The Mediterranean Basin, possessing 4.3% of endemics and 8.0% of all plants in the world (13,000 endemics/25,000 entities), is considered one of the most important plant biodiversity hotspots at the global scale. Within this vast area, it is possible to recognize 10 hotspots (one of which is represented by the Tyrrhenian islands), constituting about 22% (515,000 Km2) of the total surface of the Mediterranean Basin and including about 5500 narrow endemics. With a rough estimate of about 2500 vascular plants, Sardinia shows a floristic density of 103.5 species/1000 Km2, which is much higher than the average of the Mediterranean Basin as a whole (10.8 species/1000 Km2). The endemic flora is composed of 347 endemics sensu lato (14.4%), of which 155 (6.2%) are exclusive (endemics sensu stricto) to the island. The recognition of an autonomous Sardinian–Corsican province is based on the large number of endemic plants exclusively existing on the two islands, noteworthy among which are the two monotipic genera Morisia Gay and NanantheaDC. The Sardinian flora is peculiar also in terms of the originality of genetic inheritance of both types of Mediterranean species: widespread and endemic. This is a result of the unique geological history that played an essential role in shaping plant biodiversity on Sardinia and adjacent islands. The historic–genetic elements of the Sardinian flora indicate its inclusion within the biogeographical Mediterranean region and within the western Mediterranean subregion, as a consequence of a common geological history. Within the western Mediterranean subregion, Sardinia belongs to an Italo–Tyrrhenian Superprovince and to a Sardinian–Corsican province.
Here we present the global assessment of the conservation status of a narrow endemic plant from the Sardinian-Corsican biogeographic province, appling IUCN criteria and categories in relation to genetic, ecological and reproductive traits. This is first attempt to evaluate the conservation status of a narrow endemic plant from the Sardinian-Corsican biogeographic province.

 

Centaurea horrida Badarò (Asteraceae) is an endemic species of Northern Sardinia (Italy). It is protected by the Berne Convention (Appendix I) and listed as priority species by the “Habitat” Directive (92/43/EEC, Annexe II). The species appears in the 1997 IUCN Red List as “Vulnerable” (VU). Centaurea horrida is not able to carry out self pollination and its effective seed dispersal ability is limited. Its Extent of Occurrence is 172.43 km2 and its Area of Occupancy 108 km2. Population size has been estimated at 11,719 adult individuals. Significant differences in adult densities among sites have been highlighted. The structure of the population is dominated by adult individuals (on average 68%), while the proportion of seedlings is only 7% on average. Its habitat underwent an extinction rate of 18% in 50 years.

 

We investigated also the effects of the cessation of agro-sylvo-pastoral activities on Centaurea horrida. In a similar way to other cliff species, Centaurea horrida can expand its range when unsuitable surrounding habitats are disturbed. The analysis of aerial photos indicates that about 88.5% of a surface previously occupied by a Centaurea horrida dwarf community was replaced by juniper scrub over a 50-year period. In the juniper scrub Centaurea horrida density is at present 1/5 of that observed in the dwarf community. The spatial distribution of Centaurea horrida individuals was not random but concentrated in open microhabitats, more abundant in the dwarf community than in the juniper scrub. Juveniles were found mainly on bare soil: this being the only microhabitat where seedling emergence may occur. Furthermore, disturbance had a positive effect on the survival rates of saplings.

 

Genetic analyses conducted on seven populations covering the entire range of the species by means of SSR markers, showed a considerable amount of genetic variation (average He = 0.603–0.854), together with a medium-high differentiation among populations, as estimated both by FST (0.123) and RST (0.158). The results suggest that the origins of the current populations of C. horrida lie in two gene pools.

 

The spatial variability of life-history traits of Centaurea horrida, to detect the most sensitive stages to variation of site and habitat, was detected. Three main factors have been highlighted as determinants of the estimated variability in population structure and reproductive traits of C. horrida: 1) genetics; 2) site management; and 3) local adaptation. The first seems to influence some reproductive traits; site management can produce effects particularly where disturbances usually occur; finally, the greater part of the variability observed here is linked to the adaptation of single populations to local ecological conditions, depending on the combination site×habitat. Different cohorts of the same population could have experienced different ecological conditions. Variability observed in the studied traits could be, therefore, due not only to factors acting at the time of the study but also to factors that have been acting since many years before or those that used to act.

On the basis of the data gathered, the status to be assigned to Centaurea horrida is that of “Endangered” (EN). Since Centaurea horrida is a species that is linked to the initial stages of secondary succession, when patches of bare soil are available, but it is non-competitive with the late-succession species of the juniper scrub, active management practices to maintain its populations are proposed and discussed. Management and conservation strategies should be applied on two geographical units, according to population genetics results.

 

References

- Mameli G., Filigheddu R., Binelli G., Meloni M. 2008. The genetic structure of the remnant populations of Centaurea horrida in Sardinia and associated islands. Annals of Botany 101: 633-640.

- Farris E., Pisanu S., Ceccherelli G., Filigheddu R. 2009. Effects of the management regime on the performance of the endangered Mediterranean Centaurea horrida Badarò (Asteraceae). Journal for Nature Conservation 17: 15-24.

- Pisanu S., Filigheddu R. Farris E. In press. The conservation status of an endemic species of northern Sardinia: Centaurea horrida Badarò (Asteraceae). Plant Biosystems 143. DOI 10.1080/1126350090272249