Chapter 4: Biodiversity
ENDANGERED SPECIES: ON THE ROAD TO EXTINCTION
The previous section emphasizes the biological characteristics that are likely to make a species prone to extinction. Indeed, most extinct species and many species in danger of extinction have these characteristics. Unfortunately, the causes of extinction increasingly have much more to do with human activity and much less to do with the characteristics of the species. Given the scope of human activity, virtually any species can be prone to becoming extinct if it happens to be in the wrong place at the wrong time. This can be seen by examining the causes of species becoming threatened or endangered, the final steps towards extinction. Major causes examined briefly here are habitat change, contamination, introduced species, and exploitation. Usually, the decline of a species, however, has multiple causes.
In the last few decades modification of habitat by humans has clearly become the most severe threat to wild organisms and ecosystems. Drainage schemes, reservoir and dam construction, urban, industrial and agricultural development, and deforestation are examples of human induced habitat modification. The view from the window of any jetliner flying over almost any country in the world will reveal the extent to which humans have altered the landscape, with our endless fields, urban sprawl, and straightened rivers. Two examples in California of species endangered primarily due to habitat modification are winter run chinook salmon and spotted owl.
Winter run chinook salmon are unique to the Sacramento River, adapted for spawning in the cold, spring-fed water of the upper Sacramento, McCloud, and Pit rivers. When Shasta Dam was built in the 1940s, they were cut off from their historic spawning grounds, much of which were flooded by the reservoir as well. Below the dam, flows were greatly altered but increased flows in the summer duplicated the cold-water conditions the salmon needed to rear their young, so they survived the dam building. In fact, it was estimated that winter run chinook salmon populations in the Sacramento River numbered well over 100,000 fish in the mid-1960s. However, after the Red Bluff diversion dam was built in 1966, the population fell to around 2,000 fish and in subsequent years to only a few hundred fish. The new dam blocked the passage of spawning fish upstream despite the presence of salmon ladders (which were badly designed, so the fish had a hard time finding them). In addition, Shasta Dam was increasingly degrading the spawning habitat: dam operations caused the water to become warmer and the immense Shasta Reservoir prevented any gravel, needed for spawning, from washing downstream, to replace gravel removed by natural river processes. The Sacramento River below the Shasta Dam was becoming a warm stream with a solid bed of large rocks, conditions unsuitable for spawning of the few fish that made it over Red Bluff Diversion Dam. Once the salmon was listed as an endangered species, the following steps were taken: (1) the gates of Red Bluff Diversion Dam were raised to allow direct passage of the migrating fish, (2) thousands of tons of gravel were dumped into the river for spawning habitat, and (3) a multi-million dollar device was installed on Shasta Dam so cold water could be sucked from the bottom of the reservoir to lower water temperatures in the river. As a result of these actions, and others, the winter-run Chinook salmon populations seem to be increasing once again. At the present time, Battle Creek, a spring-fed tributary to the Sacramento River, is being restored as winter-run spawning habitat, largely through the removal of small dams that have prevented access to spawning areas.
Spotted owls largely depend upon large unbroken stands of old growth forest to feed and reproduce. Each pair of birds needs a large amount of this habitat to survive and reproduce, in part because they prefer to feed on a mouse that lives in big trees, the red-backed tree vole. The forest with the appropriate habitat was once widespread along the Pacific coast, from northern California to southern Alaska, where there are forests with trees up to 1,000 years old, and in the Sierra Nevada. These are uneven aged forests; ancient trees stand beside young ones. The complex ecological interactions present in these old growth forests are necessary for the survival of other many species of birds and mammals as well, for which the spotted owl stood as a surrogate. Unfortunately for the owl, old growth timber is extremely valuable and has been cut rapidly in the past century. The cutting of old growth forests resulted in the loss and fragmentation of spotted owl habitat to the point where the owl became listed as an endangered species. In addition, habitat change across the country has allowed the larger and more aggressive barred owl from the eastern USA to move into spotted owl habitats and displace them.
Pollution is a pervasive and insidious problem facing humans and all other species on this planet. There is no place on Earth that is free of contaminants. Nevertheless, it is unusual to find an example of a vertebrate species that have become extinct or endangered as the direct result of pollution, although many species have their ranges severely restricted by contaminants (e.g., fish that are absent from polluted waters). There is growing consensus, however, that many pollutants have subtle deleterious effects, weakening animals to make them more susceptible to disease or mimicking hormones to reduce reproduction. The best know contaminant problem in wildlife came close to eliminating some of our most spectacular birds. The osprey, peregrine falcon, bald eagle and brown pelican were all victims of the widespread use of DDT as a pesticide. As DDE, a derivative of DDT which cannot be further broken down, passes through the food chain it accumulates, reaching higher and higher concentrations at each step (See Chapter 10). Top predators thus receive the heaviest doses, which in this case almost led to their extinction. DDE accumulated to such high levels in these birds that it caused a hormonal imbalance resulting in eggshell thinning. Shells became so thin that they broke under the weight of the incubating parents. In some cases the eggs were produced with no shell at all. The recovery of these predatory birds can be attributed to the ban in 1972 of the use of DDT in this country.
The invasion of alien species can greatly disrupt ecosystems through predation, competition, and spread of disease (See Chapter 9). Such invasions typically go hand in hand with habitat change because humans alter habitats in ways that favor non-native species such as Norway rats, house mice, common carp, cockroaches, and starlings. They can finish the job quickly that habitat alteration has started. Thus, in the Colorado River, the unique native fishes had their populations greatly reduced by habitat changes caused by major dams. These large, long-lived (20-30 years) fishes are capable of living as adults in reservoirs behind the dams, but they can't reproduce because small alien fishes congregate in spawning areas and eat all their eggs and young. Thus extinction of some species in the wild is likely. Introduced species can be especially devastating on islands. In the absence of natural checks that keep their populations regulated on the continent, alien species often flourish on islands at the expense of native island fauna, many of which have evolved in the absence of major predators and consequently lack protective avoidance behavior. An example is the introduction of the brown tree snake to Guam which is a very effective predator on birds. The snakes, which probably arrived on Guam hidden in ship cargo from the Papua New Guinea area, have virtually wiped out the native forest birds of Guam. Nine species of birds, some found nowhere else, have disappeared from this island, and several others are close to extinction.
Even in protected natural areas alien species can create serious problems. Thus alien burros are a serious threat to the remaining populations of desert bighorn sheep in California. Burros are descendants of the African wild ass and are well adapted to arid areas. When burros escaped or were turned lose at the end of the mining era, they quickly established large populations in California, Nevada, and Arizona. Feral burros are in direct competition with the bighorn sheep for the limited amount of water available in desert habitats. The more aggressive burros often drive bighorn sheep away from prime drinking and grazing areas. The behavior of feral burros at water holes so greatly disturbs these sites that bighorn sheep can no longer use these areas to acquire water. Burros congregate around springs thereby polluting the water with feces and urine, reduce the plant cover by rapidly eating the most succulent plants in the area, and muddy the water by simply standing in it. Burros also compact the soil, which prevents plant growth and causes erosion. The resulting scarcity of water and forage weakens the bighorn sheep making them especially susceptible to disease spread by domestic animals.
Exploitation, such as the killing of bison and passenger pigeons, was a major source of extinction and endangerment in North America in the 19th century, but is less of a problem here today. Unfortunately, this is not true for the rest of the world, where increasing human populations confine animals and plants to limited areas which makes them increasingly vulnerable to killing for various reasons. Improved technology and weaponry also makes even large animals like whales, tigers, and rhinoceros vulnerable to extirpation, especially as their value as dead animals increases with their rarity. A growing problem is the overexploitation of fish populations in the seas by highly mechanized fishing gear, which is changes ecosystems (as fish populations collapse and as huge nets rip up habitats) and endangers marine mammals and birds through the loss of their food supply.
Historically, the distribution of life on earth was largely determined by interactions through evolutionary time with climate, geography, and other organisms. Now we humans have become the dominant "force" that all creatures have to contend with, as we drive some species to extinction, introduce other species to remote parts of the globe, change whole ecosystems, and even change climates. Biodiversity is disappearing so fast that we often do not know what we are missing until it is too late. There is growing realization that protection of biodiversity has enormous benefits to humans and that we need to find ways to support, rather than eliminate, the ecosystems and species that provide these benefits. The protection of the diversity of life on Earth, however, requires both a grand global strategy and intensely local and regional strategies for day to day conservation (Wilcove 1999). This is what Conservation Biology (Chapter 8) is all about.
Table of Contents
1. Roots of the modern environmental dilemma: A brief history of the relationship between humans and wildlife
2. A history of wildlife in North America
3. Climatic determinants of global patterns of biodiversity
5. Natural selection
6. Principles of ecology
7. Niche and habitat
8. Conservation biology
9. Conservation in the USA: legislative milestones
10. Alien invaders
11. Wildlife and Pollution
12. What you can do to save wildlife
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