Putting a price tag on biodiversity

"Biodiversity has value in its own right, as people marvel at the beauty and variety of the many faces of nature," says Ando. "But those intrinsic values can be hard to quantify. In this study, we pinned down the monetary value of one particular practical service that biodiversity provides to people: carbon storage." The research team was led by Bruce Hungate, director of the Center for Ecosystem Science and Society at Northern Arizona University. The findings are published in Science Advances.

To build the model, the researchers first had to identify some measurable service of biodiversity that society has priced. Although biodiversity provides many valuable services, concern about climate change has led economists to put a dollar value on the abatement of climate-warming carbon emissions (ranging between roughly $40 and $400 per metric ton). And now there's a $175 billion global carbon market that pays for activities that remove carbon from the atmosphere.

Biodiversity could enter the game through a 4-billion-year-old form of carbon storage that plants provide: photosynthesis. Plants absorb carbon dioxide for energy and growth, storing the carbon in their leaves, stems, and roots, and later transferring it to the soil through decay. The key is to link biodiversity and carbon storage in a quantitative way. So researchers asked: Will changing the number of plant species in an ecosystem affect the amount of carbon it stores over time?

The National Socio-Environmental Synthesis Center (SESYNC) convened the team of scientists, which analyzed data from two long-term experiments in Minnesota grasslands that measured how plant and soil carbon changed with the number of plant species in a plot. Modeling results over 50 years, they estimated the "marginal" increase in carbon storage, or how much additional carbon is stored for every species added to the mix.

Each additional species in a grassland plot increased the plot's overall carbon storage, on average. One reason for this gain may be that new species can fill new niches, yielding more overall growth.

With more species came diminishing returns in cumulative carbon storage. A change from five to six species stored almost 10 times more carbon than a change from 15 to 16 species, showing that the biggest benefit came from adding species to the least diverse plots.

At small scales, about 2.47 acres, going from one to two plant species over a 50-year time period would store an additional 9.1 metric tons of carbon, potentially saving $804 per 2.47 acres based on a mid-range estimate ($137 per metric ton) of the social cost of carbon. At larger scales, cost savings could hypothetically be significant. For example, adding just one species to the approximately 29.5 million acres of cultivated lands restored to grasslands by USDA's Conservation Reserve Program could save over $700 million. The biggest cost savings come from restoring the most degraded, species-poor lands.

These numbers underestimate the total value of increased biodiversity because biodiversity confers economic value in many ways beyond storing carbon. "Biodiversity means products like wood, food, and fuel, and services like recreation, water purification, and flood protection, all of which could be quantified using our approach," says Hungate. "Money is a language that speaks, and showing the economic value of biodiversity underscores the importance of conservation and the policies that support it."

Although the value of biodiversity is more complex than just one economic measure, this new research takes a bold step toward understanding the value of nature.