Biodiversity and food web indicators of community recovery in intertidal shellfish reefs

Contains fulltext : 168969.pdf (publisher's version ) (Closed access)8 p

Foundation species enhance food web complexity through non-trophic facilitation

Food webs are an integral part of every ecosystem on the planet, yet understanding the mechanisms shaping these complex networks remains a major challenge. Recently, several studies suggested that non-trophic species interactions such as habitat modification and mutualisms can be important determinants of food web structure. However, it remains unclear whether these findings generalize across ecosystems, and whether non-trophic interactions affect food webs randomly, or affect specific trophic levels or functional groups. Here, we combine analyses of 58 food webs from seven terrestrial, freshwater and coastal systems to test (1) the general hypothesis that non-trophic facilitation by habitat-forming foundation species enhances food web complexity, and (2) whether these enhancements have either random or targeted effects on particular trophic levels, functional groups, and linkages throughout the food web. Our empirical results demonstrate that foundation species consistently enhance food web complexity in all seven ecosystems. Further analyses reveal that 15 out of 19 food web properties can be well-approximated by assuming that foundation species randomly facilitate species throughout the trophic network. However, basal species are less strongly, and carnivores are more strongly facilitated in foundation species' food webs than predicted based on random facilitation, resulting in a higher mean trophic level and a longer average chain length. Overall, we conclude that foundation species strongly enhance food web complexity through non-trophic facilitation of species across the entire trophic network. We therefore suggest that the structure and stability of food webs often depends critically on non-trophic facilitation by foundation species

Drought, mutualism breakdown, and landscape-scale degradation of seagrass beds

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Seagrass systems under nutrient loads, hydrodynamics & green turtle grazing : do green turtles rule the seagrass world?

Contains fulltext : 101061.pdf (publisher's version ) (Open Access)Radboud Universiteit Nijmegen, 22 maart 2013Promotores : Herman, P.M.J., Roelofs, J.G.M. Co-promotor : Bouma, T.J.138 p

Megaherbivores may impact expansion of invasive seagrass in the Caribbean

1. Our knowledge of the functional role of large herbivores is rapidly expanding, and the impact of grazing on species co-existence and non-native species expansion has been studied across ecosystems. However, experimental data on large grazer impacts on plant invasion in aquatic ecosystems are lacking. 2. Since its introduction in 2002, the seagrass species Halophila stipulacea has rapidly expanded across the Eastern Caribbean, forming dense meadows in green turtle (Chelonia mydas) foraging areas. We investigate the changes in seagrass species co-existence and the impacts of leaf grazing by green turtles on non-native seagrass expansion in Lac Bay (Bonaire, Caribbean Netherlands). 3. Green turtle grazing behavior changed after the introduction of non-native seagrass to Lac Bay in 2010. Field observations, together with time-lapse satellite images over the last four decades, showed initiation of new grazing patches (65 ha, an increase of 72%). The sharp border between grazed and ungrazed seagrass patches moved in the direction of shallower areas with native seagrass species that had previously (1970-2010) been ungrazed. Green turtles deployed with Fastloc-GPS transmitters confirmed high site fidelity to these newly cropped patches. In addition, cafeteria experiments indicated selective grazing by green turtles on native species. These native seagrass species had significantly higher nutritional values compared to the non native species. In parallel, exclosure-experiments showed that non-native seagrass expanded more rapidly in grazed canopies compared to ungrazed canopies. Finally, in six years from 2011-2017, H. stipulacea underwent a significant expansion, invading 20 of 49 fixed monitoring locations in Lac Bay, increasing from 6% to 20% in total occurrence. During the same period, native seagrass Thalassia testudinum occurrence decreased by 33%. 4. Synthesis. Our results provide first-time evidence that H. stipulacea can rapidly colonize and replace native seagrasses in the Caribbean and add a mechanistic explanation for this invasiveness. We conclude that green turtle leaf grazing may modify the rate and spatial extent of this invasive species’ expansion, due to grazing preferences, and increased space for settlement. This work shows how large herbivores play an important but unrecognized role in species co-existence and plant invasions of aquatic ecosystems

Indo-Pacific seagrass beds and mangroves contribute to fish density and diversity on adjacent coral reefs

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Expansion and fragment settlement of the non-native seagrass <em>Halophila stipulacea</em> in a Caribbean bay

The non-native seagrass species Halophila stipulacea has spread throughout the Eastern Caribbean since 2002, and could potentially impact the functioning of local seagrass ecosystems. Important characteristics for invasiveness, such as dispersal, recruitment and expansion of H. stipulacea at a local scale, are unknown. We assessed H. stipulacea expansion rates within Lac Bay, Bonaire, Dutch Caribbean (7 km2), since its establishment in 2010 and tested the settlement potential of uprooted vegetative fragments of H. stipulacea. Using 49 fixed locations, we observed that between 2011 and 2015 the occurrence of H. stipulacea in the bay increased significantly from 6% to 20% while native Thalassia testudinum occurrence decreased significantly from 53% to 33%. Free-floating H. stipulacea fragments that were collected and tethered above the sediment rooted within 10 days with a settlement success rate of 100%. The growth of settled fragments was on average 0.91 shoots d−1. The ongoing shift from native T. testudinum to introduced H. stipulacea dominated meadows may have important consequences for multiple Caribbean seagrass ecosystem functions. Given the large difference in size between the two seagrass species, functions such as coastal protection, habitat structure, food availability, and the stability and resilience of these systems can be altered. The next steps towards modelling future expansion of H. stipulacea throughout the Caribbean and beyond should include the assessment of fragment viability and dispersal distance, and the impacts of natural and anthropogenic disturbance on vegetative fragment density, dispersion and settlement by this species