A review: limnological management and biomanipulation in the London reservoirs

Low algal biomasses and high water transparencies are a feature of the storage reservoirs that supply most of London's treated water. This is a result of knowledgeable limnological management and biomanipulation and despite the eutrophic nature of the River Thames with its high nutrients (7 gN m⁻³; 1 gP m⁻³) and particulate organic carbon (2 gC m⁻³). Built-in possibilities of jetting input water are managed to prevent stratification, to ensure isothermy, to mix chemicals and plankton vertically and horizontally and to manipulate the mixed-depth of the algal populations such that their potential for biomass growth is reduced by light-energy limitation. Spring algal growth is delayed and the spring peak is reduced and curtailed by the grazing impact of considerable biomasses of large-bodied daphnid populations (Daphnia magna, pulicaria & hyalina) whose development is also supported by the continuous input of high riverine algal crops. The existence of a large-bodied daphnid zooplankton in the reservoirs is associated with low levels of fish predation since the late 1960s. Variations in the intensity and nature of this vertebrate predation during the subsequent twenty years (1968–88) are illustrated by the changes that have occurred in the relationship between the phytoplankton and zooplankton biomasses of the April-May-June quarter of the year. This example of the London reservoirs serves to illustrate biomanipulation in deep water bodies by bottom-up as well as top-down effects.     

The relative importance of resources and natural enemies in determining herbivore abundance: thistles, tephritids and parasitoids

1. The relative importance of host-plant resources and natural enemies in influencing the abundance of insect herbivores was investigated in potted plant and natural population experiments, using tephritid (Diptera: Tephritidae) flies, their host plant, creeping thistle Cirsium arvense, and their Hymenoptera parasitoids. 2. Experimental manipulation of host-plant quality (i.e. levels of host-plant nutrients) and resource availability (i.e. the number of buds) increased tephritid abundance. There was no evidence that the seed-feeding tephritid fly Xyphosia miliaria preferentially oviposited on fertilized C. arvense. 3. At low thistle densities, X. miliaria showed a constant rate of resource exploitation. At higher thistle densities, a threshold was detected, above which additional buds were not attacked. 4. Parasitism attack was variable across host (tephritid) densities but levels of parasitism were consistently higher on the fertilized thistles. 5. Experimental manipulation of host-plant quality and resource availability (quantity) not only directly affects the tephritid population but also, indirectly, leads to high rates of parasitism. Both chemical and physical characteristics of host plants affect the performance of natural enemies. 6. Both top-down and bottom-up forces act to influence tephritid abundance, with bottom-up influences appearing to be the most important.     

Inter-hemispheric comparison of bottom-up effects on community structure: Insights revealed using the comparative-experimental approach

The comparative-experimental approach uses identically designed, replicated experiments at different sites along environmental gradients in order to gain insight into the changing dynamics of communities with changing environmental conditions. Such studies reveal how ecological processes vary in intensity and interact to produce community structure. Early emphases were on the community consequences of shifting top-down impacts, competition and disturbance with environmental stress. Recent advances include the more precise quantification of gradients and thus a better understanding of species responses to the environment, and the revelation that bottom-up forces can vary significantly on within-region scales, with major consequences for the impact of top-down forces and thus community dynamics. Here the use of the method to examine the role of geographic location (coastal ecosystems in different hemispheres) and oceanographic conditions (upwelling vs downwelling) on these bottom-up/top-down linkages is advanced. We show that a bottom-up factor (prey recruitment) and a top-down effect (predation rate) vary consistently with oceanographic conditions within each coastal ecosystem, and also between geographic locations (New Zealand, Oregon). In general, both recruitment and predation rates are higher in Oregon. It is postulated that these differences are common responses to oceanographic variation, and that between-hemisphere differences result from the stronger and more persistent upwelling in the California Current ecosystem.     

An artificial neural network approach for studying phytoplankton succession

Artificial neural networks are used to model phytoplankton succession and gain insight into the relative strengths of bottom-up and top-down forces shaping seasonal patterns in phytoplankton biomass and community composition. Model comparisons indicate that patterns in chlorophyll aconcentrations response instantaneously to patterns in nutrient concentrations (phosphorous (P), nitrite and nitrate (NO₂/NO₃–N) and ammonium (NH₄–H) concentrations) and zooplankton biomass (daphnid cladocera and copepoda biomass); whereas lagged responses in an index of algal community composition are evident. A randomization approach to neural networks is employed to reveal individual and interacting contributions of nutrient concentrations and zooplankton biomass to predictions of phytoplankton biomass and community composition. The results show that patterns in chlorophyll aconcentrations are directly associated with P, NO₂/NO₃–N and daphnid cladocera biomass, as well as related to interactions between daphnid cladocera biomass, and NO₂/NO₃–N and P. Similarly, patterns in phytoplankton community composition are associated with NO₂/NO₃–N and daphnid cladocera biomass; however show contrasting patterns in nutrient– zooplankton and zooplankton–zooplankton interactions. Together, the results provide correlative evidence for the importance of nutrient limitation, zooplankton grazing and nutrient regeneration in shaping phytoplankton community dynamics. This study shows that artificial neural networks can provide a powerful tool for studying phytoplankton succession by aiding in the quantification and interpretation of the individual and interacting contributions of nutrient limitation and zooplankton herbivory on phytoplankton biomass and community composition under natural conditions.     

Bottom-up regulation of a pelagic community through spatial aggregations

The importance of spatial pattern in ecosystems has long been recognized. However, incorporating patchiness into our understanding of forces regulating ecosystems has proved challenging. We used a combination of continuously sampling moored sensors, complemented by shipboard sampling, to measure the temporal variation, abundance and vertical distribution of four trophic levels in Hawaii's near shore pelagic ecosystem. Using an analysis approach from trophic dynamics, we found that the frequency and intensity of spatial aggregations-rather than total biomass-in each step of a food chain involving phytoplankton, copepods, mesopelagic micronekton and spinner dolphins (Stenella longirostris) were the most significant predictors of variation in adjacent trophic levels. Patches of organisms had impacts disproportionate to the biomass of organisms within them. Our results are in accordance with resource limitation-mediated by patch dynamics-regulating structure at each trophic step in this ecosystem, as well as the foraging behaviour of the top predator. Because of their high degree of heterogeneity, ecosystem-level effects of patchiness such as this may be common in many pelagic marine systems.     

Plant and insect diversity along a pollution gradient: understanding species richness across trophic levels

We analysed species richness of plants and true bugs (Insecta, Heteroptera) along a pollution gradient in Scots pine stands in Central Germany. As a consequence of particulate deposition, pH-values of soils increased in the vicinity of the emission source. Therefore, emission increased productivity. Species richness of plants increased with decreasing distance from emission source, and thus with increasing productivity. Similarly, species richness of herbivorous Heteroptera increased with decreasing distance from emission source, whereas, surprisingly, abundance decreased. The proportion of specialised herbivorous bug species is largest in the vicinity of the emission source. Thus, the diversity pattern of herbivores may be explained by the specialisation hypothesis and not the consumer rarity hypothesis. Species richness and abundance of carnivorous Heteroptera showed no significant trend along the gradient. Overall our data favour the bottom-up control of species diversity in the analysed system.     

Crayfish in lakes and streams: individual and population responses to predation, productivity and substratum availability

1. In a correlative study, we investigated the relative importance of fish predation, refuge availability and resource supply in determining the abundance and size distributions of the introduced and omnivorous signal crayfish (Pacifastacus leniusculus) in lakes and streams. Moreover, the biomass and food selection of predatory fish was estimated in each habitat type and stable isotopes of carbon and nitrogen were measured in perch (Perca fluviatilis), the dominant predator in the lakes, and in its potential food sources (crayfish, juvenile roach and isopods). 2. In lakes, crayfish were the most frequent prey in large perch (46%), followed by other macroinvertebrates (26%, including the isopod Asellus aquaticus) and small fish (25%). Crayfish and fish dominated the gut contents of large perch with respect to biomass. Nitrogen signatures showed that perch were one trophic level above crayfish (approx. 3.4‰) and a two‐source mixing model using nitrogen isotope values indicated that crayfish (81%) contributed significantly more to perch isotope values than did juvenile roach (19%). A positive correlation was found between the abundance of crayfish and the biomass of large perch. Crayfish abundance in lakes was also positively correlated with the proportion of cobbles in the littoral zone. Lake productivity (chlorophyll a) was positively correlated with crayfish size, but not with crayfish abundance. 3. In streams, brown trout (Salmo trutta) were the most abundant predatory fish. Gut contents of large trout in a forested stream showed that terrestrial insects were the most frequently found prey (60%), followed by small crayfish (27%) and isopods (27%). In contrast to lakes, the relative abundance of crayfish was negatively correlated with the total biomass of predatory fish and with total biomass of trout. However, abundance of crayfish at sites with a low biomass of predatory fish varied considerably and was related to substratum grain size, with fewer crayfish being caught when the substratum was sandy or dominated by large boulders. The mean size of crayfish was greater at stream sites with a high standing stock of periphyton, but neither predator biomass nor substratum grain size was correlated with crayfish size. 4. Our results suggest that bottom‐up processes influence crayfish size in lakes and streams independent of predator biomass and substratum availability. However, bottom‐up processes do not influence crayfish abundance. Instead, substratum availability (lakes) and interactions between predation and substratum grain size (streams) need to be considered in order to predict crayfish abundance.     

陆地生态系统碳平衡主要研究方法评述

陆地生态系统碳平衡是全球变化科学中的核心问题之一,目前也是生态科学中的前沿与热点问题,而陆地生态系统的复杂性与不确定性决定了对陆地生态系统碳平衡估测的复杂性和不确定性。为研究这一复杂性问题,已发展了许多研究方法。可分为“自下而上”与“自上而下”两种,各种方法都有其自身的优势和劣势。相关方向也已经有了大量的研究报道,但是,不同的研究由于在方法、时间与空间尺度等存在的差异,使得许多研究结果和预测很难被有效的整合或适用于大范围甚至全球水平。综述了陆地碳平衡的主要研究方法,分析和比较了各方法的特点,指出在研究中对不同方法的结果进行分析和比较,以及采用综合方法的必要性。     

“Bottom-up” approach for implementing nano/microstructure using biological and chemical interactions

The “Bottom-up” approach for implementing nano/microstructure using biological self-assembled systems has been investigated with tremendous interest by many researchers in the field of medical diagnostics, material synthesis, and nano/microelectronics. As a result, the techniques for achieving these systems have been extensively explored in recent years. The developed or developing techniques are based on many interdisciplinary areas such as biology, chemistry, physics, electrical engineering, mechanical engineering, and so on. In this paper, we review the fundamentals behind the self-assembly concepts and describe the state of art in the biological and chemical self-assembled systems for the implementation of nano/microstructures. These structures described in the paper can be applied to the implementation of hybrid biosensors, biochip, novel bio-mimetic materials, and nano/microelectronic devices.     

Natural enemies act faster than endophytic fungi in population control of cereal aphids

1. Fast-growing populations of phytophagous insects can be limited by the presence of natural enemies and by alkaloids that are produced by symbiotic associations of many temperate grass species with endophytic fungi. It is unclear if and how acquired plant defences derived from endophytic fungi interact with natural enemies to affect phytophagous insect populations. 2. To assess the relative importance of endophytic fungi compared to that of natural enemies on the population dynamics of phytophagous insects, we carried out a fully factorial field experiment, in which the presence of natural enemies and the presence of endophytic fungi were manipulated simultaneously. Target colonies of aphids were monitored for 8 weeks starting from their natural appearance in the field to the end of the aphid season. 3. We show that on Lolium perenne increased natural enemy densities reduced the individual numbers of two common cereal aphids, Rhopalosiphum padi and Metopolophium festucae. 4. The presence of the endophytic fungi Neotyphodium lolii reduced the number of M. festucae but did not affect the number of R. padi. The reduction in R. padi numbers by predators and parasitoids was not influenced by the presence of endophytes. For adult M. festucae, however, the negative effects of natural enemies were significant only in the absence of endophytes. 5. Over the duration of the experiment, the effect of natural enemies on aphid colony growth was much stronger than the effect of the endophytic fungi N. lolii, presumably because predator and parasitoid action on aphid colonies is much faster than any effects of endophytes. 6. Our results demonstrate that with simultaneous action of acquired endosymbionts and natural enemies, both factors can control aphid colony growth but they generally act independently of each other.     

Nutrient limitation in detritus-based microcosms in Sarracenia purpurea

Most prior work on the role of top-down and bottom-up effects in aquatic communities has ignored the significant detrital component that occurs in natural systems. We investigated the effects of specific nutrients (carbon, phosphorus, and nitrogen), as well as a top predator (the mosquito Wyeomyia smithii), on the structure of the detritivore community found in the water-filled leaves of the pitcher plant Sarracenia purpurea. The concentrations of three nutrients and the presence of the predators were manipulated in a factorial design, while the response of the remaining community was quantified. Bacterial growth was found to be strongly carbon-limited and somewhat less limited by phosphorus and there was an interaction between the effects of the two nutrients. Neither carbon or phosphorus addition affected protozoan or rotifer abundance, and nitrogen had only a minor effect. The presence of the predator, however, significantly reduced the abundance of the four numerically dominant bacteriovores. There were no interactions between top-down and bottom-up effects; the strong direct reciprocal effects between adjacent trophic levels seem to be greatly attenuated as they are propagated farther up or down the food chain.     

What is a gene?—Revisited

The dialectic discourse of the 'gene' as the unit of heredity deduced from the phenotype, whether an intervening variable or a hypothetical construct, appeared to be settled with the presentation of the molecular model of DNA: the gene was reduced to a sequence of DNA that is transcribed into RNA that is translated into a polypeptide; the polypeptides may fold into proteins that are involved in cellular metabolism and structure, and hence function. This path turned out to be more bewildering the more the regulation of products and functions were uncovered in the contexts of integrated cellular systems. Philosophers struggling to define a unified concept of the gene as the basic entity of (molecular) genetics confronted those who suggested several different 'genes' according to the conceptual frameworks of the experimentalists. Researchers increasingly regarded genes de facto as generic terms for describing their empiric data, and with improved DNA-sequencing capacities these entities were as a rule bottom-up nucleotide sequences that determine functions. Only recently did empiricists return to discuss conceptual considerations, including top-down definitions of units of function that through cellular mechanisms select the DNA sequences which comprise 'genomic-footprints' of functional entities.     

Complex food webs prevent competitive exclusion among producer species

Herbivorous top-down forces and bottom-up competition for nutrients determine the coexistence and relative biomass patterns of producer species. Combining models of predator-prey and producer-nutrient interactions with a structural model of complex food webs, I investigated these two aspects in a dynamic food-web model. While competitive exclusion leads to persistence of only one producer species in 99.7% of the simulated simple producer communities without consumers, embedding the same producer communities in complex food webs generally yields producer coexistence. In simple producer communities, the producers with the most efficient nutrient-intake rates increase in biomass until they competitively exclude inferior producers. In food webs, herbivory predominantly reduces the biomass density of those producers that dominated in producer communities, which yields a more even biomass distribution. In contrast to prior analyses of simple modules, this facilitation of producer coexistence by herbivory does not require a trade-off between the nutrient-intake efficiency and the resistance to herbivory. The local network structure of food webs (top-down effects of the number of herbivores and the herbivores' maximum consumption rates) and the nutrient supply (bottom-up effect) interactively determine the relative biomass densities of the producer species. A strong negative feedback loop emerges in food webs: factors that increase producer biomasses also increase herbivory, which reduces producer biomasses. This negative feedback loop regulates the coexistence and biomass patterns of the producers by balancing biomass increases of producers and biomass fluxes to herbivores, which prevents competitive exclusion.