Environmental Problems, Their Causes, and Sustainability 1

Figure 6-20 Natural capital:some components and interactionsin a temperate desertecosystem. When these organismsdie, decomposers breakdown their organic matter intominerals that plants use. Coloredarrows indicate transfers ofmatter and energy betweenproducers, primary consumers(herbivores), secondary, orhigher-level, consumers (carnivores),and decomposers. Organismsare not drawn to scale.Red-tailed hawkYuccaJackrabbitCollaredlizardGambel'squailAgavePricklypearcactussuch as mesquite and creosotedrop their leaves to survive ina dormant state. Succulent(fleshy) plants, such as thesaguaro (“sah-WAH-ro”) cactus,have three adaptations.They have no leaves, whichcan lose water by evapotranspiration.They store waterand synthesize food in theirexpandable, fleshy tissue.And they reduce water lossby opening their pores (stomata)to take up carbon dioxide(CO 2 ) only at night. Whatagreat evolutionary solutionto a difficult problem.Some desert plants usedeep roots to tap into groundwater.Others such as pricklyRoadrunnerDiamondback rattlesnakeProducerto primaryconsumerpear (Figure 6-20) and saguaro cacti use widely spread,shallow roots to collect water after brief showers andstore it in their spongy tissue.Evergreen plants conserve water by having waxcoatedleaves that minimize evapotranspiration. Others,such as annual wildflowers and grasses, storemuch of their biomass in seeds that remain inactive,sometimes for years, until they receive enough waterto germinate. Shortly after a rain these seeds germinate,grow, carpet some deserts with a dazzling arrayof colorful flowers, produce new seed, and die, all inonly a few weeks.Most desert animals are small. Some beat the heatby hiding in cool burrows or rocky crevices by day andcoming out at night or in the early morning. Others becomedormant during periods of extreme heat ordrought.Kangaroo ratPrimaryto secondaryconsumerDarklingbeetleSecondary tohigher-levelconsumerFungiBacteriaAll producers andconsumers todecomposersSome desert animals have physical adaptationsfor conserving water. Insects and reptiles have thickouter coverings to minimize water loss through evaporation,and their wastes are dry feces and a dried concentrateof urine. Many spiders and insects get theirwater from dew or from the food they eat. Arabianoryxes survive by licking the dew that accumulates atnight on rocks and on one another’s hair.Figure 6-21 shows major human impacts ondeserts. What are the direct or indirect effects of yourlifestyle on desert biomes?Deserts take a long time to recover from disturbancesbecause of their slow plant growth, low speciesdiversity, slow nutrient cycling (because of little bacterialactivity in their soils), and lack of water. Desertvegetation destroyed by livestock overgrazing and offroadvehicles may take decades to grow back.114 CHAPTER 6 Climate and Terrestrial Biodiversity