Role of Hydrology, Aquatic Vegetation, Habitat Size and Connectivity in Shaping Food Webs in a Eutrophic Agricultural Marshland

ABSTRACT Aim Understanding food‐web responses to environmental gradients is crucial for guiding effective ecosystem management measures. This aspect remains understudied in wetlands, particularly in human‐managed marshlands, despite the fact that many environmental factors can be controlled. This study examines whether and how hydrology, aquatic vegetation, habitat size and connectivity shape food‐web structures in drainage ditches and their conservation implications. Location Marais Poitevin, the second‐largest agricultural marshland in western France (with concepts and methodologies applicable globally). Methods From 2015 to 2021, macrofauna (including amphibians, fish and macroinvertebrates) were sampled from 55 ditches across eleven hydrological blocks with distinct water management strategies. Using literature‐based trophic links, we reconstructed food webs (representing potential trophic interactions among locally co‐occurring taxa) and computed four descriptors: proportions of top predators and omnivores, connectance and the maximum trophic level. Then, we applied structural equation modelling to investigate the direct and indirect effects of environmental factors on these food‐web metrics. Results Ditches with unstable hydrology, experiencing large water level fluctuations and prolonged droughts, supported food webs with higher connectance, while the maximum trophic level remained unaffected. In contrast, wider ditches with higher water levels and complex aquatic vegetation supported food webs with longer food chains but lower connectance. Many of these effects were mediated by the proportions of top predators and omnivores, and, in fluctuating hydrological conditions, by aquatic vegetation complexity. Main Conclusions Our findings highlight the importance of maintaining key environmental conditions to support complex and diverse food webs in human‐modified ecosystems, such as in highly water‐regulated agricultural marshes. Ecosystems with food webs featuring longer trophic chains and lower connectance may be more vulnerable, as species loss can trigger cascading effects. Specifically, to sustain ecosystem integrity, conservation efforts should focus on preventing habitat contraction and simplification while mitigating hydrological fluctuations to balance food‐web stability and biodiversity.