Towards a general theory of biodiversity for the Anthropocene

Imagine a hypothetical scenario. Imagine you are traveling through space and come across Earth for the first time ... what would you be most struck by? Would it be the water that gives our planet the nickname ‘blue-marble’? I doubt it. We’ve now found water on the moon (Saal et al., 2008), on several other planets in our own solar system (Carr et al., 1998; Malin and Edgett, 2000), and a single survey of the Milky Way found >270 planets in the so-called “habitable zone,” warm enough for liquid water (Borucki et al., 2011). Would you instead be struck by the mountains, canyons and other geological features that are most visible from space? Again, it’s doubtful. Geologists tell us there are few, if any, landforms that are wholly unique to Earth (Baker, 2008; Dietrich and Perron, 2006), and you probably would have seen them all before. Based on our current understanding of the universe, the only thing a space-traveler is likely to be struck by, and the one thing that appears to be fundamentally unique to Earth, is its remarkable variety of life. Ever since the first prokaryotic cells evolved more than 3 billion years ago, the diversity of life on Earth has steadily increased, punctuated by only a handful of extinction events. Our best guess is there are perhaps 9 million forms of eukaryotic organisms on this planet (Mora et al., 2011). The number of prokaryotic organisms is largely unknown, but a single hydrothermal vent on the bottom of the ocean can harbor an astounding 37,000 unique types of microbes (Huber et al., 2007). While the great variety of life is perhaps the most striking feature of Earth, loss of this biodiversity is one of the most striking forms of environmental change in the Anthopocene. The percentage of species that have gone extinct ranges from just < 1% to 13% of described taxa depending on the group considered (Barnosky et al., 2011). But rates of extinction are occurring orders of magnitude faster than what is ‘normal’ in the fossil record. Projections suggest that if these high rates of extinction continue, biodiversity loss could equal or exceed the five prior mass extinctions (loss of 75% or more of known taxa) in 240 to 540 years (Barnosky et al., 2011). So what? What does it matter if we lose 75% of all life forms on the planet over the next few centuries? Will Earth become any less hospitable for humans? Will this planet still be able provide people with the food, water, air, and other goods and services needed to survive and prosper? Won’t evolution simply replace all of that lost diversity with life forms that are more fit for a human dominated planet? And if evolution does compensate for extinctions in the Anthropocene, what ecological roles will those newly evolved species play? These are pressing questions as we ponder what future Earth will be like. The variety of life that has evolved over 3.6-billion years is a catalog of biological resources from which we produce nearly all of the goods and services needed for humanity to prosper (Daily et al., 1997; MEA, 2005; Cardinale et al., 2012). If we are to have any hope of predicting how human domination of the planet will impact our own prosperity in an era that will have fewer biological options, we must develop a general theory of Earth’s biodiversity that can predict both the causes as well as the consequences of biological variation. Fortunately, biologists have made great strides on developing models that simultaneously explain three dimensions of biodiversity: (a) the evolutionary origin of biodiversity, (b) the ecological maintenance of biodiversity, and (c) the ecosystem-level function of biodiversity. Below I describe one set of evolutionary and ecological models that are beginning to show remarkably consistency in form. These models are by no means the only descriptions of how diversity originates, why species coexist, or how diversity influences ecosystem function. But the particular models discussed here do have a common thread that suggests biologists from different sub-disciplines are, in some instances, converging on a suite of equations, all with similar terms that collectively predict the origin, maintenance, and function of biodiversity. Domain Editor-in-Chief Donald R. Zak, University of Michigan