COS 219-2 - Improving invasion risk assessment through the lens of evolutionary ecology: Dittrichia graveolens (stinkwort; Asteraceae)

Wednesday, August 9, 2023

3:45 PM – 4:00 PM PDT

Location: D137

Presenting Author(s)

MM

Miranda K. Melen

PhD Candidate
UC Santa Cruz
Santa Cruz, California, United States

Coauthor(s)

Ingrid M. Parker

Professor
University of California, Santa Cruz
Santa Cruz, California, United States
NL

Nicky Lustenhouwer

University of Aberdeen, Scotland, United Kingdom

Abstract: A key question in forecasting the invasion risk of a newly introduced species is understanding its relationship to disturbance. In California, Dittrichia graveolens (stinkwort) was first observed in 1984 and has spread extensively along transportation corridors. It is considered a high-risk invasive species based on its rapid spread and potential impact. A key aspect of assessing invasion risk is understanding whether a species will spread away from transportation corridors and anthropogenic environments, and there are an increasing number of observations of D. graveolens in vegetated communities further from roadsides.

To assess its potential threat for expansion into wildlands, we studied the ability of D. graveolens to establish in an annual grassland in the Mount Diablo Range in California, using an experiment to test its response to disturbance. We used a randomized block design with four treatments: no disturbance (allowing grassland vegetation to grow untouched); removal of all biomass (above- and belowground); removal of only aboveground biomass; and removal of intergenerational biomass (thatch). We transplanted 8-week-old seedlings into each plot and monitored survival and growth. We harvested each plant when it reached reproductive maturity, using final aboveground biomass as a fitness proxy. D. graveolens seeds were sourced from two habitat types, roadsides and vegetated communities, at eight paired sites in Santa Clara County. This paired design allowed us to assess whether adaptation to vegetated sites had already enhanced D. graveolens’ ability to invade more vegetated communities.

D. graveolens survival and growth were dramatically increased in plots with soil disturbance and both above- and belowground biomass removal. Removal of only aboveground biomass and intergenerational biomass had a subtle, if any, effect on D. graveolens performance. Unexpectedly, removing intergenerational biomass promoted grassland vegetation growth and thus appeared to inhibit D. graveolens growth indirectly. We saw no differentiation in performance or response to treatment between seeds from roadside sites and seeds from vegetated sites, a result we confirmed with germination experiments in the lab.

This study suggests that soil disturbance is a gateway for D. graveolens to enter plant communities in California. Management efforts to reduce bare ground and increase vegetation cover may slow D. graveolens invasion along roadways. We found no evidence for adaptive divergence between D. graveolens populations originating from roadside and vegetated habitats, suggesting that evolution has not yet enhanced D. graveolens invasiveness. Rather, biotic resistance from plant competitors is a significant predictor of invasion by D. graveolens.