The role of life-history traits, tradeoffs, and habitat in the rarity of Santa Monica Mountains Dudleya species (Crassulaceae)

In this study, life-history traits, tradeoffs in those traits, and habitat characteristics of rare and common species were compared in an attempt to explain differences in species distributions. The nine Dudleya species occurring in and around the Santa Monica Mountains were studied. Five are rare narrow endemics with small localized ranges, one is rare with an intermediate range, and three are common with broader ranges. Life-history traits were studied in wild populations and in plants grown from wild collected seeds. Habitat characteristics were recorded in two or three populations of each study species. Differences that could explain why the rare species have smaller distributions than the common species were found in aspects of growth, reproduction, and habitat specialization. In regard to life-history traits and tradeoffs, rare species grew to a smaller size and reproduced earlier than common species. The small body-size of the rare species was correlated with smaller reproductive outputs than the larger-bodied common species. The rare species also tended to have lower seedling survival. Reproductive output and survival affect population size, persistence, and dispersal, all of which affect species distributions. The habitat requirements of the study species differed in terms of co-occurring vegetation, geology, and microclimate, with the rare species being more restricted compared to the common species. To further understand how habitat plays a role in limiting species distributions, the nine species were grown in an inland garden and in a coastal garden. Plants of all species grown in the inland garden were smaller in size than those in the coastal garden. Moreover, the growth disparity between plants in the two gardens was greatest for the rare species. The rare species have a lower tolerance for hot dry conditions compared to the common species. In the Santa Monica Mountains region, the habitat conditions required by the rare species are not as prevalent as those of the common species. Differences in life-histories constrained by tradeoffs affect the prevalence of the species, as well as specialization on rare habitats.