Mutualism as a constraint on invasion success for legumes and rhizobia

Abstract

Because hereditary symbiont transmission is normally absent in the mutualism of legume plants and root‐nodule bacteria (rhizobia), dispersing plants may often arrive at new habitats where mutualist partners are too rare to provide full benefits. Factors governing invasion success were explored by analysing a system of two coupled pairwise competition models: a legume invader competing with a resident non‐mutualistic plant, and a rhizobial population competing with a resident population of nonsymbiotic bacteria. The non‐linear dependence of benefits on partner abundance in this mutualism creates the possibility of two alternative population size equilibria, so that a threshold density can exist for invasion. If legumes and rhizobia exceed a critical population size, both species achieve rapid population growth, while if initial densities of both species are below their respective thresholds, they remain rare and are thus vulnerable to extinction in the presence of competitors. Overall, the results indicate that legumes may often fail at colonization attempts within habitats where mutualist partners are scarce. Data on legume prevalence in island floras and rates of geographical spread by legume weeds are consistent with this inference. Predictive insights about invasiveness may emerge from comparative research on key traits identified by the model, especially the shape of the function determining the number of nodules formed at low rhizobial density.

Reduced availability of rhizobia limits the performance but not invasiveness of introduced Acacia

“…Although seedling growth was reduced away from conspecifics, which we might expect if soil rhizobia populations decline in the absence of a host plant (Parker 2001;Thrall et al 2005), the reduction in growth was not strong or statistically significant. Other studies that have examined the influence of soil biota on species performance have found that interactions with natural enemies in the soil are generally more pronounced in species' native ranges (e.g.…”

“…Highly promiscuous plant hosts are able to nodulate with a wide range of rhizobia strains and at low bacterial densities, while less promiscuous hosts show greater strain specificity and require higher bacterial densities before they nodulate (Roughley 1987;Bhuvaneswari, Lesniak & Bauer 1988;Thrall, Burdon & Woods 2000;Thrall et al 2005Thrall et al , 2007. In addition, plant hosts themselves influence the availability of rhizobia in the soil (Thrall, Burdon & Woods 2000), with rhizobia population numbers increasing rapidly in response to compatible plant hosts (Purchase & Nutman 1957;Parker 2001) and declining when they are absent (Thrall et al 2001).…”

Interspecific interactions and range limits: contrasts among interaction types

“…There has been a 70 tendency in theoretical models to emphasize competition as a driver of range margins (Bull and Possingham 1995;Case et al 2005;García-Ramos et al 2000;Goldberg and Lande 2007;MacLean and Holt 1979;Pielou 1974), but models do exist exploring range limits generated by other kinds of biotic interactions such as predation (Holt and Barfield, 2009) and mutualism (Afkhami et al 2014;Hutson et 75 al. 1985;Parker 2001). The effects of asymmetric interactions such as amensalism and commensalism on range margins are mentioned less frequently (Colwell and Rangel 2009;Hirzel and Le Lay 2008;Lavergne et al 2010).…”

Invasive Lespedeza cuneata and native Lespedeza virginica experience asymmetrical benefits from rhizobial symbionts

“…The legume Leucaena leucocephala failed to survive in its invaded range until its corresponding symbiotic rhizobia were introduced (Richardson et al 2000). What is more, a threshold density of nitrogen fixing bacteria is required for nodule forming on some legumes (Parker 2001), and some invasive legumes may be limited by the distribution of their symbionts . Thus, the symbiotic relationship with rhizobia still has important functions for plant growth after invading a new habitat.…”