Preserving biodiversity is important because species diversity affects the productivity of biological communities. Diverse communities can better use available resources and, thus, produce more biomass than species-poor communities. When diversity is high, communities are also more likely to contain very productive species which further increase biomass production.
While these positive biodiversity effects are seen across diverse ecosystems, from tropical forests to agricultural fields, the general mechanism through which biodiversity increases biomass production remains unclear. Energy is what fuels biological production but very few studies have directly measured energy fluxes and even fewer the effects of biodiversity on energy production. Furthermore, biodiversity effects are not fixed but change as communities grow older. So how does diversity affect the relationship between energy and biomass production over time?
We answer this question using marine phytoplankton in a laboratory study. Phytoplankton are an extremely diverse group of unicellular algae of great ecological importance because they sustain 50% of global oxygen production and carbon uptake. Using five phytoplankton species with different characteristics, we set up a total of 50 cultures across three levels of biodiversity (species alone, in pairs and in communities with all five species) and compare their energy and biomass production for ten days. Since phytoplankton reproduce daily, our experiment covers roughly ten generations.
Diversity initially boosts both energy and biomass production, so that five-species communities produce and accumulate more biomass than species alone or in pairs. But as biomass grows, energy production is limited by competition. This limitation occurs in all cultures but is stronger in diverse communities. Therefore, the positive effects of biodiversity decline over time as communities grow older, see below.
In nature, the positive effects of biodiversity might be sustained over much longer periods of time than what we observe because ecosystems are continuously disturbed by storms, arrival of new species or changes in nutrient availability. Since disturbances are so widespread our results help to compare the functioning of ecosystems of different age and with different levels of diversity.