Foraging theory predicts predator-prey energy fluxes

1. In natural communities, populations are linked by feeding interactions that make up complex food webs. The stability of these complex networks is critically dependent on the distribution of energy fluxes across these feeding links. 2. In laboratory experiments with predatory beetles and spiders, we studied the allometric scaling (body-mass dependence) of metabolism and per capita consumption at the level of predator individuals and per link energy fluxes at the level of feeding links. 3. Despite clear power-law scaling of the metabolic and per capita consumption rates with predator body mass, the per link predation rates on individual prey followed hump-shaped relationships with the predator-prey body mass ratios. These results contrast with the current metabolic paradigm, and find better support in foraging theory. 4. This suggests that per link energy fluxes from prey populations to predator individuals peak at intermediate body mass ratios, and total energy fluxes from prey to predator populations decrease monotonically with predator and prey mass. Surprisingly, contrary to predictions of metabolic models, this suggests that for any prey species, the per link and total energy fluxes to its largest predators are smaller than those to predators of intermediate body size. 5. An integration of metabolic and foraging theory may enable a quantitative and predictive understanding of energy flux distributions in natural food webs.     

Microbial Food Webs in an Artificially Divided Acidic Bog Lake

The microbial loop of a naturally acidic bog lake, Große Fuchskuhle (Northeastern Germany), that had been artificially divided into 4 basins, was investigated. In the northeast (NE) and southwest (SW) basins, which differ strongly in chemistry and primary production, we conducted intensive studies of the main carbon fluxes through microbial food webs. In the less acidic, NE basin, much higher phytoplankton as well as bacterial biomass and production were found in parallel with negligible numbers of larger zooplankters. Weakly top-down controlled populations of protists were characterized by an exceptionally low numerical proportion of heterotrophic nanoflagellates (HNF) to ciliates (-1.5-3.5). The ciliate community was dominated by a scuticociliate, Cyclidium sp. (>95% of total ciliates), with an estimated grazing rate equal to 46–80% of heterotrophic bacterial production. In contrast, in the more humic, SW basin, both phyto- and bacterioplankton dynamics seemed to be top-down controlled by abundant populations of small fine-filter feeding cladocerans, Ceriodaphnia quadrangula and Diaphanosoma brachyurum. Consequently, ciliates disappeared from the food web structure of the SW basin, HNF dropped to negligible numbers and bacteria showed very uniform morphology, dominated by small cocci or short rods. Our investigations have shown that the division of the lake into separate compartments can lead to very different microbial food web structures with extreme species compositions.     

Cascading effects of the flathead grey mullet Mugil cephalus in freshwater eutrophic microcosmos

Flathead grey mullet (Mugil cephalus L.) stocked in fish ponds were long considered to feed primarily on detritus. However, recent research has found that they obtain much of their food from plankton and that they have a detrimental effect on pond zooplankton and large phytoplankton, whilst enhancing small phytoplankton. It has been suggested that flathead grey mullet may also increase the internal phosphorus loading of the ecosystem, which would also increase phytoplankton density. To test whether zooplankton removal or nutrient release from the sediment is the better explanation for phytoplankton enhancement in the presence of flathead grey mullet, the ecosystems of fish-less tanks, tanks with a 60 m mesh filter and tanks stocked at a fish density of 243 g m^-3 were compared. In the presence of flathead grey mullets, cladocerans, ostracods and chironomid larvae became scarcer than in the control tanks, while there were more small phytoplankton and mud snails. The green algae Cladophora sp. did not occur at all. The presence of a mechanical filter also reduced cladoceran, ostracod and chironomid densities and increased phtyoplankton and mud snail density. However, the situation observed in filter tanks was intermediate between that observed in the fish tanks and the control tanks, due to the lower filtering efficiency of the mechanical filter. The organic matter content of the sediment decreased throughout the experiment in the control and filter tanks, but remained stable in fish tanks. Phosphorus and nitrogen concentrations were not affected by any treatment. These results showed that flathead grey mullet enhanced phytoplankton development due to the removal of large cladocerans and not as a consequence of nutrient release. Furthermore, the flathead grey mullet strongly modified the benthic community, probably due to direct predation.     

Modifying soil to enhance biological control of belowground dwelling insects in citrus groves under organic agriculture in Florida

An emerging organic citrus industry in Florida could benefit greatly from effective, non-conventional methods to mitigate losses from pests and diseases. We studied part of a soil food web in an organic orchard to learn ways to conserve and enhance biological control of insect pests by native entomopathogenic nematodes (EPNs). We evaluated two OMRI (Organic Materials Review Institute) approved cultural practices: (i) a mulch of commercially pelleted chicken manure, (ii) a commercial formulation of Purpureocillium lilacinus, and (iii) an un-amended control. Several soil nutrients (i.e. nitrogen, phosphate, and potassium) were affected by the amendments, but initial equilibrium values (T0) were restored by the last sampling time (T12). The plant parasitic nematode Tylenchulus semipenetrans increased in both treatments compared to the untreated control at T3 (P < 0.05). The oomycete Phytophthora nicotianae increased in the P. lilacinus plots at T1, marginally at T12, but decreased at T6 and T9. Steinernema diaprepesi, Heterorhabditis indica and Heterorhabditis zealandica were the only EPNs regularly detected in the orchard. Mulch increased numbers of H. zealandica at T6 and T9 (P < 0.05) and free living nematodes at T12 (P < 0.01). The nematophagous fungus (NF) P. lilacinus persisted in plots where it was augmented (P < 0.05), reaching a maximum level at T3 that was 17.5-fold greater than that in controls. Numbers of Paenibacillus sp. were directly related to both those of S. diaprepesi and Acrobeloides-group nematodes (P < 0.01), but inversely to the FLN counts (P < 0.05). The application of these two amendments did not produce strong changes in the EPN community but decreased the emergence from soil of adult Diaprepes abbreviatus, a root weevil pest. Thus, both amendments might contribute to citrus pest management under organic production.     

Quantitative food webs of dipteran leafminers and their parasitoids in Argentina

The quantitative structure of two host–parasitoid communities based on leaf-mining flies (Diptera, Agromyzidae) in Argentina is described. The two communities consisted of 29 and 27 hosts, 46 and 40 parasitoids, and 193 and 179 recorded host–parasitoid associations. Also, food webs were constructed for one community based solely on samples taken in the wet and dry seasons. Data were expressed as quantitative food webs, and the manner in which food web properties, such as connectance and compartmentalization, were influenced by sampling intensity was explored. The potential importance of indirect effects between hosts mediated by parasitoids (e.g. apparent competition) was assessed using quantitative parasitoid overlap diagrams. The studys results suggest that indirect effects are likely to be important in these highly connected communities. The limitations of the studys analysis, and how the conclusions can be tested experimentally, are discussed.     

桉树同安钮夜蛾生物学特性及防治的研究

同安钮夜蛾(Anua indiscriminata Moore)是近年来桉树产区出现的重要害虫之一。该虫在广东蕉岭1年发生4代,以蛹越冬,幼虫6-7龄。每年仅有1个发生高峰期,一般在9月出现。室内药效测定表明:高效氯氰菊酯、保尔、锐劲特、阿维菌素、印楝素、鱼藤氰和吡虫啉对同安钮夜蛾幼虫的毒杀效果均达100%;而Bt和灭幼脲Ⅲ号的杀虫效果分别为96.3%和92.6%。林间试验表明:高效氯氰菊酯、阿维烟剂和Bt对同安钮夜蛾幼虫的杀死率分别为100%、96.6%和89.7%;用烟雾机喷施阿维烟剂10倍柴油溶液,其杀虫率为82.2%。     

Alternative Dynamic Regimes and Trophic Control of Plant Succession

Ecological succession has been the subject of intense study and debate throughout the history of ecology as conceptualizations of process were proposed and refined. Modern concepts view ecological succession as largely driven by bottom-up resource competition for light and nutrients. However, growing evidence shows that top–down effects of consumers can govern succession. These contrasting perspectives require synthesis. We offer such a synthesis by revitalizing the hypothesis that succession proceeds by abrupt transitions to alternative states or dynamic regimes. We present evidence from field sampling along two successional gradients in a New England old field aimed at identifying pattern, and from experimentation in the same field aimed at identifying process. Field sampling revealed that a competitive dominant plant existed in a mosaic with two distinct patch types of relative abundances (≤ 25% and > 30%). Competitive dominant plant abundance varied systematically with plant species diversity (evenness), and resource supply (light and soil nitrogen). The six-year field experiment tested for alternative regimes by systematic removal and staggered reintroduction of top predators. Long-term predator removal caused an abrupt and irreversible shift from a top–down to a bottom–up controlled regime with a breakpoint at approximately 25% relative abundance of the competitive dominant plant. This caused significant shifts in plant species evenness and resource supply (solar radiation and N mineralization rate). Moreover, the competitive dominant abundance, species evenness and resource supply in the two dynamic regimes matched levels in the different patch types in the field. We conclude that a single ecosystem can display both top–down and bottom–up control. Abrupt shifts in trophic control lead to abrupt changes in the rate of development of ecosystems consistent with a working hypothesis that succession proceeds via abrupt regime shifts.     

A review of the salt sensitivity of the Australian freshwater biota

In Victoria, Australia, both dryland salinity and salinity in irrigation regions are serious agricultural problems. One option to control the latter is to pump groundwater to maintain it below the surface. However, this leaves a saline wastewater for disposal, probably into local streams or wetlands. This review of the salt sensitivity of the biota of Australian streams and wetlands gives information of interest to those responsible for developing controls on these discharges. The review addresses the lethal and sub-lethal effects of salinity on microbes (mainly bacteria), macrophytes and micro-algae, riparian vegetation, invertebrates, fish, amphibians, reptiles, mammals, and birds. Data suggest that direct adverse biological effects are likely to occur in Australian river, stream and wetland ecosystems if salinity is increased to around 1 000 mg L⁻¹. The review highlights a general lack of data on the sensitivity of freshwater plants and animals to salinity increases.     

Sex ratios in natural populations ofAphelinus mali (Hym.: Aphelinidae) in relation to host size and host density

In an apple orchard at Armidale, the Northern Tablelands of NSW, population sex ratios ofAphelinus mali (Haldeman), an endoparasitoid of the woolly apple aphid,Eriosoma lanigerum (Hausmann) varied from 0.51 (proportion of males) at low host densities to female-biased at high host densities (proportion of males ranged from 0.35–0.39). This shift in sex ratio seems to be caused by the differences in allocation of sons and daughters to hosts of different sizes. In the fieldA. mali parasitizes all life stages (four nymphal instars and adult) of the woolly aphid upon encountering. According to Hughes'(1979) optimal diet model, such general host acceptance seems to be the best strategy. However, it allows the host nymphs or adults to continue to develop or reproduce until about to mummify (pupate). No mortality was observed when first or second-instar hosts were parasitized in the laboratory. Field collected small mummified hosts yielded male-biased sex ratios whereas large mummified hosts produced mainly females. In the laboratory, progeny from smaller hosts (first to third-instars) produced sex ratios which were not significantly different from 0.5 whereas progeny from larger hosts (third and fourth-instars) produced female-biased sex ratio. During winter (June–August) and early spring (September–October) when the host populations in the orchard were predominantly nymphs, the parasitoid tended to allocate equal resources to male and female offspring. In contrast, at peak population densities in summer and autumn (December–May) when larger hosts were available, the sex ratios were female-biased. The host size ofE. lanigerum andA. mali is, therefore, an important component in the dynamics of host-parasitoid interactions.