Effects of fish in river food webs

Experimental manipulations of fish in a Northern California river during summer base flow reveal that they have large effects on predators, herbivores, and plants in river food webs. California roach and juvenile steelhead consume predatory insects and fish fry, which feed on algivorous chironomid larvae. In the presence of fish, filamentous green algae are reduced to low, prostrate webs, infested with chironomids. When the absence of large fish releases smaller predators that suppress chironomids, algal biomass is higher, and tall upright algal turfs become covered with diatoms and cyanobacteria. These manipulations provide evidence that the Hairston, Smith, Slobodkin-Fretwell theory of trophic control, which predicts that plants will be alternately limited by resources or herbivores in food webs with odd and even numbers of trophic levels, has application to river communitics.     

Fishing down marine food webs

The mean trophic level of the species groups reported in Food and Agricultural Organization global fisheries statistics declined from 1950 to 1994. This reflects a gradual transition in landings from long-lived, high trophic level, piscivorous bottom fish toward short-lived, low trophic level invertebrates and planktivorous pelagic fish. This effect, also found to be occurring in inland fisheries, is most pronounced in the Northern Hemisphere. Fishing down food webs (that is, at lower trophic levels) leads at first to increasing catches, then to a phase transition associated with stagnating or declining catches. These results indicate that present exploitation patterns are unsustainable.     

Scale and structure in natural food webs

The degree to which widely accepted generalizations about food web structure apply to natural communities was determined through examination of 50 pelagic webs sampled consistently with even taxonomic resolution of all trophic levels. The fraction of species in various trophic categories showed no significant overall trends as the number of species varied from 10 to 74. In contrast, the number of links per species increased fourfold over the range of species number, suggesting that the link-species scaling law, defined on the basis of aggregated webs, does not reflect a real ecological trend.     

Tracing food webs with stable hydrogen isotopes

The hydrogen isotopic content of an animal's food, not water, determines that animal's hydrogen isotopic content. Liver and muscle tissue from mice reared on a diet such that the ratio of deuterium to hydrogen (DIH) of their food and water was kept constant, have the same average D/H ratio as the food source. In a simple, natural population of snails and their possible algal diets, Littorina obtusata (northern Atlantic intertidal snails that feed almost exclusively on the brown alga Fucus vesiculosus) has the same D/H ratio as Fucus vesiculosis and not that of the other algae available to the snails.     

The movement of aquatic mercury through terrestrial food webs

Mercury has contaminated rivers worldwide, with health consequences for aquatic organisms and humans who consume them. Researchers have focused on aquatic birds as sentinels for mercury. However, trophic transfer between adjacent ecosystems could lead to the export of aquatic mercury to terrestrial habitats. Along a mercury-contaminated river in Virginia, United States, terrestrial birds had significantly elevated levels of mercury in their blood, similar to their aquatic-feeding counterparts. Diet analysis revealed that spiders delivered much of the dietary mercury. We conclude that aquatic mercury pollution can move into terrestrial habitats, where it biomagnifies to levels in songbirds that may cause adverse effects. Rivers contaminated with mercury may pose a threat to the many bird species that feed on predatory invertebrates in adjacent riparian habitats.     

Engagement for transformation: Value webs for local food system development

Engagement happens when academics and non-academics form partnerships to create mutual understanding, and then take action together. An example is the “value web” work associated with W. K. Kellogg Foundation’s Food Systems Higher Education–Community Partnership. Partners nationally work on local food systems development by building value webs. “Value chains,” a concept with considerable currency in the private sector, involves creating non-hierarchical relationships among otherwise disparate actors and entities to achieve collective common goals. The value web concept is extended herein by separating the values of the web itself, such as the value of collaboration, from values “in” the web, such as credence values associated with a product or service. By sharing and discussing case examples of work underway around the United States, the authors make a case for employing the value webs concept to represent a strategy for local food systems development, specifically, and for higher education–community partnerships, generally.

Multiple stable isotopes used to trace the flow of organic matter in estuarine food webs

The use of a combination of the stable isotopes of sulfur, carbon, and nitrogen allows the flow of organic matter and trophic relations in salt marshes and estuaries to be traced while eliminating many ambiguities that accompany the use of a single isotopic tracer. Salt-marsh grasses take up the isotopically light sulfides formed during sulfate reduction, and the transfer of this light sulfur through the marsh food web is illustrated with data on the ribbed mussel (Geukensia demissa) from various locations in a New England marsh. The multiple isotope approach shows that this filter feeder consumes both marsh grass ( Spartina) detritus and plankton, with the relative proportions of each determined by the location of the mussels in the marsh.     

Carbon-13 and carbon-14 abundances in alaskan aquatic organisms: delayed production from peat in arctic food webs

Inputs of terrestrial peat carbon to the nearshore Alaskan Beaufort Sea from erosion and fluvial transport are of the same magnitude as in situ primary production within 10 kilometers of shore. Nevertheless, carbon-13/carbon-12 ratios and carbon-14 abundances in marine organisms show that only small amounts of the terrestrial carbon are transferred beyond the microbial level. Freshwater organisms, however, are heavily dependent on peat, as shown by pronounced seasonal radiocarbon depressions in resident fish and ducks. Tundra ponds and lakes are areas where accumulated terrestrial peat carbon is apparently transferred to aquatic insect larvae and passed on to higher organisms. The lack of functionally analogous abundant marine prey organisms may explain why peat carbon is not efficiently transferred to apical food web species in the marine environment.     

Resilience in the US red meat industry: the roles of food safety policy

We use the case of red meat food safety to illustrate the need to problematize policy. Overtime, there have been numerous red meat scandals and scares. We show that the statutes and regulations that arose out of these events provided the industry with a means of demonstrating safety, facilitating large-scale trade, legitimizing conventional production, and limiting interference into its practices. They also created systemic fragility, as evidenced by many recent events, and hindered the development of an alternative, small-scale sector. Thus, the accumulated rules help to structure the sector, create superficial resilience, and are used in place of an actual policy governing safety. We call for rigorous attention to not only food safety, but also the role and effect of agrifood statutes and regulations in general, and engagement in policy more broadly.

扎龙保护区丹顶鹤的食物量与其种群数量的季节性波动

通过观察和样方取样法,确立了扎龙保护区内,丹顶鹤的主要植物性食物为芦苇根及嫩芽、碱蓬种子及其嫩芽、苔草的根茎和坚果.水生动物性资源主要有鲫鱼、泥鳅、麦穗鱼、葛氏鲈塘鳢(俗称老头鱼)等鱼类,中国林蛙、黑斜线蛙、花嘴蟾蜍等两栖类.底栖动物主要为昆虫的幼虫和螺类.应用ERDAS软件,Landset7图像转化处理,再通过ArcGIS软件进行栅格插值,计算出扎龙保护区内丹顶鹤三个不同季节的食物量分别为:4月食物总量为2.7×106 kg.6~7月和8~9月食物总量分别为3.01×108 kg和3.57×108 kg;水生动物总量分别为2.67×108 kg和3.51×108 kg,底栖食物总量分别为1.35×107 kg和3.56×108 kg.丹顶鹤在扎龙保护区进行繁殖和生活期间,食物总量随季节变化而呈显著的增加趋势.丹顶鹤种群数量也随季节的变化出现波动,即每年迁徙越冬的数量显著大于迁徙繁殖的数量.     

Vibration signal transmission in spider orb webs

Vibration transmission from the prey-catching region to the hub of the unloaded orb web of Nuctenea sclopetaria was measured by laser vibrometry. Compared to transverse or lateral vibrations, longitudinal vibration shows less attenuation and contains more directional information. It is transmitted well throughout the entire frequency range measured (1 to 10,000 hertz).     

Reconciling food production and biodiversity conservation: land sharing and land sparing compared

The question of how to meet rising food demand at the least cost to biodiversity requires the evaluation of two contrasting alternatives: land sharing, which integrates both objectives on the same land; and land sparing, in which high-yield farming is combined with protecting natural habitats from conversion to agriculture. To test these alternatives, we compared crop yields and densities of bird and tree species across gradients of agricultural intensity in southwest Ghana and northern India. More species were negatively affected by agriculture than benefited from it, particularly among species with small global ranges. For both taxa in both countries, land sparing is a more promising strategy for minimizing negative impacts of food production, at both current and anticipated future levels of production.     

Changes in climatic water balance drive downhill shifts in plant species' optimum elevations

Uphill shifts of species' distributions in response to historical warming are well documented, which leads to widespread expectations of continued uphill shifts under future warming. Conversely, downhill shifts are often considered anomalous and unrelated to climate change. By comparing the altitudinal distributions of 64 plant species between the 1930s and the present day within California, we show that climate changes have resulted in a significant downward shift in species' optimum elevations. This downhill shift is counter to what would be expected given 20th-century warming but is readily explained by species' niche tracking of regional changes in climatic water balance rather than temperature. Similar downhill shifts can be expected to occur where future climate change scenarios project increases in water availability that outpace evaporative demand.     

粮农生物多样性保护及可持续利用的挑战与对策

探讨了粮农生物多样性的重要性和作用,分析了世界粮农生物多样性面临的挑战,阐述了中国保护粮农生物多样性的对策措施。结果表明,粮食和农业生物多样性的很多关键组成部分都在逐步缩减,而数据缺乏却往往限制了有效补救措施的规划与先后排序。提出了中国开展粮农生物多样性保护及可持续利用的对策。     

中国疾病预防控制行业网站评估研究

为了掌握中国疾病预防控制行业网站的建设及服务情况,中国疾病预防控制中心信息中心制定了较为客观、全面和科学的评估指标体系,并与第三方专业测评机构于2008年对各省、计划单列市疾病预防控制中心及中国疾病预防控制中心直属单位的网站进行了评估。结果显示,国内疾病预防控制行业的网站建设已初具规模,但仍须从实际应用和信息保障机制方面加强建设,提升本行业网站的绩效。