1. Several species worldwide show rapid range retraction due to habitat degradation, and some of them have restricted distribution and specific resource needs. Such cases deserve particular attention and need urgent conservation actions to avoid extinction, and one way is to facilitate colonisation of new habitats by resource supplementation. 2. Here, we investigate the changes in range distribution, during the last decade (2007–2016), of an endangered endemic damselfly, Calopteryx exul Selys, and assess the importance of its favourite host-plant (Potamogeton nodosus) in colonisation and population dynamics in the last existing population of Algeria. 3. We first used dynamic occupancy models to assess range distribution dynamics and we found that both occupancy and colonisation probabilities of the species were positively dependent on the occurrence of P. nodosus. There was also evidence that extinction probability increased with habitat disturbance but decreased with the occurrence of P. nodosus. Our experimental restoration showed that the augmentation of patches of P. nodosus increased the total number of individuals, the number of reproductive events and philopatry. 4. Our study highlights the importance of insect–plant relationship in the establishment of effective restoration plans because of their implication in colonisation and extinction processes and population dynamics. 5. Since most insect species from different orders and ecosystems are ecologically dependent on plants, our restoration approach may benefit a large range of threatened species and improve their conservation status.