Surviving in a co-sexual world: A cost-benefit analysis of dioecy in tropical trees
Dioecy has an important demographic disadvantage compared to other plant breeding systems like hermaphroditism: only about half of the reproductive adults in dioecious populations produce seeds. It is theorized that dioecious species must therefore have fitness advantages to compensate for this cost, through mechanisms that increase survival or reproduction. However, a complete and quantitative evaluation of all demographic consequences of dioecy is lacking. We performed a cost-benefit analysis across the entire life cycle to quantify all costs and benefits associated with dioecy in a tropical tree community. To do so, we analyzed long-term demographic data on seeds, seedlings and trees from Barro Colorado Island (Panama). These data were complemented with newly collected reproductive data on 25 hermaphrodite and 19 dioecious species and gender information on 8 dioecious species. Reproduction, growth and survival were modelled as functions of individual size, species breeding system and three species functional traits (seed mass, wood density and adult stature), and then combined into an Integral Projection Model. We used this general IPM to study the population-level fitness consequences of the effects of breeding system on all vital rates.
We found that dioecious females produce almost twice as many seeds as hermaphrodite individuals, after controlling for seed mass, and that dioecious trees had higher tree survival and growth rates. We also found some evidence for additional costs of dioecy in lower seedling establishment and growth. Overall, the costs of having males in the population were smaller than generally expected due to compensating benefits as well as the low elasticity of population dynamics for changes in reproduction. We used a life cycle approach to quantify the demographic costs and benefits associated with dioecy, focusing on all life stages, and controlling for variation in three important functional traits. We showed that dioecy in tropical trees is associated with several costs and benefits, affecting different life stages, and that the net cost of being dioecious is almost zero.