NEW HOT PAPERS

Why do you think your paper is highly cited?

This paper helps resolve a debate that has been brewing since the 19th century. Darwin proposed more than a hundred years ago that the diversity of species in a community can control how efficiently plants and animals capture biologically essential resources and convert those resources into new biomass.

The capture of resources and production of biomass are two of the most fundamentally important processes on the planet. Together, they regulate the amount of food, fiber, and biofuel that natural habitats produce, control the removal of pollutants like carbon dioxide from air or nutrients from water, and influence the frequency of pest and disease outbreaks.

Although the idea that species diversity might control the efficiency and productivity of ecosystems is very old, it wasn’t until the 1990s that researchers began to seriously test this hypothesis with well-coordinated experiments (Figure 1).

Our paper represents one of the first attempts to summarize the results of several decades of experiments that have examined the ecological consequences of species extinction. Our summary gives us a rare glimpse of what the world may become in an era where loss of biological diversity is one of the most pronounced forms of environmental change.

Does it describe a new discovery, methodology, or synthesis of knowledge?

This paper presents the results of a meta-analysis, which is a formal statistical summary of the results of many independent experiments. This summary was very much a group effort. It took my co-authors and I more than a year to collect and analyze data from 111 experiments in which researchers carefully manipulated the number of species of bacteria, fungi, plants, or animals in a wide variety of habitat types (forests, grasslands, streams, lakes, marine coastal zones, etc). Researchers then examined how diversity impacts the efficiency by which communities capture essential resources (for example, nutrients or prey) and convert those into new biomass (for example, more plants or predators).

We were fortunate that the authors of many of the original studies graciously agreed to share their datasets with us, allowing us to publish a large database alongside our paper. Not only does this mean that anyone can critically examine our analyses and conclusions, it also means our database can be used as a template for future research.

Would you summarize the significance of your paper in layman’s terms?

Our meta-analysis simply shows that communities composed of fewer species are less efficient at capturing resources and converting those into new biomass (Figure 2). On average, systems that are severely depleted in species achieve just one-half the biomass, and capture just one-half of the resources as do more natural, diverse communities. These results hold true across both aquatic and terrestrial ecosystems, and are similar for organisms ranging from plants to predators. This suggests that species extinction may have very general impacts on the ability of ecosystems to produce food, fiber, and biofuels, to take carbon dioxide out of the atmosphere and nutrient pollutants out of water, and to control pestilent species (Figure 3).

Our study also confirms an intuitive point that not all species are equal. Extinction of certain species leads to large changes in resource capture and biomass while extinction of other species leads to small changes. This suggests that both the number of species going extinct and the particular types of species going extinct are important to consider. This point is perhaps best illustrated with a sports analogy.

Natural communities are much like a soccer team that is composed of both star players and supporting players. You probably can't win many games if you lose your top striker because s/he is the most productive player and can dominate a game. At the same time, your top striker isn’t much good without a mid-fielder who can supply great passes. Although some players have a more dominant role, each player makes an important contribution to the productivity of the team. Similarly, ecosystems are composed of both star players and supporting players. Some species are so productive that they dominate ecological processes. But supporting species can play a complementary role that enhances the productivity of the ecosystem as a whole.

How did you become involved in this research, and were there any problems along the way?

While I was a Ph.D. student, I performed an experiment in which I tried to understand which factors most limit the restoration of streams that have been degraded by human activities. During this study, I found that important ecological processes such as community production (the uptake of CO₂ and release of O₂) and respiration (uptake of O₂ and release of CO₂) recovered much faster than the diversity of species in the stream.

I was putting these results together for publication at the same time as David Tilman of the University of Minnesota Department of Ecology, Evolution, and Behavior and Shahid Naeem of the Columbia University Department of Ecology, Evolution, and Environmental Biology were publishing their classic biodiversity studies showing that species diversity directly controls the amount of biomass that plant communities produce in terrestrial habitats. But this was not what I was seeing in my research where the recovery of production was decoupled from the recovery of diversity. So I began to ask myself: does diversity generally control ecological processes or not?

For almost 10 years, I struggled to find a clear answer to this question. Some of my earliest work led me to believe that each species plays a unique and complementary role in the environment, and that one species often facilitates the productivity of other species. But along the way I also performed studies that led me to conclude that it is not diversity per se that controls processes, but rather, it is that diverse communities are more likely to contain species that have the greatest impacts on ecological processes.

These contrasting results are exactly why I consider meta-analyses to be so valuable. When this recent paper in Nature is coupled with a follow-up paper that my co-authors and I published in 2007 (Proceedings of the National Academy of Science 104: 18123-8), the balance of evidence clearly shows that species diversity does indeed control important ecological processes because species capture resources in ways that are complementary to one another.

Where do you see your research leading in the future?

The goal of my research is to figure out how many different kinds of species this planet needs to support people in a way that is sustainable. Every person consumes items produced by nature (gas, food, etc.) and, in turn, produces wastes as a result of their consumption (CO₂, excess nitrogen from fertilizer, etc). I want to know how many species are needed to continually provide products to society while, at the same time, cleanup our wastes. Although this sounds like a lofty goal, I honestly believe that we are within 10-15 years of knowing the answer for certain types of ecosystems. Once we know the answer for a few examples of ecosystems, we will be in a much better position to make educated guesses about other ecosystems, and to prioritize our conservation efforts accordingly.

However, when it comes to conservation, I tend to side with the view of many experts who believe that our current policies will set aside too little land, and be too late to preserve most of the world’s species. Because of this, I believe that environmental scientists will increasingly be called on to restore ecosystems to a more natural state.

My research is now leading me to ask: how many species need to be restored to a system in order to restore key ecological processes? Can we simply restore the diversity of the historic communities, or do hystereses (i.e., a system where diversity loss and diversity gain have differing impacts) and alternative stable states force us to restore even more species than originally went extinct? These are important applied questions for which we don’t yet have an answer.

Do you foresee any social or political implications for your research?

Absolutely! I think everyone can appreciate the fact that natural habitats clean up our water, scour greenhouse gases and other pollutants from the atmosphere, and play some role in controlling the dynamics of pest species. Our meta-analysis simply shows that these "services" depend on the variety of species that comprise natural habitats. To the extent that we value these services over other alternatives like shopping malls, parking lots, and housing complexes, we need to impose policies which preserve natural habitats and their biological diversity.

To be frank, developed and developing countries alike have put very little money or effort into environmental research and conservation. As an example, consider that NASA has an annual operating budget of $16+ billion dollars. In contrast, the US National Science Foundation (NSF), which funds nearly all other non-medical research in the social, physical, and biological sciences, has an annual operating budget of just $6 billion. Of this, only a tiny fraction goes towards research on any form of environmental science, including any study geared towards understanding the causes or consequences of modern diversity loss.

Our society and administration seem far more interested in exploring other planets than in understanding how to live sustainably on our own. In my professional opinion, research dollars would be far better spent learning how to manage the pressures we place on our limited natural resources than to trust that technology might someday lead us to a greener place.

Bradley J. Cardinale, Ph.D.
Department of Ecology, Evolution & Marine Biology
University of California-Santa Barbara
Santa Barbara, CA, USA