|
Sustaining Life: The Origin, Diversity, and Extinction of Species
There are dominant modes of behavior to describe biodiversity. Long term gains over eons are driven by new capabilities, by ‘genetic learning’ of how to survive in new habitats. Medium term oscillations are driven by climatic and topological changes, resulting primarily from plate tectonics and secondarily from variations in solar output and orbital mechanics. Short term variations are forced by variations in carrying capacity at the individual level. This paper is intended to build a theoretical model, a framework for understanding more particular situations.
Over geological time periods, there are always as many individuals as the available resources (energy, water, raw materials) available. The number of individuals per species has a strong effect on the survivability of that species. Two forces are at work: plate tectonics and genetics. Changes that happen over more than ten million years will leave no potential habitats unexploited. Changes that happen in less time will, given constant solar output and a steady-state orbit, and an absence of catastrophes:
Plate tectonics:
_ Via continental drift changes the number of areas, land and sea, & their size.
_ Via uplift, subsidence, and erosion, changes the typical climates and productivity, and thus habitats and species best suited for a particular area
_ Via continental drift, varies the latitude and thus the energy available in a particular area
_ A change in the sea level changes the habitat topology -- the number of areas, as well as total area.
All affect the potential number of species.
Genetics:
_ Via evolution, adapts existing species to optimally use their current habitat (incremental extinction);
_ Via evolution, stores the capability for currently unexpressed forms (genetic learning);
_ Via extinction, frees current habitats for use by other species, active or potential; and
_ Via extinction, depletes the pool of potential species.
Several patterns are discernible in the geological record.
Spatially:
_ For a given total area of habitat, two small areas will have more species than one large area
_ For a given number of areas, more total area means more species
These two patterns are aspects of the species/area effect, described by a number of people of the years (see Rozenweig, p. 8, for a partial listing). Only a spike in carrying capacity, however, can cause a mass extinction beyond ~ 30%
_ For a given total area of habitat, lower latitudes will have more species than higher latitudes.
_ Water, energy, and raw material availability will determine productivity for an area and thus habitat and species diversity.
_ Small areas with frequent disruptions have more species than corresponding areas without disruptions..
Temporally:
_ Diversity is steady for long periods of time.
_ There are a few occasions in the geologic record where diversity increases as substantially new habitats are exploited, e.g., the benthic bottom and the land.
_ There are occasions when land areas collide and separate, thus changing total diversity in the areas.
_ Periodic extinctions radically reduce biodiversity, with a recovery to previous levels taking on the order of 10 million years. The Terminal Permian Extinctions were exacerbated by Pangaea -- all the eggs were in one basket, and there were few refugia for species to survive in.
Dynamically, the base level is that a certain number of actual species exist. Species are increased by originations and decreased by extinctions. Unexpressed genes in the genomes provide a pool of potential species based on the actual species. Potential species are increased, after a delay, by the origination of new species. They are decreased by extinctions, however, with no delay. The geological aspects of the environment -- topology, relief, climate, nutrients, and water -- define a number of potential habitats, which the current species completely fill at equilibrium. The number of potential habitats is driven by geological processes, primarily tectonic. As environmental conditions change, so do the number of potential and, after a time, actual, species. But in the aftermath of a severe extinction, the gene pool may not have enough potential species to repopulate all potential habitats, thus ensuring a much longer period of time where actual species are well below potential levels. The number of potential habitats is determined by geography, topology, and resource availability. Potential species are determined by the number of species and potential habitats through evolution.
The base case is that potential habitats are stable. Species originations and extinctions are in balance. Evolution creates no new potential species. The maximum number of potential habitats is occupied, and any speciation is by replacement. This characterization applies to long stretches of geologic time – indeed, to most of geological time.
The next case is that of an evolutionary breakthrough: a new adaptation allows the exploitation of a previously unexploited class of habitats. The habitats in a sense were always there; life has just figured out how to use them. An evolutionary breakthrough creates an increase in the number of potential species. Speciation then converts them into actual species.
In response to plate tectonics, land areas can combine or separate. When land areas separate, the number of potential habitats decreases, because each separate area is smaller than the combined area of the two. But the continuous originations and extinctions in the two areas are necessarily distinct, and net speciation is the result as different species result in the same habitats on the two new areas. The number of potential species increases accordingly. When two areas combine, more habitats are possible. But, unless the separation was quite recent, there are two species for each habitat. The net result is a reduction of species, and of potential species.
More complicated is a quick reduction in carrying capacity -- whether by terrestrial or extraterrestrial means. A reduction in solar input by 90% for five years by, for instance, a cometary impact would drastically reduce the number of potential habitats for those years, causing the extinction of most species. Although the number of potential habitats and even the supportable biomass might recover within a decade, the recovery of species diversity would lag by orders of magnitude, as the gene pool is depleted by the same catastrophe that kills the species. The evidence is that it would take on the order of ten million years for a complete recovery, after which the ecosystem would look radically different.
So much for natural causes of variations in biodiversity. Now let’s look at cultural forces acting on biodiversity. The basic mode is that people are just another species in the context of other actual species, potential species, and potential habitats. But people have had several other impacts:
_ Reduce available area for other species. Somewhat analogous to sea level rises or crustal subsidence, between residential, industrial, and agricultural uses, people have drastically reduced (by approximately 50%) the area available for use by animals larger than 10kg in mass.
_ Reduce the number of distinct areas. By introducing species from one island or continent to another, people have created bridges, literal or figurative, for species to migrate from one area to another. Airplanes, ships, and roads provide avenues for unintentional introductions of species from one area to another. Intentional introductions augment this reintegration of Pangaea. This differs from the natural process in that the introductions are more selective than a land bridge, and possibly has a different effect, but this issue requires more study.
_ Reduce potential habitats. The creation of vast areas of monocultural agriculture, and the selective and systematic reduction of several habitats -- forests, particularly old-growth forests; prairies; and wetlands -- leave a smaller pool of habitats for species to exploit. This has no direct natural analog.
_ Increase extinction rates. People have directly extinguished a large number of mammal and bird species, and a smaller number of other vertebrates, invertebrates, and plants, by hunting and poisoning. This, too, has no direct analog in nature.
_ Reduced potential species. Agricultural breeding programs explicitly reduce the diversity in species, whether in wheat, cattle, sheep, or trees. The occasional creation of new breeds is far outweighed by the elimination of the far wider species-level variation.
_ Nuclear winter. Though the threat of nuclear war has receded, the bottleneck in carrying capacity caused by a full-scale nuclear war would create a drastic reduction in the number of potential habitats for several years. This does have natural antecedent -- the mass extinctions at the end of the Devonian, Cretaceous, and Permian, when 50-90% of all species were extinguished.
The outcome of all these policies is a reduction in biodiversity that will take millions of years to restore -- if the policies stop. If they do not stop? It seems unlikely even now that people could practically kill all life on Earth, even with another one hundred years of these policies and a nuclear war. But the return time to current levels of diversity could grow to tens or even hundreds of millions of years.
As it is unlikely that people could kill all life, it is only slightly more likely that people could kill all people on Earth. Humans are all over the planet, and in six billion individuals there is a lot of genetic diversity. But it is possible to end civilization as we know it. A world with microbes, insects, humans, and their commensals is a far higher-risk world than the one we live in.
Only a decision to limit ourselves addresses that risk. Only a consensual set of goals for sustaining other forms of life and their habitats will keep that risk from growing into a certainty.
_ Limiting human habitat
_ Preserving existing non-human habitats
_ Ending directly human-caused extinctions
_ Adding balance to species introductions. As ‘unrealistic’ as the preceding goals may be, they are far more realistic than stopping the introduction of exotic species around the world. It can be slowed, however, and research done into the gradual introduction of suites of species to create a more balanced, if more uniform, global distribution of species.
References
Carson, Rachel. 1962. Silent Spring, New York, NY: Houghton Mifflin
Crosby, Alfred. 1986. Ecological Imperialism: The Biological Expansion of Europe, 900 -- 1900, Cambridge, UK: Cambridge University
Erwin, Douglas. 1993. The Great Paleologic Crisis. New York, NY: Columbia University
Martin, Paul, & Klein, Richard (eds.) 1984. Quaternary Extinctions: A Prehistoric Revolution, Tuscon, AZ: University of Arizona
McPhee, John. 1971. Encounters with the Archdruid. New York, NY: The Noonday
Pielou, E. C. 1991. After the Ice Age, Chicago, IL: The University of Chicago
Ponting, Clive. 1991. A Green History of the World, New York, NY: St. Martin’s
Raup, David M. 1991. Extinction: Bad Genes or Bad Luck? New York, NY: W. W. Norton
Reisner, Marc. 1986. Cadillac Desert, New York, NY: Penguin Books USA
Rosenzweig, Michael. 1995. Species Diversity in Space and Time. Cambridge, UK: Cambridge University
Stanley, Steven M. 1987 Extinction, New York, NY: Scientific American
Ward, Peter. 1994. The End of Evolution- New York, NY: Bantam
Wilson, Edward O. 1992. The Diversity of Life, New York, NY: W. W. Norton
 |