Demographic vulnerability in cliff-dwelling Sonchus species endemic to the western Mediterranean

Abstract

Species of Sonchus section Pustulati (Sonchus masguindalii, Sonchus fragilis and Sonchus pustulatus) constitute a group of endemic cliff plants in the Mediterranean region, restricted to narrow non-overlapping areas within the Baetic–Rifan hotspot of plant species diversity. S. pustulatus occurs both in SE Spain and N Africa, whereas the other related species are exclusive to N Africa. We characterized all the extant populations of Sonchus species (section Pustulati) in 2008 by recording population size, demographic structure and reproductive success, and we estimated the population trends for the critically endangered Spanish S. pustulatus by repeating censuses in 2013. We also calculated the stochastic population growth rate (λS) and modelled future viability (PVA) of one of the Spanish S. pustulatus populations by using matrix models derived from detailed demographic monitoring over a six-year period (2003–2008).Population sizes ranged between 100 and 22,000 reproductive individuals. In spite of differences in plant size, population protection and anthropogenic disturbance, the life history stage structure of all populations was similar and characterized by a low abundance of seedlings and juveniles, suggesting low recruitment. The population growth rate of Spanish S. pustulatus populations ranged between 0.91 and 1.01, and the matrix model showed significant population decline (λS=0.9042; 95% CI: 0.9041–0.9043). The PVA projected that this population would shrink to a few individuals in approximately forty years under present conditions. Since high temperatures and drought negatively affected the dynamics of this population, ongoing climatic change will jeopardize its future persistence.