Novelty Can’t Last Forever–Rapid Evolution in the Face of Invasion
The introduction of a novel organism into a community has many consequences, including generating novel evolutionary relationships between species. Considering the ubiquity of invasive species in ecosystems around the world, examining the evolutionary relationships between native and invasive species and how they can affect ecological patterns is of great interest. Richard Lankau recently published a paper in PNAS titled “Coevolution between invasive and native plants driven by chemical competition and soil biota.” In the paper, Lankau addresses pairwise coevolution between invasive and native plant competitors.
Coevolution (ongoing reciprocal evolutionary responses between interacting species) is thought to have a major influence on ecological patterns. Despite this, coevolution has not been well-studied among plant competitors—given the large number of species that can interact and exert selection pressure on each other in a community, it is not so surprising that coevolution can be difficult to parse out. However, Lankau takes advantage of the qualities of an invaded system to examine coevolution in action. An invaded system is advantageous because when an invader comes to dominate an area, this likely exerts intense selection pressure on remaining native plants in the habitat. The native plant community likely returns the favor and exerts selection on the invader (sketch 1).
One of the main questions Lankau addresses in the paper is: Has an invasive plant entered into a coevolutionary relationship with a native plant?
It’s a short question, but there is a lot of evidence that needs to be gathered to address it. First, Lankau has to test (i) whether a trait of the invader varied in response to interactions with native species. Then he has to (ii) test whether traits of a native plant varied in response to the trait of the invader, and (iii) show whether these trait distributions are adaptive when the invasive and native species interact.
The invasive plant in this study is garlic mustard (Alliaria petiolata), a European biennial forb that can form dense stands in forest understories in the eastern United States (this is the fate of garlic mustard that dares invade my backyard: http://www.mnn.com/food/recipes/photos/6-edible-invasive-species-recipes/garlic-mustard-pesto). Garlic mustard produces sinigrin (an allelochemical exudes from its roots), which interferes with the growth of competing plants. In his study, Lankau used plants that originated from communities with varying amounts of garlic mustard and native plant cover. Here’s a quick summary of his methods and what he found:
(i) Populations of garlic mustard differed in their amount of root sinigrin production, and this was positively correlated with cover of heterospecific plants growing with each population (ie garlic mustard growing with more non garlic mustard plants produced more sinigrin). The graph explains it best:
This suggests an evolutionary response of garlic mustard to interspecific competition.
(ii) In a greenhouse experiment, Lankau found that a native plant (Pilea pumila) varied in response to garlic mustard competition, depending on P. pumila’s source population. P. pumila from source populations with high levels of garlic mustard were more tolerant to garlic mustard competition (see overly-simplified sketch). Plants from these populations were also better able to maintain AMF root colonization in the presence of garlic mustard (and its chemical weapon), which could explain their success in competition.
(iii) Lankau performed a reciprocal transplant experiment with six populations of P. pumila that varied in level of garlic mustard invasion (from no invasion to highly invaded). He found that P. pumila that originated from highly invaded populations had the highest fitness in highly invaded environments, but had the lowest fitness in non-invaded environments, indicating a cost of tolerance to garlic mustard competition (sketch 2). This suggests tolerance to garlic mustard competition is adaptive.
These results provide tentative evidence to suggest that coevolution is occurring. Covariation between garlic mustard population sinigrin concentrations and P. pumila population competitive ability can be interpreted, with some caution (as it is a correlative result) as evidence for coevolution. The results do suggest that a native species (P. pumila) is evolving in response to garlic mustard. What the results don’t show is whether garlic mustard is evolving in response to a particular native species or to net selection imposed by interspecific competition.
Why I think this paper is so cool:
Successful plant invaders are thought to have an advantage in their invasive habitats due to evolutionary novelty. However, this paper shows that novelty can’t last forever, in that native plants might evolve in response to invaders. These new coevolutionary relationships could, in time, act to integrate invasive plants into native communities. And that makes me feel all warm and fuzzy inside.