, in contrast with the current belief, aquatic herbivores accumulate on average three times as much biomass as do terrestrial herbivores for a given level of primary production and, as a consequence, turn over the ingested carbon only slightly faster than do terrestrial herbivores. Conversely, aquatic detritus consumers generally accumulate a much lower biomass (i.e. over ten times lower) than their terrestrial counterparts for a given level of primary production and, thus, they turn over the ingested carbon much more quickly (i.e. over ten times faster). Because the detrital pathway generally dominates the trophic flow of carbon in both aquatic and terrestrial ecosystems, carbon also tends to flow through the total compartment of first order‐consumers (both herbivores and detritus consumers) at a much faster rate in aquatic than in terrestrial ecosystems. Thus, aquatic systems, because of faster carbon recycling rates through their basal and first‐order levels of the food chain, appear to have a lower capacity than do terrestrial systems for retaining carbon under natural or anthropogenic increases in photosynthetic fixation.     

底栖分解类群对金佛山森林溪流凋落物混合分解的影响

凋落物分解对于维持源头溪流生态系统碳和养分平衡有重要意义。以亚热带典型源头溪流金佛山溪流为代表,选取3种河岸带常见凋落叶为分解对象,设计3个单种和4个混合物种的凋落物组合,在原位放置3种孔径的分解袋（0.05mm、0.25mm和2mm）,探讨混合凋落物的性状与底栖分解类群对叶片质量损失和混合效应的影响。结果表明：（1）微生物在凋落物分解过程中相对贡献均大于50%,小型和大型底栖动物进一步加速了凋落物的分解过程。（2）单种凋落物分解速率存在显著差异：八角枫（Alangium chinense,质量损失率为53.05%）>缺萼枫香（Liquidambar acalycina,30.00%）>薄叶润楠（Machilus leptophylla,12.63%）。（3）混合凋落物中仅微生物参与的处理均表现为负的非加和效应,其中八角枫+缺萼枫香、八角枫+薄叶润楠、八角枫+缺萼枫香+薄叶润楠三个处理的效应显著;小型底栖动物加入后均表现为正的非加和效应,但不显著;在微生物、小型和大型底栖动物的共同作用下,缺萼枫香+薄叶润楠和八角枫+缺萼枫香+薄叶润楠的两个处理的正的非加和效应显著。亚热带源头溪流中凋落物分解功能与河岸植物和分解者类群的复杂性密切相关。     

Global climate change and litter decomposition: more frequent fire slows decomposition and increases the functional importance of invertebrates

Of the many mechanisms by which global climate change may alter ecosystem processes perhaps the least known and insidious is altered disturbance regimes. We used a field-based experiment to examine the climate change scenario of more frequent fires with altered invertebrate assemblages on the decomposition of Eucalyptus leaves. Our design comprised three fire regimes [long-term fire exclusion (FE), long-term frequent burning (FB) and FE altered to FB (FEFB)] and two litter bag mesh sizes (8.0 and 0.2 mm) that either permitted or denied access to the leaf litter by most invertebrates. We found a significant interaction effect between fire regime and mesh size in losses of litter mass and net carbon (C). Compared with the regime of FE, with more frequent burning (FB and FEFB) the pace of decomposition was slowed by 41% (when access to litter by most invertebrates is not impeded). For the regime of FE, denying access to leaf litter by most invertebrates did not alter the pace of decomposition. Conversely, under regimes of frequently burning, restricting access to the litter by most invertebrates altered the pace of decomposition by 46%. Similar results were found for net C. For net losses of nitrogen (N), no interaction effects between fire regime and mesh size were detected, although both main effects were significant. Our results show that by modifying disturbance regimes such as fire frequency, global climate change has the potential to modify the mechanism by which ecosystems function. With more FB, decomposition is driven not only by fire regime induced changes in substrate quality and/or physiochemical conditions but through the interaction of disturbance regime with animal assemblages mediating ecosystem processes.     

Northward range extension of an endemic soil decomposer with a distinct trophic position

Ecological niche theory asserts that invading species become established only if introduced propagules survive stochastic mortality and can exploit resources unconsumed by resident species. Because their transportation is not controlled by plant health or biosecurity regulations, soil macrofauna decomposers, including earthworms are probably introduced frequently into non-native soils. Yet even with climatic change, exotic earthworm species from southern Europe have not been reported to become established in previously glaciated areas of northern Europe that already have trophically differentiated earthworm communities of ‘peregrine’ species. We discovered established populations of the earthworm Prosellodrilus amplisetosus (Lumbricidae), a member of a genus endemic to southern France, in six habitats of an urban farm in Dublin, Ireland, about 1000 km north of the genus''s endemic range. Not only was P. amplisetosus the dominant endogeic (geophagous) earthworm species in two habitats, it also occupied a significantly different trophic position from the resident species, as evinced by stable isotope ratio analysis. The suggested ability of this non-native species to feed on and assimilate isotopically more enriched soil carbon (C) and nitrogen fractions that are inaccessible to resident species portends potential implications of decomposer range expansions for soil functioning including C sequestration.