Trophic‐level dependent effects on CO2 emissions from experimental stream ecosystems

Concern over accelerating rates of species invasions and losses have initiated investigations into how local and global changes to predator abundance mediate trophic cascades that influence CO₂ fluxes of aquatic ecosystems. However, to date, no studies have investigated how species additions or losses at other consumer trophic levels influence the CO₂ flux of aquatic ecosystems. In this study, we added a large predatory stonefly, detritivorous stonefly, or grazer tadpole to experimental stream food webs and over a 70‐day period quantified their effects on community composition, leaf litter decomposition, chlorophyll‐a concentrations, and stream CO₂ emissions. In general, streams where the large grazer or large detritivore were added showed no change in total invertebrate biomass, leaf litter loss, chlorophyll‐a concentrations, or stream CO₂ emissions compared with controls; although we did observe a spike in CO₂ emissions in the large grazer treatment following a substantial reduction in chlorophyll‐a concentrations on day 28. However, the large grazer and large detritivore altered the community composition of streams by reducing the densities of other grazer and detritivore taxa, respectively, compared with controls. Conversely, the addition of the large predator created trophic cascades that reduced total invertebrate biomass and increased primary producer biomass. The cascading effects of the predator additions on the food web ultimately led to decreased CO₂ emissions from stream channels by up to 95%. Our results suggest that stream ecosystem processes were more influenced by changes in large predator abundance than large grazer or detritivore abundance, because of a lack of functionally similar large predators. Our study demonstrates that the presence/absence of species with unique functional roles may have consequences for the exchange of CO₂ between the ecosystem and the atmosphere.     

The ecological drivers and consequences of wildlife trade

Wildlife trade is a key driver of extinction risk, affecting at least 24% of terrestrial vertebrates. The persistent removal of species can have profound impacts on species extinction risk and selection within populations. We draw together the first review of characteristics known to drive species use – identifying species with larger body sizes, greater abundance, increased rarity or certain morphological traits valued by consumers as being particularly prevalent in trade. We then review the ecological implications of this trade-driven selection, revealing direct effects of trade on natural selection and populations for traded species, which includes selection against desirable traits. Additionally, there exists a positive feedback loop between rarity and trade and depleted populations tend to have easy human access points, which can result in species being harvested to extinction and has the potential to alter source–sink dynamics. Wider cascading ecosystem repercussions from trade-induced declines include altered seed dispersal networks, trophic cascades, long-term compositional changes in plant communities, altered forest carbon stocks, and the introduction of harmful invasive species. Because it occurs across multiple scales with diverse drivers, wildlife trade requires multi-faceted conservation actions to maintain biodiversity and ecological function, including regulatory and enforcement approaches, bottom-up and community-based interventions, captive breeding or wildlife farming, and conservation translocations and trophic rewilding. We highlight three emergent research themes at the intersection of trade and community ecology: (1) functional impacts of trade; (2) altered provisioning of ecosystem services; and (3) prevalence of trade-dispersed diseases. Outside of the primary objective that exploitation is sustainable for traded species, we must urgently incorporate consideration of the broader consequences for other species and ecosystem processes when quantifying sustainability.     

Complex indirect effects of epiphytic bromeliads on the invertebrate food webs of their support tree

Ecosystem engineers are species that affect others through the provision of habitat rather than consumptive resources. The extent to which ecosystem engineers can indirectly affect entire food webs, however, is poorly understood. Epiphytic tank bromeliads (Bromeliaceae) are ecosystem engineers that are common throughout the Neotropics, and are associated with a variety of predatory arthropods. Here, we examine if bromeliads, by increasing predator densities, indirectly benefit their support tree through reduction in herbivorous insects and leaf damage. We observed and manipulated bromeliad densities in Costa Rican orange orchards, and measured impacts on leaf damage and arboreal and bromeliad invertebrate communities in two different seasons. Our results show that bromeliads are associated with predatory and herbivorous invertebrates but not leaf damage. Bromeliads were correlated with increased densities of their associated predators, especially ants and hunting spiders, but we could not confirm a causal link. Associations with bromeliads changed over time, with seasonal shifts interfering with responses to our manipulations. Bromeliads had a reduced association with predators in the dry season. Moreover, a null association between bromeliads and herbivorous invertebrates in the dry season unexpectedly became positive in the wet season. In summary, we have only limited evidence that bromeliads indirectly promote the top‐down control of arboreal herbivores; instead, our manipulations suggest that bromeliads increase herbivore densities in the wet season. This research suggests that although bromeliads may act as ecosystem engineers, indirectly influencing the invertebrate food web on support trees, their effects are trophically complex and seasonally dependent. Abstract in Spanish is available with online material.     

Predator effects on herbivore and plant stability

Humans are rapidly altering the diversity and composition of ecological communities by accelerating rates of species extinctions and introductions. These changes in diversity are not random and disproportionately involve the addition or extinction of predators. Theoretical and microcosm studies suggest predator removal may either increase or decrease ecosystem stability. Here we test whether the addition or removal of predators affects aggregate biomass stability in 40 experiments carried out in six different ecosystems. Predators did not alter the temporal variability of autotroph biomass, but significantly destabilized herbivore biomass. The effects of predators on herbivore biomass stability varied significantly among ecosystems, with benthic and pelagic lake systems showing the greatest shifts. Consequently, the addition of predators to communities, as occurs in many conservation efforts, biological control programmes and species introductions, may lead to more variable system dynamics.     

Disturbance,species loss and compensation: Wildfire and grazing effects on the avian community and its food supply in the Serengeti Ecosystem,Tanzania

An important question in biodiversity studies is whether disturbances in ecosystems will cause a net loss of species or whether such losses can be compensated by replacement of other species. We use two natural disturbances, fire and grazing, to examine the response of bird and arthropod communities in grasslands of Serengeti, Tanzania. Both burning and grazing by migrant ungulates take place at the end of the rains in June–July. We documented the communities before disturbance, then 1, 4 and 20 weeks after disturbance on three replicate plots and compared them with three undisturbed plots. Birds were recorded by observation, arthropods from pitfall, tray trap and sweepnet samples. We expected that as the grass biomass was reduced by either disturbance, bird communities would change with concomitant change in arthropod food abundance. Alternatively, bird communities would change not with the absolute amount of food but with the greater accessibility of food as the grass structure changed from long to short grass. Results showed first that both bird species richness and abundance increased after both types of disturbance, but burnt sites showed a greater increase than that for grazed sites. Second, there was a change in bird species composition with disturbance. The functionally equivalent athi short‐toed lark (Calandrella athensis) was replaced by the red‐capped lark (Calandrella cinerea). Third, the abundance of most groups of arthropods was lower on disturbed sites than those on undisturbed sites, and the reduction of arthropod numbers was greatest on burnt sites. These results imply that bird abundance did not occur through an increase in arthropod abundance but rather through a change in the grass structure making food more accessible; and the higher predation could have caused the lower arthropod abundance. In addition, some bird species replaced others thus functionally compensating for their loss.     

短期氮素添加和模拟放牧对青藏高原高寒草甸生态系统呼吸的影响

氮沉降和放牧是影响草地碳循环过程的重要环境因子,但很少有研究探讨这些因子交互作用对生态系统呼吸的影响。在西藏高原高寒草甸地区开展了外源氮素添加与刈割模拟放牧实验,测定了其对植物生物量分配、土壤微生物碳氮和生态系统呼吸的影响。结果表明:氮素添加显著促进生态系统呼吸,而模拟放牧对其无显著影响,且降低了氮素添加的刺激作用。氮素添加通过提高微生物氮含量和土壤微生物代谢活性,促进植物地上生产,从而增加生态系统的碳排放;而模拟放牧降低了微生物碳含量,且降低了氮素添加的作用,促进根系的补偿性生长,降低了氮素添加对生态系统碳排放的刺激作用。这表明,放牧压力的存在会抑制氮沉降对高寒草甸生态系统碳排放的促进作用,同时外源氮输入也会缓解放牧压力对高寒草甸生态系统生产的负面影响。     

Research progress on the effects of grazing on grassland ecosystem

全球草地占据30%左右的陆地面积, 在全球气候变化、碳氮及养分循环、保持水土、调节畜牧业生产等方面具有重要的作用。目前草地的主要利用方式之一就是放牧, 不同的牲畜种类、放牧强度、年限、历史和制度等, 会影响草地植物群落、生物多样性及土壤微生物, 进而影响草地生态系统结构、功能和过程。该文围绕放牧对草地生态系统结构、功能和过程的影响, 1)回顾了20世纪50年代到现在各个历史阶段放牧对草地生态系统影响的研究; 2)利用文献计量分析的方法, 剖析了放牧对草地影响研究的热点内容、重要区域和关键词等; 3)阐明了放牧对草地植物生长、群落特征、碳氮及养分循环、生产力及土壤质量等的各方面影响的研究进展及国内相关研究的优势及存在的主要问题和不足; 4)基于上述分析, 从草地放牧精准管理、经典假说验证、放牧和全球变化研究相结合等方面, 提出未来研究的前沿方向和优先领域。该文在系统总结放牧对草地生态系统影响的研究进展、研究优势及存在问题的基础上, 提出未来的研究应与全球变化相结合, 为我国的草地放牧生态学研究、适应性管理和可持续利用等提供科学基础。     

放牧对草地生态系统影响的研究进展

全球草地占据30%左右的陆地面积, 在全球气候变化、碳氮及养分循环、保持水土、调节畜牧业生产等方面具有重要的作用。目前草地的主要利用方式之一就是放牧, 不同的牲畜种类、放牧强度、年限、历史和制度等, 会影响草地植物群落、生物多样性及土壤微生物, 进而影响草地生态系统结构、功能和过程。该文围绕放牧对草地生态系统结构、功能和过程的影响, 1)回顾了20世纪50年代到现在各个历史阶段放牧对草地生态系统影响的研究; 2)利用文献计量分析的方法, 剖析了放牧对草地影响研究的热点内容、重要区域和关键词等; 3)阐明了放牧对草地植物生长、群落特征、碳氮及养分循环、生产力及土壤质量等的各方面影响的研究进展及国内相关研究的优势及存在的主要问题和不足; 4)基于上述分析, 从草地放牧精准管理、经典假说验证、放牧和全球变化研究相结合等方面, 提出未来研究的前沿方向和优先领域。该文在系统总结放牧对草地生态系统影响的研究进展、研究优势及存在问题的基础上, 提出未来的研究应与全球变化相结合, 为我国的草地放牧生态学研究、适应性管理和可持续利用等提供科学基础。     

短期围封对西藏北部高寒草甸土壤线虫群落的影响

为了解放牧对高寒草甸的影响,以及围封对高寒草甸的恢复作用,2013年5月、8月、10月,分别对藏北短期围封高寒草甸和自由放牧高寒草甸不同深度土层的土壤线虫群落及土壤理化性质等进行了调查。调查结果表明,3a围封提高了土壤全氮、磷、钾及土壤有机质含量,土壤保水能力也明显提高;围封使土壤线虫个体密度下降,但物种多样性和丰富度却提高了;围封使土壤线虫群落中的植食性线虫所占比重呈下降趋势,食真菌线虫比重加大。短期围封有利于高寒草甸生态系统的正向演替,使之向更稳定的方向发展,同时,土壤线虫数量对土壤理化性质变化有明显的响应,通过土壤线虫群落的群落特征可以反映放牧干扰及围封对高寒草甸生态系统的影响。     

Long-term effects of sheep grazing on giant hogweed (Heracleum mantegazzianum)

A meadow dominated by a mature, flowering stand of Giant Hogweed (Heracleum mantegazzianum) was grazed by sheep in the years 1987–1993. After two years Giant Hogweed cover was much reduced and a typical meadow vegetation was established. By 1993 Giant Hogweed was completely eliminated though total species diversity was much reduced. Soil sampled from the grazed area developed no Giant Hogweed seedlings in a germination test and contained no viable seeds of the species. In contrast soil from adjacent stands produced numerous seedlings with a peak emergence from samples taken after the winter. Seeds collected in October showed a viability in tetrazolium test of 88%. Germination averaged 22% after storage at room temperature and 25% following three weeks treatment at –18°C. It was concluded that the persistence of Giant Hogweed seeds in Danish meadow soils is less than seven years.     

Constrating short-term and long-term effects of predation risk on consumer habitat use and resources

I contrasted the short-term and long-term effects of predationrisk on snail habitat use and resource dynamics. Pulmonate snails(Physella gyrina) were placed into experimental pools and exposedto four levels of predation risk while holding their densityconstant. Periphyton resources were made available in two habitats:open and covered. I hypothesized that a behavioral responseby snails to predation risk would influence periphyton standingcrop in the open and covered habitats. Snails responded to increasingpredation risk by moving into safer (covered) habitats, andthe magnitude of their response was sensitive to the actuallevel of risk: intermediate levels of risk resulted in intermediatehabitat use. However, use of the risky (open) habitat by snailswas time dependent Snails initially responded strongly to predationrisk, but they exhibited similar patterns of habitat use atall risk levels by the end of the experiment Periphyton standingcrop was positively related to predation risk. In contrast tosnail habitat use, this response was initially weak and becamestronger as the experiment progressed. Thus, the short-and long-termresponses of snail habitat use and periphyton standing cropcontrasted sharply. I suggest that the changing patterns ofsnail habitat use over time are consistent with the idea thatsnails balance predation risk against foraging gains when selectinghabitats and that the manner in which they balance foraginggains and predation risk determines the pattern of periphytonstanding crops across habitats     

土壤动物多样性及其生态功能

土壤无脊椎动物生物量通常小于土壤生物总生物量的10%,但它们种类丰富,取食行为及生活史策略多种多样,且土壤动物之间,土壤动物与微生物之间存在着复杂的相互作用关系。土壤动物的生态功能主要通过取食作用(trophic effect)和非取食作用(non-trophic effect)来实现。原生动物数量大、周转快,故原生动物本身的代谢活动(即取食作用)对碳氮矿化的贡献可以接近甚至超过细菌的贡献;然而大多数中小型土壤动物的本身代谢过程对碳氮矿化的贡献远低于土壤微生物,但它们可以通过取食作用来调节微生物进而影响碳氮的矿化。大型节肢动物中的蜘蛛和地表甲虫等捕食者经常活跃于地表,它们常常会通过级联效应对土壤生态系统产生重要的影响。蚯蚓、白蚁等大型土壤动物除可以通过取食作用以外,还可以通过非取食作用调控土壤微生物,进而显著影响土壤碳氮过程。土壤动物取食行为的多样性和复杂的非营养关系的存在造就了多维度的土壤食物网,给土壤动物的生态功能研究带来了巨大的挑战。介绍了土壤动物的多样性及主要的生态功能,并对研究的热点和前沿问题进行了探讨,以期引起关于土壤动物多样性及其生态功能的深入思考。     

Niche differentiation among invasive crayfish and their impacts on ecosystem structure and functioning

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Comparative analysis of European wide marine ecosystem shifts: a large-scale approach for developing the basis for ecosystem-based management

Abrupt and rapid ecosystem shifts (where major reorganizations of food-web and community structures occur), commonly termed regime shifts, are changes between contrasting and persisting states of ecosystem structure and function. These shifts have been increasingly reported for exploited marine ecosystems around the world from the North Pacific to the North Atlantic. Understanding the drivers and mechanisms leading to marine ecosystem shifts is crucial in developing adaptive management strategies to achieve sustainable exploitation of marine ecosystems. An international workshop on a comparative approach to analysing these marine ecosystem shifts was held at Hamburg University, Institute for Hydrobiology and Fisheries Science, Germany on 1-3 November 2010. Twenty-seven scientists from 14 countries attended the meeting, representing specialists from seven marine regions, including the Baltic Sea, the North Sea, the Barents Sea, the Black Sea, the Mediterranean Sea, the Bay of Biscay and the Scotian Shelf off the Canadian East coast. The goal of the workshop was to conduct the first large-scale comparison of marine ecosystem regime shifts across multiple regional areas, in order to support the development of ecosystem-based management strategies.     

Impact of grazing on germination trait selection in an alpine grassland on the Tibet Plateau

放牧对青藏高原高寒草地种子萌发性状选择的影响 以前的研究表明放牧能够引起草地生物与非生物环境的显著变化,但这种变化影响草地群落对种子萌发特征的选择机制尚不清楚。因此,我们旨在回答：放牧是否对草地群落中萌发特征的组成和多样性产生显著影响。我们在实验室检测了研究草地群落内主要物种种子的萌发特性,并比较了这些植物在放牧和非放牧草地上的表现。在此基础上,比较了放牧草地和非放牧草地的各萌发性状的群落加权平均值和萌发性状多样性,从而了解放牧草地和非放牧草地是否存在不同的萌发性状结构。研究结果表明,在物种水平上,放牧和非放牧草地各物种的多度变化与物种的萌发性状无显著关系。但在群落水平上,与非放牧草地相比,放牧草地的物种普遍具有较高的种子萌发率;放牧草地种子萌发对变温的正响应显著大于非放牧草地,而且放牧草地种子萌发温度生态位宽度小于非放牧草地。与非放牧草地相比,放牧草地种子萌发性状多样性增加,萌发性状均匀度降低。放牧可以改变微生境,从而通过环境过滤改变草地群落对萌发性状的选择,导致草地群落的萌发性状于放牧前后在群落水平上存 在显著差异。