Invertebrate colonization of a new, man-made stream in southern Sweden

SUMMARY.

• 1 The invertebrate colonization of a man-made stream, Flugströmmen, in southern Sweden was monitored for 18 months in 1988 and 1989. Benthic samples were taken on twelve occasions from three sites (upstream, middle and downstream) and community structure was compared with that at ten natural, permanent reference sites nearby.
• 2 The number of species colonizing increased rapidly during the first 3 months. The increase was most rapid upstream but levelled off during the second year at this site, while numbers continued to increase downstream.
• 3 Simuliid species were the earliest colonizers and reached high densities at upstream and middle sites during the first year. Ephemeropteran and plecopteran species also occurred early on, whereas Coleoptera, Odonata and Trichoptera were, on average, slower to colonize. Blackfly densities decreased upstream after the first year and hydropsychids became numerically dominant.
• 4 The colonization order of functional feeding groups was as predicted: filter feeders first, grazers/collectors intermediate, predators and shredders last.
• 5 After a year, the community structure in Flugströmmen closely resembled that in lake-outlet streams situated in the area, although communities at the three sites within the stream were most similar to one another.
• 6 The possible role of competitive and predatory processes in determining the observed successional patterns are discussed.

     

Membrane Lipids in Senescing Green Tissues2

At a late stage in sensescence cucumber cotuledons lose freshweight rapidly; at the same time there is an increase in apparentfree space and large quantities of electrolyte leak out whendiscs of cotyledon tissue are floated on water. It is concludedthat tonoplast and plasma membrane become leaky at this time. Phosphatidyl choline, the major phospholipid present, beginsto disappear once the cotyledons reach maximum fresh weight;by the time rapid water loss starts, 56 per cent has gone, andphosphatidyl ethanolamine and inositol start to disappear. Onlyat maturity is there enough of these phospholipids to furnishmore than two complete membranes around each cell; it is suggestedthat the decline in phospholipid level at senescence destroysmembrane integrity and allows leakage. The glyohpids begin to disappear at the same time as chlorophyll,2 weeks before weight loss starts. The minor lipid phosphatidyl glycerol is the first to disappearfrom the cotyledons.     

Differences in insect species richness and faunal composition of birch seedlings, saplings and trees: the importance of plant architecture

Abstract. 1. Data are presented on the species richness and faunal composition of herbivorous insects on birch seedlings, saplings and trees at one site in Northern England.
2. Species richness of insect herbivores in equal-sized samples from birch seedlings and trees was similar through most of the season.
3. Effects of plant architecture were confined to the first sampling date, when seedling faunas were species poor compared with trees – possibly due to safe overwintering sites on the extensive bark, twigs and buds of trees.
4. The faunal composition of birch seedlings, saplings and trees was also similar. Out of a total of 112 recorded species of herbivores, only one aphid species was confined to seedlings.
5. Similarly, no evidence for clear-cut vertical stratification of insects within trees was found.
6. Species turnover as host plants mature ('horizontal' stratification) and vertical stratification within trees add little to the high overall species richness of birch-feeding insects in Britain, contrary to the predictions of Lawton (1983).     

Evidence of parasite-mediated disruptive selection on genetic diversity in a wild fish population

Identifying the processes maintaining genetic variability in wild populations is a major concern in conservation and evolutionary biology. Parasite-mediated selection may strongly affect genetic variability in wild populations. The inbreeding depression theory predicts that directional selection imposed by parasites should act against the most inbred hosts, thus favouring genetic diversity in wild populations. We have tested this prediction by evaluating the strength and shape of the relationship between the load of a harmful fin-feeder ectoparasite ( Tracheliastes polycolpus ) and the genome-wide genetic diversity (i.e. heterozygosity measured at a set of 15 microsatellites) of its fish host, the rostrum dace ( Leuciscus leuciscus ). Contrary to expectation, we found a nonlinear relationship between host genetic diversity and ectoparasite load, with hosts that were either homozygous or heterozygous harbouring significantly fewer parasites than hosts with an intermediate level of heterozygosity. This relationship suggests that parasites could increase the variance of global heterozygosity in this host population through disruptive selection on genetic diversity. Moreover, when genetic diversity was measured at each locus separately, we found two very strong positive associations between host genetic diversity and the ectoparasite load. This latter result has three main implications: (i) genome-wide effect cannot alone explain the nonlinear relationship between global heterozygosity and ectoparasite load, (ii) negative non-additive allelic interactions (i.e. underdominance) may be a mechanism for resisting ectoparasite infection, and (iii) ectoparasites may favour homozygosity at some loci in this host population.     

Macroinvertebrate communities in streams in the Himalaya, Nepal

• 1 Macroinvertebrates were sampled in the riffles of fifty-eight streams from three regions of the Himalaya (Anapurna, Langtang and Everest) in Nepal. A semi-quantitative method with identification to family level was used to describe communities on-site.
• 2 Stream physicochemistry was assessed and the community structure of macroinvertebrates was related to chemistry, physiography (substratum composition, altitude and size), geographical location and the dominant land use in each catchment (terraced agriculture, forest or scrub). Community data were analysed by ordination (DECORANA) and classification (TWINSPAN).
• 3 The concentration of cations in stream water decreased significantly with altitude. Chemistry also differed between regions; sites from Anapurna had a higher pH and conductivity than those in the other two areas.
• 4 Communities were dominated by aquatic insect larvae, with Ephemeroptera, in particular the Baetidae, most numerous across sites.
• 5 There were, nevertheless, differences in community structure between sites, which were related closely to stream physicochemistry. Ordination scores were strongly correlated with altitude, magnesium concentration and substratum composition. Classification was also linked to altitude and chemistry, differentiating high-altitude sites with low silica concentrations from others. Sites from the Anapurna and Everest regions, with their contrasting chemistry, were also separated.
• 6 Community structure was also related to land use: streams draining catchments dominated by terraced agriculture had different communities from those in scrub or forest. This result was confounded, however, by the strong relationship between land use, altitude and chemistry; sites in terracing were at lower altitude, had higher concentrations of silica and a higher proportion of fine sediments than those in the other land-use types.
• 7 Overall, our data indicate that natural features of the relief and geology in the Himalaya create strong gradients in their invertebrate faunas, but that activities of man may have an effect on stream structure and ecology through catchment management.

     

An Inhibitor of Mitochondrial Oxidations in Extracts Prepared from Senescing Leaves

Aqueous extracts were prepared from leaves of Ricinus Communis,L. by centrifuging homogenates at 18000 g for 20 min. Extractsprepared from yellow or brown leaves inhibited the oxidationof succinate by mitochondria prepared from Ricinus endosperm.Extracts became less inhibitory when dialysed or treated withbovine serum albumen. The inhibitory properties of the extractsare attributed to their fatty acid content.     

Preferences and apparent digestibilities of sugars by fruit damaging birds in Japan

Gray starlings Sturnus cineraceus, azure-winged magpies Cyanopica cyana and brown-eared bulbuls Hypsipetes amaurotis are among the main bird pests in commercial fruit orchards in central Japan. Recently Brugger & Nelms (1991) suggested that developing high-sucrose fruit cultivars could reduce crop damage, because some pest birds lack the enzyme sucrase and can develop an aversion to sucrose. Preferences for, and digestibilities of, the monosaccharides glucose and fructose and the disaccharide sucrose by these pests species were therefore studied to assess whether this idea would be applicable in central Japan. Gray starlings and brown-eared bulbuls were able to detect glucose, fructose, a mixture of glucose and fructose, and sucrose at a concentration of 12% w/v. Azure-winged magpies also detected glucose and fructose, but failed to detect sucrose at the same concentration. In pairwise preference trials gray starlings and azure-winged magpies selected the monosaccharides over sucrose, but brown-eared bulbuls did not. To estimate the digestibility of the sugars the apparent assimilated mass coefficient, AMC*, was calculated for each species eating each sugar by measuring intake and faecal output. Monosaccharides had mean AMC*s of 0.77, 0.96 and 0.92 when consumed by gray starlings, azure-winged magpies and brown-earned bulbuls respectively. AMC* values for sucrose were 0.82 and 0.49 for brown-eared bulbuls and azure-winged magpies respectively, but gray starlings were shown to be unable to digest sucrose. As AMC* values varied from 0.75 to 0.97, consumption rates of sugars increased as digestibility decreased. Although increasing sucrose contents of commercial fruits may deter sucrase-deficient birds such as gray starlings from depredating fruits, it may also lead to increased crop damage by species such as the azure-winged magpie and brown-eared bulbul which may have to consume more of the less digestible fruit in order to meet their energy requirements.     

Buzzards Buteo buteo and Ravens Corvus corax in the uplands of Britain: limits to distribution and abundance

The current breeding ranges of Buzzards Buteo buteo and Ravens Corvus corax in Britain are restricted to the west, and both are relics of former widespread but not necessarily even distributions. Moore (1957) attributed the Buzzard's contraction in range to persecution by gamekeepers, his evidence being the complementary nature of the Buzzard's distribution with those of gamekeepers in the lowlands and grouse moors in the uplands; others have attributed the Raven's decline to the same cause. Is Moore correct and, if so, are these species' ranges still limited by persecution? Here we adopt an approach similar to Moore's, whereby we compare the current geographical patterns of abundance of Buzzards and Ravens in the British uplands with that of an index of grouse moor density. Unlike Moore, however, we take into account climate, land cover, topography and sheep stocking density, as well as grouse moor distribution, using multiple logistic regression modelling. We show that in the uplands the distribution of grouse moors strongly limits that of Buzzards and Ravens but are unable to conclude that this is a result of persecution because there are several competing hypotheses (moor management, food and nest-site availability) between which we are unable to distinguish.     

The role of leaf wounding and an epigeal predator on caterpillar damage to tomato plants

The effects of leaf wounding and the presence of a New Zealand carabid predator, Megadromus antarcticus, on the mortality of, and leaf damage caused by, Spodoptera litura on tomatoes were examined in glass houses. The presence of the non-climbing carabid increased S. litura mortality with a subsequent decrease in leaf damage. Leaf wounding produced a possible decrease in herbivory but did not affect S. litura mortality. Neither leaf wounding or the presence of carabids affected the vertical distribution of leaf damage on the tomato plants. The role of M. antarcticus as a biocontrol agent for tomato pests in New Zealand is briefly discussed.