Important Environmental Variables (important + environmental_variable)

Distribution by Scientific Domains


Selected Abstracts


Determining the important environmental variables controlling plant species community composition in mesotrophic grasslands in Great Britain

APPLIED VEGETATION SCIENCE, Issue 4 2009
Veronika Kalusová
Abstract Question: What is the relative importance of local site effects and selected important environmental variables in determining plant species composition? How do species respond to these environmental variables? Location: Ten mesotrophic grassland sites of high conservation value in southern England. Methods: Species cover was assessed in between 10 and 25 randomly selected 1-m2 quadrats at each site. At each quadrat degree of waterlogging (W), soil Olsen extractable phosphorus (P) and soil pH were measured. Variation partitioning was used to separate site and soil effects, and HOF (Huisman, Olff & Fresco) modelling was used to produce response curves for the major species on soil gradients, based on coenoclines derived from partial canonical correspondence analysis (pCCA). Results: Variation partitioning identified Site as the most important environmental variable (34.6%). Only 18.7% was accounted for by the three soil variables together; W (degree of waterlogging), P and pH accounted for 11.1%, 5.7% and 4.3%, respectively in raw form with 2.4% shared. However, when Site and the other soil variables were removed the variation explained reduced to 2.3% for W, 1.1% for P and 1.0% for pH. The species responses to each of these soil environmental factors could be separated into four types on each gradient. Most species were abundant at low W, low soil P and intermediate pH. Conclusions: Site-based factors were more important than the three soil variables, which were assumed to be directly or indirectly associated with productivity. This implies that each site has unique properties that are more important than the soil variables. The three soil factors were, however, significant and the groups of the most common species, based on significant response curves, can be used as a first approximation of indicators of environmental conditions in British mesotrophic grasslands for conservation. However, W accounted for most variation, and the current reliance on soil available P and soil pH for assessing conservation/restoration potential should be viewed with caution. [source]


The Effects of Temperature and Salinity on Early Life Stages of Black Sea Bass Centropristis striata

JOURNAL OF THE WORLD AQUACULTURE SOCIETY, Issue 3 2004
David L. Berlinsky
Along the Atlantic coast black sea bass occur from the Gulf of Maine to Florida and support important commercial and recreational fisheries. Interest in commercial production of black sea bass has increased in recent years due to high demand and limited seasonable availability. Efforts towards large-scale production have been hampered by a high incidence of early larval mortality. Two of the most important environmental variables affecting hatchery production of marine finfish larvae are temperature and salinity. In the wild, larval black sea bass are found in waters with temperatures of 12,24 C and salinity levels of 30,35 ppt. Studies were conducted to define the temperature and salinity ranges that support growth and development of black sea bass during early life stages. Three developmental phases were investigated: 1) fertilization to hatch: 2) hatch through yolk sac absorption: and 3) during the initial exogenous feeding stage (5,14 days post hatch: DPH). Fertilized eggs were obtained by manual spawning of fish following administration of LHRHa. Fertilized eggs were transferred to 300-mL glass Petri dishes or 500-mL beakers to assess the effects of salinity and temperature through hatch and yolk sac absorption, respectively. To determine environmental effects on growth and survival during initial exogenous feeding 400 actively feeding larvae were cultured in green water and fed enriched rotifers for a 9-d period. For investigation of the effect of salinity, sea water (35 ppt) was diluted gradually to 15, 20, 25, and 30 ppt and maintained at 21 C. For examination of the effect of temperature, seawater was adjusted from 21 C to 12, 15, 21, 27, or 30 C at a rate of 3 C/h. No eggs hatched at 12 C or when salinity was maintained at 0 or 5 ppt. Hatching was uniformly high (, 85%) at temperatures between 15 and 27 C and at salinities , 15 ppt. Survival through yolk sac absorption was greatest at temperatures between 18 and 27 C and at salinities , 20 ppt. Survival through first feeding stage was highest at temperatures , 18 C and 30 ppt salinity. Larval growth through first feeding was not significantly affected by salinity level but did increase with rearing temperature. The results indicate that survival and development of black sea bass during early life stages are most favorable at temperatures >18 C with salinity levels approaching full strength seawater. [source]


Scale-dependence of vegetation-environment relationships in semi-natural grasslands

JOURNAL OF VEGETATION SCIENCE, Issue 1 2008
Inger Auestad
Abstract Questions: Which environmental and management factors determine plant species composition in semi-natural grasslands within a local study area? Are vegetation and explanatory factors scale-dependent? Location: Semi-natural grasslands in Lćrdal, Sognog Fjordane County, western Norway. Methods: We recorded plant species composition and explanatory variables in six grassland sites using a hierarchically nested sampling design with three levels: plots randomly placed within blocks selected within sites. We evaluated vegetation-environment relationships at all three levels by means of DCA ordination and split-plot GLM analyses. Results: The most important complex gradient determining variation in grassland species composition showed a broad-scale relationship with management. Soil moisture conditions were related to vegetation variation on block scale, whereas element concentrations in the soil were significantly related to variation in species composition on all spatial scales. Our results show that vegetation-environment relationships are dependent on the scale of observation. We suggest that scale-related (and therefore methodological) issues may explain the wide range of vegetation-environment relationships reported in the literature, for semi-natural grassland in particular but also for other ecosystems. Conclusions: Interpretation of the variation in species composition of semi-natural grasslands requires consideration of the spatial scales on which important environmental variables vary. [source]


On the validity of habitat as a predictor of genetic structure in aquatic systems: a comparative study using California water beetles

MOLECULAR ECOLOGY, Issue 3 2009
A. E. Z SHORT
Abstract Among freshwater organisms, water flow is frequently considered to be one of the most important environmental variables affecting life-history traits such as dispersal abilities and therefore genetic structure. Recent studies have suggested that habitat type alone as defined by water flow is predictive of genetic population differentiation, while others have advocated against broad generalizations in favour of more conservative, species-specific conclusions. If aquatic habitat type is predictive of population differentiation, then one would expect sympatric taxa that occupy the same aquatic habitat to converge on a similar genetic structure. We tested this prediction by examining the haplotype diversity, phylogeographical concordance, population connectivity and population isolation of three lotic water beetle species in southern California: Anacaena signaticollis, Eubrianax edwardsii and Stictotarsus striatellus. In addition to coarse habitat and geography, we also controlled for the potentially confounding factors of range size, method of dispersal and clade independence. Together, the species spanned extremes of genetic and phylogeographical structure in all measures examined, suggesting that a coarse dichotomy of aquatic habitat type is not predictive of genetic structure. While there is little question that water flow plays a major role in shaping the life-history traits of freshwater organisms, it is perilous to confer predictive properties to an artificially simplistic dichotomy or use it as a surrogate for other unmeasured variables. [source]


Determining the important environmental variables controlling plant species community composition in mesotrophic grasslands in Great Britain

APPLIED VEGETATION SCIENCE, Issue 4 2009
Veronika Kalusová
Abstract Question: What is the relative importance of local site effects and selected important environmental variables in determining plant species composition? How do species respond to these environmental variables? Location: Ten mesotrophic grassland sites of high conservation value in southern England. Methods: Species cover was assessed in between 10 and 25 randomly selected 1-m2 quadrats at each site. At each quadrat degree of waterlogging (W), soil Olsen extractable phosphorus (P) and soil pH were measured. Variation partitioning was used to separate site and soil effects, and HOF (Huisman, Olff & Fresco) modelling was used to produce response curves for the major species on soil gradients, based on coenoclines derived from partial canonical correspondence analysis (pCCA). Results: Variation partitioning identified Site as the most important environmental variable (34.6%). Only 18.7% was accounted for by the three soil variables together; W (degree of waterlogging), P and pH accounted for 11.1%, 5.7% and 4.3%, respectively in raw form with 2.4% shared. However, when Site and the other soil variables were removed the variation explained reduced to 2.3% for W, 1.1% for P and 1.0% for pH. The species responses to each of these soil environmental factors could be separated into four types on each gradient. Most species were abundant at low W, low soil P and intermediate pH. Conclusions: Site-based factors were more important than the three soil variables, which were assumed to be directly or indirectly associated with productivity. This implies that each site has unique properties that are more important than the soil variables. The three soil factors were, however, significant and the groups of the most common species, based on significant response curves, can be used as a first approximation of indicators of environmental conditions in British mesotrophic grasslands for conservation. However, W accounted for most variation, and the current reliance on soil available P and soil pH for assessing conservation/restoration potential should be viewed with caution. [source]