How predatory mites cope with the web of their tetranychid prey: a functional view on dorsal chaetotaxy in the Phytoseiidae

It has been proposed from behavioural observations that the ability of Phytoseiidae to penetrate the web is related to chaetotaxy. As phytoseiid species differ widely in chaetotaxy, as well as in their ability to cope with complex webs, this hypothesis was tested by studying relations between these two variables. In this paper we develop a simple method to quantify the degree to which the soma is protected from contact with the silk and apply it to phytoseiid species for which behavioural performance in the web is known from the literature. It is found that the better protected species experience less hinder from complex webs, such as produced byTetranychus spp. Moreover, they appear to be more successful in exterminating local populations ofTetranychus spp. Thus, setal patterns in the Phytoseiidae may well be a good indicator for selecting candidates for biological control ofTetranychus spp. Finally, we propose a parsimonious scenario for the evolution of dorsal chaetotaxy in the Phytoseiidae.     

Root turnover as determinant of the cycling of C,N, and P in a dry heathland ecosystem

Root production and turnover were studied using sequential core sampling and observations in permanent minirhizotrons in the field in three dry heathland stands dominated by the evergreen dwarfshrub Calluna vulgaris and the grasses Deschampsia flexuosa and Molinia caerulea, respectively. Root biomass production, estimated by core sampling, amounted to 160 (Calluna), 180 (Deschampsia) and 1380 (Molinia) g m^-2 yr^-1, respectively. Root biomass turnover rate in Calluna (0.64 yr^-1) was lower compared with the grasses (Deschampsia: 0.96 yr^-1; Molinia 1.68yr^-1)). Root length turnover rate was 0.75–0.77 yr^-1 (Deschampsia) and 1.17–1.49 yr^-1 (Molinia), respectively. No resorption of N and P from senescing roots was observed in either species. Input of organic N into the soil due to root turnover, estimated using the core sampling data, amounted to 1.8 g N m^-2 yr^-1(^Calluna), 1.7 g N m^-2 yr^-1 (Deschampsia) and 19.7 g N m^-2 yr^-1 (Molinia), respectively. The organic P input was 0.05, 0.07 and 0.55 g P M^-2 yr^-1, respectively. Using the minirhizotron turnover estimates these values were20–22% (Deschampsia) and 11–30% (Molinia) lower.When the biomass turnover data were used, it appeared that in the Molinia stand root turnover contributed 67% to total litter production, 87% to total litter nitrogen loss and 84% to total litter phosphorus loss. For Calluna and Deschampsia these percentages were about three and two times lower, respectively.This study shows that (1) Root turnover is a key factor in ecosystem C, N, and P cycling; and that (2) The relative importance of root turnover differs between species.     

Germination and early establishment of lower salt-marsh species in grazed and mown salt marsh

Abstract. Lower salt-marsh species occur more in the grazed than in the mown sites of a salt marsh in Schiermonnikoog in the Netherlands. This was demonstrated by a sowing experiment which determined which characteristic of the stand structure, height of the canopy or percentage of bare soil, is responsible for this difference. The total number of seedlings which emerged was negatively related to the height of the canopy and positively to the percentage of bare soil. The survival of seedlings of Suaeda maritima and Plantago maritima could be explained by the height of the canopy and for the latter species also by the percentage of bare soil. The survival of Aster tripolium and Plantago maritima was higher in grazed than in mown sites. Since the amount of bare soil was higher than seemed necessary for germination and survival, it was concluded that the taller canopy was responsible for the absence of lower salt-marsh species in the mown sites.     

Phytoplankton/zooplankton relationships during spring in the brackish Lake Grevelingen 1976–1978 (III)

Summary Rotifer species of the genusSynchaeta represent the dominating zooplankton organisms during early spring (BAKKERet al., 1977).Reproductive capacity of the rotifers (sensu EDMONDSON, 1965) was always significantly correlated with algal densities, cryptomonad flagellates being the main food. The slope of the curve representing the feeding relationSynchaeta-Cryptomonas was significantly steeper than forSynchaeta and the diatomSceletonema. Positive influences of increasing temperature forSynchaeta cf.vorax were only found within the range of 2–7°C. When temperature rose above 10°CS. vorax was succeeded byS.triophthalma. Negative temperatures, occurring in brackish lakes during periods of strong frost, caused a decrease of the egg ratio (numbers of eggs per animal) and of the population density.Development times ofS. vorax eggs were determined (VAN DAMet al., 1978) in order to calculate birth rates and production. Egg development time amounted to 3 days at 5°C and to 2 days at 10°C. Birth rates declined strongly when maximum population densities were reached. Mortality was approached by subtracting the actual rate of increase of population density from the birth rate. Sometimes negative mortalities were found, presumably caused by the development of resting eggs.Population dynamics of the rotifers were related to phytoplankton biomass and to primary productivity. Grazing effects of the rotifers on the algal population became evident during the final stage of the spring bloom (April). In the beginning (February–March) the mortality of the algal population could not be explained by grazing, other factors (cf. JASSBY and GOLDMAN, 1974) being responsible.     

Genetic analysis of nurse dams selected for six-week body weight or postweaning gain in mice.

A modified crossfostering technique was developed to compare the performance of nurse dams in selected and control populations of mice. The H6 and M16 populations were selected for increased 6-week body weight and 3- to 6-week postweaning gain, respectively, while the C2 and ICR populations were the respective controls. Crossfostering was performed using H6, M16 and their reciprocal F1 crosses as nurse dams in the selected crossfostering group and C2 ICR and their reciprocals in the control group. Measurements recorded for nurse dams included mean body weight of 8 young within a nursed litter at birth (MWB) and 12 days of age (MW12). The latter was used as a measure of postnatal maternal performance. Other traits recorded for nurse dams were number born (NB), body weight at parturition (DWP) and 12 days postpartum (DW12), and weight gain (DWG), feed intake (FED) and efficiency (EFF = DWG/FED) for the first 12 days of lactation. The correlated response in MW12 was negative (P less than .01) for M16 and essentially zero for H6. Both lines exhibited positive (P less than .01) correlated responses in DWP and DW12 and no change in EFF. Only the H6 line increases significantly in DWG and FED as a result of selection. NB increased in M16 and H6, but was significant for the latter population only. Population differences in selection response [(M16-ICR)-(H6-C2)] were significant for FED only, primarily due to average direct genetic effects. Direct comparisons of M16 and H6 indicated that M16 was larger in DWP and DW12 but smaller in DWG and EFF. Average direct genetic effects favored M16 for NB, DWP, and DW12, whereas average maternal genetic effects favored H6 for NB, DWP, DW12 and FED. Percent direct heterosis, in F1 crosses of selected populations was significant for MW12 (13.7%) ,FED (10.8%) and NB (11.4%). Direct heterosis in F1 crosses of the controls was significant for MW12 (9.4%), NB (6.6%), DWP (3.5%), DW12 (3.3%) and FED (4.4%). The effects of MW12, DWG and metabolic body size (MBS) accounted for 47% of the variation in FED, pooled within populations. Of these variables, MW12 accounted for the highest proportion (32%) of variation in total feed intake.     

Surface potential and the interaction of weakly acidic uncouplers of oxidative phosphorylation with liposomes and mitochondria

The pH dependence of the binding of weakly acidic uncouplers of oxidative phosphorylation to rat-liver mitochondria and liposomes is mainly determined by the pK_a of the uncoupler molecule.

The absorption and fluorescence excitation spectra of the anionic form of weakly acidic uncouplers of oxidative phosphorylation are red-shifted upon interaction with liposomal or mitochondrial membranes. The affinity for the liposomes, as deduced from the red shift, is independent of the degree of saturation of the fatty acid chains of different lecithins. The intensity of the spectra at one pH value is strongly dependent upon the surface charge of the liposomes. With positively charged liposomes the results obtained can be almost quantitatively explained with the Gouy-Chapman theory, but with negatively charged ones deviations are observed. At a particular pH, the divalent ion Ca²⁺ strongly influences the intensity of the spectra in the presence of negatively charged liposomes, but has no effect with neutral liposomes.

With mitochondrial membranes an effect of Ca²⁺ similar to that with negatively charged liposomes is observed. Depletion of the phospholipids of the mitochondria and subsequent restoration of the mitochondrial membrane with lecithin, strongly diminishes this effect, but restoration with negatively charged phospholipids does not influence it.

From these observations it is concluded that the anionic form of the uncoupler molecule when bound to mitochondria is located within the partly negatively charged phospholipid moiety of the membrane, with its anionic group pointing to the aqueous solution.     

Elimination of African Onchocerciasis: Modeling the Impact of Increasing the Frequency of Ivermectin Mass Treatment

The African Programme for Onchocerciasis Control (APOC) is currently shifting its focus from morbidity control to elimination of infection. To enhance the likelihood of elimination and speed up its achievement, programs may consider to increase the frequency of ivermectin mass treatment from annual to 6-monthly or even higher. In a computer simulation study, we examined the potential impact of increasing the mass treatment frequency for different settings. With the ONCHOSIM model, we simulated 92,610 scenarios pertaining to different assumptions about transmission conditions, history of mass treatment, the future mass treatment strategy, and ivermectin efficacy. Simulation results were used to determine the minimum remaining program duration and number of treatment rounds required to achieve 99% probability of elimination. Doubling the frequency of treatment from yearly to 6-monthly or 3-monthly was predicted to reduce remaining program duration by about 40% or 60%, respectively. These reductions come at a cost of additional treatment rounds, especially in case of 3-monthly mass treatment. Also, aforementioned reductions are highly dependent on maintained coverage, and could be completely nullified if coverage of mass treatment were to fall in the future. In low coverage settings, increasing treatment coverage is almost just as effective as increasing treatment frequency. We conclude that 6-monthly mass treatment may only be worth the effort in situations where annual treatment is expected to take a long time to achieve elimination in spite of good treatment coverage, e.g. because of unfavorable transmission conditions or because mass treatment started recently.     

Life‐history constraints in grassland plant species: a growth‐defence trade‐off is the norm

Plant growth can be limited by resource acquisition and defence against consumers, leading to contrasting trade‐off possibilities. The competition‐defence hypothesis posits a trade‐off between competitive ability and defence against enemies (e.g. herbivores and pathogens). The growth‐defence hypothesis suggests that strong competitors for nutrients are also defended against enemies, at a cost to growth rate. We tested these hypotheses using observations of 706 plant populations of over 500 species before and following identical fertilisation and fencing treatments at 39 grassland sites worldwide. Strong positive covariance in species responses to both treatments provided support for a growth‐defence trade‐off: populations that increased with the removal of nutrient limitation (poor competitors) also increased following removal of consumers. This result held globally across 4 years within plant life‐history groups and within the majority of individual sites. Thus, a growth‐defence trade‐off appears to be the norm, and mechanisms maintaining grassland biodiversity may operate within this constraint.