Site occupancy, recruitment and extinction thresholds in grassland plants:an experimental study

There is a great need for simple but realistic methods for assessing status and management prescriptions for declining plants in fragmented landscapes . In this study we discuss a method for estimating extinction thresholds fo r metapopulations, based on analyses of site occupancy and recruitment of 1 8 species characteristic of semi-natural grasslands in Scandinavia. The ext inction threshold refers to the fraction of remaining suitable habitat at w hich a species becomes extinct. Site suitability was estimated from sowing experiments and occupancy was estimated from site inventories. A comparison between the estimated extinction thresholds and remaining suitable habitat s indicated that a decline in species diversity would accompany a further r eduction of suitable habitat. An alternative 'quasi-equilibrium' extinction threshold was also tested based on the assumption that there is a delay in population decline. With this approach, eight of the 18 selected species p resently live 'below' the extinction threshold. In order to estimate the ac curacy of the methods, we made a tentative comparison of the results with k nowledge of species status from other sources of information. We suggest th at three species, Agrimonia eupatoria, Antennaria dioica, and Gentianella c ampestris may experience further decline in Scandinavia in the near future. Studies of occupancy, recruitment and extinction thresholds provide a usef ul framework for community-wide surveys of species in declining habitats. ( C) 1998 Elsevier Science Ltd. All rights reserved.