Co‐introduction vs ecological fitting as pathways to the establishment of effective mutualisms during biological invasions

Abstract

Contents 1354I.1354II.1355III.1357IV.1357V.13591359References1359 Summary Interactions between non‐native plants and their mutualists are often disrupted upon introduction to new environments. Using legume–rhizobium mutualistic interactions as an example, we discuss two pathways that can influence symbiotic associations in such situations: co‐introduction of coevolved rhizobia; and utilization of, and adaptation to, resident rhizobia, hereafter referred to as ‘ecological fitting’. Co‐introduction and ecological fitting have distinct implications for successful legume invasions and their impacts. Under ecological fitting, initial impacts may be less severe and will accrue over longer periods as novel symbiotic associations and/or adaptations may require fine‐tuning over time. Co‐introduction will have more profound impacts that will accrue more rapidly as a result of positive feedbacks between densities of non‐native rhizobia and their coevolved host plants, in turn enhancing competition between native and non‐native rhizobia. Co‐introduction can further impact invasion outcomes by the exchange of genetic material between native and non‐native rhizobia, potentially resulting in decreased fitness of native legumes. A better understanding of the roles of these two pathways in the invasion dynamics of non‐native legumes is much needed, and we highlight some of the exciting research avenues it presents.

Cointroductions of Australian acacias and their rhizobial mutualists in the Southern Hemisphere

“…Such HGT between non‐native Australian and native resident bradyrhizobia has previously been reported in Portugal (Rodríguez‐Echeverría, ). Horizontal gene transfer may translate into more severe acacia impacts if, for example, it leads to reduced chemoattraction of rhizobia by native legumes and thus lowered symbiotic effectiveness (Le Roux et al, ).…”

“…It has been suggested that when introduced species are cointroduced with their mutualists they may become more invasive and have greater ecological impact than those that establish mutualisms by ecological fitting (Le Roux et al, ). Novel associations may not be optimal and thus limit the performance of non‐native plants.…”

“…Thus, non‐native plants that form novel mutualist associations may experience substantially longer lag phases between the time from first introduction to becoming widespread. The severity of ecological impacts caused by non‐native plants is expected to be higher when they are cointroduced with their mutualists, because these familiar associations typically involve positive feedbacks between co‐invading partners (Le Roux et al, ).…”

“…Such exchanges mean that rhizobial strains receiving mobile elements, while retaining their core genetic identity (based on non‐mobile housekeeping genes), may obtain new genetic capabilities for nodulation. This can have consequences under cointroduction where nodulation genes can be transferred between introduced rhizobial strains and resident native rhizobial strains (Le Roux et al, ). Therefore, while the identities of rhizobia are important, the true determinants of effective nodulation lie in the nodulation genes carried by them.…”

Complexity and stability of ecological networks: a review of the theory

“…The stability of the plant-legume mutualism was also explained by this cost/reward process. Plants that are selected to supply preferentially more resources to nodules that are fixing more N 2 can be crucial to the establishment of effective legumerhizobium mutualisms during biological invasions (Le Roux et al 2017).…”