Habitat diversity determines ghyll woodland species diversity. Adding complexity to the species area relationship, a new predictive model?

Ghylls are linear valley features cut into the sandstone beds of the Weald of south-eastern England. The indigenous ghyll woodlands are highly species rich, supporting distinctive assemblages of cryptogamic plants. We assessed the ability of the species-area relationship, the Choros model and a modified version of the Choros model to describe variations in higher plant diversity. Principal component analysis (PCA) was used to produce proxy environmental variables, multi-regression analysis of which allowed factors determining diversity to be investigated. Field layer diversity varied substantially between ghylls. The data did not fit the species area relationship. A statistically weak fit was provided by the standard Choros model. The modified Choros model, incorporating a PCA generated proxy variable for habitat diversity, provided a good fit. The results suggest that ghyll field layer diversity is primarily determined by habitat diversity and to a lesser extent by ghyll isolation, length and area.