causes of extinction chapter 7 · extinctions. the major causes of extinction today are the...

Advanced Biology: Bahe & Deken

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Causes ofExtinction

Chapter 7


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7.1 Mass Extinctions

The Earth has more species occupying ittoday than at any other period in its history,yet the current rate of extinction of species isgreater now than at any time in the past.Species are being driven to extinctionbecause communities and ecosystems arebeing degraded and destroyed. The speciesthat remain are losing genetic diversity aspopulation numbers dwindle, uniquepopulations and subspecies are destroyed,and remaining populations becomeincreasingly isolated from one another.

The diversity of Earth has been increasingsince life first originated about 3.5 billionyears ago. This increase has not beensteady; conversely, it has been characterizedby rapid rates of speciation followed byperiods of minimal change and episodes ofmass extinction. Theevidence for this pattern isvisible in the fossil record,especially in the marineanimals fossil record. Marineanimals first arose about 600million years ago during thePaleozoic era. New familiesof marine animals appeared inrapid and steady successionduring the next 150 millionyears. For the 200 millionyears that followed, thenumber of families was more or lessconstant at around 400. For the last 250million years of the Mesozoic and Cenozoiceras, the diversity of families has steadilyincreased to its present number of over 700families. The fossil record of marineanimals demonstrates the slow pace ofevolution, with new families appearing at arate of about one per one million years.

There have been five episodes of naturalmass extinction in the fossil record. Belowis a table showing that during each of the

five episodes of natural mass extinction alarge percentage of species disappeared. Asixth episode, beginning around 30,000years ago up to the present time,incorporates the effects of hunting andhabitat loss as human populations havespread across the continents.

PeriodMillionsof years

ago

Groups experiencingmass extinction

Pleistocene 0.01large mammals and

birdsCretaceous 65 Reptiles (dinosaurs)

Triassic 180 35% of animal families

Permian 25050% of all animal

families, including over95% of marine species

Devonian 345 30% of animal families

Ordovician 500 50% of animal families

It is quite likely that some massivedisturbance, such as widespread volcaniceruptions, a collision with an asteroid, orboth, caused the dramatic change in theEarth’s climate that resulted in the end of somany species.

The first noticeable effects of human activityon extinction rates can be seen in theelimination of large mammals fromAustralia and North and South America atthe time humans first colonized thesecontinents tens of thousands of years ago.


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Shortly after humans arrived, 80% of themegafauna – mammals weighing more than100 lbs – became extinct. On all continents,paleontologists and archaeologists havefound an extensive record of prehistorichuman alteration and destruction of habitatcoinciding with high rates of speciesextinctions. One set of estimates based onthe best available evidence indicates, forexample, that about 77 species of mammalsand 129 species of birds have becomeextinct since the year 1600, representing1.6% of known mammals species and 1.3%of known birds.

In the past two centuries, human populationgrowth has accelerated and so has the rate ofextinctions. The major causes of extinctiontoday are the introduction of nonnativespecies, over-harvesting of species,pollution, and the destruction of habitats.Scientists recently examined the records of1,880 threatened and endangered wildspecies in the United States and found thathabitat loss was involved in 85% of thecases. Exotic species had a hand in nearly50%, pollution a factor in 24%, andoverexploitation in 17%.

7.2 Island Biogeography Model

A species found naturally in a singlegeographical area and no other place isendemic to that location. Endemism is anextremely important factor in a species’ riskof extinction. If the population of anendemic species on any isolated island goesextinct the species will be globally extinct.In contrast, mainland species often havemany populations distributed over a widearea, so the loss of one population is not ascatastrophic for the species.

Remote islands, old lakes, and solitarymountain peaks often have high percentagesof endemic species. The biota of the entirecontinent of Australia has evolved in almostcomplete isolation, with 94% of its nativeplant species endemic. In the United Statesit is not surprising that the geographicallyisolated Hawaiian islands have a largenumber of endemic species. Areas that arenot geographically isolated typically have alow number of endemic species. Missourishares most its species with adjoining areas.One of the largest concentrations of endemicspecies is on the island of Madagascar,where the moist tropical forests areextremely rich in endemic species: 93% ofmammal species, 99% of frogs, and over90% of the 15,000 plant species are foundnowhere else but on Madagascar. It is scaryto think that about 80% of Madagascar hasbeen destroyed by human activity puttingthe endemic species in danger of extinction.

American ecologists Robert MacArthur andE. O. Wilson developed a general model ofisland biogeography to explain and predictthe effects of distance from the mainlandand size of an island on communitydiversity. The area occupied by acommunity can have a profound effect on itsdiversity.

Imagine two new islands that do not haveany species occupying them yet. One ofthese new islands is near the mainland that is


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rich in biodiversity, while the other is fartheraway. Which island will receive moreimmigrants from the mainland? Most likelyit would be the one closest to the mainlandbecause it’s easier for immigrants to getthere.

Similarly, imagine two new islands ofdifferent sizes. Which island will be able tosupport a greater number of species? Thelarger island should support more speciessince it will most likely have a greateramount of resources available to supportmore populations and it is a bigger target fordispersing species.

MacArthur and Wilson hypothesized thatthe number of species occurring on an islandrepresents a dynamic equilibrium betweenthe arrival of new species (and the evolutionof new species) and the extinction rate ofexisting species. Starting with anunoccupied island, the number of specieswill increase over time, since more specieswill be arriving (or evolving) than are goingextinct, until the rates of extinction andimmigration are balanced. The immigrationrate will be higher for large islands thansmall islands because large islands representa larger target for immigrating species andare more likely to have suitable niches tofill. The extinction rate will be lower onlarge islands than small islands becauselarge islands have greater habitat diversityand a greater number of populations. Therate of immigration of new species will behigher for islands near the mainland than forislands farther away, since mainland speciesare able to disperse to near islands moreeasily than to distant islands. The modelpredicts that for any group of organisms,such as birds or trees, the number of speciesfound on large islands near a continent willbe greater than on small islands far from acontinent. To provide evidence for theirhypothesis MacArthur and Wilson studiedthe diversity found on island chains,

including the West Indies, and found thatspecies richness does positively correlatewith island size and nearness of large areaswith diverse communities.

The work by MacArthur and Wilson notonly applies to water surrounded islands butalso to distinct land-based islandssurrounded by mountains, farms, towns, andcities. Conservationists argue that the modelof island biogeography suggests that thelarger the conserved area, the better thechance of preserving more species.

Conservationists also know that they canincrease the amount of space for biodiversitywithout using more area. The spatialheterogeneity model holds that if theenvironment has patches, the greater thenumber of habitats and the greater thediversity. For example, gardeners areencouraged to create patches in their yards ifthey wish to attract more butterflies andbirds. Stratification is also a way tointroduce patchiness. Just as a high-riseapartment building allows more humanfamilies to live in an area, so stratificationwithin a community provides more anddifferent types of living space for differentspecies. Stratification of its canopy(treetops) is a reason a tropical rainforest hasmany more species than a coniferous forest.

The fact that space is limited suggests thatthere must be an end point beyond whichcommunity richness cannot increase. Therewill be a species richness equilibrium point

( ) for four types of islands (i.e., near andlarge, near and small, far and large, far andsmall). Notice in the figure on the next pagethat the equilibrium point is highest for alarge island that is near the mainland. Anequilibrium is reached when the rate ofspecies immigration matches the rate ofspecies extinction. The equilibrium couldbe dynamic (new species keep on arriving,and new extinctions keep on occurring), or


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the composition of the community couldremain steady unless disturbed. Thisfundamental concept is used throughoutecology and conservation efforts.


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Questions

1. Explain the role of humans as agents ofextinction from prehistoric times to thepresent.

2. According to island biogeographictheory, the number of species in an isolatedarea of habitat is a balance between whattwo factors?

3. On a graph of colonization and extinctionrates against the number of species on anisland, what is the point where the twocurves cross called?

4. If two islands, one large and one small,are equidistant from a continent, whichisland would have more species diversity?Name two specific factors that might haveled to the unequal distribution of speciesrichness.

5. If you found yourself isolated on adeserted island and you notice there are fewplants and animals on this island, what couldyou conclude about the location of thisisland in relation to a continent?

Name:


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7.3 Vulnerability to Extinction

When environments are damaged by humanactivity, the ranges and population sizes ofmany species will be reduced, and somespecies will go extinct. Rare species mustbe carefully monitored and managed inconservation efforts. Ecologists haveobserved that particular categories of speciesare most vulnerable to extinction. The fivecategories most frequently used inconservation planning are as follows:

• Species with a narrow geographical range

• Species with only one or a fewpopulations

• Species with small population sizes

• Species in which population size isdeclining

• Species that are hunted or harvested bypeople

The following categories of species havealso been lined to extinction, though they arenot considered as all encompassing as theprevious five categories:

• Species that need a large home range

• Animal species with a large body size

• Species that are not effective dispersers

• Seasonal migrants

• Species with little genetic variability

• Species with specialized nicherequirements

• Species that are characteristically found instable, pristine environments

• Species that form permanent or temporaryaggregations

• Species that have not had prior contactwith people

• Species that have closely related speciesthat are recently extinct or are threatenedwith extinction

7.4 Conservation Categories

Extinct – The species is no longer known toexist.

Extinct in the Wild – The species existsonly in cultivation, in captivity, or as anaturalized population well outside itsoriginal range.

Critically Endangered – Species that havean extremely high risk of going extinct inthe wild.

Endangered – Species that have a very highrisk of extinction in the wild.

Vulnerable – Species that have a high riskof extinction in the wild.

Near Threatened – The species is close toqualifying for a threatened category.

Regionally Extinct – The species no longerexists within the region, but is present inother parts of the world.

7.5 Exotic & Invasive Species

Scientists categorize plants and animalsamong three categories: native, introduced,and invasive species. Native speciesevolved in a place over thousands of yearsand are thus established within anecosystem. Exotic species (a.k.a.introduced species) have been brought toan ecosystem, intentionally or otherwise,through human activity. Invasive speciesare “invasive” because they disruptecological processes and harm humansystems.

While only a minority of exotic speciescause large disruptions and harm, they canbe found in almost all regions of the UnitedStates. Many exotic species are beneficial to


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humans or, at the very least, harmless. Youmay be surprised at how many “everyday”species are exotics. Almost all NorthAmerican crops, livestock, and domestic andgame animals, many sport and aquaculturalfish, numerous horticultural products, andmost biological control organisms wereintroduced from other continents. Exoticspecies are essential to North Americanindustry, providing economic, recreational,and social benefits.

But even everyday exotic species – speciesthat are at present considered beneficial orharmless – have ecological impacts, and thefull range of these impacts will take years tounderstand. While everyday exotic speciesmay be beneficial in the short term, thelong-term impacts of a small fraction mayturn out to cause some harm in the future.The human activity of introducing speciesfrom one continent to another, from oneecosystem to another within a continent, andfrom one habitat to another within anecosystem, is changing environmentalsystems and processes on a global scale.Scientists’ ability to accurately predict suchimpacts is limited. Successful animalinvaders are usually small, highly mobile,have high birth rates, are generalist eaters,and can adapt to a wide variety of ecologicalniches.

While only a minority of exotic speciescause harm, those that do establishthemselves in their new homes can increasein abundance at the expense of nativespecies. These invasive species maydisplace native species through competitionfor limiting resources. Introduced animalspecies may prey upon native species to thepoint of extinction, or they may alter thehabitat so that many natives are no longerable to persist. The thousands of invasivespecies in the United States are estimated to

cause damages and losses amounting to$137 billion per year.

The United States currently has more than20 species of exotic mammals, 97 species ofexotic birds, 70 species of exotic fish, 88species of exotic mollusks, 2000 species ofexotic plants, and 2000 species of exoticinsects. Exotic perennial plants completelydominate many North American wetlands:purple loosestrife from Europe dominatesmarches in eastern North America, whileJapanese honeysuckle forms dense tanglesin bottomlands of the southeastern UnitedStates. Introduced annual grasses now coverextensive areas of western North Americarangelands and increase the probability ofground fires in the summer. Invasivespecies are many of the most seriousagricultural weeds, costing farmers tens ofbillions of dollars a year in lost crop yieldand extra weed control and herbicideexpenses. Insects introduced deliberately,such as European honeybees and the weevil,and accidentally, such as fire ants and gypsymoths, can build up huge populations. Theeffects of such invasive insects on the nativeinsects can be devastating. At somelocalities in the southern United States, thediversity of insect species declined by 40%following the invasion of exotic fire ants,with a similarly large decline for nativebirds. In areas of human settlement,domestic cats may be one of the mostserious predators of birds and smallmammals: a placid house cat may be afearsome hunter when outside.


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Read the descriptions of the following animals and then answer the questions on the next page.

American bullfrog - must live in water and are therefore usually found near some source ofwater, such as a lake, pond, river, or bog. Warm, still, shallow waters are preferred. Increasedwater temperatures and increased aquatic vegetation, which are common factors of lakes pollutedby humans, favor bullfrogs by providing suitable habitats for growth, reproduction, and escapefrom predators. Breeding takes place in May to July in colder climates, and from February toOctober in warmer climates. Fertilization is external, with the females depositing as many as20,000 eggs in a foamy film in quiet, protected waters. Tadpoles remain in the tadpole stage foralmost 3 years before transforming into frogs. Adults reach sexual maturity after 3 to 5 years.

Loggerhead sea turtles - named for their huge heads and powerful jaws, loggerhead turtles arethe largest hard-shelled sea turtles alive today. Their preferred habitat changes throughout thelife cycle. Just before the nesting season, male loggerhead sea turtles migrate to mating grounds,which are usually located offshore from nesting beaches. They wait for females to begincourtship and mating. Adult females go ashore to lay eggs and seem to prefer steeply sloped,high-energy beaches. When hatchlings emerge from the nest, they head for the ocean. Youngjuveniles are typically found among drifting algae mats in warm ocean currents. Older juvenilesand adults are most often found in coastal waters and tend to prefer a rocky or muddy substrateover a sandy one. They may also be found near coral reefs and venturing into salt marshes,brackish lagoons, and the mouths of rivers.

Monarch butterfly - a predominantly open country, frost intolerant species whose range ofbreeding habitats is greatly dependent upon the presence of asclepiad flora (milkweeds). Themonarch requires dense tree cover for overwintering, and the majority of the present sites inCalifornia are associated with Eucalyptus trees, specifically the blue gum, Eucalyptus globulus.These trees were introduced from Australia and have filled the role of native species that havebeen reduced by logging.

Asian elephant - mainly use scrub forest, although their habitat can vary. They can be found inthe jungle, but generally on the edge where open, grassy areas are accessible. They prefer areasthat combine grass, low woody plants, and forest. Elephants rarely forage in one area for morethan a few days in a row. Female Asian elephants bear a single calf (usually) after a gestation ofmore than a year and a half (18 to 22 months). In the past these animals migrated seasonally, buthuman activities such as agriculture have now made this virtually impossible. Asian elephantshave been domesticated for centuries. Individuals can be trained to reliably perform a widevariety of tasks. They are used as draft animals, for hunting, and for transportation. Ivory fromtheir tusks is used in the manufacture of a number of items, including jewelry.

Indian python - found in a variety of habitats including rainforests, river valleys, woodlands,scrublands, grassy marshes, and semi rocky foothills. They are usually found in habitats withareas that can provide sufficient cover. This species is never found very far from water sources,and seems to prefer very damp terrain. They reach sexual maturity between 2-3 years of ageprovided the proper body weight is met. Females can lay up to 100 eggs. During colder months,starting in October and ending in February, Indian pythons stay hidden and will usually enter abrief period of hibernation until the temperature rises again. There is a high amount ofexportation for the pet trade. The skin of Indian pythons is highly valued in the fashion industrydue to its exotic look. In its native range it is also hunted as a source of food.


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Questions1. Which species on the previous page are vulnerable to extinction? Name all that apply and listthe characteristics described that makes them vulnerable.

2. Which species on the previous page are most capable of becoming exotic species (in realitythey already are)? Name all that apply and list the characteristics described that makes themcapable of becoming an exotic.

Name:


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7.6 Overexploitation

People have always hunted and harvestedfood and other resources they need tosurvive, and as long as human populationswere small and the methods of collectionunsophisticated, people could sustainablyharvest and hunt the plants and animals intheir environment. However, as humanpopulations have increased, our use of theenvironment has escalated, and our methodsof harvesting have become dramaticallymore efficient. Overexploitation occurswhen the number of individuals taken froma wild population is so great that thepopulation becomes severely reduced innumbers. In the United States in the 1800sand 1900s, 2 billion passenger pigeons werehunted to extinction and the bison washunted nearly to extinction. Thousands ofrare species worldwide are harvested andsold for use as pets, houseplants, wood,food, or herbal medicine.

The pattern of overexploitation of plants andanimals in many cases is very similar. Awildlife resource is identified, a commercialmarket is developed, and the local humanpopulation begins to extract and sell thatresource. The initial sales are used to buyguns, boats, trucks, tools, and whatever elsewill help extract the resource more quickly.A transportation network is built to connectharvesters, buyers, and stores. As the supplybegins to diminish the price rises anddemand increases. This creates a strongdemand for overexploitation of the resource.The resource is extracted so thoroughly thatit becomes rare or even extinct, and then themarket turns to some other wildlife resourceto exploit. Commercial fishing and whalingdemonstrates this pattern well, with theindustry working one species after anotherto the point of diminishing returns, a processsometimes termed “fishing down the foodchain”.

Many countries now have laws to regulatehunting, fishing, harvesting, and trade ofwildlife. In an attempt to regulate andrestrict trade, many declining species arelisted as protected under the Convention onInternational Trade in EndangeredSpecies (CITIES). Listing species withCITIES has often protected species orgroups of species from further exploitation.However, these activities continue illegally,a crime known as poaching. In poorcountries especially, local species are anobvious source of food, medicine, orincome. Moreover, not all threatenedspecies are legally protected.

Governments and industries claim that theycan avoid overexploitation by using modernmanagement practices such as maximumsustainable yield (MSY): the greatestamount of resource that can be harvestedeach year and replaced through populationgrowth without detriment to the population.Under the assumption of logistic growth, themaximum sustainable yield is exactly halfthe carrying capacity of the species, as thisis when the population growth is highest(MSY = K/2). However, in many real-worldsituations, harvesting a species at thetheoretical maximum sustainable yield is notpossible because of factors such as weatherconditions, disease outbreaks, illegalharvesting, and damage to stock duringharvesting.


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Questions

1. Using the Internet as a resource, brieflydescribe how and why the Great Auk wasoverexploited. What was it and when did itbecome extinct?

2. On the diagram below use an arrow toindicate the population point that should beused if you wanted to harvest the specieseach year without detriment to the species.What is this practice called?

Name:


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7.7 Pollution

Pollution is increasingly becoming animportant factor in the detriment of wildlifeand biodiversity. Pesticides, cleaningagents, drugs, carbon emissions, and otherchemicals used by humans are making theirway into the food webs of many ecosystemsaround the globe. The long-term use ofmany chemicals may not be known formany years to come. The bald eagle is awell-known example of a species that wasendangered because of a pesticide known asDDT.

In the 1940s, DDT was hailed as a miraclepesticide because of its ability to kill widerange of insects. It was cheap and muchmore effective than other insecticides usedat the time. It saved millions of livesworldwide because of its ability to killmosquitoes that spread malaria and savedmillions of people from starvation by killingcrop pests. Paul Müller of Switzerland wonthe Nobel Prize for discovering it.

In the 1950s and 1960s, Rachel Carson andmany others started noticing a severe declinein the populations of several predatory birds,particularly fish-eaters such as bald eagles,cormorants, ospreys, and brown pelicans.Although DDT was suspected, the levels

that the birds where exposed to werenowhere near high enough to have killedthem. When the bird’s bodies wereanalyzed, they were found to have up to onemillion times the concentration of DDT intheir bodies than was being sprayed in thesurrounding waters. This led to thediscovery of bioaccumulation, which is thesteady increase in concentration of acontaminant with increasing level in thefood chain. DDT is not metabolized anddoes not break down in the body and since itis not excreted, the carnivore accumulatesmost of the DDT that was present in all ofthe prey organisms. DDT interferes withcalcium deposition in eggshells, causingthem to be thin, fragile, and often crushed bythe parents in the nest. Although DDT isnow illegal to use in the United States, it isstill manufactured here and used around theworld.

Acid deposition is the falling of acids andacid-forming compounds from theatmosphere to Earth’s surface. Aciddeposition includes acid rain, acid snow,acid smog, and other acidic material.Ironically, acid deposition was made worsein the 1960s and 1970s when tallsmokestacks were constructed to ease localair pollution. With tall smokestacks and theburning of fossil fuels nitrogen oxides and

sulfur oxides are combinedwith local air masseswhere they combine withwater to form nitric acidand sulfuric acid and driftto neighboringcommunities. Thus, airpollution can occur inareas where there is noheavy industry.

Acid deposition hasdrastically affected forestsand lakes in northern


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Europe, Canada, and the northeastern UnitedStates because their soils are naturally acidicand their waters contain little limestone, anatural buffer. The forests in these areas aredying and acidification of lakes directly killsalgae, invertebrates, amphibians, andultimately fish. Acid rain dissolves nutrientsin the soil, including calcium and potassium,rendering them unavailable to plant life. Italso kills microorganisms, preventingdecomposition from returning nutrients tothe soil. Acid deposition also reducesagricultural yields and corrodes marble,metal, and stonework, an effect that isnoticeable in cities.

As we have learned eutrophication is anincrease in the amounts of nutrients in anaquatic ecosystem. Eutrophication isaccelerated by runoff that carries sewage,fertilizers, or animal wastes from land intobodies of water. With the increase ofnutrients, especially nitrogen andphosphorus, there is a dramatic increase inplant and algae life (algal bloom) and in,eventually, plant and algae death.Decomposing bacteria use the dissolvedoxygen and reduce the amount of oxygenkilling all organisms that require oxygen tolive. The result is a greatly impoverishedand simplified community, a “dead zone”consisting of only those species tolerant ofpolluted water and low oxygen levels.

The ozone layer that surrounds the Earth atan altitude of 12 to 35 miles protects theEarth from the sun’s ultraviolet (UV) rays.The ozone layer absorbs UV rays andprevents it from reaching the life formsliving on Earth. In humans, UV rays causesunburn and skin cancer, weakenedimmune responses, and increase theincidence of cataracts. Amphibians haveseen a remarkable decline in populationnumbers due to UV rays destroying theireggs and creating genetic mutations among

the survivors. Phytoplankton (the majorproducers in the ocean and in lakes) aredying from increased UV exposure and arethereby disrupting the base of many foodchains.

Ozone (O3) is an oxygen gas that isproduced during lightning storms in theatmosphere. Ozone is made in thestratosphere when ordinary oxygenmolecules (O2) are hit with UV rays fromthe sun. UV radiation breaks the O2

molecules apart to form free oxygenmolecules (O). Some of the free oxygenmolecules recombine with O2 to form O3.Although UV radiation continually createsan abundance of ozone molecules, a varietyof processes can also destroy them.

In the 1960s, a group of gases calledchlorofluorocarbons (CFCs) became verypopular for use in refrigerators, airconditioners, and as propellants in aerosolspray cans. CFCs are great for thesepurposes because they didn’t becomeinvolved in unwanted chemical reactions;unfortunately, when released the gases riseinto the stratosphere where UV rays break


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them apart. The free chlorine atoms destroyozone causing a reduction in the ozonelayer. Each chlorine atom produced iscapable of destroying 100,000 ozonemolecules. Many nations have agreed tosharply reduce the production of CFCs overthe next decade.

The Earth’s atmosphere acts like a glass in agreenhouse. Sunlight penetrates through theatmosphere and heats up the Earth and thisheat radiates up from the Earth’s surface.Some of the radiated heat escapes into spaceand some is absorbed by gases in thetroposphere and warms the air. Thisprocess of heat absorption is calledthe greenhouse effect. Thegreenhouse effect is essential forsurvival; without heat trapping gases,our planet would be extremely coldand lifeless.

Not every gas in the atmosphereabsorbs heat in this way. The gasesthat do are called greenhouse gases.The major greenhouse gases arecarbon dioxide, water vapor, methane,CFCs, and nitrous oxide. Of these gases,water vapor and carbon dioxide account formost of the absorption of heat in theatmosphere.

When fossilfuels areburned, carbondioxide isreleased intothe air. Thecarbon dioxidecontent in theatmosphere hasincreased sincethe IndustrialRevolution.Withincreasing

greenhouse gases in the atmosphere therehas been an increase in the averagetemperature on the Earth. This increase inglobal temperature is known as globalwarming. The impacts of global warmingcould include a number of potentiallyserious environmental problems. Theseproblems range the disruption of globalweather patterns and a global rise in sealevels to adverse impacts on human health,agriculture, and animal and plantpopulations.


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Questions

1. Ospreys and other top predators are mostseverely affected by pesticides such as DDTbecause they

a. are especially sensitive to chemicals.b. have rapid reproduction rates.c. have bery long life spans.d. store pesticides in their tissues.e. consume prey in which pesticides are

concentrated.

2. Which of the following is a greenhousegas that may contribute to global warming?

a. methaneb. carbon dioxidec. CFCsd. all of the abovee. both a and c

3. The basic cause of eutrophication of mostfreshwater lakes is

a. increased consumption of oxygen bydecomposers.

b. the introduction of exotic blue-greenalgae.

c. the addition of carbon in sewagepollution.

d. the introduction of large amounts ofphosphorus by pollution.

Essays

4. What is the so-called greenhouse effect?How is it important to life on Earth?

5. How has the deforestation of large areascontributed to an increase in atmosphericCO2? THINK: There are two major reasonsthat are necessarily in this chapter. . .

6. (Essay con’t) Many factors – both naturaland caused by humans – affect thegreenhouse effect. State whether you thinkeach of the following would tend to increaseor decrease the possibility of globalwarming, and why.

a. Conversion from coal-fired to solarand hydroelectric power plants.

b. Large-scale tree-planting programs.

c. Slowing world population growth.

Name:


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7.8 Habitat Loss

Habitat loss is by far the greatest cause ofmost extinctions occurring today. In theUnited States only 42% of the naturalvegetation remains. The principle threats tohabitat that affect biodiversity, in order ofdecreasing importance, are agriculture,commercial developments, water projects,

outdoor recreation, livestock grazing,pollution, infrastructure and roads,disruption of fire ecology, and logging. Inaddition to outright destruction of habitats,habitats that formerly occupied wide,unbroken areas are now often divided intopieces by roads, fields, towns, and a broadrange of other human constructs. Habitatfragmentation occurs when a large,

The figure above depicts the reduction of tropical forest in Sao Paulo state, Brazil, from year 1500 with81.8% forest coverage to year 1973 with 8.3% forest coverage. By the year 2000, only 3% forest coverremains. Sao Paulo state is about the size of West Germany or the state of Oregon. (From Oedekoven,1980.)


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continuous area of habitat is both reduced inarea and divided into two or morefragments. Habitat loss has occurred in allecosystems but is especially devastating intropical rainforests and coral reefs wherebiodiversity is high.

A sequence of events in Brazil offers a fairlytypical example of how rainforests areconverted to land uninhabitable for wildlife(see figure on the previous page). First therewas a major highway that went through theinterior of the forest. Small towns andindustries then sprung up along the highwayand roads branching off from the mainhighway. The result was fragmentation of aonce immense rainforest. The governmentoffered subsidies to anyone willing to moveinto the rainforest and take up residencethere. The new residents cleared the landand burned trees in patches in order to grazetheir cattle. However, the rainforest’s soilcontains limited nutrients that support lushvegetation for only about three years. Oncethe land was degraded the farmers moved onto the next patch of forest and started again.

Loss of habitat also severely affectsfreshwater and marine biodiversity.Tropical reefs contain an estimated one-thirdof the ocean’s fish species in only 0.2% ofits surface area. Coastal degradation ismainly due to the large numbers of peopleliving on or near the coast. Over 60% ofcoral reefs have been destroyed or are on theverge of destruction; it is very possible thatwithin the next 40 years all coastal reefs willbe gone. Mangrove forest destruction is alsoa problem; Indonesia, with the mostmangrove acreage, has lost 45% of itsmangroves, and the percentage is evenhigher in tropical countries. Wetland areas,estuaries, and seagrass beds are also beingrapidly destroyed by the actions of humans.

When a habitat is destroyed, a patchwork ofhabitat fragments may be left behind. Thefragments are often isolated from oneanother by a highly modified or degradedlandscape, and their edges experience analtered set of conditions, referred to as theedge effect. Fragmentation almost alwaysoccurs during a high degree of habitat loss,but it can also occur when area is reduced toonly a minor degree if the area is divided byroads, railroads, canals, power lines, fences,oil pipelines, fire lanes, or other barriers tothe free movement of species. In manyways, the habitat fragments resemble islandsof original habitats in an inhospitable,human-dominated landscape. This is aserious threat to biodiversity.


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Habitat fragments differ from the originalhabitat in three important ways: 1)fragments have a greater amount of edge forthe area of habitat, 2) the center of eachhabitat fragment is closer to an edge, and 3)a formerly continuous habitat with largepopulations is divided into pieces withsmaller populations.

Fragmentation may limit a species’ potentialfor dispersal and colonization by creatingbarriers to normal movements. Many bird,mammal, and insect species of the forestinterior will not cross even very shortdistances of open space. If they do ventureinto the open, they may find predators suchas hawks, owls, flycatchers, and cats waitingon the forest edge to catch and eat them.With fragmentation there is also restrictedaccess to food and mates and it sets up adivision among populations that may lead to

inbreeding depression, genetic drift, andother problems associated with smallpopulation size.

With edge effects comes four importantchanges: 1) microclimate changes since treeare moved that filter sunlight and absorbmoisture, 2) increased incidence of firebecause of increased wind, lower humidity,and higher temperatures, 3) increasedvulnerability for invasion by exotic species,and 4) increased potential for disease withwild populations of animals in closerproximity to domestic animals that spreaddisease.

Edge Effects - The smaller a habitat patch, the greater the proportion that isinfluenced by conditions in the surrounding environment


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Questions

1. What factors have caused habitatfragmentation in Missouri?

2. In what ways are habitat fragmentsdifferent from the original habitat?

3. What is an edge effect? And whathabitat changes are caused by it?

Name:


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7.9 Problems with Small Populations

Large populations are needed to protectmost species from going extinct. Smallpopulations are in real danger of goingextinct for three main reasons:

1. Loss of genetic variability andrelated problems of genetic drift,inbreeding depression, andoutbreeding depression.

2. Sex ratios random fluctuate due torandom variations in birth and deathrates.

3. Ecosystems fluctuate due to variationin predation, competition, disease,and food supply; and naturalcatastrophes that occur at irregularintervals, such as fires, storms, ordroughts.

Populations adapt to changing environmentsthrough its genetic variability. Within apopulation, particular alleles may vary infrequency from common to very rare. Thisprocess is known as genetic drift andalthough genetic drift occurs in both largeand small populations, a larger population ismuch less affected because within largepopulations the chances of losing aparticular allele is small. When an alleleoccurs at a low frequency in a smallpopulation, it has a significant chance ofbeing lost in each generation due to chance.

In the wild a variety of mechanisms preventsinbreeding, mating among close relatives,in most natural populations. Individuals donot normally mate with close relatives; thetendency to mate with unrelated individualsof the same species is termed outbreeding.Individuals often leave their place of birth orare restrained from mating with relatives bybehavioral inhibitions, unique individualodors, or other sensory cues. However,when a population is small and no othermates are available, these mechanisms fail

to prevent inbreeding. Inbreeding can resultin inbreeding depression, a condition thatoccurs when an individual receives twoidentical copies of a defective allele fromeach of its parents. Inbreeding depression ischaracterized by high infant mortality, weak,and/or sterile offspring. These factors leadto even fewer individuals in the nextgeneration, leading to more pronouncedinbreeding depression.

Strong ecological, behavioral, physiological,and morphological isolating mechanismsprevent individuals of different species frommating in the wild. However, when aspecies is rare or its habitat is damagedoutbreeding may occur with an individual ofa related species. The resulting offspringsometimes exhibits outbreedingdepression, a condition that results inweakness, sterility, or lack of adaptability tothe environment. Hybrid offspring areunlikely to have the precise mixture of genesthat allows individuals to survive andreproduce successfully in a particular set oflocal conditions.

How many individuals are needed tomaintain genetic diversity? It is commonlythought that 50 to 500 reproductiveindividuals is the minimum numbernecessary to avoid short-term inbreedingdepression and maintain genetic variation ina population. This range of values has beenreferred to as the 50/500 rule. However,many individuals in a population do notreproduce due to factors such as age, poorhealth, sterility, malnutrition, small bodysize, or social structures that prevent findingmates. Because many individuals are notcapable of breeding or reproducing at anygiven time, the effective population size –the number of breeding individuals – isoften substantially less than the actualpopulation size. If sex ratios becomeseverely uneven then the effective


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population size drastically decreases. Thiswill in turn reduce the effective populationfurther resulting in a smaller population andreduced genetic variability.

Some species have large fluctuations in theirpopulation sizes generation to generationnaturally. In populations with these extremesize fluctuations, the effective populationnumber is much nearer the lowest than thehighest number of individuals. If apopulation gets so small that rare alleles arelost, then the population is said to have gonethrough a population bottleneck, or to haveexperienced the bottleneck effect, asdescribed in 6.1. The bottleneck effectprevents the majority of genotypes fromparticipating in the production of the nextgeneration. The most notable way thebottleneck effect occurs is through areduction in population numbers as a result

of a natural disaster or through humaninterference. A special category of thebottleneck effect is the founder effect,which occurs when a few individuals leaveone population and establish another newpopulation. The new population typicallyhas less genetic variability than the larger,original population.


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Questions

1. Why is genetic variability important to aspecies?

a. It prevents inbreeding.b. It reduces gene flow.c. It permits a species to adapt to

changing environments.d. It reduces heterozygosity.

2. The term genetic bottleneck refers toa. a gene resistant to mutation, which

creates new alleles.b. a reproductive mode that prevents

outcrossing and fosters inbreedingdepression.

c. a decline of a population to a very lowlevel, at which most of its genetic variabilityis lost.

d. the mating of genetically incompatibleindividuals, leading to failure in genetransfer to later generations.

3. Inbreeding depression results froma. excessive rates of mutation.b. the loss of genes from the gene pool.c. the pairing of recessive deleterious

alleles.d. the decline of polymorphism of genes.

Essays

4. How are the 50/500 rule and the effectivepopulation size related?

5. What is the difference between thebottleneck effect and the founder effect?

Name:


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causes of extinction chapter 7 · extinctions. the major causes of extinction today are the...

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