Boraginaceae Varronia rupicola (Urb.) Brtton : biogeography, systematic placement and conservation genetics of a threatened species endemic to the Caribbean

Abstract

In the Caribbean region, Varronia rupicola (Boraginaceae) is a medium to large, woody shrub endemic to the Puerto Rican Bank where it is threatened with extinction due to its limited area of occupancy, small populations and on-going threats. The greatest of these is currently loss of suitable habitat through development and degradation. These are caused by human activities that are expected to continue and possibly worsen. The species is also threatened by sea level rise and drought as well as natural disasters, particularly hurricanes and tsunamis. Combined, the effects of anthropogenic and climate change induced threats could push the species to extinction over the coming century. Through interrogation of the findings of cyto-, phylo- and population genetic as well as biogeographical research, it is clear that V. rupicola is a distinct species that is endemic to the islands of Puerto Rico, Vieques and Anegada where five populations were detected. The species has lost genetic diversity in the wild through a reduction in population size with allelic diversity proportional to the size of the population. The five populations were found to have lower than expected levels of heterozygosity as well as significant genetic differentiation and inbreeding. Varronia rupicola plants were found in an extremely limited area of intact habitat (<90 km2) overlying substrates that cover <200 km2 across the three islands. Protected areas contain less than a third (<30 km2) of the remaining intact habitat that supports the species and established ex-situ collections capture less than half of the private alleles found in the wild. An integrated approach to the species conservation is needed to maximise genetic diversity and potentiality allow adaptation of V. rupicola to environmental change and new threats.