Causes of Extinction #1: Habitat Loss
Habitat loss, degradation and fragmentation is universally accepted as the primary threat to biodiversity. Agriculture, forestry, mining, and urbanization have disturbed over half of Earth’s vegetated land. Inevitably, species disappear and biodiversity declines.
Conversion for agriculture is a major reason for habitat loss. Within the past 100 years, the area of land cultivated worldwide has increased 74%; grazing land increased 113%. Agriculture has cost the United States 50% of its wetlands and 99% of its tallgrass prairies. Native prairie ecosystems (Figure below) - which comprise of thick, fertile soils, deep-rooted grasses, a colorful diversity of flowers, burrowing prairie dogs, owls and badgers, herds of bison and pronghorns, and booming prairie chickens, - are virtually extinct.
Figure 18.13
Habitat loss is the #1 cause of extinction today. In the U.S., over 99% of tallgrass prairies have been eliminated in favor of agriculture. Big bluestem grasses as tall as a human (center) and (clockwise from top) prairie chickens, prairie dogs, burrowing owls, yellow and purple coneflowers, blue grama grass, and bison make up part of the prairie community.
The largest cause of deforestation today is slash-and-burn agriculture (Figure below), used by over 200 million people in tropical forests throughout the world. Depletion of the surprisingly thin and nutrient-poor soil often results in abandonment within a few years, and subsequent erosion can lead to desertification. Half of Earth’s mature tropical forests are gone; one-fifth of tropical rain forests disappeared between 1960 and 1990. At current rates of deforestation, all tropical forests will be gone by 2090.
Figure 18.14
Slash-and-burn agriculture is practiced by over 200 million people throughout the world; this photo was taken in Panama. Because of thin, nutrient-poor soils, plots are abandoned within just a few years. Experts predict that if current rates continue, all tropical forests will be gone by 2090.
Poverty, inequitable land distribution, and overpopulation combine in third world countries to add pressure to already stressed habitats. Use of firewood, charcoal, crop waste, and manure for cooking and other energy needs further degrade environments, threatening biodiversity through habitat loss.
Causes of Extinction #2: Exotic (Alien or Invasive) Species
Technology has made the human species the most mobile species of any which has ever lived. Both intentionally and inadvertently, humans have extended their mobility to a great number of other species, as well. Ships from Polynesian times (as long ago as 3500 BP) to the present have transported crop species and domesticated animals as well as stowaway rats and snakes. Recently, cargo ships have transported Zebra Mussels, Spiny Waterfleas, and Ruffe deep into the Great Lakes via ballast water. Europeans brought Purple Loosestrife and European Buckthorn to North America to beautify their gardens. Shakespeare enthusiast Eugene Schieffelin imported the now-ubiquitous European Starling to Central Park in the 1890s because he thought Americans should experience every bird mentioned in the works of Shakespeare. Australians imported the Cane Toad in an attempt to control the Cane Beetle, a native pest of sugar cane fields. The Brown Tree Snake (Figure below) may have hitchhiked in the wheel-wells of military aircraft to Guam - and subsequently extirpated most of the island’s “naïve” vertebrate species.
Figure 18.15
Many scientists consider exotic species to be the #2 cause of loss of biodiversity. One of the most infamous, the Brown Tree Snake (left), hitch-hiked on aircraft to Pacific Islands and caused the extinctions of many bird and mammal species which had evolved in the absence of predators. The Nile Perch (right) was intentionally introduced to Lake Victoria to compensate for overfishing of native species. The Perch itself overfished smaller species, resulting in the extinction of perhaps 200 species of cichlids.
Many of these exotic (non-native) species, away from the predation or competition of their native habitats, have unexpected and negative effects in new ecosystems. Freed from natural controls, introduced species can disrupt food chains, carry disease, out-compete natives for limited resources, or prey on native species directly - and lead to extinctions. Some hybridize with native species carefully tuned to local climate, predation, competition, and disease, resulting in genetic pollution which weakens natural adaptations. Others change the very nature of the habitats they invade; Zebra Mussels, for example, colonize most manmade and natural surfaces (including native mussels), filter-feeding so intensely that they increase water clarity and enrich bottom habitats with their waste.
Globalization and tourism are increasing the number of exotics which threaten biodiversity throughout the world, breaking down geographic barriers and threatening the wisdom of millions of years of evolution and natural selection. If current trends continue, our increasingly interconnected world will eventually be dominated by just a few fast-growing, highly adaptable, keenly competitive “super-species” rather than the rich diversity we have today. Some biologists, noting that invasive exotics closely resemble what we consider to be “weed” species, have concluded that the world’s #1 weed species is – did you guess it? – none other than Homo sapiens.
Causes of Extinction #3: Overexploitation
The modern equivalent to overkill, overexploitation threatens fisheries, tropical rain forests, whales, rhinos, large carnivores and many other species. Practices such as clear-cutting old growth forests, strip mining, and driftnet fishing go beyond harvesting of single species or resources to degrade entire ecosystems. Technology-aided over-harvesting has reduced one of the richest fisheries in the world - the Grand Banks off the coast of Newfoundland – to an estimated 1% of what they were in 1977 (Figure below). In 2003 in the journal Nature, Canadian biologists published an analysis of data showing that “industrialized” fishing has reduced large predatory fish worldwide by 90%. Some species' stocks are so depleted that less desirable species are illegally sold under the names of more expensive ones; in 2004, University of North Carolina graduate students tested DNA from fish sold as “red snapper” from eight states and found that different species made up 77% of the fish tested! Overexploitation happens on the level of genes and ecosystems as well as individual species. Forest plantations, fish hatcheries and farms, and intensive agriculture reduce both species diversity and genetic diversity within species.
Figure 18.16
Overexploitation of Atlantic cod threatens one of the worlds most productive fisheries: the Grand Banks off the coast of Newfoundland.
Causes of Extinction #4: Global Climate Change
Our increasing reliance on fossil fuels is altering the Earth’s atmosphere and climate. The effects include acid rain, breaks in the ozone layer shielding us from ultraviolet radiation, and greenhouse gases which raise the Earth’s air and ocean temperatures and sea levels. Burning tropical rain forests compounds the effect, releasing carbon as CO2 and eliminating the forest’s ability to sequester carbon – remove carbon as CO2 from the atmosphere - via photosynthesis. Inevitably, changing air and water temperatures, rainfall patterns, and salinity threaten species adapted to pre-warming conditions, and biodiversity declines globally. This concern is the topic of the Climate Change Lesson .
Causes of Extinction #5: Overpopulation
In 1960, Earth’s human population stood at 3 billion. By 1999, we had grown to 6 billion. This unprecedented growth, together with developments in technology, has added immense pressure to resource and land use. Overpopulation compounds all of the aforementioned threats to biodiversity, and unequal distribution of resources extends the consequences to social and political instability. Human population growth continues (see the chapter on Biology of Populations). Growth rates vary – ominously, from a biodiversity perspective: the highest rates are in third world tropical countries where diversity is also highest. We have already seen how slash-and-burn agriculture and Lake Victoria fisheries connect socioeconomic changes to loss of biodiversity.
Causes of Extinction #6: Pollution
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sp; Pollution adds chemicals, noise, heat or even light beyond the capacity of the environment to absorb them without harmful effects on life. To a certain extent, pollution has not kept pace with population growth, at least in Europe and the US. Startling events such as the oil-and-debris-covered and lifeless Cuyahoga River catching fire in 1969 finally provoked the U.S. to stop viewing air and waterways as convenient dumping grounds for waste. Environmental legislation, including the establishment of the Environmental Protection Agency (EPA) has improved both water and air quality. Heeding the warning provided by the extirpation of the Peregrine Falcon from the Eastern U.S., scientists discovered that many synthetic chemicals concentrate as they move through the food chain (biological magnification), so that toxic effects are multiplied. DDT – the cause of the Peregrine’s decline – was banned in the U.S., and regulation of pesticides was transferred from the Department of Agriculture to the EPA.
And yet, pollution continues to contribute to habitat degradation worldwide, especially in developing countries.
Air Pollution: Knows no boundaries and growing concern about its effects on climate earn this topic two lessons later in this chapter. Acid rain, ozone depletion, and global warming each affect diversity.
Water Pollution: Especially from threatens vital freshwater and marine resources in the US and throughout the world. Industrial and agricultural chemicals, waste, acid rain, and global warming threaten waters which are essential for all ecosystems. Threats to water resources are discussed in Lesson 2.
Soil Contamination: Toxic industrial and municipal wastes, salts from irrigation, and pesticides from agriculture all degrade soils - the foundations of terrestrial ecosystems and their biodiversity. These and other threats to soils are discussed in Lesson 2, Natural Resources.
Outside the developed world, pollution controls lag behind those of the U.S. and Europe, and developing nations such as China are rapidly increasing levels of pollution. Many pollution problems remain in industrialized countries, as well: industry and technology add nuclear waste disposal, oil spills, thermal pollution from wastewater, light pollution of the night skies, acid rain, and more to the challenges facing Earth’s biodiversity. Many will be discussed in the following lesson on Natural Resources, and you can certainly research more about those which interest or concern you. Our next task will be to switch from the doomsday report of problems and causes to a discussion of what WE – ordinary citizens – can do to help protect Earth’s biodiversity.
Protecting Biodiversity
Consider the following facts from the American Museum of Natural History’s Center for Biodiversity and Conservation (AMNH-CBC) and the Environmental Protection Agency (EPA):
Every year, Americans:
Throw away at least 2 billion disposable razors
Discard enough paper and wood to heat 5 million homes for 200 years
Drink more than two billion gallons of bottled water, costing 900 times more money than tap water – not counting the energy and toxics involved in packaging and shipping
Retire up to 130 million cell phones, containing toxic metals such as arsenic, cadmium, and lead
Generate about 3 million tons of toxic electronics waste (e-waste), and recycle only about 11%
Do any of these everyday experiences apply to you? You may be surprised to learn there is quite a lot you can do to help. Read carefully through the suggestions below, noting those that appeal to you strongly and those which seem most feasible. Many involve little more than awareness in decisions you already or will soon make.
Consume Thoughtfully and Wisely: Reduce Your Consumption Where Possible. Re-use, and Recycle. Make Durability and Efficiency Your Criteria for Product Purchases.
In general, when you buy:
Buy locally whenever possible to reduce transportation costs for you and for the environment.
Be aware of the natural resources used to make and transport any product you buy.
Substitute other materials for plastics - which are made from petroleum and produce toxic waste.
Figure 18.17
Eat with the environment and your health in mind! In the United States, the Department of Agriculture (USDA) sets standards for organic products and certification. The green-and-white seal identifies products which have at least 95% organic ingredients. The program is helpful to consumers, but not without controversy (read Barbara Kingsolvers and/or Michael Pollans ).
When you buy food plan your diet for your own health and that of the environment.
Eat low on the food chain. Top carnivores get the least energy and the most poison.
Buy local produce in season – to reduce transportation costs and the need for pesticides.
Buy at farmers’ markets or a Community Supported Agriculture (CSA) programs to support local farmers and reduce demand for energy-consuming and polluting large-scale agriculture and marketing.
Choose organic produce - for your own health and to protect the environment from excessive nutrients and pesticides (Figure above).
When you buy fish for food or for your aquarium
Check to be sure that commercial species are not from overharvested areas,
Verify that tropical saltwater fish were not collected using cyanide.
When you need paper products, be sure they are made of recycled fiber.
Or consider alternative materials such as hemp, kenaf, cornstarch, or old money or maps.
Replace paper napkins and paper towels with cloth.
Reuse envelopes and boxes. Wrap gifts in the comics or reusable cloth gift bags.
When you buy products for cleaning,painting,or washing your car,check the ingredients to be sure you are not exposing yourself and the environment to unnecessary toxins. Vinegar and baking soda work wonders!
When you buy wood or wood products be sure harvesting followed sustainable forest management – practices which ensure future productivity, biodiversity, and ecosystem health.
Look for SmartWood, FSC (Forest Stewardship Council) or similar labels.
Consider recycled or salvaged wood.
Figure 18.18
One drop per second from a dripping faucet wastes 2,700 gallons of water per year and adds to sewer and/or septic costs, as well.
When You Use Water, Remember Its Importance To All Life
Check for water leaks and repair drips with new washers (Figure above).
Use low-flush toilets and low-flow faucets and shower heads.
Have your tap water tested; use filters or refillable delivery if needed, rather than bottled water.
When You Must Use Energy, Consider Consequences and Choose Your Source Carefully
Unplug electronic equipment such as fax machines, power tools, and anything connected to a remote control.
Turn off power sources and lights when not in use.
Use your bicycle, and support bike-friendly cities and roads.
Walk! It’s good for you, as well as the environment.
Use public transportation, and support its expansion.
Make energy-efficiency your #1 priority when you purchase appliances.
Make fuel-efficiency your #1 priority if you purchase a car.
Turn down your thermostat, especially at night. Just 2oF saves 500 pounds of greenhouse-inducing CO2!
Weatherstrip and caulk doors and windows.
Replace incandescent with fluorescent light bulbs, which are four times as efficient and last far longer.
The EPA Energy Star Logo helps consumers to identify energy-efficient products. The less fossil fuel energy we use, the fewer greenhouse gases we release, reducing the threat of climate change.
Figure 18.19
Computer equipment becomes obsolete quickly and contains toxins such as lead and mercury. Consider donating your obsolete equipment, and if you must discard it, be sure you follow specific guidelines for recycling and hazardous waste disposal.
When You Must Dispose of Waste, Learn the Best Practice for Its Disposal
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educe or eliminate your use of plastic bags, sandwich bags, and six-pack plastic rings (and don’t release balloons!) so that endangered sea turtles do not mistake these for their favorite food - jellyfish.
Minimize and compost food waste.
Recycle motor oil and unused paint.
Use appropriate local hazardous waste facilities for recommended chemicals and medicines.
Donate obsolete computers and other electronic equipment – or if you cannot, recycle such “e-waste” properly (Figure above).
Don’t Contribute to the Burgeoning Problem of Exotic Species
(The following points reference Figure below.)
Don’t release aquarium fish, turtles, birds, or other pets into the wild.
Figure 18.20
Exotic (invasive or alien) species are often considered the #2 cause of extinction. Learn how to avoid transporting them!
Clean your boat thoroughly after use, and avoid traveling with wild plants and animals.
Your pet is also considered to be an exotic species. Don’t let your pets hunt birds or wild animals.
Practice Sustainable Management on Your Own Land, Even If it is “Only” a Small Yard
CK-12 Biology I - Honors Page 82