nutrients from different depths in the soil, or microbes that specialize on different substrates.

It is likely that diversity influences ecosystem function through a wide range of mechanisms. Some are structural (rooting depth), some behavioral (feeding time), and some biochemical (optimum temperature for an enzyme).

Human activities have tended to reduce the diversity of organisms and ecosystems, in some cases dramatically. The widespread use of limited numbers of cultivars has rendered many of our agricultural crops potentially very sensitive to disease outbreaks or climate fluctuations. The same concern exists for commercial livestock. Now, in an era of rapid global changes driven by human actions, the role of diversity and the processes that maintain diversity take on dramatic new importance. Many kinds of human impacts on the natural world, especially landscape fragmentation, climate change, pollution of air, soil, and water, and stimulation of biological invasives, threaten to decrease biological diversity. At the same time, the novel habitats these impacts create may need the maximum possible biodiversity if they are to cope effectively in the novel conditions. The Intergovernmental Panel on Climate Change (IPCC) estimates that a global average warming of as little as 2°C could commit 30 percent of the world’s species to extinction. We do not know the mechanism, the timetable, or the consequences, nor do we know the features that make some communities extremely resilient, while others collapse with only modest forcing.

Our ability to sequence DNA rapidly and inexpensively is increasing exponentially. This promises to provide the capability to extensively characterize the diversity of species and to allow predicting its functional consequences. We have begun to be able to determine the DNA sequences of complete simple ecosystems, and this will inevitably progress to more complete and interesting communities and organisms. These tools, combined with advanced techniques for analyzing the physical and modeling tools of ecology, offer the potential for huge breakthroughs in coming decades. A clearer picture of the role of diversity in ecosystem functioning will make it feasible to specify the diversity level required to secure sustainable provision of key services. A clearer understanding of existing diversity and the factors that lead to extinction should enable a suite of strategies for protecting diversity in key areas.

For this challenge, there are opportunities for the biological and physical sciences to interact at a range of levels. Physics will be able to extend the tools needed to quantify diversity quickly and cheaply, which is crucial for understanding the consequences of altering diversity for ecosystem functioning.

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