Abstract Pentachaeta lyonii A. Gray is a state- and federally-listed endangered species, endemic to heavily invaded southern California grasslands. Recent population extirpations resulting in a decrease in range size have prompted investigation into the effects of invasive annual plants on this species. The goals of this study were (1) to examine the impacts of competition from non-native species from three different functional groups (annual grasses, early-season forbs, and late-season forbs) on P. lyonii success in the field and in pots, (2) to determine which non-native species/functional groups have the greatest competitive effect on P. lyonii, and (3) to evaluate the environmental conditions that contribute to the displacement of P. lyonii by non-native plants. In the field, at two sites over two years, control plots were paired with plots in which non-native competitors were clipped at the soil surface. In pots, individual P. lyonii plants were grown in competition with all three groups of non-nativ...