RoseCh15_0104043_437-474_2p 11/18/04 2:32 PM Page 437 15 The feeding relationships between species can often be complicated. Communities and Ecosystems hen scientists first began studying bio- dioxide levels, which are covered in Chapter 16 logical communities, they were so fasci- (The Biosphere and the Physical Environment). Wnated with the interactions and The coordination and integration of biological dependencies between species that they saw the bi- communities has vast implications for the Earth. ological community as a superorganism. Whole For this reason, there are few biological topics as species were viewed as organs that performed spe- important for the future of life on Earth as the func- cific functions for the complete ecological superor- tioning of ecosystems. In this chapter, we survey ganism. The integration and communication how ecosystems function, from the flow of energy in between these “organs” was thought to be deliber- Module 15.1 (Energy Flow) and the recycling of nu- ate and well tuned. One way to think of this idea is trients in Module 15.15 (Ecosystems) to the porten- to imagine a stitched-together Frankenstein, each tous problem of the fragility of ecosystems. In sewn-on body part a distinct species. Modules 15.8 (Community Organization) and 15.4 Today biologists find the analogy between bio- (Equilibrium and Nonequilibrium Communities), logical communities and organisms superficial. To we consider the factors that determine the number be sure, there are populations within communities of species in a community. Surprisingly, in some that are highly dependent on each other. And it is communities predation and environmental distur- also true that biological communities and their bance may promote increased species diversity. Is- physical environments support all life on Earth by lands represent interesting communities, because such processes as recycling nutrients. The impact virtually all species on an island must travel there of this recycling can be profound. For instance, at- from some larger mainland. Species diversity on is- mospheric levels of carbon dioxide depend on lands is a consequence of dynamic processes. Under- plant photosynthesis and the respiration of all aer- standing these forces has important practical obic organisms. Global temperatures and weather applications for the design of ecological preserves, to are in turn dependent on atmospheric carbon be covered in Chapter 17 (Conservation). ❖ 437 RoseCh15_0104043_437-474_2p 11/18/04 2:32 PM Page 438 ENERGY FLOW 15.1 The flow of energy is a central organizing theme in community ecology We have all strolled through forests or walked along the seashore or lakeside. Even the untrained person will notice a variety of plants in a forest or the many insects and birds near lakes and oceans. These interacting plant and animal popula- tions are part of a biological community. The members of such a community will be apparent from their associations or their geographic location. As we have seen in the previous FIGURE 15.1A The Initial chapter, some plants and animals may interact very closely Stages of Succession in a and affect each other’s evolution. While the details of process- Temperate Forest . es such as coevolution were unknown to early ecologists, there was a strong sense that there was a mutual interdepend- thermodynamic perspective, emphasizing the transfer of en- ence among the members of a community. ergy. This thermodynamic perspective and the importance of food chains were both embraced by Raymond Lindeman in Communities Early in the twentieth century, F. E. 1942. For his Ph.D. thesis, Lindeman studied the feeding rela- Clements developed some of the first ideas about communi- tionships in a bog community in Minnesota. He simplified ties. If a tract of land is cleared but then left undisturbed, it the analysis of energy flow in this community by focusing on will be recolonized by plants over time. This recolonization, organisms that were at a similar position in the food chain. or succession, may follow a predictable pattern, with some Such positions are referred to as trophic levels. species appearing early in the sequence of recolonization, but In most ecosystems, the lowermost or first trophic level is later giving way to different species. (Figure 15.1A shows one made up of the primary producers or plants, such as the example of succession.) In studying ecological succession, mosses in Figure 15.1B. These organisms depend on sunlight Clements thought that the species that appeared during suc- for their energy. The next trophic level up consists of the her- cession made up a superorganism, with strong interdepen- bivores, organisms that consume plants, such as the herbivo- dencies much like the organs of a single plant or animal. We rous rotifers in Figure 15.1B. The biomass or energy available will cover succession in more detail in Module 15.7. to herbivores comes directly from the primary producers. It is In the 1920s, Charles Elton developed a more sophisticat- also affected by the efficiency of conversion of energy. Con- ed view of communities, one that still persists today. He stud- sumers of herbivores—such as the ducks in Figure 15.1B— ied a tundra community on Bear Island in the North Atlantic. are at the next trophic level, and so on. Elton’s focus was on feeding. Which species feeds on which is Lindeman noted that the dependence of each trophic level one of the most important interactions in an ecosystem. on the one below it suggests that the amount of energy con- Figure 15.1B shows some of the results from Elton’s study. tained in each level (for example, as plant or animal biomass) These feeding relationships also reveal a directional flow of should decline as one moves from the lower to the higher energy. Moss captures energy from the sun. Energy in the trophic levels. Lindeman called this natural progression the mosses is then consumed by herbivorous rotifers that are ul- Eltonian pyramid. In timately eaten by ducks. Diagrams that show energy flows are Modules 15.2 and 15.3, called food chains. Long-Tailed Duck we will study in more The nature of an ecological community is not solely a detail what is known function of the organisms that make up the community. The today about the energy physical environment also influences the numbers and types relationships within of organisms in a community. Likewise, photosynthesis, res- communities and the piration, and decomposition affect the physical environment. factors that affect ener- Rotifers In 1935 the English plant ecologist A. G. Tansley coined the gy transfer from one term ecosystem to describe ecological communities and their trophic level to the associated physical environment. In Module 15.15, we will next. ❖ discuss the interactions between biological communities and the physical environment. Moss Trophic Levels In 1925 A. J. Lotka published his book The Elements of Physical Biology. Influenced by his training in FIGURE 15.1B Some Feeding Relationships from Elton’s study of chemistry, Lotka advocated the study of communities from a Bear Island 438 Chapter 15 Communities and Ecosystems RoseCh15_0104043_437-474_2p 11/18/04 3:07 PM Page 439 Raymond Lindeman (1915–1942) During his short life and even shorter academic career, Lindeman managed to write six scientific papers. One of these appeared in the journal Ecology after his death, with the title,“The Trophic-Dynam- ic Aspect of Ecology.” This paper is credited with influencing many ecologists to look at the energy and feeding relationships among organisms as an important aspect of community structure. Linde- man received his Ph.D. from the University of Minnesota in 1941. Shortly afterward he moved to Yale University, where he began postdoctoral work with G. Evelyn Huchinson. The original version of Lindeman’s trophic-dynamic paper was rejected by the journal Ecology. It was only after an appeal by Hutchinson that the editor of Ecology, Thomas Park, agreed to publish the paper. FIGURE 15.1C Raymond Lindeman Energy Flow 439 RoseCh15_0104043_437-474_2p 11/18/04 2:32 PM Page 440 15.2 In most biological communities, all energy comes from the sun Biological systems are complicated, but they must follow the What causes this variation? The variation seen in same laws of thermodynamics that physical systems obey. Figure 15.2A is a function of environmental factors. Tun- The first law of thermodynamics tells us that energy can be dra has low primary productivity due to the short growing neither created nor destroyed. Energy can be changed, how- season near the Earth’s North Pole. On the other hand, se- ever, from one form into another. In biological communities, vere water shortage keeps the productivity of deserts low. almost all energy originates from the sun. Green plants cap- The open ocean has plenty of light near the surface and ture solar energy and turn it into chemical energy. Because of mostly benign temperatures, but nutrients such as phos- this special function, green plants are called primary produc- phorus are in short supply, limiting plant growth. Plants ers. The chemical energy is stored by plants as bonds holding and animals on the ocean surface die and settle to the bot- organic molecules together. Not all the captured energy from tom of the ocean, where their decomposition does not im- the sun is stored as chemical energy. Plants use some energy mediately return nutrients to the surface. In some parts of for metabolic maintenance, and some is lost as heat. the ocean, currents carry water from great depths up to the All trophic levels above plants gain energy by feeding on surface. These upwelling currents (see Module 11.4) are members of other trophic levels. Herbivores feed on plants and important for supplying the surface waters of the oceans derive their energy from the energy stored in plant tissue.