Environmental Problems, Their Causes, and Sustainability 1

Natural Capital DegradationLarge desert citiesSoil destruction by off-roadvehicles and urbandevelopmentDesertsSoil salinization from irrigationDepletion of undergroundwater suppliesLand disturbance and pollutionfrom mineral extractionStorage of toxic andradioactive wastesLarge arrays of solar cells andsolar collectors used toproduce electricityFigure 6-21 Natural capital degradation: major human impactson the world’s deserts.6-5 GRASSLAND, TUNDRA, ANDCHAPARRAL BIOMESWhat Are the Major Types of Grasslands?Hot, Mild, and ColdGrasslands have enough precipitation to supportgrasses but not enough to support large stands oftrees and are found in tropical, temperate, and polarregions.Grasslands, or prairies, are regions with enough averageannual precipitation to support grasses (and insome areas, a few trees). Most grasslands are found inthe interiors of continents (Figure 6-16).Grasslands persist because of a combination ofseasonal drought, grazing by large herbivores, and occasionalfires—all of which keep large numbers ofshrubs and trees from growing. The three main typesof grasslands—tropical, temperate, and polar (tundra)—resultfrom combinations of low average precipitationand various average temperatures (Figures 6-17and 6-22). Take a close look at the graphs in Figure 6-22.What Are Tropical Grasslands and Savannas?Hot with On-and-Off RainSavannas are hot places that have rain, exceptduring dry seasons, and enormous herds of hoofedanimals occupying different ecologicalniches.One type of tropical grassland, called a savanna,usually has warm temperatures year-round, two prolongeddry seasons, and abundant rain the rest of theyear (Figure 6-22, left). African tropical savannas containenormous herds of grazing (grass- and herbeating)and browsing (twig- and leaf-nibbling) hoofedanimals that feed on a wide variety of savanna plants,as shown in Figure 6-23 (p. 116). In this diagram, notethe number of different niches that allow these animalsto coexist.As part of their niches, these and other large herbivoreshave evolved specialized eating habits that minimizecompetition between species for vegetation. Forexample, giraffes eat leaves and shoots from the tops oftrees, elephants eat leaves and branches further down,Thompson’s gazelles and wildebeests prefer shortgrass, and zebras graze on longer grass and stems.Mean monthly temperature°C °F in. mm100 153537530 9014 35025 8013 32520 7012 30015 6011 27510 5010 2505 409 2250 308 200–5 32°F Freezing point207 175–10–15106 150–20 05 125–25 –104 100–30 –203 75–35–40–45–30–40–5021050250JanFebMarAprMayJunJulAugSepOctNovDecMonthsTropical grassland (savanna)(Harare, Zimbabwe)Mean monthly precipitationMean monthly temperature°C °F1003530 9025 8020 7015 6010 505 400 30–5 20–10–1510–20 0–25 –10–30 –20–35 –30–40 –40–45 –5032°F Freezing pointJanFebMarAprMayJunJulAugSepOctNovDecMonthsTemperate grassland(Lawrence, Kansas)in.1514131211109876543210mm3753503253002752502252001751501251007550250Mean monthly precipitationMean monthly temperature°C °F10035309025 8020 7015 6010 505 400 30–5 20–10–1510–20 0–25 –10–30 –20–35 –30–40 –40–45 –5032°FFreezing pointJanFebMarAprMayJunJulAugSepOctNovDecMonthsin.1514131211109876543210Polar grassland (arctic tundra)(Fort Yukon, Alaska)mm3753503253002752502252001751501251007550250Mean monthly precipitationFigure 6-22 Climate graphs showing typical variations in annual temperature (the graphed line) and precipitation(the shaded area on the graph) in tropical, temperate, and polar (arctic tundra) grasslands.http://biology.brookscole.com/miller14115