Thesis

Effects of invasive plants on the endangered sunflower, Pentachaeta Lyonii Gray

Invasive plants threaten native biodiversity and ecosystem function. Non-native plants can out-compete native plants for resources, reducing population sizes. For rare species, this can increase the chances of extinction. Pentachaeta lyonii is an endangered, endemic sunflower, currently ranging entirely within the urbanized Santa Monica Mountains and Simi Hills. Its former range and number of populations have been reduced in recent decades due to pressures from urbanization, and the remaining populations are in decline. This study examined the effects of competition from invasive plants as a possible cause of declines by evaluating both effects from competition and effects from community alteration. Three invasive plant groups (annual grasses, Erodium spp., and Centaurea melitensis) were studied in (1) direct competition experiments in the field and in pots, (2) observational studies comparing sites where P. lyonii is extant and extirpated, and (3) manipulative community-level experiments. In the field and pot competition experiments, all three invasive groups competitively reduced the reproductive capacity of P. lyonii, and had differing effects on P. lyonii height. Observational studies showed that the presence of annual grasses and its associated litter were correlated with extirpation, and retention of bare ground was correlated with P. lyonii persistence. Restoring P. lyonii habitat to pre-invasion conditions by removing non-native plants, scraping the soil surface, and adding cryptobiotic crust increased native species richness and reduced the cover of annual grasses. Seeding P. lyonii increased its density in existing sites, and was successful in establishing plants in new sites. Removal of invasive plants and their associated litter in P. lyonii habitat, and seeding existing and new populations are recommended for restoration and recovery of the species.

Relationships

Items