Effect of annual and perennial cropping regimes in microplots on the population density and vertical distribution of Xiphinema diversicaudatum and on soil porosity

Long-term trends in populations of Xiphinema diversicaudatum were measured in two microplot experiments in which annual and perennial crops of seven plant families were grown in monoculture. Numbers of X. diversicaudatum increased under perennial ryegrass, raspberry and strawberry but decreased under hop, spring barley, potato, sugar beet, cabbage, winter wheat and winter beans; these effects probably reflected differences in the inherent host status of the crops. There were fewer X. diversicaudatum at 40–60 cm below annual crops than below perennial crops, and there were differences between perennial crops in nematode vertical distribution. Soil pore space did not limit nematode population density; total pore space was uniformly large under all crops and there was no detectable instability to water slaking. Under ryegrass a greater proportion of soil volume was occupied by pores too small to accommodate X. diversicaudatum, but since ryegrass supported the second largest nematode population, it seems unlikely that this was seriously limiting.     

A Reproducible Colonial Morphology Formed by Individual Pluripotent Embryonal Carcinoma Cells in Culture and Selection of Variants

Embryonal carcinoma cells are stem cells equivalent to those of the early embryo which can be grown in vitro and which under certain conditions can differentiate into many cell types. Events in this differentiation process are numerous and complex, thus a system for the analysis of clonal differentiation is essential. In this paper I report that individual pluripotent embryonal carcinoma cells can each give rise to colonies, in the absence of a feeder layer but in the presence of β-mercaptoethanol, that show a distinctive and reproducible gross morphology. Embryonal carcinoma cell lines can be derived from the stem cells in these colonies. Furthermore, variant cell lines can be derived from those colonies showing an altered gross morphology. These lines when cloned as above give rise to colonies showing a gross colonial morphology different to that of wild-type. These variant lines have been shown to be embryonal carcinoma cell lines. These findings indicate that genetic and cell lineage analysis of embryonal carcinoma cell differentiation might be possible.     

Condition‐dependent mutation rates and sexual selection

‘Good genes’ models of sexual selection show that females can gain indirect benefits for their offspring if male ornaments are condition‐dependent signals of genetic quality. Recurrent deleterious mutation is viewed as a major contributor to variance in genetic quality, and previous theoretical treatments of ‘good genes’ processes have assumed that the influx of new mutations is constant. I propose that this assumption is too simplistic, and that mutation rates vary in ways that are important for sexual selection. Recent data have shown that individuals in poor condition can have higher mutation rates, and I argue that if both male sexual ornaments and mutation rates are condition‐dependent, then females can use male ornamentation to evaluate their mate’s mutation rate. As most mutations are deleterious, females benefit from choosing well‐ornamented mates, as they are less likely to contribute germline‐derived mutations to offspring. I discuss some of the evolutionary ramifications of condition‐dependent mutation rates and sexual selection.     

Frequency-dependent selection on aposematic prey: some experiments

Experiments to measure the strength of frequency-dependent selection on dimorphic populations of distasteful, brightly coloured prey were carried out, using pastry cylinders as prey and wild birds as predators. When the two forms were haphazardly intermingled, the birds took a relative excess of whichever form happened to be the rarer if all prey in the population were distasteful but selected the forms independently of frequency if they were all neutral in flavour. They did not take an excess of the rarer form from populations of distasteful prey when that form was distributed in a single clump rather than evenly dispersed among the commoner form. This confirms the expectation that rarer forms of aposematic animals are at a frequency-dependent disadvantage when they are intermingled with commoner forms but not when they occur in clusters.     

Seasonal resource tracking by Amazonian hummingbirds

Many ecological theories predict a close match between food resources and the number and diversity of consumers but comparatively few empirical studies have simultaneously quantified seasonal variation in an entire assemblage and its resource base. Hummingbirds provide an excellent model system to examine this relationship because their primary food resource is conspicuous and easily quantified. Over a 2-year period I used mist-netting and direct observation of birds feeding at flowering plants to record the abundance of 16 species of hummingbird in lowland Amazonian rainforest. Although there was very little annual variation in temperature and daylength, both rainfall and river level showed significant seasonal variation. At the study site this resulted in an annual flood lasting for over 5 months, inundating much of the area to a depth of 2 m. I recorded marked seasonal patterns in flowering phenology and arthropod abundance, which correlated with river level and rainfall, respectively. In turn, these seasonal fluctuations in resource abundance correlated very closely with variation in hummingbird species richness, hummingbird abundance and the total biomass of hummingbirds in the assemblage. This study is the first to demonstrate tracking of seasonal resource variation in equatorial rainforest and provides evidence that hummingbirds are sensitive to resource availability at spatial and temporal scales much greater than previously documented.     

Predator regime influences innate anti-predator behaviour in the freshwater gastropod Lymnaea stagnalis

1. Predation incurs high fitness costs in aquatic organisms either through direct consumption or through avoidance responses that reduce time for activities such as feeding and reproduction. Hence, avoidance responses of aquatic organisms should vary to match closely the predation threat in their environment.
2. The freshwater gastropod Lymnaea stagnalis occurs in a variety of environments which vary in the presence or absence of predatory fish. We used naïve snails reared from six populations of this species experiencing different predator regimes (three co-occurring with molluscivorous fish and three without) to assess whether populations differed in the type and degree of their avoidance behaviours. Innate behavioural responses to four treatments (control, conspecific alarm cues, fish kairomones and fish kairomones paired with alarm cue) were compared in laboratory trials.
3. The primary anti-predator behaviour of L. stagnalis in response to fish kairomones was to crawl out of the water rather than seek refuge under water. This response was strongest when fish kairomones were paired with alarm cues, and varied depending on population origin; snails reared from populations co-occurring with predatory fish showed a stronger response than those raised from populations not experiencing such predators. In addition, populations co-occurring with predatory fish responded to the fish kairomones presented alone.
4. Our findings suggest that the degree of innate anti-predator behaviour shown by L. stagnalis , in terms of both the level of risk to which it responds and the degree of response, varies depending on the predator regime experienced by field populations. Together with previous work on cue association, this demonstrates that this gastropod is able to match its avoidance behaviour very closely to short and long term predation threats within its habitat.     

Temporal partitioning of a floral resource by territorial hummingbirds

Most studies of territoriality in hummingbirds have focused on intraspecific competition for resources and the consequences for the spatial distribution of individuals within a habitat. As a result, we know little of the effects of interspecific competition for resources and less still of temporal resource partitioning. Here I describe the interactions of four species of tropical hummingbird which defended the same territory at different stages in the flowering period and at different times of the day. The pattern of territory defence was greatly influenced by the dominance hierarchy between species and the costs and benefits of territory ownership. I used a simple economic model to calculate the predicted territory size based on four potential strategies. Hummingbirds appeared to be defending territories of the smallest economical size, agreeing with two hypotheses: (1) that hummingbirds minimize the cost of territory ownership and (2) that hummingbirds maximize the time spent sitting. The model predicted accurately the observed pattern of territory acquisition; hummingbirds initiated defence as soon as the territory contained sufficient resources and were either displaced by a larger species or replaced by a smaller one as the value of the territory changed.     

Emission of methane from chalk streams has potential implications for agricultural practices

1. The emission of biogenic gases, particularly methane, is usually associated with wetlands rather than clean streams. Here, we investigated methane production from a southern English chalk stream, where increased sedimentation, compounded by extensive macrophyte growth, may have altered ecosystem function. 2. Cover of the channel by the dominant macrophyte, Ranunculus penicillatus, peaked in August, when plant beds were associated with low water velocity and the accumulation of sediment (<2000 μm) dominated by the sand‐sized fraction (63–1000 μm). 3. Over spring and summer there was a marked increase in the silt/clay fraction of the sediment, a concomitant drop in mean particle size and, hence, inferred permeability. At the same time there was an increase in CH₄ transport through Ranunculus stems and an increase in water column CH₄ concentration, while the sediment CH₄ concentration increased 100‐fold between February and April. A marked seasonal enrichment in the δ¹⁵N of N₂ dissolved in the pore water correlated with CH₄ flux and, coupled to the shift in particle size, suggested a transient input of organic matter, possibly of terrestrial origin. 4. Potential areal methane production and measured efflux were similar to that from some U.K. peatlands and represent one of the first accounts of significant methanogenesis to be measured in a stream channel. Most (>90%) of the methane flux is transported to the atmosphere through the Ranunculus stems. 5. Although the total flux of methane from U.K. chalk streams is probably relatively modest (estimated at 3.2 × 10⁻⁶ Tg CH₄ year⁻¹), this phenomenon changes our perception of the health of these ecosystems and indicates another deleterious side effect of agriculture.