Bradley J. Cardinale talks
with ScienceWatch.com and answers a few questions
about this month's New Hot Paper in the field of
Environment/Ecology. The author has also sent along images
of their work.
Article Title: Effects of biodiversity on the
functioning of trophic groups and ecosystems
BJ;Srivastava, DS;Duffy, JE;Wright,
JP;Downing, AL;Sankaran, M;Jouseau, C
Year: OCT 26 2006
* Univ Calif Santa Barbara, Dept Ecol Evolut & Marine
Biol, Santa Barbara, CA 93106 USA.
* Univ Calif Santa Barbara, Dept Ecol Evolut & Marine
Biol, Santa Barbara, CA 93106 USA.
(addresses have been truncated; see full
Why do you think your paper is highly
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
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
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?
is now leading me to ask: how many
species need to be restored to a
system in order to restore key
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
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
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
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
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