Associate Professor Saint Martin's University Lacey, Washington, United States
Abstract: Invasive plants are expected to evolve to become more competitive. They may do this through increased fecundity, faster growth and increased seed size among other means. In the absence of antagonistic relations, invasive plants are expected to invest more in growth and reproduction. Indeed, in the invasive shrub Cytisus scoparius (Scotch broom), larger seeds have been seen in invasive populations (Buckley et al 2003, Bode and Black 2022) and natural selection has been hypothesized to select against these larger seeds (Paynter et al 2015). In the Pacific Northwest of the United States, the heterogeneous distribution of biocontrol species allowed the testing of how selective pressure by biocontrols influenced seed size. Initial measures showed that sites with higher biocontrol attack rates had smaller seeds than those with lower biocontrol attack rates. I hypothesized that natural selection would exist against larger seeds, that sites with a history of high biocontrol attack rates would have smaller seeds. Since there are two species of biocontrol, I also hypothesized that the attack rates of each would be negatively correlated, showing interspecific competition. To test this thoroughly, I measured seeds per pod, seed mass and biocontrol attack rates at 18 sites over the course of four years. I also grew plants from seeds collected at the sites to investigate heritability.
The biocontrol species were easily distinguished from one another, and the species dominant at each site varied, as did the relative rates at which each species destroyed seeds. There was a negative correlation between the biocontrol species, indicating competition. The direction and magnitude of selection varied but was overall negative, favoring smaller seeds. However, the variation in seed mass was more positively correlated with annual rainfall than the level of selective pressure. Rainfall and temperature were major factors in biocontrol attack rates, pod length, seed count and seed mass. Although early findings supported the narrative that selection by biocontrol species determines seed mass, the broader findings were that plant fecundity traits are determined by weather. Since higher precipitation yielded both larger seeds and lower biocontrol success, it may give a false indicator of selective pressures. This study has implications for niche partitioning between biocontrol species and for niche match between the target and the biocontrol species.
