Reseating of ecological gradients. III. The effect of scale and niche breath measurements

Abstract

Abundances of eleven Sphagnum species in 800 sample plots are used to investigate the effect of DCA rescaling on Levins' measure of niche breadth relative to three partitions of the water‐table gradient in a boreal Norwegian mire: (1) sample plots classified into 15 categories, each spanning an interval of 2 cm vertical extent, (2) as (1), but sequence of categories rescaled by DCA and sample plots reorganized into 15 categories with uniform beta diversity, and (3), sample plots ordinated by DCA and classified into 15 categories with uniform beta diversity by subdivision of an ordination axis highly correlated with median water‐table. Habitat niche breadth is shown to be dependent on four issues (in order of supposedly decreasing importance): (1) scale, (2) noise level of data, (3) homogeneity of individual samples, and (4), weighting function. Six problems relevant to interpretation of measurements of niche breadth are discussed: (1) range of measures, (2) spacing of categories, (3) scale, (4) choice of gradients, (5) number of samples, and (6), comparability of studies. For measures of habitat niche breadth to be biologically meaningful, four conditions have to be satisfied: (1) the gradients studied have important impact on the studied species, (2) sampling is adequate, (3) scaling of gradients is in compositional turnover, and (4), comparability is demonstrated prior to comparison with other studies. Revisions of current methods are proposed. The role of DCA in niche studies is particularly emphasized.