The role of watercourse versus overland dispersal and niche effects on ostracod distribution in Mediterranean streams (eastern Iberian Peninsula)

The processes behind the heterogeneous distribution of species involve a combination of environmental and spatial effects. In the spatial context, stream networks constitute appropriate systems to compare the relative importance of two dispersal modes in aquatic organisms: overland and watercourse dispersal. In the present study, we analyzed the distribution of ostracod species in a river network in the eastern Iberian Peninsula, with variation partitioning between environmental and spatial factors, using Moran and Asymmetric Eigenvector Maps (MEMs, AEMs) as spatial variables. Our aims were to determine the relative importance of environmental and spatial control and to compare the importance of overland and watercourse dispersal for species distribution of passively-dispersing aquatic organisms. Our results suggest that watercourse was the most important dispersal mode, favoring mass-effects. The role of species sorting was significant and related to temperature, stream width and water quality, measured as a biotic index (IBMWP). These results stress the major importance of connectivity, besides niche-related factors, in structuring riverine communities of passively-dispersing aquatic organisms.     

武汉东湖轮虫种类组成及其分类讨论

作者根据Ｋｏｓｔｅ（１９７８）提出的轮虫分类系统对武汉东湖４个站１９９１年轮虫种类进行了分类。同时,对我国以往的轮虫分类中存在的若干问题进行了讨论。１９９１年作者共观察到轮虫６９种,隶属于１９个科,２９个属,新记录１２种,其中,采自水样的轮虫种类有５９种,沉积物中休卵萌发的种类４１种,两者相同种类３１种,休眠卵萌发特有种类１０种,其中的盘状鞍甲轮虫、侧刺伏嘉轮虫、侧扁棘管轮虫等在６０年代的东湖水体     

Evolutionary patterns in tropical marine reef fish feeding

The majority of tropical reef fishes are acanthopterygians. Most of them are percomorphs and thus are likely monophyletic. In accordance to modern systematics, the primitive types among the latter are large-mouthed suction feeders. Species from advanced families often have biting oral jaws with a reduced number and complexly shaped teeth. Mouth sizes decrease from the primitive towards the advanced reef fishes when ranked according to increasing family numbers (Nelson 1984). To create a functional resource axis, Randall's (1967) and Hobson's (1974) data on tropical reef fish feeding were re-interpreted by ranking food items from mobile to sessile prey. The primitive paracantho pterygian and acanthopterygian reef fishes are large-mouthed, suction-feeding predators on mobile prey. Most of the advanced, small-mouthed species are browsers and grazers, but often feed on mobile prey too. Obligatory specialists (monophagous and unable to switch) seem to be relatively rare among modern reef fishes. The trends stated above indicate a wealth of parallel developments in many advanced families of reef fishes towards small, often biting oral jaws. This parallelism may be the result of comparable regimes of selection pressures in reefs and of the need for newly evolved species to establish themselves within the already existing guilds.     

Polychaetes as surrogates for marine biodiversity: lower taxonomic resolution and indicator groups

Due to conservation needs, reliable rapid-assessment methods for mapping of biodiversity are needed. One approach is to use surrogates, i.e. quantities that correlate strongly with the number of species, but are easier to obtain. The purpose of this paper is to test two polychaete surrogates, one for higher taxa and one for indicator groups, that will facilitate prediction of species richness in marine soft-bottom communities. Soft sediment is an important habitat which covers most of the ocean bottom. Data on polychaetes from the North Atlantic were used since polychaetes are often numerically dominant in the benthic assemblages, both with regard to the number of species and their abundance. In the polychaete assemblages along the Norwegian coast, richness at the genus, family and order level were significantly, linearly correlated to total species richness (r 0.92). Polychaetes in the order Terebellida were found to be a good indicator of polychaete species richness and to a lesser extent also of whole benthic assemblages. The group Terebellida is potentially well suited as an indicator group, since it contains long-lived, large species that are easy to sort from the sediment and it is well defined taxonomically. Although promising as proxies for species richness in marine biodiversity studies, the use of lower taxonomic resolution and indicator groups requires further investigations in more local areas where there are conservation issues.     

Thirteen years of wetland vegetation succession following a permanent drawdown,Myrkdalen Lake,Norway

Myrkdalen Lake in Western Norway was subjected to a 1.4 mdrawdown in June 1987. Plant establishment and vegetation succession on theexposed sediments of a fluvial delta plain was monitored through 2000. Theinvestigated area extended from the original Equisetumfluviatile zone to the new lake edge. The substrate was homogeneousand consisted mainly of minerogenous fluvial sediments. Vegetation data weresampled within continuous quadrats along transects perpendicular to the shore,and they dropped 93 cm in elevation. Detrended CorrespondenceAnalysis and Canonical Correspondence Analysis confirmed that time sincedrawdown and elevation of the quadrats appeared to be ofnearly equal importance in explaining succession. Plant establishment was rapidon the exposed sediments due to a seed bank and to rapid invasion of plants.Thesuccession includes both floristic change as a function of time and a spatialseparation in relation to the water level. The species succession was marked bya growth form progression: mosses and annuals non-clonal perennialsclonal perennials. After one month, the annual Subulariaaquatica and small acrocarpous mosses dominated the site. Dominancethen shifted to Deschampsia cespitosa, Juncus filiformis, Blasiapusilla and Polytrichum commune. Subsequentlythere was an increase of Carex vesicaria in the lower zoneand Calamagrostis purpurea in the upper zone,while Phalaris arundinacea was common over most of theelevational gradient. Equisetum declined where it haddominated before drawdown, but it expanded gradually towards the new shoreline.The vegetation remained dynamic after 13 years and it is not considered to beinequilibrium with the new environmental gradient yet. However, annual changesmeasured by DCA scores have slowed and two vegetation zones have developed. Themajor vegetational differences along the elevational gradient can be explainedby the height of the mean June water limit. This example of species turnover inspace and time may be a model for other successions that occur along a stronggradient.     

HOW CRITICAL IS THE CRITICAL N:P RATIO?1

The value of cellular N:P that corresponds to co‐limitation by N and P, the critical (R_crit) or optimum ratio, has been used to infer the competitive advantage of phytoplankton growing in P‐impoverished systems. Using a revised quota model, with a normalized quota function and capable of simulating surge transport, the interactions between the minimum P‐quota (PCo), the shape of the P‐quota–cell growth relationship (affected by constant KQP), and transport kinetics in affecting the utility of R_crit are considered. For a low PCo to endow an organism with a high R_crit over a wide range of growth rates, the P‐quota curve must be more hyperbolic (KQP low) rather than linear (KQP high). PCo and KQP also affect the half saturation constant for growth, KgP ; this and the capacity to transport nutrients at rates above those required to sustain steady‐state growth endows a competitive advantage. However, the kinetics of transport into the organism have a greater potential for affecting KgP than changing the kinetics of internal P usage. Thus, the value of R_crit is not a critical factor affecting competition except in extreme oligotrophic conditions. For competition between species, nutrient transport, accumulation, and resource utilization are all important. However, the efficiency of internal resource utilization is of lesser importance, and certainly not of greater importance, than resource acquisition. Multinutrient models intended to describe competition need to recognize these interactions; the traditional quota model is poorly equipped to do so.     

Reduced community diversity in semi-natural meadows in southern Sweden, 1965–1990

Two data sets, one from the 1960s and one from 1990, from continuously managed semi-natural meadows (semi-natural grasslands used for hay-cutting) in Småland, southern Sweden, were analysed to describe the vegetation and in an attempt to characterise changes that have occurred in the vegetation. Based on a classification of the data set, nine plant communities were recognised. The main vegetational differences, as revealed by an ordination, were due to variation in soil moisture, which ranged from wet to dry.During the investigation period, the amount of hay meadow area decreased, particularly the area of wet-moist meadows. In addition, the total variation in the vegetation diminished, and three plant communities more or less disappeared. The turnover index of species in the data set was 36%, and most of the species that were lost had initially been uncommon. Lost species included rare taxa, such as Gentianella campestris and Linum catharticum, while the new species tended to be common in the region. Annual and biennial species accounted for a greater proportion of the lost taxa than did perennial species. The mean cover of species per plot and the relative abundance of graminoids per plot increased.The recorded changes in the vegetation may be related to the increased loads of nitrogen pollutants as well as to a decrease in management intensity. The diminished area is related to changes in land-use. The meadow sites in Småland have been part of a landscape rich in grasslands, but today they have a more or less relict status. They differ from grasslands found elsewhere in Sweden and Scandinavia. In Sweden, the majority (68%) of endangered vascular plants belong to the agricultural landscape. To ensure the survival of individual species it is important to preserve all types of meadows, not only a few selected ones, since no two meadows are alike.     

Latitudinal patterns in European ant assemblages: variation in species richness and body size

Using published distributions of 65 species from the British Isles and northern Europe, we show that ant assemblages change with latitude in two ways. First, as commonly found for many types of organisms, the number of ant species decreased significantly with increasing latitude. For Ireland and Great Britain, species richness also increased significantly with region area. Second, although rarely demonstrated for ectotherms, the body size of ant species, as measured by worker length, increased significantly with increasing latitude. We found that this body-size pattern existed in the subfamily Formicinae and, to a lesser extent, in the Myrmicinae, which together comprised 95% of the ant species in our study area. There was a trend for formicines to increase in size with latitude faster than myrmicines. We also show that the pattern of increasing body size was due primarily to the ranges of ant species shifting to higher latitudes as their body sizes increased, with larger formicines becoming less represented at southerly latitudes and larger myrmicines becoming more represented at northerly latitudes. We conclude by discussing five potential mechanisms for generating the observed body-size patterns: the heat-conservation hypothesis, two hypotheses concerning phylogenetic history, the migration-ability hypothesis, and the starvation-resistance hypothesis.     

Guilds and assembly rules in lawn communities

Abstract. The community structure of 11 lawn sites in New Zealand and Fiji was examined in terms of guilds, seeking assembly rules based on guild proportionality. First, associations were analysed, using a new patch model which examined the mean of associations within patches of about 4 cm x 4 cm. As expected from the previously-demonstrated existence of niche limitation at this scale in lawns, the majority of associations between individual species were negative. Even in a lawn only four months old there were significant associations (mostly negative, and one positive). At some sites those species with the most negative associations were those whose morphology might be expected to restrict co-existence with other species. Much ecological theory is based on the assumption that there are limitations to coexistence, related to the resource usage of the species, such that species that are too similar in resource use are less likely to coexist. This theory was tested by looking for evidence of guild proportionality. We defined two guilds, graminoids and forbs. Using these guilds, three of the sites showed significant evenness in proportional representation from the two guilds in quadrats containing four species, i.e. there was less variation in the graminoid:forb ratio than expected on a random basis. Inclusion of species-poor quadrats in a joint analysis over all richness categories overwhelmed this effect, though there was still a strong trend for two of the New Zealand sites: the two species-rich grass lawns. Preliminary analyses of alternative guild delimitations gave no indication that other guild delimitations would have revealed stronger community structure. There was a strong and significant tendency in most sites for the graminoid guild to be more strongly represented in species-poor quadrats, an effect caused by species frequencies.     

How mosquitoes fly to man

1. A model is presented to estimate the mosquito density and radius (r) of the attraction range of man as the bait.
2. Man walked to a spot in the woods and sampled mosquitoes flying to him with a net continuously. The arrival pattern was constant in each mosquito species, and in all species, the number of mosquitoes sampled per five min became constant after ten minutes sampling.
3. The mosquitoes gathered to man were composed of D: those waiting alight, I: those intruding the attraction range and F: those following man to the spot. They were estimated by means of continuous sampling sets and densities of D and I were calculated.
4. r s of female and male of A. albopictus and A. japonicus were 4.0, 5.6 and 9.0 m respectively. In these species, most individuals were D, although, in T. bambusa all individuals were I, flying to search for hosts.
5. How mosquitoes and hosts encounter, is discussed.