Persistence of a fig wasp population and evolution of dioecy in figs: a simulation study

The relationship between fig trees and their pollinator wasps is a well-kno wn example of species-specific obligate mutualism. In this article we prese nt a stochastic model of this mutualistic system, referring to data on a di oecious fig (Ficus schwarzii) in Borneo, and examine the conditions for the persistence of a wasp population for a given period. (1) When the average duration of the flowering interval of fig trees is short, even a small fig population can sustain a wasp population successfully. A population whose a verage period of flowering cycle is half that of another population can sus tain a wasp population with a number of trees less than half of the other p opulation. (2) The wasp survival rate (WSR) is higher when (a) the variatio n of the interval periods of fig flowering is smaller, (b) the fig populati on size is larger, and (c) figs can prolong their receptivity to wait for a wasp if no wasps are available. (3) WSR is predictable from the average pr oportion of the fig's receptive phases, in which wasps are available, to th eir total receptive phases. (4) The persistence period of a wasp population increases exponentially with the number of fig trees. Based on these resul ts we propose a new hypothesis, as a possible scenario, on the evolution of dioecy from monoecy in Ficus.