Scale dependency and area of occupancy: Tortula freibergii in north-west England

Abstract

Area of occupancy (AOO) is used commonly as a measure of species range size and the IUCN Red List Criteria provide thresholds of AOO for determining the extinction risk of species. Tortula freibergii is a rare moss globally and is considered to be a priority for conservation in the UK. This study provides the results of a comprehensive survey of the distribution of the species in north-west England, at a linear resolution of 0.5 km, which shows it to be considerably more widespread than understood previously. The data are used to create a 'species-area curve' that spans the scales of 0.1 to 10 km and examine how measures of AOO change according to the scale of measurement. AOO values declined sharply as the scale of measurement reduced, as a result of the linear and frequently fragmented distribution of the species. Implications for the application of the IUCN Red List Criteria and the monitoring of species are discussed.

Keywords:

• IUCN CRITERIA
• RED LIST
• RANGE SIZE
• AOO
• SPECIES-AREA CURVE
• EXTINCTION RISK