APES Ch 5 PDF

A f r i c a n Savanna Throughout these regions, we find differ- on othergrasslands. Later, they cleared...

A f r i c a n Savanna Throughout these regions, we find differ- on othergrasslands. Later, they cleared patches of forest to expand farmland and cre- ent types of ecosystems, vegetation, and ani- d eventually towns and cities. T h e e a r t h has a great diversity of spe- ated villages an mals adapted to the various climate Today, vast areas of African savanna have cies and habitats, or places where these conditions. For example, in tropical areas, We been plowed up and converted to cropland species can live. Some species live in terres- find type of grassland called a savanna. a or used for grazing fivestock. Towns are also trial, or land, habitats such as grasslands This biome typically contains scattered trees (see chapter-opening photo), forests, and expanding there, and this trend will continue and usually has warm temperaturesyear- as the human population in Africa?the con- deserts. These three major types of terres- round with alternating dry and wet seasons. tinent with the world?s fastest population tral ecosystems are called biomes. They Savannas in East Africa are home to grazing represent one of the four componentso f growth increases. Asa result, populations (primanily grass-eating) and browsing (twig- biodiversity (Figure 4.4, p. 95), £0 of elephants, lions, and other animals that which 1s the basis for one of the 2Y : and leaf-nibbling) hoofed animals. They roamed the savannas for millions of years include wildebeests, gazelles, antelopes, three scientific principles o f have dwindled. Many of these animals face zebras, elephants (Figure 5.1), and giraffes sustainability. extinction in the next f e w decades because (chapter-opening photo), as well as their Why do grasslands grow on some areas of the loss of their habitats and because peo- predators such as lions, hyenas, and humans. of the earth?s land while forests and deserts ple kill them for food and their valuable parts form in other areas? The answer lies largely Archeological evidence indicates that our such as the ivory tusks of elephants. in differences in climate, the average short- species emerged from African savannas. In this chapter, w e distinguish Early humans lived largely in trees but even- term weather conditions in a given region between weather and climate and exam- over at least three decades to thousands of tually came down to the ground and ine the role that climate plays in the loca- learned to walk upright. This freed them to years. Differences in climate result mostly tion and formation of the major terrestrial use their hands for using tools such as clubs from long-term differences in weather, ecosystems, called biomes. We also begin based primarily on average annual precipi- and spears. Much later, they developed bows and arrows and other weapons that the study of human impacts on these tation and temperature. These differences enhanced their abilities to hunt animals for important ecosystems. @ lead to three major types of cimate?tropical food and clothing made from animal hides. (areas near the equator, receiving the most After the last ice age, about 10,000 years intense sunlight), polar (areas near the ago, the earth?s climate warmed and humans earth?s poles, receiving the least intense began their transition fromhunter-gatherers Why is it important to k n o w the sunlight), and temperate (areas between to farmers growing food on the savanna and difference between weather and climate? the tropical and polar regions). y 3 3 2 3 3 5 = 3 z FIGURE 5.1 Elephants ona tropical African savanna. 1 2 0 @ CHAPTER S CLIMATE AND TERRESTRIAL BIODIVERSITY  5.1 W H A T FACTORS INFLUENCE clouds at different altitudes. Gradually, t h e clouds t h i c k e n , descend to a l o w e r altitude, and often release t h e i r mois- WEATHER? t u r e as rainfall. A c o l d f r o n t (Figure 5.2, right) is the leading edge of C O N C E P T 5.1 Key f a c t o r s t h a t influence w e a t h e r arem o v i n g an advancing mass o f cold air. Because cold air is denser masses of w a r m a n d c o l d air, c h a n g e s in a t m o s p h e r i c pressure, t h a n w a r m air, an advancing cold f r o n t stays close to the a n d occasional shifts in m a j o r winds. g r o u n d a n d wedges beneath less dense w a r m e r air. It pushes this w a r m , moist air up, w h i c h produces r a p i d l y m o v i n g , t o w e r i n g clouds called thunderheads. A s it passes W e a t h e r Is A f f e c t e d b y M o v i n g Masses t h r o u g h , it can cause h i g h surface w i n d s and t h u n d e r - o f W a r m o r Cold A i r storms, f o l l o w e d by cooler temperatures a n d a clear sky. W e a t h e r is the set of physical conditions of the lower at- mosphere, i n c l u d i n g t e m p e r a t u r e , precipitation, h u m i d i t y , W e a t h e r Is A f f e c t e d b y Changes w i n d speed, cloud cover, a n d other factors, i n a given area in A t m o s p h e r i c Pressure a n d W i n d over a period of hours to days. The most i m p o r t a n t factors Patterns i n the w e a t h e r i n a n y area are atmospheric temperature and precipitation. A t m o s p h e r i c p r e s s u r e r e s u l t s f r o m m o l e c u l e s o f gases i n Meteorologists use e q u i p m e n t mounted on w e a t h e r the atmosphere (mostly nitrogen and oxygen) zipping b a l l o o n s , a i r c r a f t , s h i p s , a n d s a t e l l i t e s , as w e l l as r a d a r a n d a r o u n d at v e r y h i g h speeds a n d b o u n c i n g o f f e v e r y t h i n g s t a t i o n a r y sensors, t o o b t a i n data o n w e a t h e r variables. t h e y e n c o u n t e r. A t m o s p h e r i c p r e s s u r e is g r e a t e r n e a r t h e They feed these data into computer models to draw e a r t h ' s s u r f a c e b e c a u s e t h e m o l e c u l e s i n t h e a t m o s p h e r e are w e a t h e r maps f o r various parts of t h e w o r l d. O t h e r com- squeezed together u n d e r t h e w e i g h t o f t h e air a b o v e t h e m. p u t e r m o d e l s p r o j e c t u p c o m i n g w e a t h e r c o n d i t i o n s based A n a i r mass w i t h h i g h pressure, called a high, c o n t a i n s o n p r o b a b i l i t i e s t h a t a i r masses, w i n d s , a n d o t h e r factors cool, d e n s e a i r t h a t descends s l o w l y t o w a r d t h e earth?s will change in certain ways. surface and becomes w a r m e r. Because of this w a r m i n g , M u c h of t h e w e a t h e r w e experience results f r o m inter- w a t e r m o l e c u l e s i n t h e a i r d o n o t f o r m d r o p l e t s ? a process a c t i o n s b e t w e e n t h e l e a d i n g edges o f m o v i n g masses o f c a l l e d condensation. T h u s c l o u d s , w h i c h are m a d e o f d r o p - w a r m a i r a n d c o l d a i r ( C o n c e p t 5.1). W e a t h e r c h a n g e s lets, u s u a l l y d o n o t f o r m i n t h e p r e s e n c e o f a h i g h. F a i r w h e n o n e a i r m a s s replaces o r meets another. T h e m o s t w e a t h e r w i t h c l e a r skies f o l l o w s as l o n g as t h i s h i g h r e m a i n s dramatic changes i n w e a t h e r occur along a front, the o v e r t h e area. b o u n d a r y b e t w e e n t w o a i r masses w i t h d i f f e r e n t t e m p e r a - A low-pressure air mass, called a low, contains l o w - tures and densities. density, w a r m air at its center. This air rises, expands, and A w a r m f r o n t is t h e b o u n d a r y b e t w e e n a n a d v a n c i n g cools. W h e n its temperature drops b e l o w a certain level, w a r m a i r m a s s a n d t h e c o o l e r o n e i t is r e p l a c i n g ( F i g u r e 5.2, called the dew point, moisture i n the air condenses and l e f t ). B e c a u s e w a r m a i r is less d e n s e ( w e i g h s less p e r u n i t forms clouds. The condensation process usually requires o f v o l u m e ) t h a n c o o l a i r , a n a d v a n c i n g w a r m a i r mass rises that the air contain suspended t i n y particles o f dust, smoke, u p o v e r a m a s s o f c o o l a i r. A s t h e w a r m a i r rises, its m o i s - sea salts, o r volcanic ash, called condensation nuclei, a r o u n d t u r e begins c o n d e n s i n g i n t o droplets, f o r m i n g layers of w h i c h water droplets can form. If the droplets i n the clouds Avil t o p Coot ? ? _ » Pe " ~ ope air mass on ? / , Cold? Fro, ? W a r m air mass 1 a nt 7 A Suri ~~ Cold air mass FIGURE 5.2 W e a t h e r fronts: A w a r m f r o n t (left) occurs w h e n a m o v i n g mass of w a r m air meets and rises up over a mass o f denser cool air. A cold f r o n t (right) forms w h e n a m o v i n g mass o f cold air w e d g e s beneath a mass o f less dense w a r m air. 121  coalesce i n t o larger d r o ps o r snowflakes heavy least two-thirds of the globe (Figure 5. 4 ) ? e s p e c i a l l y on fall f r o m t h e sky, low t e r e to p r e c i p i t a t i o n occurs. Thus, a l o w tends to the coasts of the Pacific and I n d i a n Oceans, p r o d u c e c l o u d y a n d s o m e t i m e s s t o r m y weather. A n ENSO is a 1- to 2-year n a t u r a l w e a t h e r event, ; M o v e m e n t of these air masses is i n f l u e n c e d strongly by A l t h o u g h it is not a climate event, It can r a i s et h e earth?s Jet s t r e a m s ? p o w e r f u l w i n d s t h a t circle the globe near the top average temperature by as m u c h as 0.25 c (0.45°F) for a o f t h e t r o p o s p h e r e. T h e y are like f a s t - f l o w i n g rivers of air year or two. A s a result, it can affect the climate by tempo- m o v i n g west to east, o n e i n each hemisphere somewhere rarily increasing the earth?s average temperature. ENSOs can a b o v e a n d b e l o w t h e e q u a t o r. T h e y f o r m because of the be extreme in their effects. One such super ENSO occurred in t e m p e r a t u r e difference b e t w e e n the e q u a t o r a n d t h e poles, 1997 and 1998. This 2-year period of extreme weather, in- w h i c h causes air to m o v e. A s t h e air moves a w a y f r o m the cluding severe storms, flooding, and temperature extremes, e q u a t o r , n o r t h a n d south, i t is deflected by the earth?s rota- caused $4.5 billion in damages and 23,000 deaths. t i o n a n d f l o w s g e n e r a l l y west to east. The greater the tem- La N i t i a , t h e r e v e r s e of El N i f i o , c o o l s s o m e c o a s t a l s u r - p e r a t u r e difference, t h e faster the f l o w of these winds. Jet face w a t e r s. T h i s n a t u r a l w e a t h e r e v e n t a l s o o c c u r s e v e r y streams can i n f l u e n c e w e a t h e r b y m o v i n g moist air masses f e w y e a r s a n d i t t y p i c a l l y leads t o m o r e A t l a n t i c Ocean f r o m o n e area to a n o t h e r ( C o n c e p t 5.1). W e e x a m i n e these hurricanes, colder w i n t e r s i n Canada and t h e northeastern air f l o w p a t t e r n s i n m o r e d e p t h l a t e r i n this chapter. U n i t e d States, a n d w a r m e r a n d d r i e r w i n t e r s i n t h e s o u t h - E v e r y f e w years, n o r m a l w i n d p a t t e r n s i n t h e Pacific eastern and southwestern U n i t e d States. I t a l s o u s u a l l y O c e a n ( F i g u r e 5.3, left) are d i s r u p t e d a n d this affects leads t o w e t t e r w i n t e r s i n t h e Pacific N o r t h w e s t , t o r r e n t i a l w e a t h e r a r o u n d m u c h o f t h e globe. This change i n w i n d rains i n S o u t h e a s t Asia, a n d s o m e t i m e s m o r e w i l d f i r e s in p a t t e r n s is called t h e E l Nifio-Southern Oscillation, o r ENSO F l o r i d a. S c i e n t i s t s d o n o t k n o w t h e e x a c t c a u s e s o f these ( F i g u r e 5.3, r i g h t ). w e a t h e r e v e n t s o r w h e n t h e y are l i k e l y t o o c c u r , b u t t h e y I n a n E N S O , o f t e n called s i m p l y E/ Nifio, w i n d s that d o k n o w h o w t o detect a n d m o n i t o r t h e m. u s u a l l y b l o w m o r e - o r - l e s s c o n s t a n t l y f r o m east to w e s t w e a k e n o r reverse d i r e c t i o n. T h i s a l l o w s the w a r m e r Tornadoes a n d Tropical Cyclones w a t e r s o f t h e w e s t e r n Pacific to m o v e t o w a r d t h e coast of A r e V i o l e n t W e a t h e r Extremes S o u t h A m e r i c a. A h o r i z o n t a l z o n e of g r a d u a l t e m p e r a t u r e c h a n g e c a l l e d t h e thermocline, separating w a r m a n d cold Sometimes w e experience weather extremes. Two examples w a t e r s , s i n k s i n t h e e a s t e r n Pacific. These changes result i n are v i o l e n t storms called tornadoes ( w h i c h f o r m o v e r land) d r i e r w e a t h e r i n s o m e areas a n d w e t t e r w e a t h e r i n o t h e r and tropical cyclones ( w h i c h form o v e r w a r m ocean water areas. A s t r o n g E N S O c a n a l t e r w e a t h e r c o n d i t i o n s o v e r at a n d sometimes pass over coastal l a n d areas). 0 D _ f ? o y : a r a M e e r ener n a g a ing updrafts Drought in and storms Surface winds Australa and Southeast Asia blow westward ATOR =f a oe EQUATOR _ + - ? ? - ~~ - > - ? ? ,,~ Warm water flow SOUTH ?4 Warm waters reversed i e AUSTRALIA _ stopped or AMERICA ? pushed westward ?. ~ Cold water EI Nifio Conditions N o r m a l Conditions FIGURE 5.3 Nifio: Normal trade w i n d s blowing east to west cause shore upwellings of cold, EI the tropical Pacific Ocean near the coast of Peru (left). A zone of nutrient-rich b o t t o m w a t e r in dual temperature change called the thermocline separates the warm and cold water. Every few gra in trade winds known as the El Nifio-Southern Oscillation (ENSO) disrupts this pattern years, a shift i (right) f o r 1 to 2 years. CHAPTER 5 CLIMATE AND TERRESTRIAL BIODIVERSITY 122  F I G U R E 5. 4 Typical g l o b a l w e a t h e r e f f e c t s o f an El N i n o - Southern Oscillation. Q u e s t i o n : H o w m i g h t an ENSO affect t h e w e a t h e r w h e r e you live o r g o t o school? (Compiled by the authors using data from United Nations Food and Agriculture Organization and U.S. Weather Service.) BREE Drought | | Unusually high rainfall Zz Unusually warm periods Tornadoes, o r twisters, are swirling, funnel-shaped clouds the large w a r m f r o n t m o v e s r a p i d l y o v e r the denser cold- t h a t f o r m o v e r land. T h e y can destroy houses and cause air mass, i t rises s w i f t l y a n d f o r m s strong vertical convec- o t h e r serious damage i n areas w h e r e t h e y t o u c h down. tion currents that suck air u p w a r d (Figure 5.5). Scientists The U n i t e d States is the world?s most t o r n a d o - p r o n e coun- hypothesize that the i n t e r a c t i o n o f the cooler air n e a r e r try, f o l l o w e d by Australia. ? t h e g r o u n d a n d t h e r a p i d l y r i s i n g w a r m e r a i r a b o v e causes Tornadoes in the plains of the Midwestern United a spinning, vertically rising air mass, o r vortex. M o s t t o r n a - States o f t e n o c c u r w h e n al arge, dry, cold f r o n t m o v i n g does i n the A m e r i c a n M i d w e s t o c c u r i n the spring a n d s o u t h w a r d f r o m C a n a d a r u n s i n t o a large mass o f w a r m s u m m e r w h e n cold f r o n t s f r o m t h e n o r t h p e n e t r a te deeply h u m i d air m o v i n g n o r t h w a r d f r o m the Gulf o f Mexico. A s into the Great Plains a n d t h e M i d w e s t. F I G U R E 5. 5 Formation of a Descending t o r n a d o , or t w i s t e r. T h e m o s t cool air active t o r n a d o season in t h e U n i t e d States is u s u a l l y M a r c h t h r o u g h August. Severe thunderstorm Rising w a r m air Severe thunderstorms can trigger a n u m b e r of smaller tornadoes Tornado forms when cool downdraft and warm updraft of air Rising meet and interact updraft of air 123  Rising winds exit =. from the storm a t , ? h g ha l t i t u d e s g , ? - ee L e * es ? The calm central © eye usually 1s a b o u t EE 24 kilometers (15 miles) wideg wos, te) 1, nos ~. *. Gales circle the eye at spec. = > a of up to 320 kilometers F N " g, @ (200 miles) per hour ? a s p i r a l in. t o w a r d t h e c e n t e r o f t h e storm. +e. Re P R I FIGURE 5.6 Formation of a tropical cyclone. Those forming in the Atlantic Ocean arec a l l e d h u r r i c a n e s. T h o s e f o r m i n g in t h e Pacific O c e a n are c a l l e d t y p h o o n s. longer a tropical cyclone stays o v e r w a r m waters, the Tropical cyclones ( F i g u r e 5.6) are s p a w n e d by t h e f o r m a - stronger it gets. Significant h u r r i c a n e - f o r c e w i n d s can ex- t i o n o f l o w - p r e s s u r e cells o f a i r o v e r w a r m tropical seas. tend 6 4 - 1 6 1 kilometers ( 4 0 - 1 0 0 miles) f r o m t h e center, or Hurricanes are tropical cyclones that form in the Atlantic eye, of a tropical cyclone. O c e a n. T h o s e f o r m i n g i n t h e Pacific Ocean usually are c a l l e d typhoons. H u r r i c a n e s a n d t y p h o o n s k i l l a n d i n j u r e p e o p l e , d a m a g e p r o p e r t y , a n d h i n d e r food p r o d u c t i o n. fa e e e e e s e rbe Coe U n l i k e t o r n a d o e s , h o w e v e r , t r o p i c a l cyclones take a long t i m e to f o r m a n d g a i n s t r e n g t h. T h i s a l l o w sm e t e o r o l o g i s t s +1. Explain h o w tornadoes form. t o t r a c k t h e i r p a t h s a n d w i n d speeds, a n d t o w a r n people Explain how cycloneso r hurricanes form._ i n areas l i k e l y to be h i t b y these v i o l e n t s t o r m s. ?orm, the temperature of ocean F o r a t r o p i c a l c y c l o n e to f w a t e r has to b e at least 27°C (80°F) t o a d e p t h o f 46 meters u r e o v e r these w a r m ocean ( 1 5 0 feet). A r e a s of l o w press s.2 W H A T FACTORS INFLUENCE w a t e r s d r a w i n a i r f r o m s u r r o u n d i n g higher-pressure areas. T h e earth?s r o t a t i o n m a k e s these w i n d s spiral c o u n t e r c l o c k - CLIMATE? wise i n the northern h e m i s p h e r e a n d c l o c k w i s e i n the s o u t h e r n h e m i s p h e r e. M o i s t air, w a r m e d by the heat of the CONCEPT 5.2. Key factors that influence an area's climate o c e a n , rises i n a v o r t e x t h r o u g h t h e center o f t h e s t o r m are incoming solar energy, the earth?s rotation, global patterns u n t i l i t becomes a t r o p i c a l c y c l o n e ( F i g u r e 5.6). of air and water movement, gases in the atmosphere, and the T h e i n t e n s i t i e s o f t r o p i c a l c y c l o n e s a r e rated i n differ- earth?s surface features. e n t c a t e g o r i e s. based o n t h e i r sustained w i n d speeds. The CHAPTER 5 CLIMATE AND TERRESTRIAL BIODIVERSITY 424  qu erent we {___] Polar (ice) HEM Subarctic (snow) ~~ J Cool temperate H B Highland ~«???? W a r m ocean current ~ ~ River H E Warm temperate Dry [EEEB Tropical a Major upwelling zones ~ ~ Cold ocean current F I G U R E 5. 7 N a t u r a l c a p i t a l : This generalized m a p o f t h e earth?s c u r r e n t climate zones also shows t h e m a j o r o c e a n c u r r e n t s a n d u p w e l l i n g areas ( w h e r e currents bring nutrients f r o m t h e ocean b o t t o m to t h e surface). Q u e s t i o n : Based on this map, w h a t is t h e general type o f c l i m a t e w h e r e you live? curs w h e n the sun warms the air a n d causes some o f it to Several Factors A f f e c t Regional Climates rise, while cooler air sinks i n a cyclical pattern called a c o n - It is i m p o r t a n t to understand the difference between weather v e c t i o n cell. and climate. Weather is the set of short-term atmospheric For example, the air over an ocean is heated w h e n the conditions over hours to days to years, whereas c l i m a t e is sun evaporates water. This transfers moisture and heat from the general pattern of atmospheric conditions i n a given area the ocean to the atmosphere, especially near the h o t equa- over periods ranging from at least three decades to thou- tor. This w a r m , moist air rises, then cools and releases heat sands of years. Weather often fluctuates daily, from one and moisture as precipitation (Figure 5.8, right side and top, season to another, and f r o m one year to the next. However, center). Then the cooler, denser, and drier air sinks, w a r m s climate tends to change slowly because it is the average of up, and absorbs moisture as i t flows across the earth?s surface l o n g - t e r m atmospheric conditions over at last 30 years. (Figure 5.8, left side and bottom) to begin the cycle again. Climate varies among the earth?s different regions pri- A second m a j o r climatic factor is the uneven heating of the m a r i l y because of global air circulation and o c e a n c u r - earth's surface by the sun. A i r is heated m u c h m o r e at the equa- r e n t s , o r mass m o v e m e n t s o f ocean water. Global winds tor, w h e r e the sun?s rays strike directly, than at the poles, and ocean currents distribute heat and precipitation u n - where sunlight strikes at an angle a n d spreads o u t over a evenly b e t w e e n the tropics and o t h e r parts of t h e world. m u c h greater area (Figure 5.9, left). Thus, solar heating varies Scientists h a v e described the various regions of the earth w i t h l a t i t u d e ? t h e location between the equator a n d one of according to t h e i r climates (Figure 5.7). the poles. Latitudes are designated by degrees (X°) n o r t h o r Several majo r factors help determine regional climates. south. The equator is at 0°, the poles are at 90° n o r t h a n d The first is the cyclical movement o f air driven by solar energy. It is 90° south, and areas between range f r o m 0° to 90°. a form of c o n v e c t i o n , the m o v e m e n t of matter (such as gas The i n p u t of solar energy i n a given area, called i n s o - o r water) caused w h e n the w a r m e r and less dense part of a l a t i o n , varies w i t h latitude. This p a r t l y explains w h y body of such matter rises while the cooler, denser part of the tropical regions are hot, p o l a r regions are cold, a n d tem- fluid sinks due to gravity. I n the atmosphere, convection oc- perate regions g e n e r a l l y alternate b e t w e e n w a r m and c o o l 125  temperatures (Figure 5.9, right).T h e a m o u n t of solar radia. tion reaching the earth typicallyvaries about every 11 year. because of changes in solar magnetic a c t i v i t y that can w a r m or cool the planet. , A third m a j o r factor is the wit o f the earth's axis ang resulting seasonal changes. The earth?sa x i s ? a n i m a g i n a r y line Low connecting the north and south p o l e s ? i s tilted w i t h Tespect HIGH PRESSURE Heat released PRESSURE to the sun?s rays. Asa result, regions n o r t h and south of the Cool, radiates t o space Condensation equator are tipped toward o r a w a y from the sun atd i f f e r e n t times, as the earth makes its a n n u a l r e v o l u t i o n a r o u n d the dry air and Preapitation sun (Figure 5.10). This means most areas of the world experience widely varying amounts o f solar energy, ang Falls, Rises, thus very different seasons, t h r o u g h o u t the year. This in 1s Compressed, expands, t u r n leads to seasonal changes in t e m p e r a t u r e and precipj. i warms cools tation in most areas of the globe, and o v e r three or more decades these changes help determine regional climates, -_. Warm,. t A fourth m a j o r climatic factor is the rotation of the earth | Gyan : Hot, te on its axis. As the earth rotates to the east (to t h e right, look. ing at Figure 5.9), the equator spins faster t h a n the regions F l o w s t o w a r d l o w pressure, picks u p m o i s t u r e a n d heat to its n o r t h and south. This means that air masses moving to the n o r t h or south from the equator are deflected to the HIGH PRESSURE Moist s Low east, because they are also m o v i n g east faster t h a n the land ist Surface warmed by sun PRESSURE b e l o w them. This deflection of an object's p a t h due to the rotation of the earth is k n o w n as t h e C o r i o l i s e f f e c t. Some of this high, moving mass of w a r m air cools as it flows northeast o r southeast from the equator. It becomes more dense and heavier and sinks t o w a r d the earth?s surface at about 30° north and 30° south (Figure 5.9, left). Because F I G U R E 5. 8 Convection cells play a key role in transferring energy (heat) and moisture through the atmosphere from place to place on t h e planet. M o s t air rises, ~ < ? ? cools, and releases y moisture as rain Air cools and < ? ? ? ? ? _ descends at The highest solar e Temperate deciduous tower latitudes. energy input 1s at re? forest and grassland t h e equator. Hot desert Hadley cells, Warm air rises and < ? ? _ moves toward the poles. Hadley cells. H o t desert T; e m p e r a t é d e c i d u o u s , A i r cools and forest and grass{ai < ? ? ? ? ? _ descends at F I G U R E 5. 9 Global air f. rises lower latitudes. circulation: Air and falls in giant convection cells (right). Air flow- 60°S Gold g eserts i ~~ ing away from the equator is deflected to the Polar c a p east and air flowing toward the equator is deflected west, due to the Coriolis effect. This creates global patterns of prevailing winds (left) that help to distributeh e a t a n d moisture in the atmosphere, which leads to the earth's variety of forests, grasslands, and deserts (right). 126 CHAPTER 5 CLIMATE ANO TERRESTRIAL BIODIVERSITY  Spring Winter (sun aims d i r e c t l y (northern hemisphere at e q u a t o r ) tilts a w a y f r o m sun) radiation Cold, salty, deep current ? - Summer (northern hemisphere t i l t s t o w a r d sun) F I G U R E 5. 1 1 A c o n n e c t e d loop of d e e p a n d shallow ocean Fall currents transports w a r m a n d cool w a t e r t o various parts o f t h e (sun aims directly at equator) earth. F I G U R E 5. 1 0 The earth's axis is tilted a b o u t 2 3. 5 ° w i t h respect to t h e p l a n e o f t h e earth?s path a r o u n d t h e sun. The c o l d o c e a n c u r r e n t s. T h e y are d r i v e n b y p r e v a i l i n g w i n d s resulting v a r i a t i o n s in solar e n e r g y reaching t h e n o r t h e r n a n d s o u t h e r n hemispheres t h r o u g h o u t a year result in seasons. a n d t h e earth?s r o t a t i o n ( t h e C o r i o l i s e f f e c t ) , a n d c o n t i n e n t a l C r i t i c a l t h i n k i n g : H o w m i g h t y o u r life be d i f f e r e n t if t h e coastlines c h a n g e t h e i r directions. As a result, b e t w e e n t h e earth's axis was n o t tilted? continents, t h e currents f l o w in r o u g h l y circular p a t t e m s , called g y r e s , w h i c h m o v e c l o c k w i s e i n t h e n o r t h e r m h e m i - sphere a n d c o u n t e r c l o c k w i s e i n the s o u t h e r n h e m i s p h e r e. it is part of a convection cell (Figure 5.8), it starts flowing O t h e r l o n g - t e r m f a c t o r s a f f e c t i n g t h e earth?s c l i m a t e are back t o w a r d the equator in w h a t is k n o w n as a Hadley cell. ( 1 ) s l i g h t c h a n g e s i n t h e s h a p e of t h e Earth?s o r b i t a r o u n d Because of the Coriolis effect, this air m o v i n g toward the the sun f r o m m o s t l y r o u n d to m o r e elliptical o v e r a 100,000 equator curls i n a westerly direction. In the n o r t h e r n hemi- y e a r cycle, ( 2 ) s l i g h t c h a n g e s i n t h e t i l t o f Earth?s a x i s o v e r a sphere, it thus flows southwest from northeast. In the south- 4 1 , 0 0 0 - y e a r cycle, a n d ( 3 ) s l i g h t c h a n g e s i n Earth?s w o b b l y ern hemisphere, it flows northwest from southeast. o r b i t a r o u n d t h e s u n o v e r a 2 0 , 0 0 0 - y e a r cycle. T h e s e t h r e e These w i n d s are k n o w n as the northeast trade winds l o n g - t e r m f a c t o r s are k n o w n as t h e M i l a n k o v i t c h cycles. ( n o r t h of the equator) and t h e southeast trade winds (south W a t e r also m o v e s v e r t i c a l l y i n t h e o c e a n s as d e n s e r of t h e equator). T h e y w e r e named long ago w h e n sailing w a t e r s i n k s w h i l e less d e n s e w a t e r rises. T h i s c r e a t e s a c o n - ships used t h e m to m o v e goods i n trade b e t w e e n the con- nected loop of deep and shallow ocean currents (which tinents. T h e y are examples of prevailing w i n d s ? m a j o r sur- are s e p a r a t e f r o m t h o s e s h o w n i n F i g u r e 5. 7 ). T h i s l o o p face w i n d s t h a t b l o w almost continuously. acts s o m e w h a t l i k e a g i a n t c o n v e y e r b e l t t h a t m o v e s h e a t The w a r m air t h a t does n o t descend i n the Hadley cells f r o m t h e s u r f a c e t o t h e d e e p sea a n d t r a n s f e r s w a r m a n d at 30° n o r t h and 30° s o u t h continues m o v i n g t o w a r d the cold w a t e r b e t w e e n t h e t r o p i c s a n d t h e p o l e s ( F i g u r e 5.11). poles and c u r v i n g to t h e east due to t h e Coriolis effect. These p r e v a i l i n g w i n d s that b l o w generally from t h e west G r e e n h o u s e Gases W a r m t h e L o w e r i n temperate regions o f t h e globe are k n o w n as westerlies Atmosphere (Figure 5.9, left). This c o m p l e x m o v e m e n t o f air results in six h u g e re- As energy flows f r o m the sun to the earth, some of it is gions b e t w e e n the e q u a t o r and the poles i n w h i c h w a r m reflected by the earth?s surface back i n t o the atmosphere. air rises a n d cools, t h e n falls a n d heats u p again i n great Molecules of certain gases i n the atmosphere, i n c l u d i n g rolling patterns (Figure 5.9, right). The t w o nearest the w a t e r vapor (H20), carbon d i o x i d e (CQ2), m e t h a n e (CH,), e q u a t o r are t h e H a d l e y cells. These c o n v e c t i o n cells and a n d n i t r o u s oxide ( N , 0 ) , absorb some of this solar energy the resulting p r e v a i l i n g w i n d s distribute heat and moisture a n d release a p o r t i o n of it as i n f r a r e d r a d i a t i o n (heat) that over the earth?s surface, t h u s helping to d e t e r m i n e w a r m s t h e l o w e r atmosphere and the earth?s surface. regional climates (Concept 5.2). These gases, called g r e e n h o u s e gases, play a role i n A f i f t h m a j o r factor determining regional climates is ocean d e t e r m i n i n g the l o w e r atmosphere?s average temperatures currents (Figure 5.7). T h e y help to redistribute heat f r o m the and thus t h e earth?s climates. sun, thereby influencing climate and vegetation, especially T h e earth?s surface absorbs m u c h of t h e solar e n e r g y near coastal areas. This solar heat, along w i t h differences i n that strikes it a n d t r a n s f o r m s i t i n t o l o n g e r - w a v e l e n g t h water density (mass p e r unit volume), creates w a r m and i n f r a r e d r a d i a t i o n , w h i c h t h e n rises i n t o t h e l o w e r 127  Prevailing winds On the leeward side of the mountain On the windward side of a Pick Up moisture mountain range, air rises, range, air descends, warms, and f r o m an ocean releases iittle m o i s t u r e , c a u s i n g rain cools, a n d releases m o i s t u r e m W e y s h a d o w effect ?7 F I G U R E 5. 1 2 Ther a i n s h a d o w e f f e c t is a r e d u c t i o n o f rainfall a n d loss of moisture f r o m t h e landscape on the lee w a r d s i d e o f a m o u n t a i n. W a r m , moist air in onshore w i n d s loses m o s t o f its moisture as fain and snow that fall on the wi iIndward slopes of a mountain range. This leads to semiarid and arid conditions o n the leeward side of the mountain range and on the land beyond. a t m o s p h e r e. S o m e o f t h i s h e a t escapes i n t o space, b u t the f l o w of prevailing surface w i n d s and the m o v e m e n t of s o m e is a b s o r b e d b y m o l e c u l e s o f g r e e n h o u s e gases and storms. W h e n moist air f r o m an ocean b l o w s inland ang e m i t t e d i n t o t h e l o w e r a t m o s p h e r e as even l o n g e r - reaches a m o u n t a i n range, it is forced upward. As the air w a v e l e n g t h i n f r a r e d r a d i a t i o n (see F i g u r e 2.12, p. 4 8 ). rises, i t cools, expands, a n d loses most of its moisture as S o m e o f t h i s released e n e r g y radiates i n t o space, and rain a n d snow that fall o n the w i n d w a r d slope of the s o m e adds t o t h e w a r m i n g o f t h e l o w e r a t m o s p h e r e and mountain. t h e e a r t h ' s s u r f a c e. T o g e t h e r , these processes result i n a A s s h o w n i n Figure 5.12, w h e n the d r i e r air mass n a t u r a l w a r m i n g o f t h e t r o p o s p h e r e , called the g r e e n - passes over the m o u n t a i n t o p s , it flows d o w n t h e leeward h o u s e e f f e c t (see F i g u r e 3.3, p. 6 8 ). W i t h o u t t h i s n a t u r a l slopes (facing a w a y f r o m the w i n d ) and w a r m s up. This w a r m i n g effect, t h e e a r t h w o u l d be a v e r y cold a n d w a r m e r air can h o l d m o r e m o i s t u r e , b u t i t t y p i c a l l y does m o s t l y lifeless planet. n o t release m u c h o f it. This tends to d r y out plants and H u m a n a c t i v i t i e s such as t h e p r o d u c t i o n and b u r n i n g soil below. This process is called t h e r a i n s h a d o w e f f e c t. o f fossil fuels, c l e a r i n g o f forests, a n d g r o w i n g o f crops re- O v e r m a n y decades, it results i n semiarid o r arid condi- lease large q u a n t i t i e s of t h e g r e e n h o u s e gases carbon diox- tions on the leeward side of a h i g h m o u n t a i n range. ide, m e t h a n e , a n d n i t r o u s oxide i n t o the atmosphere. Sometimes this effect leads to the f o r m a t i o n of deserts A c c o r d i n g to a c o n s i d e r a b l e b o d y o f scientific evidence, w e such as Death Valley, a p a r t o f the M o j a v e Desert, which a r e e m i t t i n g g r e e n h o u s e gases i n t o the atmosphere faster lies w i t h i n the U.S. states of C a l i f o r n i a , Nevada, Utah, t h a n t h e y can be r e m o v e d b y the earth?s carbon a n d nitro- a n d Arizona. g e n cycles (see Figures 3.15, p. 78, a n d 3.16, p. 79). Cities also create distinct m i c r o c l i m a t e s based on C l i m a t e research and climate models indicate that these t h e i r w e a t h e r averaged o v e r three decades or more. emissions h a v e played a n i m p o r t a n t role i n w a r m i n g the Bricks, concrete, asphalt, and o t h e r b u i l d i n g materials e a r t h f o r almost 50 years a n d thus are helping t o change its absorb and hold heat, and b u i l d i n g s b l o c k w i n d. M o t o r c l i m a t e. I n o t h e r words, h u m a n activities are enhancing the vehicles and the heating and c o o l i n g systems of build- earth?s natural greenhouse effect. If the earth?s average atmo- ings release large quantities of heat a n d pollutants. A s a spheric t e m p e r a t u r e continues to rise as projected, this will result, cities on average t e n d t o have m o r e haze and a l t e r t e m p e r a t u r e a n d precipitation patterns, raise average smog, higher temperatures, a n d l o w e r w i n d speeds than sea levels, a n d shift areas w h e r e w e can g r o w crops and t h e s u r r o u n d i n g countryside. These factors m a k e cities w h e r e m a n y types o f plants a n d animals (including humans) heat islands, can live. W e discuss t h i s issue m o r e fully i n Chapter 20. T h e Earth?s S u r f a c e F e a t u r e s A f f e c t C H E C K P O I N T FOR U N D E R S T A N D I N G 5. 2 Local Climates 1. Identify the four factors that determine regional climates Various topographic features of the earth?s surface can around the world. create local c l i m a t i c c o n d i t i o n s that d i f f e r f r o m the general Ra Explain h o wa rain shadow desert forms. c l i m a t e i n s o m e regions. For e x a m p l e , m o u n t a i n s i n t e r r u p t 128 CHAPTERS CLIMATE AND TERRESTRIAL BIODIVERSITY Acres t o Hectares It is important in science to be able to convert between En glish and metric units, and also to move easily through the metric system. Rather than writing out ali the zeroes in ve ty large or very small numbers, it is often easier to express them using exponential nota- tion and to calculate values by simply addi ng and subtracting exponents. FRQ A p p l i c a t i o n To explore the relationship between acres and metric acres , or hectares, begin by using exponents to make conversions between ? given areas. C o n v e r s i o n Factors: 1 km? = 1,000,000 m?, which is a square with sides 1000 m long, or 10? meters long. 1 hectare (ha) = 10,000 m?, which is a square with sides 100 meters long, or 10? meters long. 1 ha = 2.477 acres 1 acre = 43,560 ft? 1000 m 10? m 1,000,000 6 2 1000 m sq m e t e r s 103 m l e m 1 square kilometer = 1000 m xX 1 0 0 0 m = 1 , 0 0 0 , 0 0 0 square meters OR 103 m x 103 m = 10° m2 H o w m a n y hectares in a s q u a r e k i l o m e t e r ? 1 ha 106 m? x = 10? ha, o r 100 hectares 10* m? Notice how the rules o f multiplying and dividing with exponents have been illustrated in the calculations above. The area of a square kilometer is 10? x 103 square meters = 103+? m2, or 10° m?. The area of a hectare is 10? x 102 square meters = 102+? m?, or 10* m?. The number of hectares is given by 106 m? + 104m? = 106-4, or 102 hectares. A hectare (ha) is a metric acre. Using dimensional analysis, calculate how many hectares there are in 62,500 square feet. a c?r e - ? | ZA7t hacres a =5 2 , 5 0 _ o0c gh1 ha a f t 2 xX ?Za?se0ft 62,500 ~ 107,593.2 By dividing 62,500 by (43,560 x 2.47), the result is 0.581 ha. Note in the dimensional analysis how the ft? and acre units cancel. 5.3 H O W DOES CLIMATE AFFECT Climate Helps to D e t e r m i n e W h e r e THE NATURE A N D LOCATION Terrestrial O r g a n i s m s Can Live OF BIOMES? Differences in climate (Figure 5.7) help to explain why. one area of the earth?s land surface is a desert, another a CONCEPT 5.3 Desert, grassland, and forest biomes can be grassland, and another a forest. Different climates based tropical, temperate, or cold depending on their climate and on long-term average annual precipitation and tempera- location. tures, global air circulation patterns, and ocean currents, lead to the formation of tropical (hot), temperate (moderate), 129  a n d p o l a r ( c o l d ) d e s e r t s , g r a s s l a n d s , a n d f o r e s t s , as s u m - T h e r e are also differences in v e g e t a t i o n a l o n g t h e t r a n s i t i o n yne o r e c o t o n e b e t w e e n a n y t w o d i f f e r e n t e c o s y s t e m s or m a r i z e d i n F i g u r e 5.13 ( C o n c e p t 5.3). ze F i g u r e 5. 1 4 s h o w s h o w scientists h a v e d i v i d e d t h e w o r l d biomes. i n t o b i o m e s ? t a r g e terrestrial regions, each characterized b y a c e r t a i n t y p e of c l i m a t e a n d d o m i n a n t f o r m s o f p l a n t l i f e. T h e v a r i e t y o f b i o m e s a n d a q u a t i c s y s t e m s is o n e o f t h e Three Types of Deserts f o u r c o m p o n e n t s o f t h e earth?s b i o d i v e r s i t y (see F i g u r e 4. 4 , In a desert, a n n u a lp r e c i p i t a t i o n is l o w a n d o f t e n scattered unevenly t h r o u g h o u t the year. D u r i n g the day, theb a k i n g p. 9 5 ) ? a v i t a l p a r t o f t h e earth?s n a t u r a l c a p i t a l. F i g u r e 4. 7 sun warms the g r o u n d and evaporates w a t e r f r o m plant (p. 9 6 ) s h o w s h o w m a j o r b i o m e s a l o n g t h e m i d s e c t i o n o f t h e U n i t e d States are r e l a t e d t o d i f f e r e n t c l i m a t e s. leaves and f r o m the soil. Atn i g h t. m o s t of the heat stored in the ground radiates q u i c k l y i n t o the atmosphere, This O n maps such as the o n e i n Figure 5.14, biomes are s h o w n w i t h sharp boundaries, and each b i o m e is covered explains w h y i n a desert, y o u m i g h t roast d u r i n g the day w i t h o n e general t y p e o f v e g e t a t i o n. See S u p p l e m e n t 5, but shiver at night. F i g u r e 2, p. $ 1 0 f o r a m a p s h o w i n g the v e g e t a t i o n features A c o m b i n a t i o n of l o w rainfall a n d v a r y i n g average o f d i f f e r e n t parts o f t h e earth. I n reality, biomes are n o t temperatures o v e r m a n y decades creates a variety of des- uniform. T h e y consist o f a variety of areas, each w i t h ert t y p e s ? t r o p i c a l , temperate, a n d cold (Figures 5.13 and s o m e w h a t d i f f e r e n t biological c o m m u n i t i e s b u t w i t h simi- 5.14 and Concept 5.3). Tropical deserts (Figure 3.15, top larities t y p i c a l of the b i o m e. These areas occur because of photo) such as the Sahara and the N a m i b of Africa are hot t h e i r r e g u l a r d i s t r i b u t i o n of the resources needed by and d r y most of t h e year (Figure 5.15, t o p g r a p h ). They p l a n t s a n d a n i m a l s a n d because h u m a n activities have have few plants and a hard, w i n d b l o w n surface strewn r e m o v e d o r altered t h e n a t u r a l vegetation i n m a n y areas. w i t h rocks and sand.. 1 3 Natural! capital: ital: Average precipitation ipitati and average tempera ture, acting together a r n i t i n g factors over a long time, help to determine the type of desert, grassland, or forest in any particular area, and thus the types of plants, animals, and decomposers found in that area (assuming it has not been disturbed by human activities). 1 3 0 @ CHAPTER 5 CLIMATE AND TERRESTRIAL BIODIVERSITY  Tropic o f Cancer ?Bn High mountains O Polar ice Arctic tundra (cold grassland) , Temperate grassland Tropic o f a a Tropical grassland (savanna) Capricorn i Chaparrat | Coniferous forest | | Temperate deciduous forest | Temperate rain forest Tropical rain forest Tropical dry forest Desert * FIGURE 5. 1 4 Natural capital: The earth's major biomes result primarily from differences in climate. In temperate deserts ( F i g u r e 5.15, center p h o t o ) , d a y - The lack of v e g e t a t i o n , especially i n t r o p i c a l a n d p o l a r time t e m p e r a t u r e s are h i g h i n s u m m e r a n d l o w i n w i n t e r deserts, also makes t h e m v u l n e r a b l e t o h e a v y w i n d e r o - and there is m o r e p r e c i p i t a t i o n t h a n i n tropical deserts s i o n f r o m sandstorms. (Figure 5.15, center g r a p h ). The sparse vegetation consists mostly of w i d e l y dispersed, d r o u g h t - r e s i s t a n t shrubs a n d cacti or o t h e r succulents adapted to the d r y conditions Three Types o f Grasslands and t e m p e r a t u r e v a r i a t i o n s. Grasslands occur p r i m a r i l y i n the i n t e r i o r s of c o n t i n e n t s i n In cold deserts such as the Gobi Desert i n M o n g o l i a , veg- areas that are t o o moist f o r deserts to f o r m a n d t o o d r y f o r etation is sparse (Figure 5.15, b o t t o m photo). W i n t e r s are forests to g r o w (Figures 5.13 and 5.14). Grasslandsp e r s i s t cold, summers are w a r m o r hot, and p r e c i p i t a t i o n is l o w because of a c o m b i n a t i o n of seasonal d r o u g h t , grazing by (Figure 5.15, b o t t o m g r a p h ). I n all types of deserts, plants large herbivores, and occasional f i r e s ? a l l o f w h i c h keep and animals h a v e evolved adaptations t h a t help t h e m to shrubs and trees f r o m g r o w i n g i n large n u m b e r s. T h e t h r e e Stay cool a n d to get e n o u g h w a t e r to s u r v i v e (Science m a i n types of g r a s s l a n d ? t r o p i c a l , temperate, a n d c o l d Focus 5.1), (arctic t u n d r a ) ? r e s u l t f r o m l o n g - t e r m c o m b i n a t i o n s of Desert ecosystems are vulnerable to disruption be- l o w average p r e c i p i t a t i o n a n d v a r y i n g average t e m p e r a - cause they have slow plant growth, low species diver- tures (Figure 5.16) ( C o n c e p t 5.3). sity, slow nutrient cycling, l o w bacterial activity in their One m a j o r type o f tropical grassland is savanna ( C o r e soils, and very little water. It can take decades to centu- Case S t u d y and Science Focus 5.2). It contains w i d e l y ties for their soils to recover from disturbances such as scattered clumps of trees a n d u s u a l l y has w a r m t e m p e r a - off-road v e h i c l e traffic, w h i c h can also d e s t r o y t h e h a b i - tures y e a r - r o u n d w i t h a l t e r n a t i n g d r y a n d w e t seasons lats f o r a variety of a n i m a l species t h a t live u n d e r g r o u n d. (Figure 5.16, top g r a p h ). Herds of grazing a n d b r o w s i n g 131  a Tropical d e s e r t i t Freezing p o i n t i =O J F M A M 3 $ A $ O N D Month Cold desert Freezing p o i n t F I G U R E 5. 1 5 These climate graphs track the typical variations in annual temperature (red) and precipitation (blue) in tropical (savanna), temperate, and cold deserts. Top photo: a tropical desert in Morocco. Center photo: a temperate desert in southeastern California,w i t h Saguaro cactus, a p r o m i n e n t species in this ecosystem. Bottom photo: a cold desert, Mongolia?s Gobi Desert. D a t a a n a l y s i s : Which month of t h e year has the highest temperature and which month has the lowest rainfall f o r each of t h e three types of deserts? 1 3 2 @ CHAPTER 5 C L I M A T E A N D TERRESTRIAL BIODIVERSITY  Staying Alive in the Desert wildflowers and grasses store much of Adaptations for survival in the desert their biomass in seeds that remain inac- have two themes: beat the heat and tive, sometimes for years, until they re- every drop o f water counts. ceive enough water to germinate. Shortly Desert plants have evolved a number after a rain, these seeds germinate, grow, of strategies based on such adaptations. and carpet some deserts with dazzling ar- During long hot and dry spells, plants rays of colorful flowers (Figure 5.A) that such as mesquite and creosote drop their leaves to survive in a dormant state. Suc- last for up to a few weeks. Most desert animals are small. Some culent (fleshy) plants such as the saguaro beat the heat by hiding in cool burrows or ("sah-WAH-ro") cactus (Figure 5.A and rocky crevices by day and coming out at Figure 5.15, middie photo) have no night or in the early morning. Othersb e - leaves that can lose water to the atmo- come dormant during periods of extreme sphere through transpiration. They also store water and synthesize food in their heat or drought. Some larger animals such as camels can drink massive quantitieso f expandable, fleshy tissue and they reduce water when it is available and store it in water loss by opening their pores only at their fat for use as needed. In addition, night to take up carbon dioxide (CO). the camels thick fur helps it keep cool be- The spines of these and many other des- FIGURE 5. A After a brief rain, cause the air spaces in the fur insulate the ert plants guard them from being eaten these wildflowers bloomed in this camel's skin against the outside heat. in by herbivores seeking the precious water temperate desert in Picacho Peak addition, camels do not sweat, which State Park in the U.S. state of Arizona. they hold. reduces their water loss through evapora- Some desert plants use deep roots to tion. Kangaroo rats never drink water. | tap into groundwater. Others such as insects get their water from dew or from They get the water they need by breaking prickly pear and saguaro cacti use widely the food they eat. down fats in seeds that they consume. spread shallow roots to collect water after brief showers and store it in their insects and reptiles such as rattle- spongy tissues. Some desert plants conserve water snakes have thick outer coverings to min- imize water loss through evaporation, T O and their wastes are dry feces and a dried What are three steps you w o u l d take to by having wax-coated leaves that re- concentrate of urine. Many spiders and survive in the open desert if you had to? duce water loss. Others such as annual availability. Savanna plants, like those i n deserts, are animals migrate across the savanna to find w a t e r and food adapted to s u r v i v e d r o u g h t and e x t r e m e heat. M a n y h a v e in response to seasonal and year-to-year variations i n rain- deep roots that tap into g r o u n d w a t e r. fall (Figure 5.16, blue areas i n top graph) and food Ina temperate grassland, w i n t e r s can be b i t t e r l y cold, s u m - CONSIDER THIS... mers are h o t and dry, and a n n u a l precipitation is sparse a n d CONNECTIONS Savanna Grassland Niches and falls unevenly throughout the year (Figure 5.16, center graph). Feeding Habits Because the aboveground parts of most of t h e grasses die As an example of differing niches, some large herbivores have and decompose each year, organic m a t t e r accumulates t o evolved specialized eating habits that minimize competition produce deep, fertile topsoil. This topsoil is h e l d i n place b y among species for the vegetation found on the savanna (Core a t h i c k n e t w o r k of the grasses? i n t e r t w i n e d roots unless t h e Case Study). For example, giraffes eat leaves and shoots from topsoil is plowed up, w h i c h exposes it to h i g h w i n d s f o u n d the tops of trees, elephants eat leaves and branches farther down, wildebeests prefer short grasses, and zebras graze on in these biomes. This biome?s grasses are adapted to d r o u g h t s longer grasses and stems. a n d to fires that b u m the p l a n t parts above the g r o u n d b u t do not h a r m the roots, f r o m w h i c h n e w grass can g r o w.  r a a a 1 Tropical grassland (savanna) ® ' > o n e a l eel oo * ane 2 O e CN eee a oe S e e [aeton nme > f e te f ~ a r n F O , R E e e , _ 5 ietmartarig =. i 2 1 :. sen ll ad oe t} ? , 1 F M A M JS J A S O N OD 1 Month Temperate grassland (prairie) 350 G z 9@ z 20 300 8 2 3 g 10 250 $ = = = ° 2 0 03 2-10 8 e x soe E 100 § § -30 2 2 50 3 40 ashes 2 ay 0 ; F M A M } J AS ON D 350 ~ z g g g 300 3 8 250 8 a A o g g Freezing point. \ | 200 9 2-10 a i -20 150 3 100 § 5 -30 3 40 7 3 F I G U R E 5. 1 6 These climate graphs track the typical variations in annual temperature (red) and precipitation (blue) in tropical, temperate, and cold (arctic tundra) grasslands. Top photo: savanna (tropical grassland) in Kenya, Africa, w i t h zebras grazing (Core Case Study). Center photo: prairie (temperate grassiand) in t h e U.S. state o f llinois. Bottom photo: arctic tundra (cold grasstand) in Iceland in fall. D a t a a n a l y s i s : Which month of the year has the highest temperature and which m o n t h has the lowest rainfall f o r each of the three types of grassland? CHAPTER 5 CLIMATE AND TERRESTRIAL BIODIVERSITY 134 FIGURE 5. 1 7 N a t u r a l c a p i t a l d e g r a d a t i o n : This intensively cultivated cropland is an example of the replacement o f b i o l o g i c a l l y diverse t e m p e r a t e grasslands (such as in t h e center p h o t o of Figure 5.16) w i t h a m o n o c u l t u r e crop. In the m i d w e s t e r n a n d w e s t e r n areas of the U n i t e d CONSIDER T H I S... States, w e find t w o types of t e m p e r a t e grasslands de- T H I N K I N G A B O U T Prairies pending p r i m a r i l y o n average r a i n f a l l : short-grass prai- Some people say t h e widespread destruction and degradation nes (Figure 5.16, center p h o t o ) and the tallgrass prairies o f prairies is justified because they believe t h a t these grasslands (which get m o r e rain). I n all prairies, w i n d s b l o w almost are being underutilized and should be p u t to use f o r humans. To others, the belief that prairies are n o t useful in their natural continuously and e v a p o r a t i o n is rapid, o f t e n leading to state reflects our lack o f understanding of h o w nature w o r k s fires in the s u m m e r and fall. This c o m b i n a t i o n of w i n d s and o f our separation f r o m the rest of nature. W h i c h v i e w do and fires helps to m a i n t a i n such grasslands by h i n d e r i n g you support? Why? tree growth. M a n y of t h e world?s n a t u r a l temperate grassl