Minimum population size for reproduction in Willamette daisy

SUMMARY

• We located and visited 20 populations of Willamette daisy with a range of population sizes (<10 to >4000), representing the majority of remaining locations of this species.
• We counted all flowering individuals in all populations (or used current population sizes as determined by others), and collected mature flower heads from each population.
• We determined the percentage of viable seeds per flower head for each population by counting filled and empty ovules from approximately 30 flower heads from each location.
• Our results indicate that populations with 20 or fewer individuals generally fail to produce viable seeds. In populations larger than 20, filled ovule production ranged between two and 24 percent, with an overall average of 9.16% of ovules filled for plants sampled in all populations.
• Our results suggest that populations of Willamette daisy with fewer than 20 flowering individuals are unable to produce viable seed, and therefore will go extinct without intervention to increase population sizes or import pollen.
• In populations larger than 20, some filled seeds are being produced, indicating that recruitment by seed is possible. However, filled seed numbers were quite low, suggesting that all populations would benefit from management activities that boost seed production and potential recruitment.
• Sampling in 2006 supports the results from 2005: Populations of Willamette Daisy that fall below a population size threshold of 20 or fewer flowering individuals experience near-total reproduction failure