Epigenetics and the success of invasive plants

Biological invasions impose ecological and economic problems on a global scale, but also provide extraordinary opportunities for studying contemporary evolution. It is critical to understand the evolutionary processes that underly invasion success in order to successfully manage existing invaders, and to prevent future invasions. As successful invasive species sometimes are suspected to rapidly adjust to their new environments in spite of very low genetic diversity, we are obliged to re-evaluate genomic-level processes that translate into phenotypic diversity. In this paper, we review work that supports the idea that trait variation, within and among invasive populations, can be created through epigenetic or other non-genetic processes, particularly in clonal invaders where somatic changes can persist indefinitely. We consider several processes that have been implicated as adaptive in invasion success, focusing on various forms of ‘genomic shock’ resulting from exposure to environmental stress, hybridization and whole-genome duplication (polyploidy), and leading to various patterns of gene expression re-programming and epigenetic changes that contribute to phenotypic variation or even novelty. These mechanisms can contribute to transgressive phenotypes, including hybrid vigour and novel traits, and may thus help to understand the huge successes of some plant invaders, especially those that are genetically impoverished. This article is part of the theme issue ‘How does epigenetics influence the course of evolution?’

Rubus alceifolius (giant bramble).

R. alceifolius is a robust, aggressive perennial scrambling shrub, spreading by long arching spiny stems, rooting at their tips, as well as by bird-dispersed seeds. It can develop dense impenetrable thickets. It is native to tropical SE Asia but has been introduced to a number of other territories, most notably the Indian Ocean island of La Réunion, where it is one of the eight most threatening plant invaders to become established on the island and occurs not only on sites disturbed by man but also in primary forest with minimal disturbance (Macdonald et al., 1991). It can behave as a liana, climbing into the canopy of forest trees and increasing the risk of wind damage. It occurs also on the islands of Mayotte, Mauritius and Madagascar (Vos, 2004; Kueffer and Lavergne, 2004a,b) and in Queensland, Australia where it is invading pastures, roadsides, creekbanks, sugarcane plantations and the edges of rainforest (Queensland Government, 2012). Holm et al. (1979) record it as a 'principal' weed in Australia, and risk assessment by the Australian method gave a score of 11 (PIER, 2012). Binggeli et al. (1998) classified it as highly invasive in the tropics. In a joint project between USDA and the Weed Science Society of America it was identified among the highest-ranked potential future invasive weeds in USA (Parker et al., 2007; WSSA, 2012).

Biotic resistance to plant invasions.

Biotic resistance to plant invasions takes many forms: consumption by native herbivores, competition with native plants and infection by native pathogens. But how often does biotic resistance prevent the damaging monocultures that typify the most problematic plant invaders, and how often is biotic resistance overwhelmed by the direct and indirect impacts of human activities? This chapter attempts to answer these questions, drawing on the long history of research into biotic resistance. We first briefly describe the major forms of biotic resistance to exotic plant invasions as an antecedent to other, more detailed chapters on competition, herbivory and pathogens. We then describe a new neutral model where variance in disturbance promotes invasions over the short term, but over longer timescales only propagule pressure drives invasions. These findings are a cautionary tale; pending increases in global trade and travel, particularly to the tropics, may provide the prerequisite disturbance and propagule pressure needed to ultimately stoke further invasions. Finally, we highlight case studies where invasions have been mitigated by restoration of biotic resistance from native herbivores and competitors. These studies provide strong empirical support that conservation of native biodiversity can be a nature-based solution to some invasions, although it remains to be seen if climate change will alter these effects over longer timescales.

Success of native and invasive plant congeners depends on inorganic nitrogen compositions and levels

Aims Successful plant invaders usually exhibit three strategies: Jack-of-all-trades (more robust in stressful sites), Master-of-some (more responsive in favorable sites), and Jack-and-master (both robustness and responsiveness). To revisit these strategies, we examined how soil inorganic nitrogen (N) compositions and levels influence the success of native and invasive plant congeners in the context of plant communities. Methods We conducted an experiment involving three fixed factors: species origin, N composition, and N level. Here we selected 21 plant species (eight pairs of invasive and native congeners and five non-congeneric natives) to assemble plant communities, which were subject to nine N environments consisting of three N compositions (3:1, 2:2, and 1:3 NO3 -/NH4 +) and three N levels (low, medium, and high N). We determined the following metrics: total biomass, relative biomass (a proxy of species success), mortality rate, and mortality time. Important Findings Across nine N environments, native and invasive congeners exhibited similar total biomass, relative biomass, and mortality time, but invaders had a marginally lower mortality rate than natives. Similar success between native and invasive congeners was linked to their similar growth and tolerance. N compositions influenced mortality time and N levels affected the total biomass and relative biomass. Importantly, species origin, N composition, and N level interactively affected the total biomass, relative biomass, and mortality time. These findings suggest that native and invasive plant congeners may be similarly successful across different N environments, and that inorganic N compositions and levels both contribute to plant invasion success.