Functionally redundant herbivores: urchin and gastropod grazers respond differently to ocean warming and rising CO2

Future ocean CO2 and temperatures are predicted to increase primary productivity across tropical marine habitats, potentially driving a shift towards algal-dominated systems. However, increased consumption of algae by benthic grazers could potentially counter this shift. Yet, the response of different grazer species to future conditions will be moderated by their physiologies, meaning that they may not be functional equivalents. Here, we experimentally assessed the physiological response of key grazers—the sea urchin Heliocidaris crassispina and 2 gastropod species, Astralium haematragum and Trochus maculatus—to predicted CO2 concentrations (400, 700 and 1000 ppm) and temperature conditions (ambient, +3 and +5°C). In line with metabolic theory, we found that urchin metabolic rate increased at future temperatures regardless of CO2 conditions, with evidence of metabolic acclimation to higher temperatures. The metabolic rate of A. haematragum was depressed only by CO2, whereas T. maculatus initially had elevated metabolic rates at moderate CO2, which were depressed by the combination of the highest CO2 concentration and temperatures. Taxa showed differential survival, with no urchin mortality under any future conditions but substantial mortality of both gastropods under elevated temperatures regardless of CO2 concentration. Importantly, all species had substantially reduced algal consumption in response to elevated CO2, though the urchins only demonstrated an energetic mismatch under combined future CO2 and temperature. Therefore, despite sharing an ecological niche, these key grazers are likely to be differentially affected by future environmental conditions, potentially reducing the strength of ecological compensatory responses depending on the functional redundancy in this grazing community.

Effect of Grazing System on Grassland Plant Species Richness and Vegetation Characteristics: Comparing Horse and Cattle Grazing

Horses are of increasing relevance in agriculturally managed grasslands across Europe. There is concern to what extent grazing with horses is a sustainable grassland management practice. The effect of longer-term horse grazing on the vegetation characteristics of grasslands has received little attention, especially in comparison to grazing cattle. Our study analyses the relative importance of grazing system (grazer species and regime) and grassland management for vegetation characteristics in grasslands as indicator for sustainable management. We monitored grassland vegetation in western central Germany and compared paddocks grazed by horses under two different regimes, continuous (HC) vs. rotational (HR), to paddocks grazed by cattle (C) under similar trophic site conditions. We observed more plant species and more High Nature Value indicator species on HC compared to C. The vegetation of C was more grazing tolerant and had higher forage value than HC. Regardless of the grazing regime, the competitive component was lower, the stress-tolerant component higher and the floristic contrast between patch-types stronger on HC and HR paddocks compared to C. Species richness was strongly influenced by the extent of the floristic contrast. Our results emphasize the potential of horse grazing for biodiversity in agriculturally managed grasslands.

Periphyton consumption by an invasive snail species is greater in simplified than in complex habitats

Habitat complexity may stabilize consumer–resource interactions and reduce the probability of invasion in aquatic habitats. We tested the hypotheses that (i) higher habitat complexity reduces resource consumption independently of grazer species, but that (ii) invasive grazers have a greater influence on decreasing resources independently of habitat complexity. We performed an experiment using artificial substrates to simulate different complexity levels. We evaluated Melanoides tuberculata (O.F. Müller, 1774) and Aylacostoma chloroticum Hylton Scott, 1954 consumption of specific algal groups and the interaction between habitat complexity and grazer species. Moreover, we evaluated grazer activity on the different substrates during the experiment. The results support only the first hypothesis and indicate lower resource consumption on complex substrates compared with simpler substrates. Additionally, the effect of the grazing of the invasive species on taxon richness was greater in simplified than in complex habitats. The grazing activity on the substrate suggests a relationship between resource exploitation and habitat complexity in which the invasive grazing species visited the simple habitat less frequently. However, the effects of invasive grazers on food resources were higher on the simple substrate. The effects of grazing activity on food resources depend on the interaction between habitat complexity and grazer species. In this way, the introduction of an invasive species may have negative impacts on the structure and function of periphytic communities, mainly in simplified aquatic ecosystems.

Grazing of the copepod Cyclops vicinus on toxic Microcystis aeruginosa: potential for controlling cyanobacterial blooms and transfer of toxins

Grazing of zooplankton on phytoplankton may contribute to a reduction of harmful cyanobacteria in eutrophic waters. However, the feeding capacity and interaction between zooplankton and toxic cyanobacteria vary among grazer species. In this study, laboratory feeding experiments were designed to measure the grazing rate of the copepod Cyclops vicinus on Microcystis aeruginosa and the potential microcystin (MC) accumulation in the grazer. Copepods were fed a mixed diet of the edible green alga Ankistrodesmus falcatus and toxic M. aeruginosa for 10 days. The results showed that C. vicinus efficiently ingested toxic Microcystis cells with high grazing rates, varying during the feeding period (68.9–606.3 Microcystis cells animal-1 d-1) along with Microcystis cell density. Microcystis cells exhibited a remarkable induction in MC production under grazing conditions with concentrations 1.67–12.5 times higher than those in control cultures. Furthermore, C. vicinus was found to accumulate MCs in its body with concentrations increasing during the experiment (0.05–3.21 μg MC animal-1). Further in situ studies are needed to investigate the ability of Cyclops and other copepods to assimilate and detoxify MCs at environmentally relevant concentrations before deciding on the biocontrol of Microcystis blooms by copepods.

Species interactions can shift the response of a maerl bed community to ocean acidification and warming

Predicted ocean acidification and warming are likely to have major implications for marine organisms, especially marine calcifiers. However, little information is available on the response of marine communities as a whole to predicted changes. Here, we experimentally examined the combined effects of temperature and partial pressure of carbon dioxide (pCO2) increases on the response of maerl bed assemblages, composed of living and dead thalli of the free-living coralline alga Lithothamnion corallioides, epiphytic fleshy algae, and grazer species. Two three-month experiments were performed in the winter and summer seasons in mesocosms with four different combinations of pCO2 (ambient and high pCO2) and temperature (ambient and +3 °C). The response of maerl assemblages was assessed using metabolic measurements at the species and assemblage scales. Gross primary production and respiration of assemblages were enhanced by high pCO2 conditions in the summer. This positive effect was attributed to the increase in epiphyte biomass, which benefited from higher CO2 concentrations for growth and primary production. Conversely, high pCO2 drastically decreased the calcification rates in assemblages. This response can be attributed to the decline in calcification rates of living L. corallioides due to acidification as well as increased dissolution of dead L. corallioides. Future changes in pCO2 and temperature are likely to promote the development of non-calcifying algae to the detriment of the engineer species L. corallioides. The development of fleshy algae may be modulated by the ability of grazers to regulate epiphyte growth. However, our results suggest that predicted changes will negatively affect the metabolism of grazers and potentially their ability to control epiphyte abundance. Here, we demonstrate that the response of marine communities to climate change will depend on the direct effects on species physiology and the indirect effects due to shifts in species interactions. This double, interdependent response underlines the importance of examining community-level processes, which integrate species interactions, to better understand the impact of global change on marine ecosystems.