Response of chironomid assemblages to environmental change during the early Late-glacial at Gerzensee, Switzerland

Prior to ca. 14,660 yr BP, during the early Late-glacial (Oldest Dryas), larval assemblages of Chironomidae (Insecta: Diptera) in Gerzensee, Switzerland, were dominated by cold stenothermic taxa as well as by taxa typical of subalpine lakes today. This was the coldest period of the entire sequence. After ca. 14,660 yr BP, in the Late Glacial Interstadial (Bølling–Allerød), a temperature increase is recorded by a sharp rise in the oxygen-isotope ratio in lake marl and by an increase in the organic-matter content of the sediments. Changes in the chironomid fauna then are consistent with rising temperatures. This warming trend is interrupted between 14,070 and 13,940 yr BP, coinciding with the GI-1d cold oscillation, but the change in the chironomid assemblage is more consistent with a response to increasing lake depth and density of aquatic macrophytes than falling temperature. A rise in cold-adapted chironomid taxa between 13,840 and 13,710 yr BP suggests that summer air temperatures may have declined. Changes in the chironomid assemblage after 13,710 yr BP suggest a decline in submerged macrophytes coupled with a rise in lake productivity and summer temperature, although the latter is not reflected in the oxygen-isotope record. This suggests that there may have been increasing seasonality during this period when summer temperatures were rising, driven by rising summer insolation, and winters becoming cooler, which is largely reflected in the oxygen-isotope record. A decline in thermophilic chironomids and a rise in cold-adapted taxa after 13,180 yr BP suggest a response to cooling at the beginning of the Gerzensee Oscillation.

Temperate Mountain Forest Biodiversity under Climate Change: Compensating Negative Effects by Increasing Structural Complexity

Species adapted to cold-climatic mountain environments are expected to face a high risk of range contractions, if not local extinctions under climate change. Yet, the populations of many endothermic species may not be primarily affected by physiological constraints, but indirectly by climate-induced changes of habitat characteristics. In mountain forests, where vertebrate species largely depend on vegetation composition and structure, deteriorating habitat suitability may thus be mitigated or even compensated by habitat management aiming at compositional and structural enhancement. We tested this possibility using four cold-adapted bird species with complementary habitat requirements as model organisms. Based on species data and environmental information collected in 300 1-km2 grid cells distributed across four mountain ranges in central Europe, we investigated (1) how species’ occurrence is explained by climate, landscape, and vegetation, (2) to what extent climate change and climate-induced vegetation changes will affect habitat suitability, and (3) whether these changes could be compensated by adaptive habitat management. Species presence was modelled as a function of climate, landscape and vegetation variables under current climate; moreover, vegetation-climate relationships were assessed. The models were extrapolated to the climatic conditions of 2050, assuming the moderate IPCC-scenario A1B, and changes in species’ occurrence probability were quantified. Finally, we assessed the maximum increase in occurrence probability that could be achieved by modifying one or multiple vegetation variables under altered climate conditions. Climate variables contributed significantly to explaining species occurrence, and expected climatic changes, as well as climate-induced vegetation trends, decreased the occurrence probability of all four species, particularly at the low-altitudinal margins of their distribution. These effects could be partly compensated by modifying single vegetation factors, but full compensation would only be achieved if several factors were changed in concert. The results illustrate the possibilities and limitations of adaptive species conservation management under climate change.

Sex determination and temperature-induced sex differentiation in three natural populations of Nile tilapia (Oreochromis niloticus) adapted to extreme temperature conditions

As a species of major interest for aquaculture, the sex determination system (SDS) of Nile tilapia, Oreochromis niloticus, has been widely investigated. In this species, sex determination is considered to be governed by the interactions between a complex system of genetic sex determination factors (GSD) and the influence of temperature (TSD) during a critical period. Previous studies were exclusively carried out on domestic stocks with the genetic and maintenance limitations associated. Given the wide distribution and adaptation potential of the Nile tilapia, we investigated under controlled conditions the sex determination system of natural populations adapted to three extreme thermal regimes: stable extreme environments in Ethiopia, either cold temperatures in a highland lake (Lake Koka), or warm temperatures in hydrothermal springs (Lake Metahara), and an environment with large seasonal variations in Ghana (Kpandu, Lake Volta). The sex ratio analysis was conducted on progenies reared under constant basal (27 degrees C) or high (36 degrees C) temperatures during the 30 days following yolk-sac resorption. Sex ratios of the progenies reared at standard temperature suggest that the three populations share a similar complex GSD system based on a predominant male heterogametic factor with additional influences of polymorphism at this locus and/or action of minor factors. The three populations presented a clear thermosensitivity of sex differentiation, with large variations in the intensity of response depending on the parents. This confirms the presence of genotype-environment interactions in TSD of Nile tilapia. Furthermore the existence of naturally sex-reversed individuals is strongly suggested in two populations (Kpandu and Koka). However, it was not possible here to infer if the sex-inversion resulted from minor genetic factors and/or environmental influences. The present study demonstrated for the first time the conservation of a complex SDS combining polymorphic GSD and TSD components in natural populations of Nile tilapia. We discuss the evolutionary implications of our findings and highlight the importance of field investigations of sex determination. (c) 2007 Elsevier B.V. All rights reserved.