Asynchrony Contributes to Tropical Biodiversity

diverse tropical forest

Original article in Nature  |  Commentary in Nature

Tropical forests harbor amazing species diversity—many times that of temperate or boreal forests. The source of this diversity has been a long-standing ecological mystery, as basic resources are limiting in the same ways (if not always to the same degree) across the latitudinal gradient. New research drawing on long-term seed production and seedling recruitment data from 10 far-flung forests points to the importance of reproductive timing in maintaining species coexistence.

In ecology, the “storage effect” refers to the ability to take advantage of favorable times and places in order to survive leaner times. When different species experience different periods of plenty, the process facilitates continued co-existence. The study’s authors present evidence that the influence of the storage effect on between-species competition (relative within-species competition) increases as one moves from the poles toward the equator—the first time an ecological process has been shown to affect the latitudinal biodiversity gradient.   

The longer growing season of tropical forests allows for greater variation between species in the timing of reproduction, so that the seeds and seedlings of each species may experience different conditions, even within the same year. This asynchrony can also be echoed in year-on-year competition and tends to emphasize within-, rather than between-species competition. In locations with shorter seasons, greater overlap between species in the seasonal timing of reproduction means that a boom year for one species is likely to also be favorable for others, reducing the number of species that can coexist.

The study used seed collection and seedling recruitment data from 10 forests (three from the LTER Network and seven from Smithsonian’s ForestGEO Network) spanning 64 degrees of latitude and up to 23 years. When explaining the latitudinal difference in biodiversity, evolutionary ecologists point to an increased rate of speciation, supported by favorable, stable tropical climates. This study emphasizes that competitive interactions are also likely to play a role and that long-term measures of ecological processes will be needed to solve the mystery.

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