Diversity and structure of macrophyte communities in two different fish pond systems: Dombes (France) and Midden-Limburg (Belgium)
Résumé
Agricultural intensification and the expansion of farmland in recent decades have contributed to a drastic decline in biodiversity. Different types of production systems, like land sharing or land sparing strategies – try to reconcile production and biodiversity conservation. Among all these agricultural landscapes, there are many in Europe with ponds (also called "pondscapes") for fish production. These fish ponds ecosystems can contain remarkable biodiversity, including rich communities of macrophytes with several rare or protected species. However, intensification of fish farming practices increasingly results in pond ecosystem deterioration and the loss of this biodiversity. The diversity and structure of communities in relation to different fish production strategies remains little studied.
Our work aims to fill this knowledge gap by investigating how fish stock management affects macrophyte community characteristics at different spatial scales. For this purpose, we used data about macrophytes communities collected in 2022 in multiple fish ponds, in two important European pondscapes (17 from Dombes in France and 21 from Midden-Limburg in Belgium) with different fish management strategies. All ponds in the Dombes region are used for extensive fish farming (land sharing principle), whereas pond management in Midden-Limburg is more variable and includes ponds for fish farming (high fish density) and ponds without fish for purposes of biodiversity conservation (land sparing principle).
Our results show a significant negative effect of higher fish density on the species diversity of macrophytes. Fish density also has an effect on community’s composition. Nestedness predominates over turnover between the different types of fish stock management. Fishless ponds help to maintain species that are not present in ponds for fish production. Despite their different communities, the two regions show similar gamma diversity.
Our results confirm the negative effect of intensive production of benthic fish on macrophytes community composition and diversity. Neither system seems to be a better alternative for macrophyte conservation. By going beyond this simple binary choice, we can say that an "ideal fish pondscape” for maintaining both productivity and biodiversity conservation should be a mixture of both strategies, to diversify conditions and habitats. Inside this landscape, fish-free managed ponds also seem essential for maintaining additional macrophytes species, not found in ponds for fish production.
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