Thin lines between native and invasive aquatic plants are common, posing challenges for response efforts

Lines separating native and invasive plant species can be thin due to
close relatedness, obscured by cryptic invasions, or breached by
hybridization. Past work suggests these phenomena are especially prevalent
in aquatic systems. This could arise from strong filters imposed by
aquatic environments magnifying the importance of
“preadaptation”—non-native species succeeding where closely related native
species occur due to shared traits. If so, there should be stronger
signals of preadaptation in aquatic than terrestrial plant invasions, with
implications for management. I tested for a stronger influence of
preadaptation in aquatic than terrestrial invasions by comparing the
relatedness of invasive and native species in aquatic vs. terrestrial
flora of the Midwestern USA using herbarium records and taxonomic and
phylogenetic analyses. I predicted that aquatic species would constitute
an evolutionarily distinct subset of plants; aquatic invaders would be
more closely related, taxonomically and phylogenetically, to native
aquatic plants than terrestrial invaders are to their native counterparts;
and aquatic invaders from the Midwest’s primary donor region (the
Palearctic) would be more closely related to Midwestern species than are
Palearctic aquatic plants that are not invasive in the
Midwest. Findings supported the evolutionary legacy of adaptation
to aquatic environments being reflected in thin lines separating invasive
and native aquatic plants. Aquatic species constituted an evolutionarily
distinct subset of plants, and invasive aquatic plants were more closely
related to native aquatic plants (taxonomically and phylogenetically) than
terrestrial invaders were to terrestrial native
species. Additionally, aquatic plants of the Palearctic that are
invasive in the Midwest were more closely related to Midwestern natives
(phylogenetically, but not taxonomically) than were their non-invasive
counterparts, reinforcing the role of preadaptation. Synthesis and
applications: Thin lines separating native from invasive aquatic plants
pose challenges for response efforts. For resource managers, the
prevalence of cryptic invasions requires dedicated, robust approaches to
surveillance. For outreach, finer distinctions between native and invasive
necessitate nuanced approaches to communication and education. When
control is needed, it should be implemented cautiously to minimize
non-target impacts to close native relatives, but thoroughly enough to
prevent native relatives being displaced by competition and/or
introgression.