Habitat deterioration affects population viability and persistence,of both plant and animal species. For plants the effect of changes in environmental conditions on population dynamics and extinction risks largely depends on the life span of the species. We used transition matrix models, a life-table response experiment, and stochastic simulations to investigate the overall effects of nitrogen enrichment (9, 6,3, and 0 g N/m2/year) and grassland management (mowing and grazing) on vital rates, population growth rate (Խ), and extinction risks of a previously common perennial herb (Primula veris [L.]). We experimentally manipulated 32 permanent plots that were monitored during a 5-year period. To study the effects of nitrogen addition and management on recruitment in more detail and to evaluate the role of seed availability, we experimentally sowed seeds at different densities. Nitrogen addition always resulted in negative population growth rates (range: 0.998 to 0.700), whereas mean population growth rates in the control situation were > 1. Interaction effects indicated that the negative effect of increasing nitrogen addition decreased population growth rate more strongly when the vegetation was mown compared with grazing. Besides significantly increased mortality rates, especially in the youngest life stages, decreased population growth rates under nitrogen enrichment were mainly the result of reduced flowering probabilities, seed production rates per plant, recruitment, and establishment of seedlings. Mowing, however, encouraged growth of vegetative adults into a flowering stage and decreased germination and seedling establishment, whereas the reverse was observed under grazing. Under the highest nitrogen levels, mean times to extinction were much shorter than would be expected based on the longevity of the species, especially under mowing. These results demonstrate that even long-lived and formerly common perennial species may quickly respond to habitat deterioration, especially when the deterioration takes place over a short period of time.
EWI Biomedical sciences