One example of an endangered species is the whooping crane, the tallest bird in North America. Due to hunting, and loss of nesting areas the total population dropped to only 16 birds by 1942. Since then, they have been protected and the wild population has slowly increased to around 350 birds.
Measuring Extinction rates
The rate of extinction in the past can be measured by examining fossils. So far there have been 5 major extinctions (Fig 14.2). The most recent, 65 million years ago, wiped out the dinosaurs and about 50% of the species on the planet. These mass extinctions were caused by climate change and asteroid impact.
Based on fossil evidence the average species lasts for about 1 million years before becoming extinct. So each year roughly 1 species in a million should die out naturally. Nowadays for a species to be considered extinct no individuals must have been seen in the wild for 50 years.
Records of mammals and birds since 1600 indicate that the current extinction rate is between 50 times and 100 times above the normal rate, and the rate has increased over the last 100 years. Fig 14.4. Currently 24% of all mammals worldwide are in danger of extinction.
Habitat Loss and food chains
Humans cut down forests for timber, or to grow crops. Many natural ecosystems have been destroyed to make way for roads and houses. As the global human population increases this habitat destruction is increasing. Fig 14.7.
The relationship between the number of species that can survive in a certain area is called the species-area curve. Fig 14.6. At current rates of cutting, tropical forests will be reduced to about 10% of their original size within the next 35 years. Using the species-area curve we can estimate that this will cause the extinction of about 50% of the original species in the rainforests.
Habitat Fragmentation
Often a natural area is divided into smaller sections by roads or development. This causes habitat fragmentation, which is a particular problem for large animals such as bears which need a very big habitat to survive.
Energy flows in one direction through the ecosystem:
typically it comes in as light and leaves as heat.
Each ecosystem usually has several trophic levels or feeding levels.
Producers, such as plants, produce their own food.
Primary consumers, or herbivores, eat producers.
Secondary consumers, or carnivores, eat herbivores.
Tertiary consumers eat secondary consumers, or carnivores.
Most of the energy that an animal gets from its food is used up by that animal. Typically only 10% of the energy is passed on to the next trophic level. So it takes a lot of producers to support a few consumers, and very little energy is available to the top trophic level. Fig 14.8.
This is called a trophic pyramid.
Humans can be primary consumers or secondary consumers. It takes a lot more energy (and land) to support a secondary consumer. In other words you could feed roughly 10 people who eat corn on the same land that feeds only one person who eats meat.
Other Human causes of Extinction
Introduced species
Introduced species that people bring from one area to another can have a devastating effect, because there are usually no predators to keep the new species in check.
Examples: Brown tree snakes in Guam have caused the extinction of many bird species, and are so common they cause electrical outages by climbing up power poles. Tree snakes
Black rats in Hawaii eat the eggs of many birds, and have caused several species of honeycreepers (birds that were only found on Hawaii) to go extinct.
In 1986 the Asian tiger mosquito was found in Jacksonville, and has since spread around Florida and the south.
Ferocious tiger mosquito.
There are 50,000 introduced species in the US, and this costs $130 billion in damage and control efforts.
Overexploitation
Overexploitation means killing more individuals than the population can replace. For example three out of the original eight species of tigers are now extinct due to hunting. Excessive fishing has greatly reduced stocks of many fish species.
For example cod and haddock, which have been fished for hundreds of years, are now in peril.
Atlantic fish stocks. A report in 2006 estimated that if current trends continue, ocean fish stocks will collapse by about 2050.
Over-hunting is a problem with several mammals. One controversial idea is to hunt exotic animals on ranches in Texas. Exotic hunting
14.2 The consequences of extinction.
Loss of Resources
Many plants and animals are important resources for us, such as trees providing wood, or wild relatives of crops which have useful genes that could improve the next generation of crops. For example teosinte from Mexico is the wild relative of corn, and it is resistant to many more diseases than regular corn plants. Fig 14.9.
A lot of medicines are derived from wild plants, such as the rosy periwinkle, which is a source of anti-cancer drugs.
Ecological Communities.
A community is all the organisms living together in a particular habitat. The plants and animals can affect each other in many ways:
Mutualism is a relationship where both species benefit. For example bees get food from plants, and in return the plant gets pollen transported from one flower to another. Fig 14.12. Bees pollinate at least 80% of all the crops in the United States, but habitat destruction and new parasites are reducing the bee population.
Predation : one organism "the predator" eats another "the prey". One example is warblers, small birds that feed on insects in forests. Fig 14.13. By eating insects, the warblers reduce damage to the trees, and help the forestry industry, which is worth $200 billion a year.
Competition: when two or more species use the same resources (food, nesting sites etc) they are in competition with each other. One species tends to drive out the other : competitive exclusion . This is used by egg producers. Fig 14.14. Many chickens are infected with Salmonella bacteria, which get into the eggs and can cause food poisoning in humans. Instead of using antibiotics, which the Salmonella bacteria can become resistant to, some egg producers deliberately infect their chickens with harmless bacteria, that prevent the Salmonella from growing.
Keystone species
Some predators are keystone predators : they keep other species in check and so allow a diverse community to survive. Removing the keystone predator results in a drop in the number of species living in an area. Wolves in Yellowstone National Park are a keystone species. Fig 14.15.
| Objective | Why bother? | Actions |
|---|---|---|
| Reduce fossil fuel use | Burning fossil fuels increases global warming | Buy energy efficient cars and appliances. Walk, bike or carpool. Buy florescent lights. |
| Reduce meat consumption | Meat requires a lot of energy and land to produce. Eating meat increases risk of heart attack and cancer. | Eat more meals without meat, or just use smaller portions of meat. Buy grass-fed or free-range meat. |
| Reduce pollution | Pollution kills animals and plants, and can affect humans too! | Recycle metal, glass and paper. Buy organic produce. Do not use pesticides or herbicides. |
