Multifunctionality in tropical dry forests

Background/Question/Methods How many species are needed to maintain ecosystem function? This question is particularly relevant when the conservation of the large diversity of tropical forests as well as the maintenance of its critical functions is at stake. Focus on a single ecosystem function is likely to support the conservation of only a small fraction of such biodiversity: only 23 species have been shown to be needed to store more than 90% of the carbon in tropical rain forests; the other 50 species can be considered redundant for that function. Focusing on a wider range of functions is likely to include a larger fraction of such biodiversity. In this work we analyze various functions, seasons and successional stages of a tropical dry forest and explore how: (i) the shape of the biodiversity vs. magnitude of ecosystem function relationship changes, and how (ii) the number of species needed to reach 90% of these functions and the number of redundant species decrease as the various aspects are simultaneously considered. The study was undertaken at a tropical dry forest in the Pacific Coast of Mexico. We analyzed aboveground carbon storage, aboveground water storage, functional biomass (e.g. above ground biomass with photosynthetically active folliage), and annual leaf litter production. Dry and wet season functional biomass was compared. Four succesional stages (early, middle, late and old growth forests) following pasture abandonment were used. Shapes of biodiversity vs. magnitude of ecosystem function were built by calculating the contribution of individual species to the function, and adding species in decreasing order of contribution to the function.

Results/Conclusions Most important contrasts in the shape of the biodiversity vs. ecosystem function were those between dry season functional biomass, for which 3 of a total of 10 species contributed to 90% of the function, and rainy season functional biomass, for which 18 of a total of 79 species were needed. Comparisons among succesional stages showed increasing magnitude of the function and increasing proportion of species needed to reach 90% of the function with increasing age. Number of species contributing to 90% of all the functions, seasons and stages increased up to 40 out of the 79 species found in this study. We expect that this number will increase as a wider diversity of functions is considered. We conclude that a multifunctionality approach is needed to understand the contribution of species diversity to the wide diversity of functions of tropical forests.