I want to end this series of lectures on the impacts of human activity on natural ecosystems by considering one final problem -- this is what is known as the "biodiversity crisis" -- basis of this crisis is that life forms (species) on planet Earth are going extinct at an ever increasing rate as a result of human activities.
Let's begin by first considering the amount of biological diversity on earth -- about 1.4 million species of living organisms have been named by biologists and many more remained to be discovered and named -- the total number of species on earth is not known with any certainty -- estimates vary between 5 - 50 million -- believed that only about 5% of the insect species have been described -- seems remarkable that while we know the number of stars in the Milky Way, we don't know the number of species on earth to within an order of magnitude.
Distribution of species (species diversity) among ecosystems is uneven:
1. tropical rainforests cover only 7% of the earth's land surface, but they are thought to contain about 70% of all species on earth
2. coral reefs make up only about 0.2% of the ocean's surface, yet that contain 1/3 of the ocean's fish species.
What do we know about the process of extinction? Extinction is a natural process and the fossil record is full of examples of entire groups of species that have gone extinct &emdash;- e.g. dinosaurs.
Fossil record can be used to estimate the "life span" of biological species and the rates at which extinction "normally" occurs --- species persistence known only for some groups of organisms for which there is an adequate fossil record (e.g. fishes, marine invertebrates) -- average species persistence = 1 - 10 million years.
For the past 600 million years, the average extinction rate for species has been 9% per million years -- for a biosphere containing 2 million species, this translates to 1-2 species lost every 5 years.
While species may go to extinction by "natural causes," human activities are accelerating the rate at which species extinction is occurring -- estimated that by the year 2000, 20% - 50% of all the species on this planet will have gone extinct.
This translates to an extinction rate of one species per hour -- this is clearly a rate of extinction far greater than that for "normal extinction" -- another aspect of this high extinction rate is that many species may go extinct before they are studied or even named.
"Extinction is forever" -- each species is a unique gene pool shaped by millions of years of evolution -- once a species is extinct it will not return.
This problem of extinction and the preservation of biological diversity is being studied by a branch of biology called "conservation biology" &emdash;- relatively new field that crosses the boundaries of various disciplines: ecology, environmental planning and policy, ethics, economics.
Causes of extinction:
The single greatest cause of species extinction is habitat destruction -- species have evolved adaptations to specific habitats and destruction of their habitat means their death.
Most of the extinctions predicted by the year 2000 will occur through the destruction of tropical rain forest since over 1/2 of the world's species live there -- in the past 30 years, over 50% of the world's tropical rain forests have been destroyed -- locally this destruction is even greater:
1. Madagascar -- possesses one of the world's most distinctive floras and faunas -- 93% of forests gone.
2. coastal Brazil -- forests that enchanted Charles Darwin on the voyage of the Beagle -- 99% gone.
At the present, less than 5% of existing rainforests are protected within preserves and parks.
Giant pandas -- endangered due to destruction of bamboo forests for agriculture in China.
Other ecosystems are also being disrupted by human activities and species are endangered
1. 93% of the ocean's coral reefs have been damaged by human activity; 10% have been destroyed -- at current depletion rate, 60% will be destroyed in the next 40 years.
2. Lake Victoria in east Africa -- once contained 300 species of endemic cichlid fish -- species only found in Lake Victoria -- introduction of Nile perch, a large game fish that feeds on the smaller cichlids, has resulted in the extinction of 200 of these species -- largest mass extinction of vertebrates in modern times.
3. nearly 50% of wetlands (marshes, estuaries, etc.) in the U.S. have been dredged, drained dammed, filled or paved.
Hunting, poaching, collecting -- another contributing factor to the diversity crisis -- species have been and are continuing to be hunted to extinction -- e.g. passenger pigeons were once so numerous in the U.S. that flocks of them filled the sky, but they were hunted to extinction -- species subject to hunting are often "high visibility" species (furry, cuddly, spectacular) &emdash;- estimated that between 15,000 and 30,000 species per year become extinct through hunting, fishing, etc. both commercially and recreationally.
Cacti &emdash;- species are endangered because they have been collected to the point of near-extinction -- both commercial and private collectors are responsible.
Today, Africa's black rhinoceros is endanger of extinction due to poaching for their horns which are used for dagger handles in the Middle East and medicine in the Far East -- similar problems face African elephants which are poached for the ivory in their tusks.
Large predators are important components of most ecosystems &emdash;- e.g. "keystone species" -- many have been hunted for sport and in the name of "predator control" -- local extinctions have resulted -- California has the grizzly bear as its state symbol, but the last California grizzly was killed in the 1920's -- similarly, the gray wolf has been eliminated from large expanses of its previous range (e.g. Yellowstone), though efforts are being made to re-introduce the species.
Other extinct California mammals: wolf (1924), jaguar (1860), bison -- currently 16 species of land mammals recognized as "threatened" or "endangered" live in California.
Why should we worry about these extinctions? So what if the California condor goes extinct? Why should we care about a little gray, nondescript bird called the least Bell's vireo here in San Diego? Put another way, do species other than our own have any value?
It has been argued that species have three different kinds of value:
1. Commodity value -- species have this if they can be made into a product that can be bought or sold in the market place -- can argue that we should prerent species from going extinct because some may be useful to humans -- 40% of all prescription medicines have active ingredients originally derived from plants -- e.g. rosy periwinkle inhabits tropical rainforest and yields powerful anti-leukemia agents -- plant-derived medicines earn $40 billion per year on the international trade market.
It is possible that new products of great potential value are being lost as species go extinct before they are even given a name -- e.g. maybe some rare plant contains the compounds needed to fight AIDS -- tragedy if the plant goes extinct before it is even discovered!!!
2. Amenity value -- species have amenity value if their existence improves our lives in some nonmaterial way -- e.g. the happiness or joy that results from sighting a hummingbird, eagle, coyote, etc. --
3. Moral value -- this concept of value is more difficult to define and more controversial -- some philosophers would say that ALL species have an intrinsic value that is not dependent upon any uses to what we put them -- as a unique entity that is the product of millions of years of evolution, each species has value in and of itself.
Conservation efforts and programs have primarily taken a "species approach" -- e.g. the U.S. Endangered Species Act protects individual species.
Many ecologists now advocate an "ecosystem approach" to conservation &emdash; they point out that endangered species are almost always highly visible indicators of natural ecosystems -- when these highly visible species become endangered or approach extinction, that, in turn, means that an entire ecosystem is endangered or facing extinction -- e.g. endangered status of the spotted owl in the Pacific northwest reflects the diminishing amount of the old-growth forest.
Note: The Endangered Species Act (ESA) only protects individual species and not entire ecosystems &emdash;- ecologists advocate an "ecosystem" approach to conservation rather than a "species" approach.
I earlier introduced the "biosphere" concept and the fact that in ecosystems, "everything is connected to everything else" -- our species is part of this biosphere and we are connected directly and indirectly to the other components -- e.g. about 75% of the oxygen we breath is produced by plants living in the ocean, tropical forests are important to the carbon cycle which, in turn, is important to global climate (greenhouse effect)
Simply put, the survival of human ecosystems is linked to the survival of natural ecosystems -- they are, in reality, our "life support system" -- as our activities degrade and destroy these natural systems, we are degrading and destroying our own life support system -- when they go, we go.
It is clearly in our own best interests to preserve, wisely manage and restore what is left of our remaining wilderness and natural ecosystems.
Next time: Exam IV.