Page 654 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

618 part IV Soils, ecosystems, and Biomes
White oak
Caterpillar and other insects on leaves
Tree (grey) squirrel
Spider
Ticks
Cottontail rabbit
Blackberry
Gopher snake
Cedar Waxwing
Yellow- bellied Sapsucker
Beetles and other insects
Mule deer
▲Figure 19.9 Temperate forest food web. Based on the text discussion, can you find the primary producers and then locate the primary, secondary, and tertiary consumers in this web? What role do the earthworms and bacteria play?
level (secondary consumers), with each trophic level hav- ing (usually) less biomass and fewer organisms than the one beneath (Figure 19.10a). Although the pattern gener- ally holds for numbers of organisms and biomass, excep- tions exist and pyramids can be inverted, such as when the number of large trees (producers) is less than the num- ber of small insects (primary consumers) or when the bio- mass of phytoplankton (which have short life spans) is less than that of the zooplankton that eat them. Only energy consistently decreases between lower and higher trophic levels, maintaining the true pyramid shape.
Food Web Efficiency In terms of energy, only about 10% of the kilocalories (food calories, not heat calories) in plant matter are passed from primary producers to pri- mary consumers. In turn, only about 10% of the energy for primary consumers is passed to secondary consum- ers, and so on. Thus, the most efficient consumption of resources happens at the bottom of the food chain where
plant biomass is higher and the energy input toward food production is lowest.
This concept applies to human eating habits and, on a broader scale, to world food resources. If humans take the role of herbivores, or primary consumers, they eat food with the highest energy available in the food chain. If hu- mans take the role of carnivores, or secondary consumers, they eat food in which the available energy has been cut by 90% (the grain is fed first to cattle, then the cattle are con- sumed by humans). In terms of biomass, 810 kg of grain is reduced to 81 kg of meat. In terms of the numbers of organ- isms, if 1000 people can be fed as primary consumers, only 100 people can be fed as secondary consumers. By the lat- ter analysis, far more people can be fed from the same land area producing grain than producing meat (Figure 19.10b).
Today, approximately half of the cultivated acreage in Canada and the United States is planted for animal consumption—beef and dairy cattle, hogs, chickens, and turkeys. Much of U.S. grain production goes to
Shelf fungus
Deer mouse
Toad
Grasses
Earthworm
Soil bacteria