Direct and indirect food web regulation of microbial decomposers in headwater streams

The direct and indirect regulation of primary productivity has been well established in autotrophic‐based ecosystems; however, less is known about the processes affecting decomposers in detrital‐based ecosystems. Because, small headwater, woodland streams are a dominate feature in most ecosystems and are tightly linked to terrestrial detritus, understanding decomposer‐mediated functions in these systems is critical for understanding carbon processes across the landscape. In this light, we conducted a microcosm and mesocosm experiment to test the direct and indirect food web effects on decomposers in small stream ecosystems. The results from the microcosm experiment supported an existing literature, demonstrating that nutrients directly stimulate decomposers and that microbivores directly reduce decomposers. Based on well‐founded food web theory in autotrophic systems, we predicted that fishes from different trophic‐functional guilds would indirectly stimulate decomposers by enhancing dissolved nutrients and by reducing microbivore densities. Our mesocosm experiment partially supported these predictions. Specifically, we found that fishes that consumed mostly terrestrial foods increased decomposers from the bottom–up by enhancing allochthonous nutrient loading into the stream ecosystems. Contrary to our predictions, however, predatory fishes that consume microbivores did not increase decomposers from the top–down. Rather, in streams with the predatory fish species, microbivores increased (rather than decreased) on leaf litter. This may have resulted from an experimental artifact associated with refuge provided by leaf packs. In conclusion, our data demonstrate that decomposers are regulated by similar direct and indirect processes important in autotrophic‐based ecosystems. This provides further evidence that food web processes can regulate leaf decomposition and flux of detrital carbon through ecosystems.     

Detritus and nutrient dynamics in the shore zone of lakes: a review

The importance of detritus varies greatly among shore zones of lakes, but in a large majority of these regions detrital pathways prevail. Aside from a great spatial and seasonal variability, macrophytes and bottom sediments appear to be dominant stores of nutrients in these habitats. Macrophytes hold a central position in nutrient cycling in the shore-littoral lake zones. They are the main source of autochthonous detritus as they prevail in the total biomass of littoral organisms, and they are only rarely available as direct food of consumers. Various processes and interactions determine the role of macrophytes in nutrients dynamics. These are: the intensity of nutrient uptake and translocation, release of nutrients by healthy plants and from decomposing plants, exchange of elements between macrophytes and their periphyton, as well as interception of seston by macrophyte stands. Particular plant species differ in their time of dying and susceptibility to decomposition. The changes in decomposing material (size structure of particles and nutrient content) mean that detritus in various stages of decomposition differs in its role in trophic dynamics of shore-littoral lake zones. Several types of shore regions as regards detritus sources and retention level are discussed.     

Meiofauna promotes litter decomposition in stream ecosystems depending on leaf species

Litter decomposition, a fundamental process of nutrient cycling and energy flow in freshwater ecosystems, is driven by a diverse array of decomposers. As an important component of the heterotrophic food web, meiofauna can provide a trophic link between leaf‐associated microbes (i.e., bacteria and fungi)/plant detritus and macroinvertebrates, though their contribution to litter decomposition is not well understood. To investigate the role of different decomposer communities in litter decomposition, especially meiofauna, we compared the litter decomposition of three leaf species with different lignin to nitrogen ratios in litter bags with different mesh sizes (0.05, 0.25, and 2 mm) in a forested stream, in China for 78 days. The meiofauna significantly enhanced the decomposition of leaves of high‐and medium‐ quality, while decreasing (negative effect) or increasing (positive effect) the fungal biomass and diversity. Macrofauna and meiofauna together contributed to the decomposition of low‐quality leaf species. The presence of meiofauna and macrofauna triggered different aspects of the microbial community, with their effects on litter decomposition varying as a function of leaf quality. This study reveals that the meiofauna increased the trophic complexity and modulated their interactions with microbes, highlighting the important yet underestimated role of meiofauna in detritus‐based ecosystems.     

Spatial stoichiometry: cross‐ecosystem material flows and their impact on recipient ecosystems and organisms

Cross‐ecosystem material flows, in the form of inorganic nutrients, detritus and organisms, spatially connect ecosystems and impact food web dynamics. To date research on material flows has focused on the impact of the quantity of these flows and largely ignored their elemental composition, or quality. However, the ratios of elements like carbon, nitrogen and phosphorus can influence the impact material flows have on food web interactions through stoichiometric mismatches between resources and consumers. The type and movement of materials likely vary in their ability to change stoichiometric constraints within the recipient ecosystem and materials may undergo changes in their own stoichiometry during transport. In this literature review we evaluate the importance of cross‐ecosystem material flows within the framework of ecological stoichiometry. We explore how movement in space and time impacts the stoichiometry of material flow, as these transformations are essential to consider when assessing the ability of these flows to impact food web productivity and ecosystem functioning. Our review suggests that stoichiometry of cross‐ecosystem material flows are highly dynamic and undergo changes during transport across the landscape or from human influence. These material flows can impact recipient organisms if they change stoichiometry of the abiotic medium, or provide resources that have a different stoichiometry to in situ resources. They might also alter consumer excretion rates, in turn altering the availability of nutrients in the recipient ecosystem. These alterations in stoichiometric constraints of recipient organisms can have cascading trophic effects and shape food web dynamics. We highlight significant gaps in the literature and suggest new avenues for research that explore how cross‐ecosystem material flows impact recipient ecosystems when considering differences in stoichiometric quality, their movement through the landscape and across ecosystem boundaries, and the nutritional constraints of the recipient organisms.     

Species richness and trophic diversity increase decomposition in a co-evolved food web

Ecological communities show great variation in species richness, composition and food web structure across similar and diverse ecosystems. Knowledge of how this biodiversity relates to ecosystem functioning is important for understanding the maintenance of diversity and the potential effects of species losses and gains on ecosystems. While research often focuses on how variation in species richness influences ecosystem processes, assessing species richness in a food web context can provide further insight into the relationship between diversity and ecosystem functioning and elucidate potential mechanisms underpinning this relationship. Here, we assessed how species richness and trophic diversity affect decomposition rates in a complete aquatic food web: the five trophic level web that occurs within water-filled leaves of the northern pitcher plant, Sarracenia purpurea. We identified a trophic cascade in which top-predators--larvae of the pitcher-plant mosquito--indirectly increased bacterial decomposition by preying on bactivorous protozoa. Our data also revealed a facultative relationship in which larvae of the pitcher-plant midge increased bacterial decomposition by shredding detritus. These important interactions occur only in food webs with high trophic diversity, which in turn only occur in food webs with high species richness. We show that species richness and trophic diversity underlie strong linkages between food web structure and dynamics that influence ecosystem functioning. The importance of trophic diversity and species interactions in determining how biodiversity relates to ecosystem functioning suggests that simply focusing on species richness does not give a complete picture as to how ecosystems may change with the loss or gain of species.     

陆地生态系统混合凋落物分解研究进展

凋落物分解在陆地生态系统养分循环与能量流动中具有重要作用,是碳、氮及其他重要矿质养分在生态系统生命组分间循环与平衡的核心生态过程。自然生态系统中,植物群落大多具有较高的物种丰富度和多样性,其混合凋落物在分解过程中也更有可能发生养分传递、化学抑制等种间互作,形成多样化的分解生境,多样性较高的分解者类群以及复杂的级联效应分解,这些因素和过程均对研究混合凋落物分解过程、揭示其内在机制形成了极大的挑战。从构成混合凋落物物种丰富度和多样性对分解生境、分解者多样性及其营养级联效应的影响等方面,综合阐述混合凋落物对陆地生态系统凋落物分解的影响,探讨生物多样性在凋落物分解中的作用。通过综述近些年的研究发现,有超过60%的混合凋落物对其分解速率的影响存在正向或负向的效应。养分含量有差异的凋落物混合分解过程中,分解者优先利用高质量凋落物,使低质量的凋落物反而具有了较高的养分有效性,引起低质量凋落物分解加快并最终使混合凋落物整体分解速率加快;而凋落物物种丰富度对土壤动物群落总多度有轻微的影响或几乎没有影响,但是对线虫和大型土壤动物的群落组成和多样性有显著影响,并随着分解阶段呈现一定动态变化;混合凋落物改变土壤微生物生存的理化环境,为微生物提供更多丰富的分解底物和养分,优化微生物种群数量和群落结构及其分泌酶的活性,并进一步促进了混合凋落物的分解。这些基于植物-土壤-分解者系统的动态分解过程的研究,表明混合凋落物分解作用不只是经由凋落物自身质量的改变,更会通过逐级影响分解者多样性水平而进一步改变分解速率和养分释放动态,说明生物多样性确实在一定程度上调控凋落物分解及其养分释放过程。     

Linking the green and brown worlds through nonconsumptive predator effects

Predators can indirectly affect lower trophic levels by either consuming their prey (consumptive effect, CE) or by changing the physiology or behavior of their prey (nonconsumptive effect, NCE). Cascading effects of predators on primary producers are common, and can be propagated by CEs, NCEs, or a combination of both mechanisms. Predator impacts in detrital food webs (the ‘brown world’) have received considerably less attention than their effects on systems with primary producers at the base (the ‘green world’), and only recently have we begun to appreciate the importance of above‐ground predators indirectly impacting below‐ground processes. Numerous studies reveal the total impact (CEs and NCEs) of predators in brown food webs, but our understanding of the role of isolated NCEs is limited. Many habitats and major taxa have not been studied, and patterns are difficult to distinguish due to frequent reporting of mixed effects. Predators play an important role as connectors between brown and green worlds when they feed from both food webs (multichannel feeding). We are only beginning to understand how NCEs influence detrital food webs, and it is unknown whether multichannel fear is an essential component of predator–prey ecology that regulates ecosystem function. Synthesis Predators have been shown to impact ecosystems through both consumptive and nonconsumptive effects on their prey Historically, herbivory‐based ‘green’ systems have been the venue for documenting these predator effects, while detritus‐based ‘brown’ systems received considerably less attention. However, similar mechanisms exist in green and brown worlds, suggesting strong parallels. We review and synthesize predator effects in detrital systems, highlighting important shortcomings in current understanding. Furthermore, we build upon the idea of multichannel feeding (i.e. consumption of prey from both green and brown food webs) to propose the existence of ‘multichannel fear’. We provide a framework for documenting multichannel fear to facilitate continued exploration of how predators link seemingly disparate systems.     

全球气候变暖对凋落物分解的影响

凋落物分解作为生态系统核心过程,参与生态系统碳的周转与循环,影响生态系统碳的收支平衡,调控生态系统对全球气候变暖的反馈结果。全球气候变暖通过环境因素、凋落物数量和质量以及分解者3个方面,直接或间接地作用于凋落物分解过程,并进一步影响土壤养分周转和碳库动态。气候变暖可通过升高温度和改变实际蒸散量等环境因素直接作用于凋落物分解。气候变暖可引起植物物种短期内碳、氮和木质素等化学性质的改变以及群落中物种组成的长期变化从而改变凋落物质量。在凋落物分解过程中,土壤分解者亚系统作为主要生命组分(土壤动物和微生物)彼此相互作用、相互协调共同参与调节凋落物的分解过程。凋落物分解可以通过改变土壤微生物量、微生物活动和群落结构来加快微生物养分的固定或矿化,以形成新的养分利用模式来改变土壤有机质从而对气候变化做出响应。未来凋落物分解的研究方向应基于大尺度跨区域分解实验和长期实验,关注多个因子交互影响下,分解过程中碳、氮养分释放、地上/地下凋落物分解生物学过程与联系、分解者亚系统营养级联效应等方面。     

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.     

Warming,eutrophication, and predator loss amplify subsidies between aquatic and terrestrial ecosystems

The exchange of organisms and energy among ecosystems has major impacts on food web structure and dynamics, yet little is known about how climate warming combines with other pervasive anthropogenic perturbations to affect such exchanges. We used an outdoor freshwater mesocosm experiment to investigate the interactive effects of warming, eutrophication, and changes in top predators on the flux of biomass between aquatic and terrestrial ecosystems. We demonstrated that predatory fish decoupled aquatic and terrestrial ecosystems by reducing the emergence of aquatic organisms and suppressing the decomposition of terrestrial plant detritus. In contrast, warming and nutrients enhanced cross‐ecosystem exchanges by increasing emergence and decomposition, and these effects were strongest in the absence of predators. Furthermore, we found that warming advanced while predators delayed the phenology of insect emergence. Our results demonstrate that anthropogenic perturbations may extend well beyond ecosystem boundaries by influencing cross‐ecosystem subsidies. We find that these changes are sufficient to substantially impact recipient communities and potentially alter the carbon balance between aquatic and terrestrial ecosystems and the atmosphere.     

Detritus, trophic dynamics and biodiversity

Traditional approaches to the study of food webs emphasize the transfer of local primary productivity in the form of living plant organic matter across trophic levels. However, dead organic matter, or detritus, a common feature of most ecosystems plays a frequently overlooked role as a dynamic heterogeneous resource and habitat for many species. We develop an integrative framework for understanding the impact of detritus that emphasizes the ontogeny and heterogeneity of detritus and the various ways that explicit inclusion of detrital dynamics alters generalizations about the structure and functioning of food webs. Through its influences on food web composition and dynamics, detritus often increases system stability and persistence, having substantial effects on trophic structure and biodiversity. Inclusion of detrital heterogeneity in models of food web dynamics is an essential new direction for ecological research.     

Brown ground: a soil carbon analogue for the green world hypothesis?

For many decades, ecologists have asked what prevents herbivores from consuming most of the plant biomass in terrestrial ecosystems, or "Why is the world green?" Here I ask the analogous question for detritivores: what prevents them from degrading most of the organic material in soils, or "Why is the ground brown?" For fresh plant detritus, constraints on decomposition closely parallel constraints on herbivory: both herbivore and decomposer populations may be controlled by plant tissue chemistry from the bottom up and predators from the top down. However, the majority of soil carbon is not plant litter but carbon that has been consumed by detritivores and reprocessed into humic compounds with complex and random chemical structures. This carbon persists mainly because the chemical properties of humic compounds and interactions with soil minerals constrain decomposition by extracellular enzymes in soil. Other constraints on decomposers, such as nutrient limitation of enzyme production and competition with opportunistic microbes, also contribute to brown ground. Ultimately, the oldest soil carbon persists via transformation into complex molecules that are impervious to enzymatic attack and effectively decoupled from processing by the soil food web.     

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.     

Stoichiometry and food-chain dynamics

Traditional models of chemostat systems looking at interactions between predator, prey and nutrients have used only a single currency, such as energy or nitrogen. In reality, growth of autotrophs and heterotrophs may be limited by various elements, e.g. carbon, nitrogen, phosphorous or iron. In this study we develop a dynamic energy budget model chemostat which has both carbon and nitrogen as currencies, and examine how the dual availability of these elements affects the growth of phytoplankton, trophic transfer to zooplankton, and the resulting stability of the chemostat ecosystem. Both species have two reserve pools to obtain a larger metabolic flexibility with respect to changing external environments. Mineral nitrogen and carbon form the base of the food chain, and they are supplied at a constant rate. In addition, the biota in the chemostat recycle nutrients by means of respiration and excretion, and organic detritus is recycled at a fixed rate. We use numerical bifurcation analysis to assess the model's dynamic behavior. In the model, phytoplankton is nitrogen limited, and nitrogen enrichment can lead to oscillations and multiple stable states. Moreover, we found that recycling has a destabilizing effect on the food chain due to the increased repletion of mineral nutrients. We found that both carbon and nitrogen enrichment stimulate zooplankton growth. Therefore, we conclude that the concept of single-element limitation may not be applicable in an ecosystem context.     

Does the strength of cross‐ecosystem trophic cascades vary with ecosystem size? A test using a natural microcosm

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Modelling food webs and nutrient cycling in agro-ecosystems

Agricultural practices affect the spatial patterns and dynamics of the decomposition of soil organic matter and the availability of plant-limiting nutrients. The biological processes underlying these patterns and dynamics are the trophic interactions among the organisms in the soil community food web. Food web models simulate nutrient flow rates close to observed rates and clarify the role of the various groups of organisms in the cycling of nutrients. Several large interdisciplinary programs are currently focusing on these interactions, with a view to developing and managing sustainable forms of agriculture.     

Towards a mechanistic understanding of temperature and enrichment effects on species interaction strength,omnivory and food‐web structure

Revealing the links between species functional traits, interaction strength and food‐web structure is of paramount importance for understanding and predicting the relationships between food‐web diversity and stability in a rapidly changing world. However, little is known about the interactive effects of environmental perturbations on individual species, trophic interactions and ecosystem functioning. Here, we combined modelling and laboratory experiments to investigate the effects of warming and enrichment on a terrestrial tritrophic system. We found that the food‐web structure is highly variable and switches between exploitative competition and omnivory depending on the effects of temperature and enrichment on foraging behaviour and species interaction strength. Our model contributes to identifying the mechanisms that explain how environmental effects cascade through the food web and influence its topology. We conclude that considering environmental factors and flexible food‐web structure is crucial to improve our ability to predict the impacts of global changes on ecosystem diversity and stability.     

Carrion cycling in food webs: comparisons among terrestrial and marine ecosystems

In light of current global changes to ecosystem function (e.g. climate change, trophic downgrading, and invasive species), there has been a recent surge of interest in exploring differences in nutrient cycling among ecosystem types. In particular, a growing awareness has emerged concerning the importance of scavenging in food web dynamics, although no studies have focused specifically on exploring differences in carrion consumption between aquatic and terrestrial ecosystems. In this forum we synthesize the scavenging literature to elucidate differences in scavenging dynamics between terrestrial and marine ecosystems, and identify areas where future research is needed to more clearly understand the role of carrion consumption in maintaining ecosystem function within each of these environments. Although scavenging plays a similar functional role in terrestrial and aquatic food webs, here we suggest that several fundamental differences exist in scavenging dynamics among these ecosystem types due to the unique selection pressures imposed by the physical properties of water and air. In particular, the movement of carcasses in marine ecosystems (e.g. wave action, upwelling, and sinking) diffuses biological activity associated with scavenging and decomposition across large, three‐dimensional spatial scales, creating a unique spatial disconnect between the processes of production, scavenging, and decomposition, which in contrast are tightly linked in terrestrial ecosystems. Moreover, the limited role of bacteria and temporal stability of environmental conditions on the sea floor appears to have facilitated the evolution of a much more diverse community of macrofauna that relies on carrion for a higher portion of its nutrient consumption than is present in terrestrial ecosystems. Our observations are further discussed as they pertain to the potential impacts of climate change and trophic downgrading (i.e. removal of apex consumers from ecosystems) on scavenging dynamics within marine and terrestrial ecosystems.     

Inter‐annual changes in detritus‐based food chains can enhance plant growth response to elevated atmospheric CO2

Elevated atmospheric CO₂ generally enhances plant growth, but the magnitude of the effects depend, in part, on nutrient availability and plant photosynthetic pathway. Due to their pivotal role in nutrient cycling, changes in abundance of detritivores could influence the effects of elevated atmospheric CO₂ on essential ecosystem processes, such as decomposition and primary production. We conducted a field survey and a microcosm experiment to test the influence of changes in detritus‐based food chains on litter mass loss and plant growth response to elevated atmospheric CO₂ using two wetland plants: a C₃ sedge (Scirpus olneyi) and a C₄ grass (Spartina patens). Our field study revealed that organism's sensitivity to climate increased with trophic level resulting in strong inter‐annual variation in detritus‐based food chain length. Our microcosm experiment demonstrated that increased detritivore abundance could not only enhance decomposition rates, but also enhance plant growth of S. olneyi in elevated atmospheric CO₂ conditions. In contrast, we found no evidence that changes in the detritus‐based food chains influenced the growth of S. patens. Considered together, these results emphasize the importance of approaches that unite traditionally subdivided food web compartments and plant physiological processes to understand inter‐annual variation in plant production response to elevated atmospheric CO_2.     

All wet or dried up? Real differences between aquatic and terrestrial food webs

Ecologists have greatly advanced our understanding of the processes that regulate trophic structure and dynamics in ecosystems. However, the causes of systematic variation among ecosystems remain controversial and poorly elucidated. Contrasts between aquatic and terrestrial ecosystems in particular have inspired much speculation, but only recent empirical quantification. Here, we review evidence for systematic differences in energy flow and biomass partitioning between producers and herbivores, detritus and decomposers, and higher trophic levels. The magnitudes of different trophic pathways vary considerably, with less herbivory, more decomposers and more detrital accumulation on land. Aquatic-terrestrial differences are consistent across the global range of primary productivity, indicating that structural contrasts between the two systems are preserved despite large variation in energy input. We argue that variable selective forces drive differences in plant allocation patterns in aquatic and terrestrial environments that propagate upward to shape food webs. The small size and lack of structural tissues in phytoplankton mean that aquatic primary producers achieve faster growth rates and are more nutritious to heterotrophs than their terrestrial counterparts. Plankton food webs are also strongly size-structured, while size and trophic position are less strongly correlated in most terrestrial (and many benthic) habitats. The available data indicate that contrasts between aquatic and terrestrial food webs are driven primarily by the growth rate, size and nutritional quality of autotrophs. Differences in food-web architecture (food chain length, the prevalence of omnivory, specialization or anti-predator defences) may arise as a consequence of systematic variation in the character of the producer community.