Most popular notions about the success of weedy invasives tend to focus on the superior traits of the plant or animal in question compared to the natives. They may grow faster, or produce more abundant seed, or are more aggressive. Conversely, native organisms, having evolved in isolation from these invasives, have not any adaptative defences against their wiles and ways. But nature can be more subtle than that. This article in PLoS Biology describes an invasive European weed, the garlic mustard, which has taken root in North American forests. Previously confined to disturbed areas and forest edges, it has lately begun spreading into closed canopy areas, suppressing the growth of understorey plants, including the seedlings of dominant tree species such as maple and ash. Garlic mustard itself does not form mycorrhizal associations with arbuscular mycorrhizal fungi (AMF); instead it may produce phytochemicals which inhibit the growth and germination of these fungi. In high growth densities, the authors of this paper have shown that it can inhibit mycorrhizal fungi to the extent that the growth native seedlings that require AMFs to thrive (much like orchids require mycorrhiza to germinate in the wild) can also be inhibited. Worryingly, this means that plants which do not rely so much on AMFs for germination and growth, which are mostly weedly plants, will be selected for in the understorey, and hence come to dominate the forest composition when the mature trees age and and fall away. As the authors put it, "In conclusion, our results reveal a novel mechanism by which an invasive plant can disrupt native communities: by virtually eliminating the activity of native AMF from the soil and drastically impairing the growth of native canopy species."
This study highlights a few things that organismal biologists always like to repeat, like some sort of mantra:
- There is so much we do not know about how organisms interact with each other in the wild. -- Although seedling dependence on mycorrhiza has been known for a long time, as well as the allelopathic properties of many phytochemicals, this is perhaps the first time that the two chains of thought have been linked in this way.
- Systematics and taxonomy are important as baseline knowledge. -- A significant clue to the mechanism of invasion was given by the taxonomic family of the garlic mustard (the Brassicaceae, or the Lettuce family), members of which are known to produce glucosinolates, "organic plant chemicals with known anti-herbivore, anti-pathogenic and allelopathic properties...." Another point which might be noticed is that the AMF is not named in this study: fungi are notoriously difficult to identify without their reproductive structures, and experts on the taxonomy of fungi are few and not growing any younger. One way to refine this study is to examine if the phytochemicals produced by the garlic mustard are specific to certain kinds of fungi or have a general effect, but this is impossible without taxonomic expertise.
- Everything is linked. -- It may not be immediately apparent how an anti-fungal agent secreted by a small weed can affect the composition of whole forests in a century's time. Because of the complexity of the webs of ecological interactions in the wild, it is nearly impossible to predict what might happen when a habitat is tampered with. It is something like twiddling with a Jenga tower: removing one block may not make much of a difference, or it may result in the whole edifice crashing down.
Although this study was carried out in the temperate New World, it might have some application to our Old World tropics as well. Weedy invasives are a problem in Singapore's small and fragmented nature reserves, notably Clidemia hirta, a prickly ground weed that originates from the Americas. Its success in invading disturbed gaps and forest fringes is probably due to its sun-loving habit and tolerance of water stress compared to native understorey plants, but are those the only reasons? As of now we can only speculate as to what lies beneath....