Environmental Problems, Their Causes, and Sustainability 1

Figure 4-22 Satellite data on the earth’s gross primary productivity in terms of ocean and land concentrationsof chlorophyll in producer organisms during the winter of 2004. On land rain forests and other highly productiveareas are dark green and the least productive (mostly deserts) are brown or white in polar areas. At sea,the concentration of chlorophyll found in phytoplankton, a primary indicator of ocean productivity, ranges fromred (highest) to orange, yellow, green, light blue, and dark blue (lowest). Where are the areas of highest andlowest productivity on land and at sea? (Image data from SeaWiFs Project/NASA and by kind permission ofORBIMAGE. All rights reserved.)It is tempting to conclude from Figure 4-24 that agood way to feed the world’s hungry millions wouldbe to harvest plants in highly productive estuaries,swamps, and marshes. But people cannot eat mostplants in these areas. In addition, these plants providevital food sources (and spawning areas) for fish,shrimp, and other aquatic species that provide us andother consumers with protein.We might also conclude from Figure 4-24 that wecould grow more food for human consumption byclearing highly productive tropical rain forests andplanting food crops. According to most ecologists, thisis also a bad idea. Here is why. In tropical rain forestsmost nutrients are stored in the vegetation rather thanin the soil, where they need to be to grow crops. Whenthe trees are removed, frequent rains and growing cropsrapidly deplete the nutrient-poor soils. Thus crops canbe grown for only a short time without massive and expensiveapplications of commercial fertilizers.Are the oceans a way out? Because the earth’s vastopen oceans provide the largest percentage of theearth’s net primary productivity, why not harvest itsprimary producers (floating and drifting phytoplankton)to help feed the rapidly growing human population?The problem is that harvesting the widelySunPhotosynthesisGross primaryproductionRespirationGrowth and reproductionEnergy lost andunavailable toconsumersNet primaryproduction(energyavailable toconsumers)Figure 4-23 Distinction between gross primary productivity andnet primary productivity. A plant uses some of its gross primaryproductivity to survive through respiration. The remaining energyis available to consumers.http://biology.brookscole.com/miller1471