Density and environmental effects on shell size in some sand dune snail populations

Populations ol three species of land snail, Helicella itala. Candidula intersecta and Cochlicella acuta are studied on sand dunes on Coll in the Inner Hebrides. Population density and mean shell size were estimated on 18 sample sites. The sites were ranked for six environmental factors. Environmental factors which are related to the dune vegetational succession account for much of the variation in the densities of C. intersecta and C. acuta. The density of H. itala, however, shows no strong association with these factors. Both snail density and shell size are reactively independent of total soil calcium levels. For each of the three species, mean shell size is negatively associated with population density; the environmental factors account for little of the variation in shell size. Four possible mechanisms are suggested to account for the variation of shell size with density. It is argued that a direct influence of density on shell size, possibly mediated by mucus conditioning of the environment is the most probable mechanism. There is some evidence to suggest an inter-specific effect whereby C. intersecta density affects H. itala shell size.     

Nitrogen translocation in a clonal dune perennial,Hydrocotyle bonariensis

Summary Hydrocotyle bonariensis, a common rhizomatous perennial of coastal North Carolina, forms extensive clones in dune systems characterized by a patchy nitrogen distribution. An experiment was conducted in which Hydrocotyle clones were grown across artificially created soil nitrogen gradients to determine: (1) the effect of soil nitrogen availability and nitrogen translocation on clonal structure and (2) the costs versus benefits of nitrogen translocation as measured by sexual and clonal reproduction. Acropetal translocation of nitrogen resulted in highly significant benefits to clones growing from areas of high N to areas of low N. Limited basipetal translocation was also demonstrated. Hydrocotyle ramets responded to increased nitrogen availability, from either intraclonal translocation or immediate uptake from the soil, by producing branches. Nitrogen level, however, had no effect on internode distances. Clonal integration of nitrogen, in tandem with a plastic morphology, allow Hydrocotyle clones to expand across a nitrogen-limited dune environment and to locally exploit nitrogen patches when they are encountered.     

An estimation of the natural value of dune habitats using Empidoidea (Diptera)

The dune landscape along the Belgian coast was evaluated on the basis of its Empidoidea fauna by investigating the faunal composition in different habitat types. The sites selected for sampling were marram dunes, dune grassland, dune slack, scrubby vegetations and three different dune woodlands. White water traps at soil surface level were used to sample five sites from the end of April until November 1989, and four sites from April to August 1992. Data on the most abundant Dolichopodidae, Empididae and Hybotidae were analysed by means of the following multivariate analyses: detrended correspondence analysis (DCA), canonical correspondence analysis (CCA) and two-way indicator species analysis (TWINSPAN). Most sites showed a distinctive species composition, and several species were confined to particular habitat types. Empidoid communities from canopied and open habitats showed the largest contrasts during 1989. Six environmental variables were measured in July and August 1992, and correlations indicated that aerial humidity and, to a lesser extent, soil moisture were probably the main factors explaining dolichopodid distributions. In Empididae, on the contrary, the presence of canopy by scrub or trees appeared to be important for the occurrence of many species. Thus species diversity and abundance increased from dry to humid sites in dolichopodids and from open to canopied sites in empidids. On the other hand, canopied sites appeared to function as home-bases from which certain empidid species colonized more sun-exposed areas. With the Dolichopodidae it was possible to examine the recorded distribution of all species throughout Belgium. Nearly 75% of the dolichpodids appeared to be typical lowland species and in eight species, largest Belgian populations were established in the dune region. Moreover, seven species could be considered as true coastal species as they were almost entirely restricted to this region. These results clearly stress that not merely the coastal dunes themselves but the full diversity of the associated habitats needs to be maintained in order to protect the characteristic dune empidoid communities.     

Agricultural development vs biodiversity conservation: the Mediterranean semiarid vegetation in El Ejido (Almería, southeastern Spain)

The development of a greenhouse agriculture in the traditionally impoverished region of Poniente de Almería, on the Mediterranean coast of Spain, has caused an enormous rise in living standards. However, the environmental impact of this badly-planned growth threatens the every dynamics of the exploitation system. A special examination must be made of the use of the three major resources responsible for the functioning of greenhouse production and its impact on the ecosystems and particularly on the vegetation. These resources are: clayey soils, sand from fossil dunes and ground water. While the use of the clayey soils and sand have negative effects on the conservation of ecologically valuable communities found nowhere else in Europe, ground water overexploitation has produced an increase in salinity in most of the aquifers. Of these, sand has been by far the best monitored resource and restoration programmes have been implemented in the extraction zones. This survey deals with the recent evolution of areas where the arto (Maytenus senegalensis subsp. europaeus) and the sabina (Juniperus phoenicea subsp. turbinata) have long been the dominant species, although the presence of the former is nowadays notoriously diminished. The study is based on aerial photographs taken in 1957, 1977 and 1985, together with our own field work. Curiously enough, all this man-made process of degradation has stimulated ornithological diversity. Finally, we propose here some measures which aim to preserve the most important enclaves of these Mediterranean shrub formations, specially those of the arto, since sabina-dominated communities already belong to existing conservation areas.     

Effects of environmental and temporal factors on Glomeromycotina spores in sand dunes along the Gulf of Valencia (Spain)

AMF symbiosis in sand dunes is the key for maintenance of stable vegetation. The main goal of this work was to determine the effects of environmental and temporal factors on AMF living in sand dunes (Gulf of Valencia, Spain). Soil samples were collected seasonally at 6 sites, during 2 yrs, from three habitats and four plant species and the frequency and relative abundance of AMF was examined. AMF were more frequent in mobile than in embryonic dunes, in spring and in sites with old vegetation. Ten AMF species were identified, their distribution depending mainly on the anthropogenic disturbance of the site. Gigasporaceae Cetraspora sp. and Dentiscutata sp. preferred undisturbed soil whereas Diversisporaceae, Glomeraceae and other Gigasporaceae were associated with recently restored soils. All AMF species were found in all plant species although Corymbiglomus corymbiforme was mainly associated with Echinophora spinosa. Our results might be of help for Mediterranean sand dune restoration.     

Effects of sand burial on purple sandgrass (Triplasis purpurea): the significance of seed heteromorphism

The effects of sand burial on seedling emergence, growth, and reproduction of the annual dune grass Triplasis purpurea (Walt.) Chapm. were examined. This species matures heteromorphic seeds on panicles enclosed by leaf sheaths along tiller nodes in a characteristic pattern: seeds at the lower nodes are heaviest, whilst those at the upper nodes are lightest. Field excavation of seedlings revealed that seedlings can emerge from seeds buried to 4 cm.In one experiment in an incubator, seeds collected from the upper and lower tiller nodes of glasshouse-grown parental plants were buried in sand at depths of 0, 3, and 6 cm. As expected, the effects of seed depth on seedling growth were due to delayed emergence for seedlings from buried seeds. However, seedlings from the larger seeds from the lower nodes of parental plants had significantly greater dry mass than those from the lighter seeds from upper nodes. Because lower node seeds are the most likely to become buried by sand in nature, the seed heteromorphism may be adaptive in the coastal environment.A second experiment involved the burial of seedlings to 0, 50, or 100% of their height. Although most seedlings did not survive complete burial (100% height), for partially buried seedlings (50% height) there was high survival and a marked stimulation of growth and eventual reproduction compared to unburied seedlings. This stimulation could be linked to increased root growth for seedlings that had been buried. The ability to respond positively to sand accumulation may represent an adaptation to the dynamic dune environment.     

The effect of resource integration on fitness related traits in a clonal dune perennial,Hydrocotyle bonariensis

Summary The costs and benefits of resource integration in Hydrocotyle bonariensis were examined by comparing severed and intact clones grown across multiple resource gradients. Basipetal movement of water, nitrogen and photosynthates was demonstrated to occur between two rhizome branch systems interconnecting hundreds of ramets within a clone. Hydrocotyle clones of this size and larger have been shown to span highly patchy and unpredictable resource conditions in coastal dune environments. The extensive movement of water and nitrogen to portions of a clone deficient in these resources, resulted in a significant net benefit to the clone in terms of fitness-related traits: total biomass, ramet proliferation and seed production. The translocation of photosynthates across light gradients allowed for sexual reproduction and clonal expansion in the shade although this incurred a net cost to the clone in terms of ramet and seed production.