We used a 12-year survey of butterflies in 24 parks in Marseille (South-East France), to highlight the overall decline of butterfly biodiversity (richness, abundance, diversity) in this Mediterranean city. This study confirms the importance of the urban gradient in the composition and structure of butterfly communities with a clear difference between the city centre and the outskirts. The study also highlights the preponderant role of local factors. While the distance from the natural environment remains a major factor, nectar resources and habitat diversity are local factors that can positively drive biodiversity. In contrast, management, as currently practised, has a negative effect on biodiversity. This implies that management, nectar resources and habitat diversity in parks are the key action levers at local scale. Thus, our results show that the response is not linear, from the most urban to the least urban, but seems to be closer to a patch/mosaic structure according to the importance of local factors. The long-term survey shows that butterfly communities show a simplified pattern: generally, parks on the outskirts tend to resemble those of the intermediate level and parks of the intermediate level tend to resemble those of the city centre. The long-term survey shows that butterfly communities have lost in complexity with the loss of Mediterranean species probably linked to the disappearance of their host plants. Finally, this long-term study suggests that it is possible to influence biodiversity through management with direct action in the management of green spaces.