Disturbance-mediated population dynamics of Pedicularis furbishiae S. Wats., a rare riparian endemic

Furbish's Lousewort is a hemiparasitic perennial herb endemic to the St. John River, mostly in northern Maine. Its ancestry and mode of arrival remain unknown. Populations grow on mid-successional eroding banks below the forest edge. P. furbishiae's distribution and habitat raise questions about population dynamics and conservation of rare species dependent on natural disturbance.; The size, glacially deposited banks, and periodic ice-drives of the undammed upper St. John form a locally unusual, ecologically open riparian habitat. Multivariate analyses of the vegetation describe a heterogeneous landscape of fugitive and disturbance-withstanding species. Individual sites had 16 to 72 species, with 207 species overall. Diversity was highest in areas judged to be about five years post-disturbance. P. furbishiae occurred on high-diversity sites, i.e. where disturbance is neither too frequent nor too rare. Large-scale variation in the vegetation is related to a site's orientation, soil moisture, and time since disturbance.; I amalgamated three years of demographic data ({dollar}>{dollar}4700 individuals in 13 field populations) into stage-based matrix models for three population types differing in soil moisture, an important demographic factor. Annual rates of increase in the absence of disturbance ranged from 1.223 for "saturated" to 1.045 for "moist" and 0.859 for "dry" populations. Because environmental stochasticity plays an overwhelming role in the species dynamics, I extended the models by adding effects of floods and ice-drives. Disturbance alone decreased estimated population growth rates and increased extinction probabilities. Succession, however, causes fecundity to peak following disturbance, then decline after several years. When fecundity is modelled in this fashion, intermediate frequencies and intensities of disturbance can maximize population growth rate. Low intensity is more important than low frequency in maximizing the population growth rate for "saturated" and "moist" populations. Only the "saturated" population with catastrophic mortality below 85-75% exhibited consistent positive growth. In reality, however, such populations can suffer 90-100% mortality. P. furbishiae thus appears to be a fugitive species dependent on a balance between patch extinction and colonization. Conservation should focus on large-scale protection of the river corridor and watershed integrity.