Enhanced competitive ability in a native and an invasive species in response to the invader Bromus inermis

Background/Question/Methods

Due to their impact, dominance and ability to change the resource conditions where they invade, invasive species can impose strong selective pressures on individuals they interact with. As a result, some invasive species have been shown to select for individuals with an enhanced competitive ability in native species. However, other invasive species present in the invaded areas could also be selected for an increased competitive ability, which could lead to an invasional meltdown. For an invasional meltdown to ensue, the increase in competitive ability of invasive species should not be only towards the invader, but they should instead become universally better competitors. Finally, these changes in the competitive ability of native and other invasive species after invasion, can feedback to impact the invader’s performance and competitive ability.

We set up a greenhouse experiment to evaluate whether Bromus inermis can select for individuals with an enhanced competitive ability of a native species (Symphyotrichum laeve) and two invasive species (Melilotus officinalis and Poa pratensis). To test for this, we grew individuals from invaded (experienced individuals) and uninvaded (naïve individuals) areas in competition against B. inermisor under intraspecific competition, in both soil types (invaded and uninvaded). 

Results/Conclusions

In general, B. inermis’ performance and competitive ability was negatively affected when grown in it’s own conditioned soil. These soil conditions and B. inermis competition seem to have lead to changes in S. laeve' and P. pratensis' competitive ability. Both species, however, developed different competitive strategies in response to B. inermis, such as tolerance and avoidance. However, although experienced individuals from the invader P. pratensis showed a higher tolerance to competition, an invasional meltdown is not very likely. P. pratensis increased tolerance is expressed only in B. inermis-conditioned soil and, more importantly, only when in competition against B. inermis. No difference among naïve and experienced individuals was observed when under intraspecific competition, which could indicate a specialized competitive response against B. inermis.  

Evaluating plant soil feedbacks and the evolutionary responses of other resident species are important to better understand the mechanisms and consequences for invasion. Invaders can have evolutionary consequences for native, as well as other invasive species present in the invaded areas. These changes in species’ competitive abilities can lead to the suppression of or coexistence with the invader, or to an invasional meltdown. Lastly, understanding how species respond to a strong competitor, can help us better understand the mechanisms behind plant-plant competition.