Abstract
Understanding the factors that determine invasion success for non‐native plants is crucial for maintaining global biodiversity and ecosystem functioning. One hypothesized mechanism by which many exotic plants can become invasive is through the disruption of key plant–mycorrhizal mutualisms, yet few studies have investigated how these disruptions can lead to invader success. We present an individual‐based model to examine how mutualism strengths between a native plant (Impatiens capensis) and mycorrhizal fungus can influence invasion success for a widespread plant invader, Alliaria petiolata (garlic mustard). Two questions were investigated as follows: (a) How does the strength of the mutualism between the native I. capensis and a mycorrhizal fungus affect resistance (i.e., native plant maintaining >60% of final equilibrium plant density) to garlic mustard invasion? (b) Is there a non‐linear relationship between initial garlic mustard density and invasiveness (i.e., garlic mustard representing >60% of final equilibrium plant density)? Our findings indicate that either low (i.e., facultative) or high (i.e., obligate) mutualism strengths between the native plant and mycorrhizal fungus were more likely to lead to garlic mustard invasiveness than intermediate levels, which resulted in higher resistance to garlic mustard invasion. Intermediate mutualism strengths allowed I. capensis to take advantage of increased fitness when the fungus was present but remained competitive enough to sustain high numbers without the fungus. Though strong mutualisms had the highest fitness without the invader, they proved most susceptible to invasion because the loss of the mycorrhizal fungus resulted in a reproductive output too low to compete with garlic mustard. Weak mutualisms were more competitive than strong mutualisms but still led to garlic mustard invasion. Furthermore, we found that under intermediate mutualism strengths, the initial density of garlic mustard (as a proxy for different levels of plant invasion) did not influence its invasion success, as high initial densities of garlic mustard did not lead to it becoming dominant. Our results indicate that plants that form weak or strong mutualisms with mycorrhizal fungi are most vulnerable to invasion, whereas intermediate mutualisms provide the highest resistance to an allelopathic invader.
Highlights
The introduction of invasive plants into local ecosystems is one of the most significant threats to natural plant and animal populations (Moser et al, 2009; Vilá et al, 2011)
To better understand how disruptions to plant–mycorrhizal mutualisms could explain variation in garlic mustard’s invasiveness, we developed an individual‐based model (IBM) to answer two questions: (a) How might the strength of the mutualism between a native plant (Impatiens capen‐ sis) and a mycorrhizal fungus affect the plant’s resistance to garlic mustard invasion? (b) Can there be a non‐linear relationship between initial garlic mustard density and establishment? We hypothesized that strong mutualisms, that is, higher nutrient exchanges between the mycorrhizal fungus and the native plant, would exhibit the highest resistance to plant invasion
What would happen if environmental conditions were to favor garlic mustard for a few generations so that it increased to a higher cover percentage? Is there a threshold beyond which garlic mustard always successfully invades? We addressed this question using the intermediate mutualism strength, the condition most resistant to garlic mustard invasion
Summary
The introduction of invasive plants into local ecosystems is one of the most significant threats to natural plant and animal populations (Moser et al, 2009; Vilá et al, 2011). We used garlic mustard (Alliaria petiolata) as the example because it is the best‐documented case of an invasive plant indirectly affecting the growth and abundance of native plants by suppressing mycorrhizal fungi (Brouwer et al, 2015; Hale et al, 2011) It is a pervasive invader with the potential to create dense monocultures in forest understories and has been a prominent challenge for land managers since it was first introduced into North America in the 1860s (Anderson, Dhillion, & Kelley, 1996; Meekins & McCarthy, 1999; Nuzzo, 1999). To better understand how disruptions to plant–mycorrhizal mutualisms could explain variation in garlic mustard’s invasiveness, we developed an IBM to answer two questions: (a) How might the strength of the mutualism between a native plant (Impatiens capen‐ sis) and a mycorrhizal fungus affect the plant’s resistance to garlic mustard invasion? We hypothesized that there would be a starting density that garlic mustard must reach before becoming invasive, which we defined as garlic mustard eventually representing more than 60% of the total plant cover
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