Population viability analysis of a rare plant species in patchy habitats with sporadic fire

Reduction and isolation of remnant natural communities and changes in disturbance regimes may threaten the persistence of many species. This study describes the effects of habitat spatial structure and fire frequency on population viability of Hypericum cumulicola, a perennial herbaceous species restricted to open sandy habitats in Florida rosemary scrub along the Lake Wales Ridge, Florida, U.S.A. A survey of species composition of 89 scrub patches at Archbold Biological Station indicated that occurrence of H. cumulicola and other open habitat specialists was positively related to patch size and patch aggregation. Metapopulation models predicted that some suitable patches will be unoccupied because of local extinctions and low rates of colonization. Transplanted H. cumulicola survived, grew, fruited, and recruited equally well in unoccupied and occupied sites, indicating that its patchy pattern of site occupancy was likely due to limited dispersal and periodic extinction. Seeds buried and later exhumed germinated after one and two years of burial, demonstrating a persistent seed bank. Analyses of the association of microhabitat variables with H. cumulicola demographic variation along a time-since-fire gradient showed that survival of this species decreased with increasing proximity of the shrub Florida rosemary and ground lichen cover indicating its dependence on sporadic fires to decrease their interference. Matrix population models were used to compare H. cumulicola demography and extinction probability under different fire scenarios. Projected finite rates of increase ranged from 0.76 to 2.56. Times-since fire and the finite rates of increase showed a significant inverse relationship. Expanding populations were predicted only in recently burned patches. Growth was the most important elasticity component in recently burned patches while survival in the seed bank had the largest elasticity in sites a decade post fire and in long-unburned patches. Simulations of population trajectories after an initial fire indicated that in the absence of migration and additional fires, even large populations may become locally extirpated within 250 years. Fire return intervals less than every 20 years, and variation in fire return intervals, are recommended as appropriate management for H. cumulicola in rosemary scrub.