Species Richness, Species Rarity and Conservation Evaluation of Rich-Fen Vegetation in Lowland England and Wales

Abstract

(1) The botanical resource of rich-fen sites in England and Wales is analysed, based on a survey between 1972-85 of most known sites. The 464 plant species recorded in the survey are segregated into those (153) for which fens are a major habitat (principal fen species), those (203) that are as well or better represented in other habitats as in fens (affiliated species) and those (108) essentially atypical of fens (accidental species). (2) The occurrence of principal fen species of low frequency (occurring in < 5% of samples) (i.e. rare species) is analysed from 2900 quadrats from herbaceous and wooded fen vegetation. Many rare species occur mainly in just a few distinctive herbaceous vegetation types of low crop mass. Most do not occur at all in fen carr. (3) Variation in species richness (number of species 100 m-2) in 2100 samples of herbaceous fen vegetation is analysed. Very species-rich stands are infrequent and referable to just a few vegetation types. These are the ones that also contain most rare species. (4) Individual fen plant species (principal and affiliated species) occupy a discrete range of vegetation species-richness (the 'associated-richness') in herbaceous fen vegetation samples. Those species which show only small variation in the associated-richness values (interquartile range <50% median associated-richness value) are 'richness indicator species' and their occurrence can be used to predict the species-richness of herbaceous fen vegetation. (5) Regressions of total number of species and number of common species against number of rare species within vegetation samples both show a strong (P < 0 0001) positive relationship. The proportion of rare species within the vegetation also increases with increasing species richness. (6) The present infrequency of species-rich vegetation (and its associated rare species) in undrained fens is probably mainly due to increase in vegetation crop mass at many sites, caused by the abandonment of traditional management regimes perhaps exacerbated by increased net productivity caused by fertilizer run-off from the catchments. (7) A few rare species are most characteristic of comparatively species-poor vegetation of fairly high crop-mass. The reasons for their current rarity are not known. (8) The data on species-richness and rarity provide a basis for conservation priorities in rich-fens. Four possible indices, based on richness and rarity, for the evaluation of fen sites for conservation are proposed and fifteen sites are evaluated using these.