Monitoring habitats and plant species listed in Annexes on Council Directive 92/43/EEC: is the use of Unmanned Aerial Vehicles (UAVs) on coastal cliffs suitable?

According to the Article 11 of the Council Directive 92/43/EEC (known as Habitat Directive, thereafter HD) Member States shall undertake surveillance of the conservation status of the natural habitats and species listed in the Annex I and Annexes II, IV and V respectively; the results of these monitoring activities concerning Annex I habitats and Annex II species represent part of the reporting on the implementation of the measures taken under this Directive that each Member State is obligated to produce every six years,according to the Article 17 of the same Council Directive. Monitoring activities in the HD contest is a very complicate task where different methods are carried out for habitats and plant species both for scientific and normative reasons. Recently, indeed, the Institute for Environmental Protection and Research (ISPRA) published different volumes concerning the monitoring of plant species and habitat of Community interest (Angelini et al., 2016, Ercole et al., 2016a), with the scientific support of Italian Society for Vegetation Science (Gigante et al, 2016) and Italian Botanical Society (Ercole et al., 2016b) for Habitat and plant species respectively. In spite of this separation, often plant species and habitats sensu HD share the same physical space, and therefore an integrated monitoring approach should be achieved. A clear example of this problem are coastal cliffs that are vertical or sub-vertical slopes often plugging directly in the sea. Cliffs play a crucial role in biogeography and conservation biology because they host a large number of rare and endemic species worldwide and in the Mediterranean region too (Soriano et al. 2012; Aronne et al., 2015). Many of these specie are listed in the annexes of the HD as well as some habitats such as 1240 (Vegetated sea cliffs of the Mediterranean coasts with endemic Limonium spp.) and Habitat 8210 (Calcareous rocky slopes with chasmophytic vegetation); the presence of both habitat and species makes mandatory to include cliffs as a phisical place where monitoring activities has to be carried out. However monitoring coastal cliffs by common sampling methods based on field survey can results a very harsh job due to the intrinsic difficulties caused by vertical and subvertica nature of these places. For studies at community level botanists should be aided by other people with expertise in climbing or they have to acquire this skill themselves; even so in some places the field sampling is unfeasible due to safety reasons and/or because the site is accessible from the sea only. Concerning habitats, the verticality of the cliffs makes ineffective the common methods (vertical aerial and satellite images) to produce vegetation maps used to quantify the occupied area of the habitat; moreover the area should be quantified as the real one and not the projected surface (Gigante et al., 2016). The use of UAVs (Unmanned Aerial Vehicles, commonly referred as drones) and digital photogrammetry software has been applied to monitor and assess geological risk in steep slope rocky cliffs (Danzi et al., 2013; Santo et al., 2017) representing an interesting alternative to the common in field methods, allowing to draw up detailed thematic maps by acquisition of accurate oblique photographs. UAVs has been successfully applied to monitoring plant communities, even if its application regards mainly horizontal plant communities (Gonçalves et al., 2015). In this paper we present the results of a research activity aimed to test the effectiveness of the UAVs to gather quantitative data on plant species and habitats of a coastal cliff in Campania (Southern Italy). The research was carried out close to Palinuro (SA) on a coastal cliff that is about 200 m in length and up to 70 m. a.s.l. and hosts