Chapter 52 An Introduction to Ecology and the Biosphere

Transcription

1 Lecture Outline Chapter 52 An Intrductin t Eclgy and the Bisphere Overview: The Scpe f Eclgy Bilgists recrd and transmit data n the annual 8,000-km migratin f gray whales that calve in Baja Califrnia, cllecting infrmatin that has helped this ppulatin recver frm the brink f extinctin. A century ag, whaling had reduced this ppulatin t a few hundred individuals. After 70 years f prtectin, mre than 20,000 gray whales travel t the Arctic each year. What envirnmental factrs determine the gegraphic distributin f gray whales? Hw d variatins in their fd supply affect the size f the gray whale ppulatin? These questins are the subject f eclgy, the scientific study f the interactins between rganisms and their envirnment. These interactins ccur n a hierarchy f scales frm rganismal t glbal. Frmatted: Hidden Cncept 52.2 Interactins between rganisms and the envirnment limit the distributin f species. Eclgists have lng recgnized distinct glbal and reginal patterns in the distributin f rganisms. Bigegraphy is the study f past and present distributins f individual species in the cntext f evlutinary thery. Eclgists ask a series f questins t determine what limits the gegraphic distributin f any species. Eclgists ask why species ccur where they d, fcusing n the factrs that determine the distributin f species. Bitic factrs include all the living rganisms in the individual s envirnment. Abitic factrs are nnliving chemical and physical factrs such as temperature, light, water, and nutrients that influence the distributin and abundance f rganisms. Fr example, red kangars are abundant in areas f Australia that have sparse and variable rainfall. An abitic factr precipitatin may determine where red kangars live. It is als pssible that climate influences red kangar ppulatins indirectly, thrugh bitic factrs such as pathgens, parasites, predatrs, cmpetitrs, and fd availability. Species dispersal cntributes t the distributin f rganisms. The mvement f individuals away frm centers f high ppulatin density r frm their area f rigin is called dispersal. Perhaps there are n kangars in Nrth America due t barriers t their dispersal.

2 The dispersal f rganisms is crucial t understanding gegraphic islatin in evlutin and the brad patterns f current gegraphic distributin f species. Fr example, cattle egrets were fund nly in Africa and suthwestern Eurpe in the early 1800s. After these birds crssed the Atlantic Ocean and clnized nrtheastern Suth America, they were able t spread thrughut Central America, reaching Nrth America by the 1950s. Tday, cattle egrets have breeding ppulatins as far west as the Pacific Cast and as far nrth as suthern Canada. One way t determine whether dispersal is a key factr limiting distributin is t bserve the results when humans have accidentally r intentinally transplanted a species t areas where it was previusly absent. Fr the transplant t be cnsidered successful, the rganisms must nt nly survive in the new area but als reprduce there. If the transplant is successful, then the ptential range f the species is larger than its actual range. In ther wrds, the species culd live in areas where it currently des nt. Eclgists rarely cnduct transplant experiments tday because species intrduced int new gegraphic lcatins may disrupt the cmmunities and ecsystems t which they are intrduced. Instead, eclgists study the utcme when a species has been transplanted accidentally r fr anther purpse. Behavir and habitat selectin cntribute t the distributin f rganisms. Smetimes rganisms d nt ccupy all f their ptential range but select particular habitats. Des behavir play a rle in limiting the distributin in such cases? Habitat selectin is ne f the least-understd eclgical prcesses, but it appears t play an imprtant rle in limiting the distributin f many species. Female insects ften depsit eggs nly in respnse t a very narrw set f stimuli, which may restrict distributin f the insect t certain hst plants. Fr example, the Eurpean crn brer can feed n a wide variety f plants but is fund nly n crn. Egg-laying females are attracted by the drs f crn plants. Bitic factrs affect the distributin f rganisms. Negative interactins with ther rganisms in the frm f predatin, parasitism, r cmpetitin may limit the ability f rganisms t survive and reprduce. Survival and reprductin may als be limited by the absence f species n which the transplanted species depends, such as pllinatrs fr flwering plants. Predatrs and herbivres are examples f bitic factrs that limit the distributin f species. In certain marine ecsystems, there is an inverse relatinship between the abundances f sea urchins and seaweeds. Sea urchins graze n seaweeds, preventing the establishment f large stands f seaweeds. W. J. Fletcher tested the hypthesis that sea urchins are a bitic factr limiting seaweed distributin by remving sea urchins frm experimental plts. He bserved a dramatic increase in seaweed cver, shwing that urchins limit seaweed distributin. The presence r absence f fd resurces, parasites, diseases, and cmpeting rganisms can act as bitic limitatins n species distributin.

3 Abitic factrs affect the distributin f rganisms. The glbal distributin f rganisms bradly reflects the influence f abitic factrs such as temperature, water, and sunlight. The envirnment is characterized by spatial hetergeneity and tempral hetergeneity. Organisms can temprarily avid stressful cnditins thrugh behavirs such as drmancy r hibernatin. Envirnmental temperature is an imprtant factr in the distributin f rganisms because f its effect n bilgical prcesses. Very few rganisms can maintain an active metablism at very high r very lw temperatures. Cells may rupture if the water they cntain freezes, and mst prteins denature at temperatures abve 45 0 C. Sme rganisms have extrardinary adaptatins that allw them t live utside the temperature range habitable fr mst ther living things, but mre rganisms functin best within a specific range f temperatures. Endtherms may expend energy regulating their internal temperature at temperatures utside that range. The variatin in water availability amng habitats is an imprtant factr in species distributin. Intertidal species may face desiccatin as the tide recedes. Terrestrial rganisms face a nearly cnstant threat f desiccatin and have adaptatins that allw them t btain and cnserve water. Desert rganisms, fr example, have a variety f adaptatins fr acquiring and cnserving water in dry envirnments. The salt cncentratin f the envirnment affects the water balance f rganisms thrugh smsis. Mst aquatic rganisms have a limited ability fr smregulatin and are restricted t either freshwater r marine habitats. High-salinity habitats such as salt flats typically cntain few species. Sunlight prvides the energy that drives nearly all ecsystems. Light intensity is nt the mst imprtant factr limiting plant grwth in mst terrestrial envirnments, althugh shading by a frest canpy creates intense cmpetitin fr light in the understry. In aquatic envirnments, light intensity limits the distributin f phtsynthetic rganisms. Every meter f water depth selectively absrbs 45% f red light and 2% f blue light passing thrugh it. As a result, mst phtsynthesis in aquatic envirnments ccurs near the surface. T much light can als limit the survival f rganisms. The atmsphere is thinner at higher elevatins, absrbing less ultravilet radiatin. The sun s rays are mre likely t damage DNA and prteins in alpine envirnments. The physical structure, ph, and mineral cmpsitin f sils and rcks limit the distributin f plants and, thus, f the animals that feed n them, cntributing t the patchiness f terrestrial ecsystems. The distributin f rganisms at extreme acidic and basic cnditins is limited. ph can als have indirect effects by altering the slubility f nutrients and txins.

4 In streams and rivers, substrate cmpsitin can affect water chemistry, thus influencing the distributin f rganisms. In freshwater and marine envirnments, the structure f substrates limits the rganisms that can attach t r burrw in thse habitats. Fur abitic factrs are the majr cmpnents f climate. Climate is the lng-term, prevailing weather cnditins in a particular area. Fur abitic factrs temperature, water, sunlight, and wind are the majr cmpnents f climate. Climatic factrs, especially temperature and water, have a majr influence n the distributin f rganisms. Climate patterns can be described n tw scales. Macrclimate patterns are n glbal, reginal, r lcal levels, and micrclimate patterns are very fine patterns, such as the cnditins experienced by a cmmunity f rganisms under a fallen lg. Glbal climate patterns are determined by sunlight and Earth s mvement in space. The sun s warming effect n the atmsphere, land, and water establishes the temperature variatins, cycles f air mvement, and evapratin f water that are respnsible fr latitudinal variatins in climate. Bdies f water and tpgraphic features such as muntain ranges create reginal climatic variatins, while smaller features f the landscape affect lcal climates. Seasnal variatin may influence climate. Ocean currents influence climate alng the cast by heating r cling verlying air masses, which may pass ver land. Castal regins are generally mister than inland areas at the same latitude. In general, ceans and large lakes mderate the climate f nearby terrestrial envirnments. In certain regins, cl, dry cean breezes are warmed when they mve ver land, absrbing misture and creating a ht, rainless climate slightly inland. This pattern draws a cl breeze frm the water acrss the land. At night, air ver the cean rises, drawing cler air frm the land back ut ver the water and replacing it with warmer ffshre air. This Mediterranean climate pattern ccurs inland frm the Mediterranean Sea. Muntains have a significant effect n the amunt f sunlight reaching an area as well as n lcal temperature and rainfall. In the Nrthern Hemisphere, suth-facing slpes receive mre sunlight than nrth-facing slpes and are therefre warmer and drier. These envirnmental differences affect species distributin. At any given latitude, air temperature declines 6 C with every 1,000-m increase in elevatin. This temperature change is equivalent t the change caused by an 880-km increase in latitude. Bilgical cmmunities n muntains are similar t thse at lwer elevatins farther frm the equatr. As mist, warm air appraches a muntain, it rises and cls, releasing misture n the windward side f the peak.

5 On the leeward side f the muntain, cl, dry air descends, absrbing misture and prducing a rain shadw. Deserts are cmmnly n the leeward side f muntain ranges. The changing angle f the sun ver the curse f a year affects lcal envirnments. Belts f wet and dry air n either side f the equatr shift with the changing angle f the sun, prducing marked wet and dry seasns at arund 20 latitude, where many trpical deciduus frests grw. Seasnal changes in wind patterns prduce variatins in cean currents, ccasinally causing the upwelling f nutrient-rich, cld water frm deep cean layers. This nutrient-rich water stimulates the grwth f surface-dwelling phytplanktn and the rganisms that feed n them. Many features in the envirnment influence micrclimates. Frest trees mderate the micrclimate beneath them. Cleared areas experience greater temperature extremes than the frest interir. Within a frest, lw-lying grund is usually wetter than high grund and tends t be ccupied by different species f trees. A lg r large stne shelters rganisms, buffering them frm temperature and misture fluctuatins. Every envirnment n Earth is characterized by a msaic f small-scale differences in abitic factrs that influence the lcal distributin f rganisms. Lng-term climate changes prfundly affect the bisphere. One way t predict the pssible effects f current climate changes is t cnsider the changes that have ccurred in temperate regins since the end f the last ice age. Until abut 16,000 years ag, cntinental glaciers cvered much f Nrth America and Eurasia. As the climate warmed and the glaciers melted, the distributin f trees expanded nrthward. A detailed recrd f these migratins is captured in fssil pllen in lake and pnd sediments. If researchers can determine the climatic limits f current gegraphic distributins fr individual species, they can predict hw that species distributin will change with glbal warming. A majr questin fr tree species is whether seed dispersal is rapid enugh t sustain the migratin f the species as climate changes. An example t cnsider is the eastern hemlck, whse mvement nrth was delayed nearly 2,500 years at the end f the last ice age. This delay in seed dispersal was partly attributable t the lack f wings n the seeds, which tend t fall clse t the parent tree. The fssil recrd can infrm predictins abut the bilgical impact f current glbal warming trends n the gegraphic range f the American beech, Fagus grandiflia. Tw different climate-change mdels are used t cmpare the current and predicted gegraphic ranges f this tree. These mdels predict that the nrthern limit f the beech s range will mve km nrth ver the next century and its suthern range will mve nrthward even farther. The beech will have t migrate 7 9 km per year t maintain its distributin in a warming climate. Hwever, since the ice age, the beech has migrated int its present range at a rate f nly 0.2 km per year. Withut human assistance, the beech will have a much smaller range and may becme extinct.

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