Changes In Species Assemblages Research Articles

Diatom responses to late-glacial and early-Holocene environmental changes at Kråkenes, western Norway

A stratigraphic diatom sequence is presented for the period from 13,870-9,170 cal BP from Krakenes Lake, western Norway. Changes in species assemblages are discussed with reference to the changing environmental conditions in the Allerod, Younger Dryas, and the early Holocene and to the development of the aquatic ecosystem. The site is sensitive to acidification, and diatom-based transfer functions are applied to estimate the past pH status. The combination of rapid sediment accumulation together with an excellent calibrated radiocarbon chronology means that the rate of inferred pH change and associated increase in [H+] can be assessed and compared with recent, anthropogenically acidified situations.

Temporal changes of microbial assemblages in the ice and snow cover of a high mountain lake

It has recently been shown that a rich community of microorganisms inhabits the slush layers of the winter cover of high mountain lakes. In this study, temporal changes in species assemblages and environmental conditions in the ice and snow cover of Lake Redó in the Pyrenees (Spain) are presented. The winter cover was a highly dynamic environment, in which major changes occurred in physical structure and chemical characteristics. The biomass and species composition of the microbial groups present (autotrophic and heterotrophic flagellates, ciliates, and bacteria) reflected these physicochemical alterations. After an initial phase, during which the ice sheet formed and the first snows accumulated with little or no development of slush layers, there were two stages of microbial assemblages, which coincided with the two main phases in the physical change of the cover: the growth period (from January to mid‐April) and the ablation period (from mid‐April to ice‐out in June). Initially, microbial biomass originated from inputs of phytoplankton‐rich (5.9 μg chlorophyll a liter−1 ) surface lake water that flooded the cover as a result of the hydrostatic adjustment induced by the progressive accumulation of snow on top of the ice sheet. This was followed by a differential growth of mixotrophic or heterotrophic flagellated species, such as Ochromonas sp., Cryptomonas spp., Monosiga ovata and Oikomonas termo, which peaked at different times. Species depended primarily on bacterivory for their growth, since during this period, light did not reach the slush layers. By April, the transition from cover growth to ablation was marked by a reduction in biomass and in the number of species. During the ablation period, the rate of change of the cover was greater, and microbial assemblages were characterized by the vertical segregation and by the appearance of new species, some of which were typically nonplanktonic. During that time, the cover was highly influenced by the large amount of water from the melting of the snowpack within the catchment. This water likely provided chemicals and organisms, while increased light availability allowed for the growth of algae (Pteromonas sp. and other volvocales) and of the associated food web in the upper slush. In deeper layers, however, a bacteria‐based food web provided prey for large ciliates (Urosoma sp., Dileptus sp., and Lacrymaria sp.), probably carried from littoral or watershed soils.

Development of community metrics to evaluate recovery of Minnesota wetlands

Monitoring wetland recovery requires assessment tools that efficiently and reliably discern ecosystem changes in response to changes in land use. The biological indicator approach pioneered for rivers and streams that uses changes in species assemblages to interpret degradation levels may be a promising monitoring approach for wetlands. We explored how well metrics based on species assemblages related to land use patterns for eight kinds of wetlands in Minnesota. We evaluated land use on site and within 500 m,1000 m, 2500 m and 5000 m of riverine, littoral, and depressional wetlands (n = 116) in three ecoregions. Proportion of agriculture, urban, grassland, forest,and water were correlated with metrics developed from plant, bird, fish, invertebrate, and amphibian community data collected from field surveys. We found79 metrics that relate to land use, including five that may be useful for many wetlands: proportion of wetland birds, wetland bird richness, proportion of insectivorous birds, importance of Carex, importance of invasive perennials. Since very few metrics were significant for even one-half of the wetland types surveyed, our data suggest that monitoring recovery in wetlands with community indicators will likely require different metrics,depending on type and ecoregion. In addition, wetlands within extensively degraded ecoregions may be most problematic for indicator development because biotic degradation is historic and severe.

Response of an Assemblage of Coccinellidae (Coleoptera) to a Diverse Agricultural Landscape

Changes in species assemblages of predatory coccinellids in response to landscape structure (habitat diversity and patchiness) was studied in a southern Michigan landscape during 1989–1990. Three sites with different mosaics of cultivated (alfalfa, corn, wheat) and uncultivated (deciduous, field succession) habitats were sampled for coccinellid species using yellow sticky traps. The landscape at each site was characterized using the Berger–Parker index for habitat diversity and a relative patchiness index for habitat fragmentation. Relative abundance, species richness, species dominance, and the Kendall coefficient were used to analyze and compare species assemblages among sites. Principal component analysis (PCA) was used to extract factor components per species and site scores, which were compared with landscape indices using the Pearson correlation coefficient. Thirteen species of coccinellids were captured during the 2 yr of weekly sampling. Of these species, Coccinella septempunctata (L.) was the dominant species in the landscape and was equally abundant in the 3 sites. Coleomegilla maculata lengi Timberlake was more abundant in the site that had a corn habitat, whereas Cycloneda munda (Say), Chilocorus stigma (Say), and Brachiacantha ursina (F.) were more abundant in the site that had a deciduous habitat. Overall, the site with a deciduous habitat had higher species richness. There were significant differences in species composition between the site with a deciduous habitat and the other 2 sites. Differences observed among sites were significantly correlated with the presence of uncultivated habitats in the landscape.

Trends in the demersal fish community off Namibia from 1983 to 1990

Demersal fish communities off southern Namibia were analysed on the basis of seven winter and five summer stratified random sampling cruises between 1983 and 1990. The 37 most abundant species selected for analysis represented 78-&ndash96 per cent of the total estimated biomass. The biomass of most of them decreased over the period of study. However, the decreases were only substantial for winter cruises; the fluctuations recorded on the summer cruises appeared to be more closely related to hydrographic conditions. In relatively cool summers the estimated biomasses of seven species (including Merluccius capensis and Lophius vomerinus) were quite similar to, or lower than, the biomasses estimated six months earlier. In contrast, the estimated biomasses of the same seven species during relatively warm summers were distinctly higher than the estimates made the previous winter. In all, 10 species, including the commercially important Austroglossus microlepis, Lophius vomerinus and Merluccius capensis, were more available to bottom trawls in summer than in winter. Multivariate analysis revealed several distinct assemblages separated by latitude and depth, but with seasonal variation. Similarity indices between cruises were high (mean Schoener index around 0,7), indicating that changes in biomass do not affect the structure of the demersal fish community. It is suggested that, although the high levels of fishing effort deployed in the region lowered the total fish biomass, community stability, expressed as absence of change in species assemblages, was significantly high.

Diatom evidence on Wisconsin and Holocene events in the Bering Sea

Previous work on surface (modern) sediments has defined diatom species which appear to be good indicators of various oceanographic/ecologic conditions in the North Pacific Ocean and marginal seas. Three long cores from the eastern and northern sides of the Aleutian Basin show changes in species assemblage which can be interpreted in terms of changes in the ocean environment during the last glaciation (Wisconsin) and the Holocene. The early and late Wisconsin maxima were times of prolonged annual sea-ice cover and a short cool period of phytoplankton productivity during the ice-free season. The middle Wisconsin interstade, at least in the southern Bering Sea, had greater seasonal contrast than today, with some winter sea-ice cover, an intensified temperature minimum, and high spring productivity. Variations in clastic and reworked fossil material imply varying degrees of transport to the basin by Alaskan rivers. The results of Jousé from the central Bering Sea generally correspond with those presented here, although there are problems with direct comparison.