What factors sustain the diversity of life on our planet? You might be surprised to learn one answer is sexual harassment. 

But in a new Journal of Ecology paper, the author proposes a new mechanism sustaining coexistence between competitive species, referred to as "sexual conflict" or "harassment". In the context of the life sciences, this is defined as when one sex of an organism -- usually male -- attempts to increase its fertilization success rate at the expense of the fertility of mating partners. In other words, it notes that evolution does not always maximize population growth rate, sometimes it stabilizes the population size according to density, and evolution of sexual harassment controls the population growth rate.

Plants are good models to study the effect of reproductive competition, according to  Kazuya Kobayashi, from KyotoU's Field Science Education and Research Center, because they cannot readily migrate or move, so they must react with the environment accordingly. Kobayashi explains that stable coexistence among competitive species typically persists because of two factors: niche partitioning and natural enemies.

A new, third factor -- attributable to sexual conflict -- also contributes

When there is a high population density of the same species, strong competition for successful mating -- fertilization -- occurs, leading to the evolution of selfish males who attempt to mate even with unwilling females. As a result, the number of possible offspring is reduced due to the stress and pressure of such conflict. In contrast, with low density populations, both males and females have few candidates for mating, so more cooperation exists and the number of offspring increases.

Kobayashi constructed a simulation model incorporating the effect of this conflict on fecundity. The model demonstrates that hundreds of competitive species without differentiation in resource usage can stably coexist over 10,000 generations, indicating that an innate conflict in fertility plays a role in sustaining a population.

Image: Journal of Ecology.

This research reveals the important role of intraspecific conflict in sustaining a complex ecosystem over a broad range of environments, and can be applied to efforts to rescue endangered species or to prevent overgrowth of invasive ones.

Citation: Kazuya Kobayashi, Sexual harassment sustains biodiversity via producing negative density-dependent population growth. Journal of Ecology, https://doi.org/10.1111/1365-2745.13088. 

Data deposited in the Dryad Digital Repository: https://doi.org/10.5061/dryad.n05rj44