Inbreeding and small population size reduce seed set in a threatened and fragmented plant species, Lupinus sulphureus ssp. kincaidii (Fabaceae) Original Research Article

Willamette Valley upland prairie in western Oregon, USA, has been reduced to less than 1% of its original historic range following European settlement in the 1850s. Lupinus sulphureus ssp. kincaidii (Kincaidʹs lupine), a threatened species and the primary larval host plant of the endangered Icaricia icarioides fenderi (Fenderʹs blue butterfly), was historically a panmictic metapopulation. Habitat fragmentation may be causing many of the Kincaidʹs lupine colonies to display typical symptoms of inbreeding depression, such as low seed production. Hand outcrosses on bagged inflorescences significantly increased seed set and seed fitness compared to open pollination and within-colony pollen treatments. Natural seed set was positively correlated with an increase in the number of Kincaidʹs lupine patches, suggesting that population size limits seed set. An increase in fruit set was positively correlated with Kincaidʹs lupine raceme number, raceme density, and the number of lupine patches, demonstrating that floral display and population size increase pollinator service. Restoration of Kincaidʹs lupine populations should consider measures that lessen the effects of inbreeding depression, especially in small, isolated populations, for the long-term persistence of the species.