Host‐plant genotype mediates supply and demand of animal food in an omnivorous insect

1. Omnivorous predators can protect plants from herbivores, but may also consume plant material themselves. Omnivores and their purely herbivorous prey have previously been thought to respond similarly to host‐plant quality. However, different responses of omnivores and herbivores to their shared host plants may influence the fitness, trophic identity, and population dynamics of the omnivores. 2. The aim of the present study was to show that an omnivorous heteropteran (Anthocoris nemorum L.) and two strictly herbivorous prey species respond differently to different genotypes of their shared host plant, Salix. Some plant genotypes were sub‐optimal for the omnivore, although suitable for the herbivores, and vice versa. 3. The contrasting patterns of plant suitability for the omnivore and the herbivores highlight an interaction between plant genotype and omnivores' access to animal food. Plant genotypes that were sub‐optimal for the omnivore when herbivores were experimentally excluded became the best host plants when herbivores were present, as in the latter situation additional prey became available. By contrast, the quality of plant genotypes that were intrinsically suitable for omnivores, did not improve when herbivores were present as these plant genotypes were intrinsically sub‐optimal for herbivores, thus providing omnivores with almost no additional animal food. 4. The differential responses of omnivores and their prey to the same host‐plant genotypes should allow omnivores to colonise sub‐optimal host plants in their capacity as predators, and to colonise more suitable host plants in their capacity as herbivores. It may thus be difficult for Salix to escape herbivory entirely, as it will rarely be unsuitable for both omnivores and pure herbivores at the same time.     

Pupal colour polymorphism in Papilio machaon L. and the survival in the field of cryptic versus non-cryptic pupae

• 1 A field experiment was carried out in a natural habitat of Papilio machaon L. in southern Sweden to assess the evolutionary significance of pupal colour polymorphism.
• 2 Cryptic and non-cryptic pupae were planted in pairs in the vegetation, and exposed to predators.
• 3 The protective coloration conferred a selective advantage approximating 1.5 on the cryptic pupae of the summer generation. In the overwintering generation no difference could be detected between the predation of cryptic and non-cryptic pupae.
• 4 The adaptive fitness of protective coloration, as determined by the different rates of elimination of the colour morphs, was greater for the green pupae than for the brown ones.
• 5 Natural selection favours the evolution of a seasonal difference in the proportion of green and brown pupae in the summer and hibernating generations of P.machaon.

     

On the mechanism of floral shifts in speciation: gained pollination efficiency from tongue- to eye-attachment of pollinia in Platanthera (Orchidaceae)

This work explores the mechanism of floral shifts that may result in speciation. The model organisms chosen were the moth-pollinated pollinator-limited orchid species Platanthera bifolia and P. chlorantha . P. bifolia exhibits tongue-attachment of pollinia on pollinators, a character state that has been found previously to be ancestral. The close relative P. chlorantha exhibits the derived state of eye-attachment of pollinia on pollinators. We reasoned that differences between the species in pollination efficiency could give insights into the mechanism of floral shifts and thus cladogenesis. Four populations per species were investigated. In three populations, where the species were growing intermixed and were sharing pollinators, there was significant difference in the pollen export and import efficiency per visit-night (night with pollen export and/or import) between the two species. P. bifolia exported pollinia more efficiently but imported pollen less efficiently than did P. chlorantha . Pollen import was 1.7–4 times faster in P. chlorantha (eye-attachment) than it was in P. bifolia (tongue-attachment). P. chlorantha had a lower risk of interference between pollen import and export. An increase in fitness through greater speed and efficiency of pollen import due to an enlargement of the stigmatic surface and a reduction in the risk of sexual interference may therefore be mechanisms of the floral shift from tongue- to eye-attachment of pollinia on pollinators.  © 2004 The Linnean Society of London, Biological Journal of the Linnean Society , 2004, 83 , 481–495.     

The species delimitation problem applied to the Agabus bipustulatus complex (Coleoptera, Dytiscidae) in north Scandinavia

Agabus bipustulatus (Linnaeus) is one of the most common aquatic beetles in Europe. Two species have been traditionally recognized within the Palaearctic Agabus bipustulatus complex (Coleoptera, Dytiscidae) in Scandinavia: a lowland form A. bipustulatus and a high-altitude form A. solieri Aube. The specific status of solieri has been debated for more than a century but no quantitative investigation has been made to evaluate the status of this taxon. In this study we show that there is no clear-cut delimitation between the two forms, either morphological or genetic, across an altitudinal gradient in north Scandinavia. Morphological differences between 22 populations were analysed separately for each sex with both thin-plate splines relative warp analysis and 'classical-length' morphometries. Genetic variation at five polymorphic enzyme loci was analysed among seven populations. The morphological studies showed gradual variation correlated with altitude, in particular in the character that is traditionally used to separate solieri and bipustulatus , and in both the beetles' morphometric size and the lateral width of the metasternal plate, which is connected to flight capacity. The genetic study indicates that the a-Gpdh enzyme locus, which is involved in the transfer of energy to the flight muscles, is evidently subject to directional selection. Only minor population differences were observed without this system. Subdivision was found in some populations and was probably caused by migration from outside or within the local population. The overall conclusion is that there is no clear-cut species delimitation between A. bipustulatus and A. solieri in north Scandinavia. This indicates that A. solieri is a cold-adapted altitudinal form of the variable A. bipustulatus; additional support for this is the finding of solieri 'look alikes' in cold springs in areas normally inhabited by bipustulatus.     

Variability in exchange of CO2 across 12 northern peatland and tundra sites

Many wetland ecosystems such as peatlands and wet tundra hold large amounts of organic carbon (C) in their soils, and are thus important in the terrestrial C cycle. We have synthesized data on the carbon dioxide (CO₂) exchange obtained from eddy covariance measurements from 12 wetland sites, covering 1–7 years at each site, across Europe and North America, ranging from ombrotrophic and minerotrophic peatlands to wet tundra ecosystems, spanning temperate to arctic climate zones. The average summertime net ecosystem exchange of CO₂ (NEE) was highly variable between sites. However, all sites with complete annual datasets, seven in total, acted as annual net sinks for atmospheric CO₂. To evaluate the influence of gross primary production (GPP) and ecosystem respiration (R_eco) on NEE, we first removed the artificial correlation emanating from the method of partitioning NEE into GPP and R_eco. After this correction neither R_eco (P= 0.162) nor GPP (P= 0.110) correlated significantly with NEE on an annual basis. Spatial variation in annual and summertime R_eco was associated with growing season period, air temperature, growing degree days, normalized difference vegetation index and vapour pressure deficit. GPP showed weaker correlations with environmental variables as compared with R_eco, the exception being leaf area index (LAI), which correlated with both GPP and NEE, but not with R_eco. Length of growing season period was found to be the most important variable describing the spatial variation in summertime GPP and R_eco; global warming will thus cause these components to increase. Annual GPP and NEE correlated significantly with LAI and pH, thus, in order to predict wetland C exchange, differences in ecosystem structure such as leaf area and biomass as well as nutritional status must be taken into account.     

Characteristics of oligotrophic hardwater lakes in a postglacial land-rise area in mid-Sweden

1. We describe some remarkable ephemeral, oligotrophic hardwater lakes formed because of land rise in the coastal areas of the Baltic Sea, that are unique in Sweden and probably also worldwide. Two younger, coastal lakes were studied by regular sampling for 1 year and compared with an older (i.e. greater altitude) lake, that passed through the oligotrophic hardwater stage some 3–4000 years ago. 2. Despite some differences in composition of the catchment, the two younger lakes were similar with regard to water chemistry and plankton community composition. The concentration of phosphorus was low while nitrogen was high, resulting in very high N/P quotients (101 and 131). Although water colour was moderate, the concentration of organic carbon was extremely high (average values of ≥ 20 μg TOC L^–1), consisting mainly of dissolved compounds (DOC). 3. While the plankton was poorly developed, sediments in both lakes were covered by a layer of photosynthesising micro‐organisms. This substantial `microbial mat', which has not been described in detail before, was up to 15 cm thick and dominated by cyanobacteria and purple sulphur bacteria. The concentration of sediment phosphorus was extremely low (352 μg g^–1 dw) in one of the lakes and dominated by organic‐bound (residual) phosphorus. 4. Deep sediments in the older lake, representing its oligotrophic hardwater period, differed in phosphorus composition from the currently oligotrophic hardwater lakes by having a strong dominance of HCl‐extractable (Ca‐bound) phosphorus. This indicates that phosphorus, initially organic‐bound within the microbial mat, is subsequently bound to calcium. We hypothesise that this is promoted by the environmental conditions created by the benthic photosynthetic activity, in combination with the prevailing hardwater conditions. 5. The rich and flourishing microbial community on the sediments may also explain the high concentration of DOC in the lake.     

Patterns of denitrification rates in European alluvial soils under various hydrological regimes

1. Denitrification in floodplain soils is one of the main biological processes emitting and reducing nitrous oxide, a greenhouse gas, and the main process responsible for the buffering capacity of riparian zones against diffuse nitrate pollution. 2. The aim of this study was to measure denitrification rates under a wide range of current climatic conditions and hydrological regimes in Europe (from latitude 64°N to latitude 42°N and from longitude 2°W to longitude 25°E), in order to determine the response patterns of this microbial process under different climatic and hydrological conditions, and to identify denitrification proxies robust enough to be used at the European scale. 3. Denitrification activity was significant in all the floodplain soils studied whatever the latitude. However, we found an increase in rates of an order of magnitude from high to mid latitudes. Maximum rates (above 30 g N m⁻² month⁻¹) were measured in the maritime conditions of the Trent floodplain. These rates are similar to mineralisation rates measured in alluvial soils and of the same order of magnitude as the amount of N stored in herbaceous plants in alluvial soils. 4. We used Multivariate Adaptative Regression Splines to relate the response variable denitrification with five relevant predictors, namely soil moisture, temperature, silt plus clay, nitrate content and herbaceous plant biomass. 5. Soil moisture, temperature, and nitrate were the three main control variables of microbial denitrification in alluvial soils in decreasing order of importance. 6. The model developed for denitrification with interaction effects outperformed a pure additive model. Soil moisture was involved in all interactions, emphasising its importance in predicting denitrification. 7. These results are discussed in the context of scenarios for future change in European hydrological regimes.     

SEED DENSITY, COVER, PREDATION AND THE DISTRIBUTION OF BIRDS IN A BEECH WOOD IN SOUTHERN SWEDEN

The distribution of six species of birds within a beech wood was analysed in two winters when beech seeds were abundant. All species were concentrated in patches where seed density was high in one winter, but only two showed a similar distribution in the second winter. The distribution of the other four species, three tits Parus spp. and the Nuthatch Sitta europaea, was positively correlated with low cover. Cover was inversely correlated with seed density. Bird predators were recorded many times during the second winter but not at all during the first winter. It is suggested that predator avoidance was more important than food density for some species in the second winter. The relationships between bird density and both food and cover were sigmoid in shape. The proportion of the seed crop consumed by the birds was estimated at about 4.5%, which was probably more than the amount consumed by mammals. The Woodpigeon Columba palumbus (which took 44% of the total seeds consumed by birds) and the Brambling Fringilla montifringilla (33%) were the most important seed consumers among the birds.