Scale insect allozyme differentiation within and between host trees

Summary Allelic frequencies and genotypic distributions in three polymorphic enzyme systems demonstrated genetic differentiation over extraordinarily short distances in a population of black pineleaf scale insects infesting ponderosa pine trees. A hierarchical analysis of the population genetic structure showed significant differences between demes on different twigs within individual host trees, between demes on neighboring trees, and between demes in pine plots on adjacent city blocks. Allelic frequencies at a malic enzyme locus were associated with deme-to-deme variation in ecological correlates of insect fitness, suggesting adaptive hypotheses about the causes of population subdivision.     

Mercury translocation in and evaporation from soil. I. soil lysimeter experiments with 203Hg‐radiolabeled compounds

Due to a considerable increase of anthropogenic mercury emissions, the mercury load of many soils has risen significantly, for instance in northern Europe. Understanding the fate of mercury in soils is a prerequisite for assessing the effects of ecotoxicological concern. This paper presents a method for obtaining qualitative and quantitative information about mercury translocation in and evaporation from soil. Soil lysimeters were treated with ²⁰³Hg‐labeled HgCl₂ and CH₃HgCl and irrigated with artificial rain. It was demonstrated that the leaching of Hg can be detected by measuring the relative y‐activity throughout the soil profile by means of Na(TI)I detectors. Furthermore, the set‐up was designed to allow detection of Hg volatilization from soil by using traps of iodized charcoal, followed by a potassium peroxodisulfate solution and measuring the γ‐activity. The amount of radioactive Hg in soil leachate was measured by a Na(Tl)I well‐type detector after upconcentration. The determination of monomethyl ²⁰³Hg was been performed by extraction procedures that isolate the methyl mercury compounds. The amount of ²⁰³Hg retained in the soil profile and the real depth of leaching were determined by stratifying the soil profile at the end of the experiment and measuring the y‐activity. With control of all pathways of Hg, the experimental design allows performance of a mass balance analysis.     

Mercury translocation in and evaporation from soil. III. quantification of evaporation of mercury from podzolized soil profiles treated with Hg Cl

Mercury evaporation from undisturbed iron‐humus podzol lysimeters was measured over 3 months after treatment with HgCl₂ spiked with radioactive ²⁰³Hg. The relative evaporation rate from HgCl₂ treated soils followed the sum of two exponential functions. Because evaporation asymptotically approaches zero with time, the integral of the fit curve represents the evaporative loss in percent of atmospheric deposition. For the soil investigated, about 5% of atmospheric Hg deposition was reemitted into the atmosphere. It is hypothesized that mercury evaporation can decrease the leaching of mercury in and from soil significantly; this effect is probably increasing with decreasing rain acidity or soil acidity. Mercury deposited as soluble salt remains susceptible to reemission to air for 300 d after incorporation into the soil matrix. Indications are found that Hg evaporation from soils in geological background areas predominantly derives from recent atmospheric Hg deposition and not from geological sources.     

Calcium limitation in Daphnia magna

The role of ambient calcium concentrations on survival, moulting,growth and egg production was assessed in the cladoceran Daphniamagna. A threshold for survival was found in the range 0.1–0.5mg Ca l^–1, even when ionic strength of the medium waskept constant. Accumulated length and length specific dry weightwas retarded at low Ca (0.5–1.0 mg Ca l^–1) at foodconcentrations above incipient limiting level. For lower foodlevels, the effect of Ca on growth was less clear. The effectof low Ca on growth rate was most manifest during the firstdays after hatching, reflecting the higher Ca demands of theearly juveniles. Age-specific egg production was strongly reducedat Ca concentrations <10 mg Ca l^–1. This was partlyan indirect effect of reduced growth rates, but could also bean effect of higher energetic costs associated with Ca uptakein a Ca-poor medium. The high Ca demands in D.magna may notbe representative of other Daphnia species. Nevertheless, highspecific Ca content seems to be a common property of Daphniaspp. and Ca deficiency could be a major determinant of speciessuccess and community structure among crustacean zooplankton;it also puts constraints on carbon sequestration in the pelagicfood web of softwater lakes.     

Particle size,resource concentration,and the distribution of net-spinning caddisflies

Summary The dimensions of net meshes constructed by hydropsychid Trichoptera vary both within and between species. Despite these catchnet differences, the diets of most Hydropsychidae studied in Utah streams were statistically indistinguishable. There was no relationship between the size of available resources and catchnet construction among species assemblages inhabiting 10 different localities. A particle-size model of caddis communities, suggesting that taxa feed selectively on particle sizes corresponding to the dimension of catchnet meshes, is not supported by these data.Diatom concentration increased regularly with downstream passage in two different drainages. The identity and number of coexisting hydropsychid species and the size of their catchnets were strongly correlated with diatom concentration. Taxa with large catchnet mesh were the only residents at sites where diatom concentration was very low; as resource concentration increased downstream, species with successively smaller mesh joined the coexisting guild. Together, the broad dietary similarities and distributional pattern from Utah streams suggest that resource concentration, rather than particle size, is the basis of community organization among the hydropsychid Trichoptera.     

Interpreting population differentiation in terms of drift and selection

Summary Many empirical studies demonstrate some degree of genetic differentiation among populations of the same species. Understanding the relative importance of the processes causing this genetic differentiation has proven to be a difficult task. In particular, population differentiation can be influenced primarily by selection, genetic drift, and migration. We review the effect of drift and migration on patterns of genetic variation, with special reference to the conditions necessary for population differentiation. Conceptually, selection may be implicated in cases of population differentiation if the effect of drift and migration can be shown to be insufficient to cause the observed patterns. We examine some of the pitfalls of this approach when used with allozyme data, and revise a previous conclusion concerning the relative importance of selection in poulations of scale insects.     

Tolerance to low ambient calciumshows inter-population differences in Daphnia galeata

The tolerance to low ambient calcium (Ca) was assessed in twopopulations of Daphnia galeata. There was a threshold for survivalbetween 0 and 2 mg Ca l^-1. However, the ability to cope withlow Ca concentrations clearly differed between the two populations,as the population from the low-Ca locality was less tolerantto low Ca. Additionally, neonate individuals had poorer survivalthan adults when reared at ~0 mg Ca l^-1, supporting a suggestedjuvenile bottleneck regarding the tolerance to low Ca concentrations.The mean specific Ca content in adult D. galeata was not differentbetween the two populations, but individuals reared in mediumwith 1 mg Ca l^-1 only had two-thirds of the Ca content of thosereared in medium with 10 mg Ca l^-1. The significant differencesbetween the two populations investigated suggest that inter-populationvariation in tolerance to low ambient Ca concentrations mightbe important to explain the success of Ca-demanding crustaceansin softwater lakes.     

The costs of moulting in Daphnia; mineral regulation of carbon budgets

1. Zooplankton differ in their elemental contents. For the essential elements calcium (Ca) and phosphorus (P), Daphnia spp. have particularly high contents, and thus high demands, compared with other cladocera and copepods.
2. There are recent indications that the growth of zooplankton, notably Daphnia , may be directly constrained by the availability of P and Ca.
3. Major pools of P and Ca are associated with the carapace, and the moulting process represents a constant drain of these elements, as well as carbon (C), from the animals.
4. The demands for P and Ca for moulting may pose constraints on the carbon accumulation in the animals, and thus the extensive needs for these elements may translate into a total loss of C transported in the pelagic food web.
5. While the costs of moulting are substantial for the zooplankton, the export of minerals bound in the moult will play only a minor role for the epilimnetic element budgets in most lakes.
     

Mercury translocation in and evaporation from soil. II. Evaporation of mercury from podzolized soil profiles treated with HgCl2 and CH3HgCl

Mercury evaporation from soil columns of an iron humus podzol treated with various amounts of HgCl₂ and CH₃HgCl was measured over 3500 h. The effects of rain acidity, rain duration, and rain intensity were investigated. Hg evaporation from CH₃HgCl‐treated soil columns seems to be a biologically determined process. Hg evaporation from HgO₂‐treated soil probably is mainly an abiotic process, following a pseudo first‐order reaction with rapidly decreasing evaporation rate due to a decreasing amount of available Hg. The added Hg compounds were transformed to highly volatile Hg⁰ and/or (CH₃)₂Hg. The Hg evaporation rate decreased with increasing rain acidity, which may cause accumulation of Hg in the soil. No effects of rain duration and rain intensity were found.