PS 34-164 - Speciation in plant-pollinator community

Background/Question/Methods

Mutualism can promote diversity by increasing the speciation rate of species involved, for example by increasing ecological opportunities (niche broadening) or through partner shift. Unfortunately, empirical examples and experimentations cannot provide a full picture about evolutionary process and the different ecological mechanisms that can drive these speciation events.

By using evolutionary game theory (EGT), more precisely Darwinian dynamics (a mix of population dynamics and evolutionary dynamics), we investigate under which conditions speciation can happen in a community initially composed of one monomorphic plant and one monomorphic pollinator species, and where both the plant and the animal have each one a trait that evolves. Plants interact with each other through intraspecific or interspecific competition (Lotka-Volterra competitive equation) and with pollinators through mutualistic interaction (similarly to type II functional response). We also consider competition between pollinator individuals, as they need to share resources provided by plants (e.g., nectar). The purpose of the model is to examine speciation in this system, and, in particular, to study how many different plant and pollinator species can co-evolve in a given niche.

Results/Conclusions

We show that speciation must happen first in the plant community, and that it can happen only if the width of the niche is larger than the range of competitive interaction between plant species. This result can be related to the concept of limiting similarity (two species too similar cannot coexist). By simulating the model, we show that the number of pollinator species cannot be greater than the number of plant species. This result can be explained by the competitive exclusion principle: as we assume that plant nectar is the only food resource for pollinator, the maximum number of pollinator species cannot be greater than the number of nectar resources, i.e., plant species. However, this result is in opposition to empirical data, where pollinator species outnumbered plant species in many cases. Competitive exclusion principle can also explain that plant species must speciate first, providing a new source of food for pollinator. The width of the niche, the range of plant competitive interaction and the range of mutualistic interaction seem to influence the final number of pollinator and plants species. Contrary to similar models, we show that speciation stops after reaching a certain number of pollinator and plant species.