IEC12 09/11/2001 11:33 AM Page 197 Chapter 12 Ecosystems: the flux of energy and matter primary producers, the second trophic level contain- 12.1 Ecosystem ecology ing the primary consumers, the third trophic level con- Ecosystem ecology is the study of the interactions of taining the secondary consumers, and on to the top organisms with the transport and flow of energy and consumers in the system. The model is an oversim- matter. Ecologists who study ecosystems ask questions plification, but it serves to illustrate the flow of energy such as: What are the feeding relationships among the and matter in ecosystems (Fig. 12.1). organisms in an ecosystem? How many different types Energy flows one way in ecosystems, with energy of feeding relationships can be supported in a system? input from the sun being captured by primary pro- Why are some systems more productive than others? ducers, and large losses of energy between each trophic How much carbon and nitrogen are stored in the level, owing to respiration and inefficient energy trans- plants in an ecosystem? How rapidly do nutrients cycle fer (Fig. 12.1). through the living organisms in an ecosystem? How Because of losses to respiration, and inefficiencies in much of a particular nutrient is lost from the system harvesting, assimilation and digestion, the amount of each year? energy in any trophic level is not completely available to the next highest trophic level. This observation has been used to explain the limits to trophic levels in 12.2 The trophic–dynamic concept of ecosystems. Rarely are there more than four or five ecosystem structure trophic levels in an ecosystem, and the number of A conceptual framework developed to explain the individuals in top trophic levels is usually limited. dynamics of energy and matter flow in aquatic ecosys- Ecologists often illustrate this pattern through the use tems was developed in 1942 by Raymond Lindemann of pyramid plots, where the total amount of energy at the University of Minnesota. This model is called in each trophic level is plotted in a series of stacked the trophic–dynamic model, and it describes the rela- boxes, starting with the first trophic level on the bottom tionships between different organisms in an ecosystem (Fig. 12.2). A trophic pyramid constructed using energy by following feeding relationships among them. The is never inverted, consumers cannot use more energy organisms in an ecosystem are divided into different than is available in their food. However, pyramids con- trophic levels, with the first trophic level containing the structed with biomass can sometimes be inverted, and Fig. 12.1 A diagrammatic representation of the trophic–dynamic concept of ecosystem Respiration structure. Light energy is captured by plants and used to build their bodies. Much of this energy is lost to respiration, but a portion Light Producers Herbivores Carnivores is passed onto the next trophic level, energy represented by herbivores. A portion of the energy in herbivores is then passed on to carnivores. When individuals in any Decomposers trophic level die, they are broken down by decomposers, which recycle nutrients back into the system for uptake by other organisms. Respiration 197 .. IEC12 09/11/2001 11:33 AM Page 198 198 Chapter 12 3rd consumer ratio of the amount of plant production they ingest to 2nd consumer the total plant production. This ratio will be dependent on the plant life form. It could be as low as a few per Primary cent in northern evergreen forests, but as high as 60% in consumer African grasslands. Of the total ingested, not all will be digested. Plant material that is high in tannins, phen- olics, hemicelluloses or lignin will be harder to assimilate PRIMARY PRODUCER than tissues low in these compounds. Once assimilated, the conversion into consumer biomass will depend on the respiratory demands of the consumer. Thus, endothermic consumers spend a much greater propor- Fig. 12.2 A diagram of the amount of energy in four trophic levels tion of their assimilated energy on respiration than do of an idealized ecosystem. Each higher trophic level contains less ectothermic consumers. An endothermic animal eating energy due to losses to respiration within each level and inefficient low-quality plant food may have a net production transfer of energy between levels. efficiency of less than 1%, while an ectothermic carni- vore that eats a high-quality animal prey may have a those constructed using numbers of organisms are often value as high as 30%. invertedafor example, there are many insect herbi- vores on a single oak tree. 12.4 Nutrient cycling and decomposer trophic levels 12.3 Differences in efficiency of energy Detritus, or dead organic matter, is another major transfer among ecosystems energy pathway in most ecosystems. Breakdown of Once solar energy is used to fix atmospheric carbon detritus is a central process in the nutrient cycles of dioxide into plant material, there are differences among ecosystems. The productivity of open ocean habitats is ecosystems in its availability to higher trophic levels. often tied to the amount of decomposition and release This is because primary producers can take many dif- of nutrients by bacteria in the upper layers of the water, ferent forms, from algae to trees, and have differences before dead organisms can sink below the zone of light in the allocation of carbon to structures, which differ penetration. The turnover rate of detritus is often used in digestibility. For example, woody plants allocate as a measure of nutrient cycling, and forms the basis a significant proportion of their carbon to woody for the high productivity of some tropical communities stems. Wood is not easily digested by animals, usually found on nutrient-poor soils (Table 12.1). The turn- requiring that they have a symbiotic association with over rate will depend on the structure and chemical micro-organisms capable of digesting wood. In con- make-up of the detritus, temperature, moisture and trast, phytoplankton in aquatic systems can be largely decomposer community. Often, large arthropods are digested by zooplankton grazing on them. In addition, needed to break dead plant material (litter) apart, allow- in forest ecosystems, the age of a forest, or successional ing bacteria and fungi greater surface areas to attack stage, will affect the proportion of biomass allocated to the tissue. wood, leaves or other parts of the plants. During earlier As a rule of thumb, the pattern and rate of nutrient stages of succession, the community will have a higher movement are more important to ecosystem function number of herbaceous species, and allocation to leaf than are the absolute amounts of nutrients in the sys- material will be high. As a forest stand matures, a higher tem. Nutrient movement between plants, animals, percentage of carbon will be tied up in woody material, decomposers, detritus and soil will depend on the dif- and there is a rise in the respiration costs of supporting ferent storage pools of nutrients, and the turnover rates this tissue. of these pools. Nutrient storage pools include the living After allocation by plants, carbon and nutrients may organisms in an ecosystem, detritus and soil. Because be harvested by herbivores. The exploitation effici- decomposer activity is affected strongly by temperature ency of these herbivores will be determined by the and humidity, moist, warm areas generally have faster .. .. IEC12 09/11/2001 11:33 AM Page 199 Ecosystems 199 Table 12.1 The half-life (time taken for half the material to be broken down) and Litter production rate −2 −1 rate of production of litter for three different Community Half-life (years) (g m year ) forest communities. Tropical rainforest 0.1 1200 Temperate deciduous forest 1 800 Northern coniferous forest 7 600 turnover rates than colder, drier ecosystems. However, for the difference by observing that at every stage in the leaf structure (evergreen or deciduous), plant life form food chain, the transfer of energy is not completely (woody or herbaceous), and soil structure also influence efficient. For example, when the mice eat the grain, nutrient cycling. Leaf tissues that are high in lignin con- some of the energy contained in the wheat is dissipated tent may be hard to break down, and nutrients may not as heat (Fig. 12.2). be released from this pool very fast. Many evergreen trees keep their leaves for several years, maintaining 12.6 Productivity, species richness nutrients in their biomass and not releasing them to the and disturbance system. The soil pool is influenced by soil structural properties such as the proportion of clay, sand and silt Chapter 2 discussed the distribution and primary pro- particles. Water in sandy soils drains easily, leaching ductivity of the major biomes in the world. Product- nutrients away with it. High clay and organic contents ivity in different biomes ranged widely, from less than increase the water and nutrient-holding capacity of 50 g to over 3000 g of carbon per square metre per soil. The presence of peat bogs, coal swamps and fossil year. There is a broad correlation between product- fuel deposits shows that decomposition has lagged ivity and the availability of resources (Fig. 12.3), both behind productivity many times and in many ecosys- within ecosystems and among ecosystems. Generally, tems during the Earth’s history. The result is that much as resources such as light and nutrients increase, plant dead organic material has remained undecomposed for productivity increases. This, in turn supports greater thousands, or even millions, of years. productivity in higher trophic levels. The relationship between resource availability and species richness is not always positive. Studies from a 12.5 Food chains and food webs variety of ecosystems have shown a peak in species The path of energy and matter from one trophic level richness at intermediate levels of resource availability.