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Earth as Biodiversity Hotspot
Environmental Stewardship in the Next Era of Conservation
V. Alaric Sample

Several years ago, astronomers Frank Drake and Carl Sagan developed what became known as the “Drake Formula,” a way of estimating the number of planets in the universe that may harbor life. Among what Sagan famously described as “billions and billions of stars” in the cosmos, they estimated that life-bearing planets could number in the millions. More recently, Peter Ward and Donald Brownlee of the University of Washington re-examined the basic assumptions in the Drake Formula. While agreeing that life probably exists in many parts of the universe—perhaps millions— Ward’s and Brownlee’s findings strongly suggest that, in most instances, these life forms have not evolved beyond the microbial level.

For reasons that they detail in their book Rare Earth and in numerous scientific papers, they make a convincing case that Earth may be one of very few planets on which complex life forms have developed at all. NASA’s Kepler spacecraft has discovered several planets orbiting in the “habitable zone” around distant stars— “Goldilocks planets” that reside where it is not too hot nor too cold for life as we know it to survive on the planet surface. But in reviewing the 4 billion-year history of factors that have allowed complex life to develop on Earth, and other factors that nearly extinguished it on more than one occasion, Ward and Brownlee have identified a lengthy sequence of critical conditions that are highly unlikely to have been repeated elsewhere and, to our knowledge, are unique to Earth:

  • Among the many types of stars found in the universe, and the different stages in their own development, our sun is unusually stable, and it has remained so for the billions of years required for complex life forms to evolve. A “habitable zone” around a star that is in the process of expanding or collapsing does not remain habitable for long.
  • We have a moon that is extraordinarily large for the size of the planet it orbits, and it acts to maintain the constant 23° tilt in the Earth’s axis that gives us seasons and maximizes the proportion of the planet that can support life. Planets without this advantage have poles that are permanently too frigid, and equatorial regions that are permanently too hot to support life on the surface.
  • We have large outer planets, primarily Jupiter and Saturn, that act as protective big brothers, continuously sweeping comets and asteroids from Earth’s orbital path, preventing a repeat of the kind of impact event 65 million years ago that caused a global mass extinction on Earth and ended the age of the dinosaurs.
  • We have oceans of liquid water in an amount great enough to stabilize global temperatures within a range suitable for life, but not so much that there are no continents that rise above the surface to provide terrestrial habitat or shallow coastal waters critical to the development of complex life. In addition to volcanism that creates new terrestrial environments, Earth is the only planet we have observed that has plate tectonics, a constant recycling of material from the Earth’s mantle that regulates the concentration of carbon dioxide in the atmosphere. The well-known “greenhouse effect” has for billions of years kept global temperatures within a range that is suitable for life on the surface.
  • We have atmospheric oxygen that was toxic to many early life forms, but which forced organisms to protectively adapt in ways that gave rise to more complex life forms and triggered the evolution toward the higher plants and animals that we see on Earth today.
We know for certain that no other planet in our solar system enjoys these advantages; nor do any of the exoplanets that have thus far been discovered orbiting around distant stars. In fact, the probability of all these factors coming together to form another planet suitable for the development of complex life is infinitesimally small. We may very well be alone in the universe.

How much more wondrous then is this abundance of life on Earth, in all its richness and diversity, in all the beauty and complexity of a single insect or flowering plant? As Ward and Brownlee write, “the possibility that animal life may be very rare in the universe also heightens the tragedy of the current rate of species extinction ... are we eliminating species not only from our own planet but also from an entire quadrant of the galaxy?”

Forests have been part of this planet for over 300 million years, and they have served as the great Noah’s Ark of plant and animal species of all kinds. Land covers only one-quarter of the Earth’s surface. Yet the majority of all known plant and animal species inhabit these terrestrial environments— and most of that diversity is found in forests. As forests are allowed to disappear, so too do species, each one of them a wondrously constructed result of millions of years of natural experimentation, trial, and adaptation.

This puts a whole new light on the responsibility we all share to conserve and sustain life on Earth, whether you believe that this Garden of Eden was created by the divine as a cradle for mankind, or that a series of factors have combined over the past 4 billion years to create and sustain complex life that is unique to what Carl Sagan referred to as the “pale blue dot” that is planet Earth. Either way, the realization that this may be one of the few places in the universe where life can survive at more than the microbial level gives new meaning to our efforts as conservationists, literally as the stewards of creation.

Just a few thousand years ago—the blink of an eye in the history of life on Earth—the number of humans scattered around the globe was less than 3 million, fewer than we now find in a single small city. In 2011, we crossed the threshold of 7 billion people, and most demographers expect human population to level off at around 10 billion by 2050. Famed biologist Edward O. Wilson has shown that the rate at which species are being lost in the present day is similar to that in the planetary mass extinctions millions of years ago that nearly snuffed out life on Earth. This time the cause is not asteroids or gamma rays, but the daily struggle by 7 billion humans to meet their own needs for food, energy, and shelter.

This then is the central challenge of our time, for humanity and especially for those of us whose chosen field is the advancement of science and practice in environmental and natural resource conservation. How can we meet the basic needs of increasing billions of human beings while still sustaining the natural environment that is the basis for what may be the only complex life among all the planets, all the stars, and perhaps all the galaxies in the universe?

 
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