Detecting the effect of intensive agriculture on Odonata diversity using citizen science data.

Agricultural areas represent one of the major ecosystems of the world. Intensification of agricultural practices produced openfields characterized by low biological diversity. Nevertheless, the distance up to which intensive agricultural fields alter surrounding natural systems is rarely quantified. We determined the spatial scale at which agricultural landscapes alter the diversity of Odonates, a key taxon in wetland ponds, and we tested to what extent citizen science data can be used reliably for this purpose. We compiled 7731 observations made in a portion of the region Centre-Val-de-Loire (France) over 10 years by naturalists on 729 water bodies to analyze the effect of agricultural landscapes (mainly wheat, rapeseed, sunflower) on the species richness of both damselflies and dragonflies in lentic systems. Sixty species were reported over the 10-year period. For dragonflies, intensive agricultural landscapes best explained their richness at the scales of 800 and 1600 m for overall and autochthonous species, respectively, when using the full dataset. The spatial scale was smaller for damselflies, at 200 m for both overall and autochthonous species. These distances were not severely impacted when constraining the data to consider several biases. Multimodel averaging showed that the proportion of intensive agriculture decreased species richness, despite the potential biases inherent to an imperfect database acquired by citizens. This imperfect citizen dataset allows to infer the lowest effect size of agriculture on species richness. Quantitatively, this effect was more important for autochthonous species. Interestingly, both relatively rare taxa and common or generalist species can be under threat in intensive agricultural landscapes, calling for more ecotoxicological studies. The influence of agricultural practices from a distance implies that conservation and management plans of wetland ponds should consider the landscape ecological characteristics and not only the pond features. Conservation efforts focusing too locally on a site may be undermined because intensive agriculture from a distance limits the potential for the site to recover highly diverse communities. These distant effects should be integrated by policy-makers when deciding which wetland pond should benefit from a conservation plan or which conservation action may be planned, implementing, for instance, buffer zones and/or ecological corridors composed of natural vegetation.