J. Robin Svensson

Assessing historical environmental conditions linked to habitat colonization is important for understanding long‐term resilience and improving conservation and restoration efforts. Such information is lacking for the seagrass Zostera marina, an important foundation species across cold‐temperate coastal areas of the Northern Hemisphere. Here, we reconstructed environmental conditions during the last 14,000 years from sediment cores in two eelgrass (Z. marina) meadows along the Swedish west coast, with the main aims to identify the time frame of seagrass colonization and describe subsequent biogeochemical changes following establishment. Based on vegetation proxies (lipid biomarkers), eelgrass colonization occurred about 2,000 years ago after geomorphological changes that resulted in a shallow, sheltered environment favoring seagrass growth. Seagrass establishment led to up to 20‐ and 24‐fold increases in sedimentary carbon and nitrogen accumulation rates, respectively. This demonstrates the capacity of seagrasses as efficient ecosystem engineers and their role in global change mitigation and adaptation through CO₂ removal, and nutrient and sediment retention. By combining regional climate projections and landscape models, we assessed potential climate change effects on seagrass growth, productivity and distribution until 2100. These predictions showed that seagrass meadows are mostly at risk from increased sedimentation and hydrodynamic changes, while the impact from sea level rise alone might be of less importance in the studied area. This study showcases the positive feedback between seagrass colonization and environmental conditions, which holds promise for successful conservation and restoration efforts aimed at supporting climate change mitigation and adaptation, and the provision of several other crucial ecosystem services.

Much of our understanding of competition comes from observations in sessile systems, such as rainforests and marine invertebrate communities. In terrestrial systems, sessile species often compete for multiple limiting resources (i.e., space, light, and nutrients), but in marine systems, space is viewed as the primary or sole limiting resource. Competition theory, on the other hand, suggests that competition for a single limiting resource is unlikely to maintain high species diversity, but manipulative tests of competition for other resources in marine benthic systems are exceedingly rare. Here, we manipulate the availability of food for a classic system, marine sessile invertebrate communities, and investigate the effects on species diversity, abundance, and composition during early succession as well as on the growth of bryozoan populations in the field. We found the number of species to be greater, available space to be lower, and the community composition to be different in assemblages subjected to increased food availability compared to controls. Similarly, laboratory‐settled bryozoans deployed into the field grew more in the presence of enhanced food. Our results suggest that food can act as a limiting resource, affecting both diversity and abundance, even when bare space is still available in hard‐substratum communities. Consequently, broadening the view of resource limitation beyond solely space may increase our understanding and predictability of marine sessile systems.

We evaluated performance of species distribution models for predictive mapping, and how models can be used to integrate human pressures into ecological and economic assessments. A selection of 77 biological variables (species, groups of species, and measures of biodiversity) across the Baltic Sea were modeled. Differences among methods, areas, predictor, and response variables were evaluated. Several methods successfully predicted abundance and occurrence of vegetation, invertebrates, fish, and functional aspects of biodiversity. Depth and substrate were among the most important predictors. Models incorporating water clarity were used to predict increasing cover of the brown alga bladderwrack Fucus vesiculosus and increasing reproduction area of perch Perca fluviatilis, but decreasing reproduction areas for pikeperch Sander lucioperca following successful implementation of the Baltic Sea Action Plan. Despite variability in estimated non-market benefits among countries, such changes were highly valued by citizens in the three Baltic countries investigated. We conclude that predictive models are powerful and useful tools for science-based management of the Baltic Sea.

Summary: Allelopathy is an important non‐resource interaction in terrestrial plant communities that may affect invasions by non‐indigenous plants. The ‘novel weapons hypothesis’ (NWH) predicts that non‐indigenous plants will become invasive if they have allelopathic compounds that assemblages in the new range are not adapted to. Recently, the non‐indigenous, chemically rich macroalga Bonnemaisonia hamifera (Hariot) has become one of the most abundant filamentous red algae in Scandinavian waters. We used B. hamifera to specifically test the aspect of the NWH that concerns invasion success based on novel allelochemicals in the invaded range. Allelopathic interactions were tested through effects on the growth rate of adult native macroalgae in co‐cultures with B. hamifera and through the settlement success of native macroalgal propagules and microalgae on surfaces coated with 1,1,3,3‐tetrabromo‐2‐heptanone. We also investigated whether 1,1,3,3‐tetrabromo‐2‐heptanone can be transferred from B. hamifera to its native host algae, as a means of pre‐emptive competition. The settlement of native macroalgal propagules and microalgae was strongly inhibited on surfaces coated with 1,1,3,3‐tetrabromo‐2‐heptanone at ecologically relevant concentrations, but there were no effects of adult B. hamifera on growth rates of adults of the six native naturally co‐occurring species. The compound was shown to be transferred from B. hamifera to the surface of its native host algae at inhibitory concentrations in both laboratory and field experiments. By inhibiting the settlement of propagules on its thallus and on surrounding surfaces, B. hamifera achieves a competitive advantage over native macroalgae, a finding that parallels previous reports on soil‐ and litter‐mediated allelopathic interactions among vascular plants. Because competition for available substrata in marine benthic systems is intense, the ability to reserve space may be vital for B. hamifera's successful invasion. This is the first example of an allelopathic compound that can be transferred by direct contact from an exotic to a native species, with an active and unaltered function. Synthesis. Our results clearly show that the main secondary metabolite of the invasive red alga B. hamifera has strong allelopathic effects towards native competitors, suggesting that its novel chemical weapon is important for the highly successful invasion of new ranges.

This chapter focuses on the recent and emerging research involving chemical defences against herbivory in aquatic primary producers. It provides an overview of plant chemical defence theories and highlights recent research on aquatic primary producers addressing a number of aspects of these theories, concluding with new chemical approaches to tackle the questions and suggestions for future research directions. It explains that aquatic primary producers are a taxonomically and functionally diverse group of organisms that includes macroalgae, microalgae, and vascular plants. It also states that despite the fact that aquatic primary producers constitute a large and diverse group of organisms that vary in their evolutionary histories, selection for chemical defences to resist or reduce grazing are commonplace across the phylogenetic boundaries.

The intermediate disturbance hypothesis (IDH) and the dynamic equilibrium model (DEM) are influential theories in ecology. The IDH predicts large species numbers at intermediate levels of disturbance and the DEM predicts that the effect of disturbance depends on the level of productivity. However, various indices of diversity are considered more commonly than the predicted number of species in tests of the hypotheses. This issue reaches beyond the scientific community as the predictions of the IDH and the DEM are used in the management of national parks and reserves. In order to compare responses with disturbance among measures of biodiversity, we used two different approaches of mathematical modelling and conducted an extensive meta-analysis. Two-thirds of the surveyed studies present different results for different diversity measures. Accordingly, the meta-analysis showed a narrow range of negative quadratic regression components for richness, but not evenness. Also, the two models support the IDH and the DEM, respectively, when biodiversity is measured as species richness, but predict evenness to increase with increasing disturbance, for all levels of productivity. Consequently, studies that use compound indices of diversity should present logical arguments, a priori , to why a specific index of diversity should peak in response to disturbance.

Bacteria associated with seaweeds can both harm and benefit their hosts. Many seaweed species are known to produce compounds that inhibit growth of bacterial isolates, but the ecological role of seaweed metabolites for the associated bacterial community structure is not well understood. In this study the response of a colonizing bacterial community to the secondary metabolite (1,1,3,3-tetrabromo-2-heptanone) from the red alga Bonnemaisonia hamifera was investigated by using field panels coated with the metabolite at a range of concentrations covering those measured at the algal surface. The seaweed metabolite has previously been shown to have antibacterial effects. The metabolite significantly affected the natural fouling community by (i) altering the composition, (ii) altering the diversity by increasing the evenness and (iii) decreasing the density, as measured by terminal restriction fragment length polymorphism in conjunction with clone libraries of the 16S rRNA genes and by bacterial enumeration. No single major bacterial taxon (phylum, class) was particularly affected by the metabolite. Instead changes in community composition were observed at a more detailed phylogenetic level. This indicates a broad specificity of the seaweed metabolite against bacterial colonization, which is supported by the observation that the bacterial density was significantly affected at a lower concentration (0.02 μg cm⁻²) than the composition (1–2.5 μg cm⁻²) and the evenness (5 μg cm⁻²) of the bacterial communities. Altogether, the results emphasize the role of secondary metabolites for control of the density and structure of seaweed-associated bacterial communities.

Empirical evidence suggests that disturbance has profound effects on the species diversity of aquatic and terrestrial assemblages. Conceptual ecological theories, such as the intermediate disturbance hypothesis (IDH), predict maximum diversity at intermediate levels of disturbance. Tests of the predictive power and generality of these models are, however, hampered by the fact that the meaning and units of “disturbance” are not clearly defined. For example, it is seldom recognized that the rate of disturbance is the product of both frequency and extent (e.g., area or volume) of disturbance events. This has important consequences for the design and interpretation of experiments on disturbance. Here we present, for the first time, an experimental design that allows for unconfounded testing of combinations of area and frequency (i.e., regimes) for a given rate of disturbance. We tested the prediction that species richness responds differently to equal rates of disturbance, depending on the specific combination of frequency and area, on marine hard‐substratum assemblages. Five different rates of disturbance and two regimes (small frequent or large infrequent disturbances) were applied at three sites. The results showed that the effect of a certain rate of disturbance (1) varies strongly among assemblages and (2) also depends on the specific combination of frequency and area of disturbance events. Maximum species richness was observed at intermediate rates of disturbance at site 1 (i.e., support for the IDH), whereas there was a monotonic decline at site 2 and there was no evident pattern at site 3. The variable responses among sites were explained by differences in degree of competitive exclusion and rates of recruitment. At the site where the IDH was supported, the regime with a large proportion of the area disturbed infrequently showed higher richness, compared to the regime with a small proportion disturbed frequently. This was likely due to a stronger decrease of dominants, which allowed for the recruitment of new colonizing species. In summary, we conclude that tests and general syntheses of models of disturbance–diversity patterns would benefit from more explicit definitions of the components of disturbance, as well as a stronger focus on the importance of variation in inherent properties of natural assemblages.

Development of a mechanistic understanding and predictions of patterns of biodiversity is a central theme in ecology. One of the most influential theories, the intermediate disturbance hypothesis (IDH), predicts maximum diversity at intermediate levels of disturbance frequency. The dynamic equilibrium model (DEM), an extension of the IDH, predicts that the level of productivity determines at what frequency of disturbance maximum diversity occurs. To test, and contrast, the predictions of these two models, a field experiment on marine hard-substratum assemblages was conducted with seven levels of disturbance frequency and three levels of nutrient availability. Consistent with the IDH, maximum diversity, measured as species richness, was observed at an intermediate frequency of disturbance. Despite documented effects on productivity, the relationship between disturbance and diversity was not altered by the nutrient treatments. Thus, in this system the DEM did not improve the understanding of patterns of diversity compared to the IDH. Furthermore, it is suggested that careful consideration of measurements and practical definitions of productivity in natural assemblages is necessary for a rigorous test of the DEM.