Like most invasive plants introduced to the U.S. from Europe and other places, garlic mustard first found it easy to dominate the natives. A new study done in 2009 shows that over time its fungus-killing toxin becomes less potent. The study suggests that evolution can alter the attributes of an invasive plant that give it an advantage over native plants. In fact, the study suggests the plant's defenses are undermined by its own success.
Most plants rely on soil fungi to supplement them with phosphorus, nitrogen and water, however, garlic mustard does not utilize any of this additional help. Instead, garlic mustard produces glucosinolates which are compounds that leach into the soil and kill off many soil fungi, especially those native to North America. This process weakens the native plants because they rely on those soil fungi. As a result, garlic mustard now grows in dense patches and stifle the growth of native plants.
The study focused on answering this question: Once garlic mustard has killed off most of its competitors, why would it invest as much energy in maintaining its toxic components? The team collected garlic mustard seeds from 44 locations, grew them in a greenhouse and tested glucosinolate levels in each. Those tests found that older populations (those that have been present in an area for more than 30 years) produced lower levels of the fungicidal compounds than younger population less than two decades old. Genetic studies suggested that these patterns were the result of natural selection. That is, the plants that produced less of the toxin were more likely to survive and reproduce in older population. The researchers then grew the garlic mustard in soil from native woodlands. After a time, they removed these plants and potted native trees in the same soil. The trees did best in pots that had held plants from older populations of garlic mustard, indicating, again, that the plants' toxin output had diminished over time, killing less of the fungus on which the native plants relied.
While this study focused on only one plant, the results indicate that some invasive plants evolve in ways that may make them more manageable over time. It also provides some information to clarify some questions we discussed about the Garlic Mustard paper in class. We were confused by figure 7 which demonstrated how some plants grew better in soil that was previously inhabited by Garlic Mustard while others didn't do so well. It could depend on how old the Garlic Mustard plants were genetically. If they originated in a area that's been dominated by Garlic Mustard for a substantial amount of time then there would be less toxin in the soil and therefore more growth would be possible.