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Marsilea villosa Kaulfuss is an endemic Hawaiian fern with a very small, fragmented natural range and an ephemeral habit that makes it difficult to assess population health. Its sporocarps are presumed to remain viable for many years, allowing it to survive periods of drought and then sexually reproduce when there is sufficient precipitation to cause them to be submerged in standing water. Surveys of plant cover at 'Ihi'ihilauākea Crater, where the largest and best-protected stand was located, have shown that vigorous growth of the species occurs after the crater floor is flooded. This study documents dramatic decline over the last 8 yr, during which growth has been largely vegetative. Analyses of rainfall records suggest that events producing long-duration floods occur on average every 6.5 yr, yet 13 yr have elapsed since the last one. Although this may in part explain the poor condition of the population, other ecological changes have occurred including decline of the dominant trees and invasion of alien grasses that may influence flooding frequency. Marsilea villosa may be able to avoid extinction because flooding caused by rare climatic events will kill off the competitors that have encroached on its former ecological space. However, it is predicted to be a less-conspicuous part of the ecosystem most of the time unless management can effectively suppress invaders.