Integrating experimental and distribution data to predict future species patterns

Predictive species distribution models are mostly based on statistical dependence between environmental and distributional data and therefore may fail to account for physiological limits and biological interactions that are fundamental when modelling species distributions under future climate conditions. Here, we developed a state-of-the-art method integrating biological theory with survey and experimental data in a way that allows us to explicitly model both physical tolerance limits of species and inherent natural variability in regional conditions and thereby improve the reliability of species distribution predictions under future climate conditions. By using a macroalga-herbivore association (Fucus vesiculosus - Idotea balthica) as a case study, we illustrated how salinity reduction and temperature increase under future climate conditions may significantly reduce the occurrence and biomass of these important coastal species. Moreover, we showed that the reduction of herbivore occurrence is linked to reduction of their host macroalgae. Spatial predictive modelling and experimental biology have been traditionally seen as separate fields but stronger interlinkages between these disciplines can improve species distribution projections under climate change. Experiments enable qualitative prior knowledge to be defined and identify cause-effect relationships, and thereby better foresee alterations in ecosystem structure and functioning under future climate conditions that are not necessarily seen in projections based on non-causal statistical relationships alone

Functional diversity of mesograzers in an eelgrass-epiphyte system

Historically, small invertebrate grazers in marine plant communities have been considered to be a relatively homogeneous group in their impact on ecosystem processes. However, recent studies propose that species composition is an important agent in determining grazer effects. We used four mesocosm experiments to test the biomass-specific and density-dependent effects of common mesograzers in temperate regions (Littorina littorea, Rissoa membranacea, Idotea baltica and Gammarus oceanicus) on epiphyte and eelgrass biomass and productivity. Mesograzer species identity strongly influenced epiphyte accumulation and eelgrass growth, where Rissoa was the most efficient mesograzer (per biomass) and Gammarus had the weakest impact. Density-dependent effects varied considerably among species. Both gastropod species reduced epiphyte accumulation in direct proportion to their density, and Littorina had the strongest negative effect on epiphyte biomass. The impact of Idotea seemed to level off to a threshold value and Gammarus had no density-dependent effect on epiphyte accumulation at all. Rissoa and Idotea increased eelgrass productivity in accordance with their effect on epiphyte accumulation, whereas Littorina showed a less positive effect than could be expected by its strong impact on epiphyte biomass. Gammarus had no significant impact on eelgrass growth. Our results show that the different functional traits of superficially similar mesograzers can have important consequences for ecosystem processes in macrophyte systems

How do nutrient conditions and species identity influence the impact of mesograzers in eelgrass-epiphyte systems?

Coastal eutrophication is thought to cause excessive growth of epiphytes in eelgrass beds, threatening the health and survival of these ecologically and economically valuable ecosystems worldwide. Mesograzers, small crustacean and gastropod grazers, have the potential to prevent seagrass loss by grazing preferentially and efficiently on epiphytes. We tested the impact of three mesograzers on epiphyte biomass and eelgrass productivity under threefold enriched nutrient concentrations in experimental indoor mesocosm systems under summer conditions. We compared the results with earlier identical experiments that were performed under ambient nutrient supply. The isopod Idotea baltica, the periwinkle Littorina littorea, and the small gastropod Rissoa membranacea significantly reduced epiphyte load under high nutrient supply with Rissoa being the most efficient grazer, but only high densities of Littorina and Rissoa had a significant positive effect on eelgrass productivity. Although all mesograzers increased epiphyte ingestion with higher nutrient load, most likely as a functional response to the quantitatively and qualitatively better food supply, the promotion of eelgrass growth by Idotea and Rissoa was diminished compared to the study performed under ambient nutrient supply. Littorina maintained the level of its positive impact on eelgrass productivity regardless of nutrient concentrations

Sea-land transitions in isopods: pattern of symbiont distribution in two species of intertidal isopods Ligia pallasii and Ligia occidentalis in the Eastern Pacific

Studies of microbial associations of intertidal isopods in the primitive genus Ligia (Oniscidea, Isopoda) can help our understanding of the formation of symbioses during sea-land transitions, as terrestrial Oniscidean isopods have previously been found to house symbionts in their hepatopancreas. Ligia pallasii and Ligia occidentalis co-occur in the high intertidal zone along the Eastern Pacific with a large zone of range overlap and both species showing patchy distributions. In 16S rRNA clone libraries mycoplasma-like bacteria (Firmicutes), related to symbionts described from terrestrial isopods, were the most common bacteria present in both host species. There was greater overall microbial diversity in Ligia pallasii compared with L. occidentalis. Populations of both Ligia species along an extensive area of the eastern Pacific coastline were screened for the presence of mycoplasma-like symbionts with symbiont-specific primers. Symbionts were present in all host populations from both species but not in all individuals. Phylogenetically, symbionts of intertidal isopods cluster together. Host habitat, in addition to host phylogeny appears to influence the phylogenetic relation of symbionts

Differences in stress tolerance and brood size between a non-indigenous and an indigenous gammarid in the northern Baltic Sea

Differences in stress tolerance and reproductive traits may drive the competitive hierarchy between nonindigenous and indigenous species and turn the former ones into successful invaders. In the northern Baltic Sea, the non-indigenous Gammarus tigrinus is a recent invader of littoral ecosystems and now occupies comparable ecological niches as the indigenous G. zaddachi. In laboratory experiments on specimens collected between June and August 2009 around Tva¨rminne in southern Finland (59°500N/23°150E), the tolerances towards heat stress and hypoxia were determined for the two species using lethal time, LT50, as response variable. The brood size of the two species was also studied and some observations were made on maturation of juveniles. Gammarus tigrinus was more resistant to hypoxia and survived at higher temperatures than G. zaddachi. Brood size was also greater in G. tigrinus than in G. zaddachi and G. tigrinus matured at a smaller size and earlier than G. zaddachi. Hence, there are clear competitive advantages for the non-indigenous G. tigrinus compared to the indigenous G. zaddachi, and these may be further strengthened through ongoing environmental changes related to increased eutrophication and a warming climate in the Baltic Sea region