The role of different reproductive barriers during phenotypic divergence of isopod ecotypes

The question of how diverging populations become separate species by restraining gene flow is a central issue in evolutionary biology. Assortative mating might emerge early during adaptive divergence, but the role of other types of reproductive barriers such as migration modification have recently received increased attention. We demonstrate that two recently diverged ecotypes of a freshwater isopod (Asellus aquaticus) have rapidly developed premating isolation, and this isolation barrier has emerged independently and in parallel in two south Swedish lakes. This is consistent with ecological speciation theory, which predicts that reproductive isolation arises as a byproduct of ecological divergence. We also find that in one of these lakes, habitat choice acts as the main barrier to gene flow. These observations and experimental results suggest that migration modification might be as important as assortative mating in the early stages of ecological speciation. Simulations suggest that the joint action of these two isolating barriers is likely to greatly facilitate adaptive divergence, compared to if each barrier was acting alone.     

Facilitation of clear-water conditions in shallow lakes by macrophytes: differences between charophyte and angiosperm dominance

A number of mechanisms result in a feedback between water clarity and macrophytes and, consequently, the occurrence of alternative stable states in shallow lakes. We hypothesize that bottom-up mechanisms and interactions within the benthic food web are more important in a charophyte-dominated clear-water state, while top-down mechanism and interactions in the planktonic food web prevail at angiosperm dominance. Charophytes, which dominate at lower nutrient concentrations and develop higher densities than most angiosperms, can have a higher influence on sedimentation, resuspension, and water column nutrients. During dominance of dense submerged vegetation like charophytes, zooplankton can be hampered by low food quality and quantity and by high predation pressure from juvenile fish, which in turn are favoured by the high refuge potential of this vegetation. Grazing pressure from zooplankton on phytoplankton can therefore be low in charophytes, but the main feedback in angiosperm-dominated ecosystems. Charophytes offer a higher surface than most angiosperms to periphyton, which favors benthic invertebrates. These support macrophytes by grazing periphyton and constitute a central link in a trophic cascade from fish to periphyton and macrophytes. To test these hypotheses, more experiments and field measurements comparing the effect of charophytes and angiosperms on water clarity are needed.     

Trophic web structure in a shallow eutrophic lake during a dominance shift from phytoplankton to submerged macrophytes

In Lake Krankesjön, southern Sweden, sago pondweed (Potamogeton pectinatus L.) and a stonewort (Chara tomentosa L.) expanded spatially during the second half of the 1980's after more than a decade of phytoplankton blooms and sparse submerged vegetation. During the expansion of submerged plants the number of resting and breeding waterfowl increased. The increase was significant for herbivorous birds such as coot (Fulica atra L.) and mute swan (Cygnus olor (Gmelin)), but also for omnivorous dabbling ducks. The shift from phytoplankton to submerged macrophytes caused structural changes on higher trophic levels, and an altered trophic web developed. The density of planktonic Cladocera decreased, which is suggested to be a result of decreased phytoplankton productivity and biomass as nutrient levels dropped. The benthic macroinvertebrate assemblage changed from low diversity and biomass dominated by Chironomidae and Oligochaeta on bare sediment, to high diversity and biomass characterized by plant-associated forms like snails and isopods in areas covered by macrovegetation. The mean size of perch (Perca fluviatilis L.) increased, probably as a result of higher availability of macroinvertebrates in the vegetation. The perch reached a mean size where the species is known to shift to a fish diet, permitting an increased top down effect on the ecosystem. The results support the idea that shallow eutrophic lakes can shift between two states, each one stabilized by feed-back mechanisms including both biotic and abiotic factors. Shifts between these states are suggested to be a possible explanation for observed drastic changes in abundance of waterfowl in shallow eutrophic lakes.     

Waterfowl,macrophytes, and the clear water state of shallow lakes

The importance of lake ecosystems for waterfowl remains a topic of debate. In order to assess how temporal variations in lake features, specifically shifts between alternative stable states, may interact with the waterfowl fauna, we performed a long-term (22 years) study of the shallow Lake Krankesjön, southern Sweden. Lower total numbers of waterfowl occurred during periods with low macrophyte cover and turbid water, than when submersed macrophytes flourished and the water was clear. Some specific functional groups of waterfowl, such as herbivores, invertebrate, and fish feeders, showed a positive relation to clear water and high macrophyte cover. Hence, our data suggest that some migratory waterfowl may select lakes based on water quality, thereby adjusting their large-scale migratory routes. On the other hand, omnivorous waterfowl exhibited their highest abundances during turbid conditions. Furthermore, waterfowl not primarily relying on food from the lake showed no response to fluctuations in turbidity or macrophyte cover, but followed regional trends in population dynamics. In our study lake, L. Krankesjön, we estimated that waterfowl remove less than 3% of the macrophyte biomass during a stable clear-water state with lush macrophyte beds. However, during transition periods between alternative stable states, when macrophyte biomass is lower and the plants already stressed, the consumption rate of waterfowl may have a stronger effect on lake ecosystem functioning.