Trophodynamics of mercury and other trace elements in a pelagic food chain from the Baltic Sea.

Mercury (Hg) and 13 other trace elements (Al, Ti, V, Cr, Fe, Mn, Co, Ni, Cu, Zn, As, Cd, and Pb) were measured in phytoplankton, zooplankton, mysis and herring in order to examine the trophodynamics in a well-studied pelagic food chain in the Baltic Sea. The fractionation of nitrogen isotopes (delta(15)N) was used to evaluate food web structure and to estimate the extent of trophic biomagnification of the various trace elements. Trophic magnification factors (TMFs) for each trace element were determined from the slope of the regression between trace element concentrations and delta(15)N. Calculated TMFs showed fundamental differences in the trophodynamics of the trace elements in the pelagic food chain studied. Concentrations of Al, Fe, Ni, Zn, Pb and Cd showed statistically significant decreases (TMF<1) with increasing trophic levels and thus these trace elements tropically dilute or biodilute in this Baltic food chain. Cu, As, Cr, Mn, V, Ti and Co showed no significant relationships with trophic levels. Hg was unique among the trace elements studied in demonstrating a statistically significant increase (TMF>1) in concentration with trophic level i.e. Hg biomagnifies in this Baltic food chain. The estimated TMF for Hg in this food chain was comparable to TMFs observed elsewhere for diverse food chains and locations.

Species turnover of a continental bird fauna: Northern Europe, 1850-1970.

Faunistic information shows that Denmark, Norway, Sweden and Finland were colonized by an average of 2.8 species (winners) and lost 0.6 species (losers) per decade and country in 1850-1970. The smallest passerines had best changes of colonization, but body size and colonization success were not related in nonpasserines; extinctions were fairly common among the largest species. Turnover was highest in a faunal group derived from steppe regions and in widely but often patchily distributed species. Population trends were generally similar over a larger region. Species turnover has changed the total number of breeding pairs only a little, but the colonization rate has been highest after 1950, when quantitative population changes have also been extensive.Extinction was most often caused by persecution and/or habitat changes by man (e.g. destruction of old forests). As large nonpasserines are often hunted or persecuted, they were frequent among the losers, many of which have specialized habitat requirements and usually migrate to the tropics.Over 50 of the 88 winners have been favoured by man's direct measures, such as introductions (3 spp.), relaxed persecution, or improved food conditions. Habitat changes (eutrophication, reduced grazing, and conifer plantations) stand out as major factors and may subtly interact with population parameters. Perturbation of tropical habitats in this century may have benefitted many Palearctic winter visitors: the frequency of tropical migrants among the winners has increased from 10% before 1900 to 38% after 1900. Population changes in central parts of the range probably led to several colonizations and extinctions at the periphery.In theoretical terms, r-selected species (small body size, large clutch size) were often winners, while K-species comprised the bulk of the losers, but this is no more than a general trend. Judged by their faunal origin, species adapted to effective dispersal were often winners, but also seemed vulnerable to environmental deterioration. Because many winners have invaded habitats that have been available for a long time, North European bird communities are probably often unsaturated.

Geographical gradients of stability in European land bird communities.

European data from 15 long-term censuses of breeding land bird communities were analysed in order to measure community stability. Stability was here operationally defined as year-to-year persistence of community structure (total density, number of species, diversity, evenness of the species-abundance distribution, species list, and frequencies of species). Central Europe and southern Scandinavia formed a zone of fairly stable communities, but more northern communities were fairly unstable. This result contrasts with several recent arguments, but accords well with the classic dogma that instability of biological communities increases northwards in the northern hemisphere.Three theoretical explanations for the difference in stability between northern and southern communities were explored, and causal mechanisms underlying the hypotheses were specified and tested. Considerable evidence supported the hypothesis that environmental (climatic) unpredictability increases northwards and mainly causes the phenomenon observed. Gradients in productivity may, perhaps in interaction with unpredictability, also contribute to geographical differences in stability, but tests suggested that low diversity is not an important cause of northern instability.