Extinction debt in a common grassland species: immediate and delayed responses of plant and population fitness

Changes in landscape structure and environmental conditions due to habitat fragmentation can have significant effects on plant populations. Decreasing genetic diversity and changing population structure can reduce plant fitness and influence the long-term persistence of populations. Dry calcareous grasslands in Estonia have witnessed a large decline in area within the last 80 years, but due to extinction debt, the species richness in these grasslands has not yet responded to this decline. In these calcareous grasslands, we studied genetic diversity, phenotypic performance and population characteristics of a common habitat-specialist grass, Briza media. A decrease in genetic diversity was associated with a decrease in plant reproductive output. In addition, we found that some fitness components of B. media showed a delayed response to landscape changes. Specifically, plant height and germination success were related to historical rather than to current landscape parameters, indicating a time-lagged response of plant performance to habitat fragmentation. Dependence on historical landscape structure may thus result in a future decline in population fitness even if habitat loss and fragmentation no longer continue. The documented effect of current environmental conditions, however, shows that fitness-related traits are already slowly adapting to the changing conditions. Our results indicate that even common habitat-specialist species can be susceptible to landscape changes and be threatened by decreased population performance in the future.