Abstract: Over 50 years ago, Robert Paine introduced the idea of keystone species to Ecology as a central principle for understanding dynamics among species. The concept of the keystone, a seemingly small component of system that has a functional significance far out of proportion to its size, helped clarify many questions in Community Ecology. More recently, it has become apparent that the keystone concept is also useful for explaining patterns and dynamics in biosphere/atmosphere exchange and in understanding biological controls over geochemical transformations. Using data from physiological studies of volatile organic carbon emissions from leaves and atmospheric studies of ozone and tropospheric reactivity, we argue that keystone molecules, ones produced at very low levels by plants, play an outsized role in the functioning of the atmosphere. Similarly, with data from studies of root exudation and nutrients, we argue that particular exudates are essential for elemental transformations far beyond what might be predicted based on their mass contributions alone. The significance of keystone molecules creates a challenge for ecological theory because two of our underlying principles, optimality and stoichiometry, are most effective when applied to atoms and molecules that are abundant. Keystone molecules allow organisms to have large impacts with small investments. This ability could be considered a trait that could, itself, be selected. An obvious example of this situation is the well-studied occurrence of high potency defensive compounds that require genetic specialization but small material investment. Our current results show that this phenomenon of genetic specialization and small material investment is also common in situations where organisms are having direct impacts on their physiochemical environment. We suggest that the ubiquity of keystone molecules requires the development of novel theory that can be used to predict occurrence and abundance across space and time. We speculate that (bio)chemical reactivity may be a unifying feature of keystone molecules and that further efforts to understand the evolution of chemical reactivity are necessary.
