Disturbance allows persistence of engagnered plants in coastal sand dunes
Many species endemic to frequently disturbed ecosystems require disturbance and early successional microhabitats for persistence. Human modification of disturbance regimes alters the availability of microhabitats and may affect the viability of species in these ecosystems. Because restoring historical disturbance regimes typically is expensive and requires action at large spatial scales, restoration projects must be justified by linking the persistence of species with successional microhabitats. Coastal sand dune ecosystems worldwide are characterized by their endemic biodiversity and frequent disturbance. Dune-stabilizing invasive plants have altered successional dynamics by reducing sand movement and the availability of early successional habitat, and threaten species in these ecosystems. We examined the distribution and population dynamics of two federally endangered plant species, the annual Layia carnosa and the perennial Lupinus tidestromii, within a dune ecosystem in northern California, USA, that has been heavily invaded by the invasive grass, Ammophila arenaria. Within the dunes we characterized the vegetation communities in early and late successional habitats. We quantified differences in the abundance of both species and the stage structure of L. tidestromii. We parameterized a matrix population model for L. tidestromii and examined the magnitude by which the successional stage of the habitat (early or late) influenced population dynamics.
Both L. carnosa and L. tidestromii had higher frequencies and L. tidestromii had a higher frequency of seedlings in early successional habitats. Lupinus tidestromii plants in early successional microhabitats had higher projected rates of population growth than those associated with stabilized, late successional habitats, due primarily to higher rates of recruitment in early successional microhabitats. These results support the idea that restoration of disturbance and maintenance of early successional microhabitat is critical in historically dynamic landscapes. Our results suggest that large-scale restorations are necessary to allow persistence of the endemic plant species that characterize these ecosystems.