Human Ecosystem Engineering: Effects and Feedback in Coastal Benthic Systems

In the attempt to enhance their environment, humans are causing changes in ecosystems worldwide, by acting as ecosystem engineers. Changes in ecosystems, caused by any ecosystem engineering species, have the potential to feedback to the engineer. Humans are experiencing such feedback, affecting ecosystem goods and services. As today’s feedback following human ecosystem engineering (HEE) are mostly negative, concerns are raising about the future of Earth's environment and its ability to provide the services required to maintain viable human civilizations. With 2.5 billion people living within 100 km of the coast, disproportionate pressure is placed on coastal ecosystems. Holding great environmental and economic value, concerns are consequently especially turned towards these coastal ecosystems. As humans are facing challenges related to climate change, biodiversity loss and increasingly threatened ecosystems, there is therefore a great need to understand human-induced pressures, effects and feedback processes in the environment.

This thesis explores the nature and consequences of HEE unintended effects on coastal ecosystems, through the study of a combination of three different areas worldwide: Sine Saloum in Senegal, West Africa, Åland Islands in Finland, Baltic Sea, and Mobile Bay in Alabama, Gulf of Mexico. It is well-known that unintended consequences following HEE can be detected using bioindicators, so in my thesis I focused on bioindicators particularly relevant in coastal ecosystems, namely macrofaunal organisms. Based on three papers, my PhD thesis first evaluates HEE effects on macrofaunal communities and ecosystem functioning by identifying effects on structural and functional properties of macrofaunal communities to different sets of anthropogenic and abiotic drivers, using a combination of taxonomic and trait-based approaches. Because macrofaunal organisms are central to several ecosystem functions and many of the ecosystem services provided by benthic macrofaunal communities are threatened directly via human-driven impacts, I also looked at how proxies for macrofauna-mediated ecosystem functions, which acts as biotic feedback for humans, responded to anthropogenic activities. I focused on macrofaunal ecological functions related to sediment reworking (i.e., bioturbation and bioirrigation potentials), to biogeochemical cycling and to their capacity to serve as essential role in energy acquisition for higher trophic levels while serving as central food resource for epibenthic predators and demersal fish.

The findings highlight that not only macrofauna was affected by the physical environment and sediment properties, but HEE was also highlighted as a significant driver. Indeed, in the three studied coastal ecosystems, drivers for macrofaunal community structure and functioning were always including variables not only describing the physical environment (e.g., salinity, temperature, hydrodynamics, depth, dissolved oxygen, season) and sediment properties (e.g., grain size, CN content, organic content) but also HEE (e.g., shellfish harvesting, heavy metals, hazardous substances, costal development and shoreline modification, eutrophication, fish farming, dredging). Results are showing that HEE is affecting macrofaunal communities (i.e., abundances, biomasses, diversity and species and trait composition) which translates to modifications in ecological functions and processes associated with macrofaunal communities, in turn having the potential to affect ecosystems services. Both spatial proximity to HEE and the type of HEE were shown to affect structure and functioning of macrofaunal communities. A concrete example of HEE effects and feedback was highlighted with shellfish harvesting in the Sine Saloum, Senegal.

This research provided additional evidence regarding the interest in using trait-based approaches in combination to the taxonomic approaches to better understand HEE effects and feedback. A novel index was found relevant to estimate the macrofaunal community quality as a food resource for consumers (e.g., benthic-feeding fish), reflecting an additional ecological function to sediment reworking. Moreover, this thesis demonstrates that the ecosystem engineering framework can be applied to the human species. The use of the HEE framework could represent a valuable step towards sustainability (e.g., helping reaching the UN Sustainable Development Goals (SDGs)). The findings of this thesis also bring a deeper understanding of HEE effects in coastal ecosystems and potential feedback for human society and economy. In order to preserve ecosystem services, particular attention should be paid towards human activities on coastal ecosystems.