Cascading predator control interacts with productivity to determine the trophic level of biomass accumulation in a benthic food web

Large-scale exploitation of higher trophic levels by humans, together with global-scale nutrient enrichment, highlights the need to explore interactions between predator loss and resource availability. The hypothesis of exploitation ecosystems suggests that top-down and bottom-up control alternate between trophic levels, resulting in a positive relationship between primary production and the abundance of every second trophic level. Specifically, in food webs with three effective trophic levels, primary producers and predators should increase with primary production, while in food webs with two trophic levels, only herbivores should increase. We provided short-term experimental support for these model predictions in a natural benthic community with three effective trophic levels, where the number of algal recruits, but not the biomass of gastropod grazers, increased with algal production. In contrast, when the food web was reduced to two trophic levels by removing larger predators, the number of algal recruits was unchanged while gastropod grazer biomass increased with algal production. Predator removal only affected the consumer-controlled early life-stages of algae, indicating that both the number of trophic levels and the life-stage development of the producer trophic level determine the propagation of trophic cascades in benthic systems. Our results support the hypothesis that predators interact with resource availability to determine food-web structure

A Pacific oyster invasion transforms shellfish reef structure by changing the development of associated seaweeds

Biological invasions are reshaping coastal ecosystems across the world. However, understanding the significance of such invasions is often hampered by the lack of process-based research, resulting in a limited mechanistic comprehension of novel ecological interactions and their consequences. The Pacific oyster (Crassostrea gigas) has invaded European coasts, resulting in an astonishing transformation of the intertidal shellfish reef communities in the Wadden Sea; from reefs constructed by blue mussels only (Mytilus edulis) to mixed reefs dominated by oysters. Shellfish reefs structure the marine vegetation on soft bottoms by accumulating seaweeds. Nevertheless, assessments of the consequences of the oyster take-over have almost exclusively focused on effects on associated fauna. By constructing small-scale reefs dominated by blue mussels or oysters and following the development of seaweeds over summer, we demonstrated that oysters promoted bloom-forming green algae communities with low primary biomass and low habitat complexity. In contrast, blue mussels promoted the development of meadow-like communities dominated by habitat forming brown seaweeds of the genus Fucus, with high primary biomass and high habitat complexity. An additional field survey showed that increasing numbers of Pacific oysters on a recently invaded natural blue mussel reef significantly decreased the development of the Fucus meadow in spring. Our results indicate that the invasion of oysters may have effects on the structure and function of intertidal reef-communities by changing energy flow and habitat-function

Cascading effects from predator removal depend on resource availability in a benthic food web

We tested joint effects of predator loss and increased resource availability on the grazers’ trophic level and the propagation of trophic interactions in a benthic food web by excluding larger predatory fish from cages and manipulating nutrients in the coastal zone of the Baltic Sea. The combination of nutrient enrichment and excluding larger predators induced an increase in medium-sized predatory fish (three-spined stickleback). The meso-predator fish in turn did not change the total abundance of the invertebrate herbivores, but did cause a substantial shift in their community composition towards the dominance of gastropods by reducing amphipods by 40–60%, while gastropods were left unchanged. The shift in grazer composition generated a 23 times higher producer biomass, but only under nutrient enrichment. Our results show that top-predator declines can substantially shift the species composition at the grazers’ level, but that cascading effects on producers by a trophic cascade strongly depend on resource availability

The Macroalgal Holobiont in a Changing Sea

When studying the effects of climate change on eukaryotic organisms we often oversee a major ecological process: the interaction with microbes. Eukaryotic hosts and microbes form functional units, termed holobionts, where microbes play crucial roles in host functioning. Environmental stress may disturb these complex mutualistic relations. Macroalgae form the foundation of coastal ecosystems worldwide and provide important ecosystem services - services they could likely not provide without their microbial associates. Still, today we do not know how environmental stress will affect the macroalgal holobiont in an increasingly changing ocean. In this review, we provide a conceptual framework that contributes to understanding the different levels at which the holobiont and environment interact, and we suggest a manipulative experimental approach as a guideline for future research.</p

Omnivory and grazer functional composition moderate cascading trophic effects in experimental Fucus vesiculosus habitats

We tested the relative strength of direct versus indirect effects of an aquatic omnivore depending on the functional composition of grazers by manipulating the presence of gastropod and amphipod grazers and omnivorous shrimp in outdoor mesocosms. By selectively preying upon amphipods and reducing their abundance by 70–80%, omnivorous shrimp favoured the dominance of gastropods. While gastropods were the main microalgal grazers, amphipods controlled macroalgal biomass in the experiment. However, strong predation on the amphipod by the shrimp had no significant indirect effects on macroalgal biomass, indicating that when amphipod abundances declined, complementary feeding by the omnivore on macroalgae may have suppressed a trophic cascade. Accordingly, in the absence of amphipods, the shrimp grazed significantly on green algae and thereby suppressed the diversity of the macroalgal community. Our experiment demonstrates direct consumer effects by an omnivore on both the grazer and producer trophic levels in an aquatic food web, regulated by prey availability. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00227-010-1602-6) contains supplementary material, which is available to authorized users

Intertidal mussel reefs change the composition and size distribution of diatoms in the biofilm

Migrating diatoms are microscopic ecosystem engineering organisms that have functional consequences on the seascape scale by significantly contributing to the microphytobenthos biofilm. The microphytobenthos biofilm is a thin photosynthesising layer that covers the sediment on intertidal flats. It fuels the food web, increases sediment stability, and enhances the deposition of particles, providing ecosystem services to coastal communities. Here we tested the effect of another ecosystem engineering habitat, intertidal blue mussel reefs, on the composition and properties of migrating diatom communities. Small-scale reefs constructed in the intertidal mimicked and reinforced the natural pattern in diatom community composition and function that we documented in the field. The field experiment adding small reefs to the intertidal ran from 30 April to 10 June 2015 and the field samples were collected around a natural blue mussel bed on the same tidal flat on 7 October 2015 (N 53.489 degrees, E 6.230 degrees). Both the constructed small-scale reefs and the natural reef changed the community composition of diatoms in the biofilm by promoting higher numbers of smaller-sized cells and species. Small diatoms have higher growth and gross photosynthesis rates, indicating that this explains the higher production and chlorophyll-a concentration of the biofilm measured on natural intertidal shellfish reefs. Our results showed that shellfish reefs have a large impact on biofilm functioning. However, biofilms are also fuel for the shellfish, indicating that the two very different ecosystem engineers may facilitate coexistence on tidal flats through a positive feedback loop

Bioengineering promotes habitat heterogeneity and biodiversity on mussel reefs

Loss of biodiversity is among the most pressing global problems. Yet, despite its pertinent nature, the biological processes involved in the maintenance of biodiversity are poorly understood. Habitat heterogeneity is widely regarded as a key factor underpinning the biodiversity of land- and sea-scapes. However, it remains unclear how species coexist in many of those ecosystems that lack conspicuous heterogeneity. We demonstrate how spatially self-organized mussel reefs create microhabitats/heterogeneity that facilitate diverse invertebrate communities. By comparing seawater filled pools with open inlets in a mussel reef, we found that natural reef pools, emerging due to the habitat engineering of the mussels, strongly increased variation in organic enrichment and promoted beta-diversity compared to the surrounding tidal flat. These findings significantly extend the scale of influence typically described for self-organized habitats and highlight the importance of bioengineering and its positive effects on habitat heterogeneity and community diversity