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 for metapopulations, based on analyses of site occupancy and recruitment of 18 species characteristic of semi-natural grasslands in Scandinavia. The extinction threshold refers to the fraction of remaining suitable habitat at which 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 habitats indicated that a decline in species diversity would accompany a further reduction 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 presently live 'below' the extinction threshold. In order to estimate the accuracy of the methods, we made a tentative comparison of the results with knowledge of species status from other sources of information. We suggest that three species, Agrimonia eupatoria, Antennaria dioica, and Gentianella campestris may experience further decline in Scandinavia in the near future. Studies of occupancy, recruitment and extinction thresholds provide a useful framework for community-wide surveys of species in declining habitats. (C) 1998 Elsevier Science Ltd. All rights reserved.