The comparative ecology and distribution and within-species variation of the lichenized Ascomycetes Ramalina cuspidata and R. siliquosa in the British Isles

Abstract

The two species are shown to be almost perfectly sympatric, but their centres of abundance on the shore are different, Ramalina cuspidata being most abundant low in the supralittoral and R. siliquosa most abundant higher in this zone. Principal component analyses show that the within-species variation, which is considerable, is continuous in both species. Correlation of the attribute scores and height above sea level of the individuals with the component scores for the individuals demonstrates that in R. cuspidata both morphological and chemical variation are related to vertical position on the shore. Only chemistry is related to vertical position in R. siliquosa, morphological variation being controlled by angle of rock surface and protection from inundation. The continuum of morphological variation is considered to be a response to a potential evapotranspiration gradient in both species. Individual ramuli of R. cuspidata frequently exhibit a wide range of the morphological continuum along their length indicating that morphological variation is not the result of genotype selection. There is no unequivocal evidence by which to refute the widely held view that the zonation of chemotypes in both species is the result of habitat selection. However, it now seems more probable that chemical variation may be explained as being the result of progressive truncation of biosynthetic pathways as a response to an increasingly stressful environment with increasing height above sea level. The concept of morphological buffering is introduced and the role of morphological plasticity in the survival strategy of lichens is discussed.