American Society of Naturalists

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“Surviving in a cosexual world: a cost-benefit analysis of dioecy in tropical trees”

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Marjolein Bruijning, Marco D. Visser, Helene C. Muller-Landau, S. Joseph Wright, Liza S. Comita, Stephen P. Hubbell, Hans de Kroon, and Eelke Jongejans

Dioecious species survive in a cosexual world due to increased fecundity and low overall costs

Old Darwinian puzzle solved: How trees compensate for the costs of males

Seed, leaf, and flower characteristics of two hermaphroditic species (<i>Gustavia superba</i> and <i>Dipteryx oleifera</i>) and two dioecious species (<i>Cecropia obtusifolia</i> and <i>Simarouba amara</i>) from Central Panama.<br />(Drawing by Marco D. Visser)
Seed, leaf, and flower characteristics of two hermaphroditic species (Gustavia superba and Dipteryx oleifera) and two dioecious species (Cecropia obtusifolia and Simarouba amara) from Central Panama.
(Drawing by Marco D. Visser)

Most tree species are hermaphroditic, meaning that individuals carry flowers with both male (stamens) and female (pistil) parts. In contrast, in dioecious species, individuals have either male or female flowers. This reduces the number of individuals in a population that can produce seeds, as only female flowers develop seeds. How dioecious species can compensate for this demographic cost has been a longstanding challenge in ecology, and was already recognized by Darwin: “There is much difficulty in understanding why hermaphroditic plants should ever have been rendered dioecious” (Darwin, 1877). Female trees must compensate for costs of having males in a population, but how do they do this?

A team of researchers from Radboud University (the Netherlands), the Smithsonian Tropical Research Institute (Panama), Yale University, and the University of California (both USA) set out to test this. They used long-term data on more than 100 tree species from a tropical tree community on Barro Colorado Island (Panama). They combined data on seeds, seedlings, saplings, and adult trees to estimate growth, survival, and reproduction across the entire life cycle.

Their results show that female trees compensate for the costs of males by producing almost twice as many seeds compared to hermaphroditic trees, perhaps because the latter do not carry the costs of male reproduction. When combining the costs and benefits of dioecy into a population model, they revealed that no net costs of dioecy existed. The model revealed another surprise: the cost of having males was far smaller than expected because tree survival rather than reproduction was overwhelmingly important for population maintenance. These results together can explain the persistence of dioecious species in a cosexual world, as well as the long-standing puzzle observed by Darwin. Read the Article