Contrasting patterns in bryophyte and vascular plant species richness in relation to elevation, biomass and Soay sheep on St Kilda, Scotland

Abstract

Background: It was hypothesised that bryophyte and vascular plant richness of vegetation should exhibit different patterns along gradients of elevation, vegetation productivity and sheep grazing pressure. Aims: The species richness of bryophytes and vascular plant was studied in relation to biomass, elevation and sheep preference on the oceanic northern Atlantic island of St Kilda, Scotland. Methods: Plant communities and biomass were sampled from productive lowlands to summit grasslands associated with decreasing sheep preference. The relationships between bryophyte and vascular plant diversity, elevation and sheep were examined using generalised linear models and smooth trend-surface analyses. Results: Bryophyte and vascular plant richness showed contrasting trends in relation to elevation, sheep preference and biomass. Bryophyte richness peaked at mid to high elevations, was negatively related to levels of sheep preference and standing litter, and showed a curvilinear relation to biomass. Vascular plant species richness had a negative relationship with elevation, negative or unimodal association with sheep preference, and negative relationship with biomass. Conclusions: Bryophyte and vascular plant richness patterns in relation to elevation, vegetation productivity and sheep grazing are not adequate surrogates for each other, and suggest that general species richness models could benefit from organism-specific adjustments.