Page 653 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 19 ecosystem essentials 617
Tertiary consumers
Secondary consumers
Primary consumers
Producer
Seals
Squid
Fish
Orca
Penguins
Baleen whales
Other seabirds
Humans
(a) Phytoplankton (bottom), the producers, use solar energy for photosynthesis. Krill and other herbivorous zooplankton eat the phytoplankton. Krill, in turn, are consumed by organisms at the next trophic level.
Nutrient cycling is continuous within a food web, aided by detritivores (also known as detritus feeders), the organisms that feed on detritus—dead organic de- bris (dead bodies, fallen leaves, and waste products) pro- duced by living organisms. Detritivores include worms, mites, termites, centipedes, snails, slugs, and crabs in terrestrial environments and bottom feeders in ma- rine environments. These organisms renew the entire system by breaking down these organic materials and releasing simple inorganic compounds and nutrients. Decomposers are primarily bacteria and fungi that di- gest organic debris outside their bodies and absorb and release nutrients in the process. The metabolic work of microbial decomposers produces the “rotting” action that breaks down detritus. Detritus feeders and decom- posers, although operating differently, have a similar function in an ecosystem.
In a food web, the organisms that feed on produc- ers are primary consumers. Because producers are al- ways plants, the primary consumer is a herbivore, or plant eater. A secondary consumer mainly eats primary consumers (herbivores) and is therefore a carnivore. A tertiary consumer eats primary and secondary con- sumers and is referred to as the “top carnivore” in the food chain; examples are the polar bear in the Arctic and the leopard seal and orca in Antarctica (discussed just ahead). Orca, an oceanic dolphin, feeds on fish, seals, penguins, and other whales and is found in both Arctic and Antarctic waters. A consumer that feeds on both producers (plants) and consumers (meat) is an omnivore—a category occupied by humans, among other animals.
the seal except for the bones and intestines, which are quickly eaten by scavenging birds (Figure 19.7c and d). Figure 19.25 at the end of the chapter shows several large Arctic mammals, and illustrates the effect of sea-ice losses on Arctic food webs.
In the Antarctic region, the food web begins with phytoplankton, the microscopic algae that harvest solar energy in photosynthesis (Figure 19.8). Herbivorous zoo- plankton, such as the shrimplike crustacean called krill (Euphausia), eat phytoplankton and are thus the primary consumers. Secondary consumers such as whales, fish, seabirds, seals, and squid eat krill, forming the next tro- phic level. Many Antarctic-dwelling seabirds depend on krill and on fish that eat krill. All of these organisms participate in other food chains, some as consumers and some being consumed.
Figure 19.9 shows an example of a temperate forest food web in eastern North America. Like the previous diagrams, the figure is simplified compared to the actual complexity of nature.
Energy Pyramids The overall amount of energy moving through trophic levels decreases from lower to higher lev- els, a pattern that can be illustrated in an energy pyramid in which horizontal bars represent each trophic level. (Ecological pyramids also include biomass pyramids, discussed ahead.)
At the bottom of the pyramid are the producers, which have the most energy and usually (but not always) the highest biomass and numbers of organisms. The next level, primary consumers, represents less energy because energy is used (by metabolism and given off as heat) as one organism eats another. Energy decreases again at the next
Sperm whales
Krill
(b) Gentoo penguins (Pygoscelis papua) on a rocky shore in Antarctica watch the water for leopard seal predators.
◀Figure 19.8 A simplified Antarctic Ocean food web. [Photo by Bobbé Christopherson.]
Several examples illustrate food webs. In the Arctic waters, bearded seals (Erignathus barbatus), with body fat often exceeding 30%, are consumers that feed on fish and clams (Figure 19.7b). In turn, the seal is preyed on by the polar bear (Ursus mari- timus), another marine mammal and the domi- nant Arctic predator. Polar bears consume most of
Krill is a shrimplike crustacean 7.5 cm long overall. “Superswarms” of krill, several kilometres wide, can weigh
up to 1.8 million tonnes.
Phytoplankton
Leopard seals
Carnivorous zooplankton
Factory ships
Herbivorous zooplankton