Human disturbance, habitat degradation and niche shift: the case of the endemic Calendula maritima Guss. (W Sicily, Italy)

Calendula maritima Guss. is a plant endemic to the coastline of W Sicily, whose distribution range experienced a dramatic shrinkage during the last centuries along with the degradation and fragmentation of the associated plant assemblages. According to that, it can represent an effective target species to test the response of coastal biota to anthropogenic disturbance. In order to better understand the niche width of C. maritima, the coastal plant communities in which it is still found have been investigated through vegetation relevés carried out on different size plots. Although the sea marigold has been considered as a pioneer species typical of shifting dunes, field investigations highlighted that it also occurs in different plant assemblages. Data concerning the smaller plots allowed the identification of three groups of relevés: one showing the highest number of trivial ruderal plants, another hosting some psammophilous species, and a third linked with rocky surfaces. These three groups do not cluster with the relevés issuing from literature, carried out some 40 years ago, that mostly focused on dune ecosystems. It is assumed that the strong floristic differences between these two different types of relevés may be linked with the destruction of shifting dunes occurred over the last decades, and which also caused the local extinction of many other plants related with this habitat. However, this could also reflect a lack of historical relevés concerning C. maritima populations growing on coastal rocky sites. Interestingly, a gradual expansion of the niche of C. maritima was highlighted, likely depending on the ability of the species to colonize new plant communities in response to increasing anthropogenic disturbance. Our results also confirm that hybridization with the contiguous congener species Calendula fulgida Raf. may represent a major threat for conservation of the remnant populations of C. maritima.