An “Evolutionary Rescue Route” Towards Coexistence of Competitive Plant Species

Okayama University press release.

In nature, plant species having the same pollinators experience “reproductive interference” owing to competition, and their coexistence is thought to be possible only through resource partitioning. However, recent studies have suggested that coexistence can occur without resource partitioning if the species evolve to self-pollinate. Now, researchers from Japan provide credibility to this hypothesis with simulations, establishing a novel mechanism for the coexistence of competing flowering species.

GFS Pollinator. Credit: Tom Breeze

Plants, just like animals, have to compete to survive in nature when multiple flowering species share the same pollinators. In turn, this can lead to a condition called “reproductive interference” which can reduce the fecundity of the species with a lower relative abundance, driving them to extinction. The only way for the vulnerable species to continue coexisting seems to be “pollination niche partitioning” wherein these flowering species accommodate different patterns of pollinator resource use among themselves.

However, recent studies have suggested an alternate hypothesis that allows for coexistence without the need for niche partitioning. Known as “the pre-emptive selfing hypothesis,” it proposes self-pollination (or selfing) as a possible survival strategy. But it is not known whether this is indeed the case. Now, a group of researchers from Japan has validated this hypothesis using computer simulations and mathematical modeling. Their findings have recently been published in the Journal of Ecology.

In their study, the researchers constructed an individual-based model to investigate the conditions for which the evolution of prior selfing favored the coexistence of plant species in the presence of reproductive interference. They considered two plant species competing via mutual reproductive interference with the potential for evolving the ability of prior selfing. “In a previous study, we found that two species of Asian dayflower, Commelina communis and C. communis f. ciliata, have flowers that look similar, share the same pollinators, and compete through pollinator-mediated reproductive interference. However, this observation had no explanation in the existing theory,” explains Dr. Koki R. Katsuhara from Okayama University, Japan, the lead scientist on the study. “Then I hit upon the pre-emptive selfing hypothesis that offered a plausible explanation for their existence and figured that this idea might apply to plant species other than Commelina.”

The researchers, through their simulations, found that the evolution of prior autonomous selfing favored the propagation of inferior competitive plant species that were otherwise driven to extinction due to reproductive interference. In turn, this “evolutionary rescue” led to a long-term coexistence of the plant species.

These findings demonstrate that the coexistence mechanisms of plant species are more complex and dynamic than imagined. In addition, the study shows that a co-evolutionary coexistence of competitive flowering plant species is possible even in the same niche. Moreover, in the presence of moderate levels of available pollinators, inbreeding depression is appreciably reduced, which improves the overall health of the gene pool, and coexistence of species with evolutionary rescue is favorably stabilized in the long term. Interestingly, increasing the level of available pollinators tended to tip the balance in favor of either the out-crossers (those relying on external agents for pollination) or the selfers depending on the inbreeding depression level in the proposed model.

“This work will lead to an improved understanding of plant species coexistence, which, in turn, could help us manage plant diversity in a sustainable way, as well as rescue native species threatened by an invasive alien species,” concludes an optimistic Dr. Katsuhara.

It seems, intense pressure, like that imposed by pollinator limitation, inbreeding depression, and reproductive interference, is not always a bad thing, especially if it leads to such evolutionary gems like co-existence of flowering plants and avoidance of extinction.

You can read the full article here:

Katsuhara, K. R., Tachiki, Y., Iritani, R., & Ushimaru, A. (2021). The eco-evolutionary dynamics of prior selfing rates promote coexistence without niche partitioning under conditions of reproductive interference. Journal of Ecology, 00, 113. https://doi.org/10.1111/1365-2745.13768

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