• Through what mechanisms are interactions among the living and nonliving parts of the Earth system coordinated, and when does that coordination fundamentally shift or disappear?

  • In an era of increasingly pervasive human influence on physical and biological components of the Earth system, what are the most effective strategies for maintaining the integrity of natural systems and the services they provide?

GRAND CHALLENGE 5.
UNDERSTANDING BIOLOGICAL DIVERSITY

Life on Earth is astoundingly diverse, a result of evolution. While such diversity has been recognized for centuries, its role in the health of a species or ecosystem has only recently begun to be studied. An understanding of diversity is becoming increasingly important as human activity has a larger and larger impact on the natural world. The modeling capabilities and tools of the physical sciences will play a critical role in such studies.

Diversity appears in the natural world at many levels. A single multicellular organism can consist of more than 1015 cells, divided into many different organs. In some organs, no two cells are identical. Within a single species, individual organisms vary extensively at the DNA sequence level, and this translates into substantial diversity in appearance and behavior. Dogs are in breeds that range from Great Danes to Chihuahuas. However, we have only rough estimates of the extent of existing diversity. While the diversity of well-studied groups like birds and mammals is generally well known, data on the diversity of insects, microorganisms, and marine invertebrates are thin. Recent estimates conclude that only 5-10 percent of such species have been classified.2 Diversity within populations and organisms is known for very few taxa.

Within a species, diversity in the sequence of genes protects against extinction by infectious agents or predators and may allow species to function efficiently across a wider range of environmental conditions. The role of diversity in the functioning of ecosystems is only beginning to be understood. A number of studies indicate that plant communities tend to be more productive, more resistant to biological invasives, or less sensitive to disturbance when they are more diverse. Some evidence supports the hypothesis that increasing diversity increases the probability that a community contains at least one well-adapted species (a sampling effect). Other evidence points to a complementarity in which diversity allows species to forage more efficiently for resources. For example, a more diverse community could contain species active at different times of the year, species that extract water and



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