The loss of biodiversity has often been seen as an aesthetic or bioethical issue. The lack of a broader understanding of the consequences of the declining diversity of our living resources has been an important gap in our scientific understanding of the world.
Results emerging from the most extensive research ever on the ecological impacts of biodiversity have begun to fill this gap. A large-scale study called the BIODEPTH project has shown that reduced plant diversity impairs important aspects of ecosystem functioning. The research demonstrated that reduced biodiversity of grassland plants also lowers the productivity of the land. These findings have important implications for agriculture, grassland management, water quality and sustainable land use.
(BIODEPTH is an acronym for BIODiversity and Ecosystem Processes in Terrestrial Herbaceous Ecosystems.)
Plant biodiversity is declining worldwide because of intensive farming, land abandonment, pollution, and other environmental change. To mimic this gradual loss of plant species, researchers created a series of small meadows that contained progressively fewer plant species. They created the miniature meadows by removing the existing vegetation and sowing different flower and grass seeds of local origin, basing the highest diversity upon the species richness of neighbouring grasslands. Five levels of diversity were sown, from the highly diverse to single species monocultures typical of modern agriculture.
A similar method was used to set up experiments at eight field sites in seven countries, embracing a wide range of climates, soil types and grasslands, creating about 500 plots of four square metres each. BIODEPTH performed the same standardized experiment at every location, making the results applicable on a large scale.
At each field site scientists monitored key ecological processes such as plant growth and harvest yield (plant productivity), the breakdown of dead leaves (decomposition), and the amounts of nutrients in plants and soils (nutrient recycling and retention). Across sites, there is a reduction of harvest yield with decreasing species richness. Each halving of the number of plant species reduced yield by approximately 10-20%.
Reduced plant diversity led to changes in the way the whole ecosystem performed. Because they grew less, they also took up less carbon dioxide from the atmosphere, showing that loss of biodiversity might increase the effects of climate change by reducing the ability of ecosystems to absorb carbon dioxide.
BIODEPTH scientists now aim to reveal the mechanisms that explain the widespread relationship between biodiversity and ecosystem function. One explanation is that plants grow better in species-rich communities because each species has its own specialized way to exploit soil nutrients, capture light, deter pests or gather other resources for growth. The plants interact with each other in complementary and positive ways. Modern monoculture crops are probably less efficient in capturing and using resources unless a farmer adds large amounts of fertilizers and pesticides, an environmentally unsustainable practice.
When there are more species, each with its own preferred resources and its own way of gathering them, the plant community has a greater overall capacity for growth. Scientific evidence for these mechanisms remains controversial because it is difficult to demonstrate. However, farmers in "traditional" agriculture sometimes practice intercropping or companion planting, where different crops are grown together to get a better yield from the land. They often use the technique to deter insect pests or increase the amount of nitrogen in the soil.
A new theory from BIODEPTH suggests that grasslands that have lost species are less resistant to changes in the environment. These findings suggest that biodiversity is useful to society in buffering extreme climatic events such as drought, flood, and fire. This "Insurance Effect" of biodiversity has yet to be properly tested by experiments.
Extreme weather, for example as seen during the regular El Nino climate phenomenon, is becoming more common as greenhouse gases accumulate in the earth's atmosphere. Investigating practical ways for ecosystems to cope with environmental change is relevant to the agricultural economy and for sustainable management of the environment. Fundamental, long-term ecological research is required for informed policy-making.