Conservation of a Rare California Wildflower:
A Case Study of the Santa Cruz Tarplant

Adelia Barber
Bachelor's of Science in Environmental Science
December 2001

Many endangered plants exist in small, declining populations and exhibit signs of poor reproductive fitness due to negative environmental pressures and/or a loss in genetic viability. The Santa Cruz tarplant (Holocarpha macradenia) is an extremely rare annual wildflower that has suffered many recent local extirpations. The tarplant currently exists at eleven isolated sites, only three of which have large, stable populations. This study hypothesized that individuals in the small tarplant populations are experiencing reduced reproductive fitness, primarily as a result of negative environmental forces. We looked for correlations between population size and both indicators of plant performance (seed number, seed weights, seed heads per plant, and plant size) and indicators of habitat quality (plant density, indicators of disturbance, and invasive plant presence). Primary experimental components included: comparing fitness traits of field plants to plants grown in a common garden environment, measuring indicators of habitat quality, testing the effects of self and cross fertilization in both types of the tarplant's dimorphic seeds, and performing a small-scale invasive grass removal experiment.

Field plants growing in small populations produced fewer seeds, fewer seed heads, and a greater proportion of dormant seeds than plants in large populations. These trends were not observed in the greenhouse populations, leading us to believe that the effect is mostly environmental. We observed a strong correlation between population size and indicators of habitat quality and a dramatically increased rate of survival in plots where invasive plants were removed. Fewer non-dormant seeds likely represent a lack in pollination, while other negative reproductive traits may be a resource limitation stemming from invasive plant competition. Overall, we find strong evidence that the small tarplant populations are suffering from adverse environmental effects including pollen and resource limitations, invasive plants, and a lack in disturbing forces; whether these are the only or original sources of population decline has yet to be determined. We cannot rule out the possibilities of genetic problems, allee effects, or intrinsic biological attributes of the tarplant as factors reducing tarplant fitness and population size. However, this study demonstrates that negative environmental pressures are likely powerful enough to cause extirpation, therefore, intensive and proactive management, and possibly even ex-situ cultivation is needed to rehabilitate the small tarplant populations.