R-Selected Traits in an Invasive Population

Abstract

I compared life-history traits and self-fertilization rates in greenhouse culture of native Spartina alterniflora with an invasive population from a low-density Pacific estuary to see whether plants in the novel r-selective regime exhibit early reproduction, greater self-compatibility and high reproductive effort putting individuals at greater risk of death. Plants were grown from seed collected from the Atlantic and Gulf coasts of North America and a population introduced approximately 100 years ago to Willapa Bay, Washington, USA that has expanded to cover over ca. 6000 hectares. Pacific marshes have almost no native emergent vascular plants in the intertidal habitat, offering a virtually empty niche for invasive S. alterniflora. The low-density plants at the leading edge of this rapidly expanding population suffer a severe Allee effect of greatly reduced fecundity, caused by pollen limitation, compared to high-density areas. I found 99% of the invasive plants initiated reproduction in the first year of this study, while only 52% of the native range plants did so in the first year, followed by 34% in the second and 14% of native plants had not initiated reproduction after three years. The invasive plants had more than twice the reproductive effort of the native plants. Plants with the highest reproductive effort in the first year of growth died regardless of coastal site of origin, although nearly all of those that died over two years were invasive Pacific plants (27%). The invaders set two times the seed as the natives under forced selfing, suggesting greater self-compatibility or reproductive effort. These results suggest either a founding and bottleneck event and/or the invasive population has evolved from the long-lived, predominantly self-incompatible, K-selected state of the probable invasive propagules originating in dense, competitive native marshes.