240 Energy Flow and Ecosystems As a result of these studies, it was recognized that recreational fishing can be managed by harvesting many of the larger fish and thus changing rates of algal growth and removing nutrients in both reservoirs and natural lakes As discussed later, this biomanipulation links fisheries management and lake management to control of nutrient cycling and plant growth Rainfall, runoff, and transport of sediments from the watershed (as well as inputs from wastewater treatment plants) can add various amounts of nutrients over time in lakes and wetlands Consequently, the rate of eutrophication varies from one watershed to another depending on land uses, slopes, and climate The concept of trophic dynamics and its focus on transfers of energy between trophic levels was not widely accepted until a new postwar influx of investigators began detailed studies of energetic and nutrient cycling Several questions have persisted for decades: How can energy flow regulate the number of trophic levels within an ecosystem? How does one group of consumers regulate the numbers of individuals and energy flow in other trophic levels? Which nutrients limit plant growth and how are consumers involved in recycling nutrients? These questions and many others were rapidly taken up by ecologists such as G Evelyn Hutchinson, Eugene Odum, and Howard Odum and their students in the 1940s and into the 1970s After considerable debate and continued developments of the concept, many ecologists today rely heavily on models and field experiments using modifications of Lindeman’s approach Some still have concerns about fundamental issues regarding how trophic levels are defined relative to the complexities of natural food webs As a result, there are various definitions of what constitutes a trophic level Some prefer to use a more general term, trophic position, to indicate relative feeding relationships but not measure transfer efficiencies across trophic levels Because most descriptive field-based studies of food webs really are studies of subwebs, and therefore are incomplete, a thorough test of predictions regarding food chain length or long-term stability relationships derived from trophic models is usually not feasible except in very simple food webs or in laboratory studies Conceptually, the predictions of how energy flow regulates trophic dynamics are relatively straightforward First, some of the initial energy entering the first trophic level is lost by reflectance from the plants, lost as heat, expended in metabolism and evapotranspiration, or lost because of a less than complete coverage of foliage (leaf area) or algal volume Thus, some warming occurs by energy absorption by the physical habitat (e.g., soil, rock, or water) Then, from constraints imposed by the second law of thermodynamics, energy is lost at each step in the flow of energy from the first trophic level to successively higher levels within food webs The use of solar energy or chemical energy by primary producers and the consumption of plants and animals at higher trophic levels are relatively inefficient because some energy is lost to metabolism and as heat at each transfer across trophic levels The total energy flow through the plant trophic level is termed gross primary productivity (GPP) Once the energetic costs of respiration are subtracted from GPP, the remaining energy is called net primary productivity (NPP) This NPP is usually a very small portion of the available solar energy that entered the first trophic level NPP is the only amount available for transfer to upper trophic levels In some ecosystems, there is an ‘‘energy subsidy’’ provided by inputs of organic matter from another ecosystem For example, leaf litter entering a stream, lake, or open cave can be an essential source of stored energy for use by detritivores in a different ecosystem than the forest ecosystem in which it was produced In all energy transfers between trophic levels, the assimilation of energy is variable but generally of low efficiency Typically, efficiencies (output:input ratios) are less than 10%, but higher values are known for some food webs The consistently low values that were first measured in ecological studies in the 1940s and 1950s led to the hypothesis that the number of trophic levels within any food web was determined primarily by the amount of incoming energy and the efficiency of energy transfers Food Webs and Trophic Levels Empirical studies demonstrate that most food webs contain fewer than four trophic levels However, the number of trophic levels is not a consistent measure because of the complexity of feeding relationships over time and space, the mobility of consumer species, and the movement of food resources across ecosystem boundaries Many species vary in how they obtain their energy and how efficient they are at different stages of their life histories and under different conditions Among consumer species, many rapidly growing juveniles or reproductive adults require high-quality, nutrient-rich foods These same individuals typically feed on lower protein foods when they become nonreproductive adults Numerous species are omnivores and feed on plants and animals from different trophic levels Because of these complexities, there has not been complete agreement on how to operationally define trophic levels The transformation of inorganic elements into organic matter requires energy to be converted into biomass by species of algae, green plants, and a few types of bacteria These microand macroautotrophs are often represented by many species The degree of similarity (niche overlap) in their abilities to produce and to store organic matter is important in predicting the consequences of any losses of species Many species have evolved into persistent assemblages that store carbon and nutrients such as nitrogen and phosphorus Energy stored in the form of plant-produced organic matter is later passed on directly to grazing species and then indirectly to predators within food webs Efficiency of energy transfer from one trophic level to the next higher level is of fundamental importance in understanding conceptually how different ecosystems function Measures of efficiency, however, are only a part of the explanation for why some ecosystems have longer food chain lengths than others Relatively ‘‘inefficient’’ food webs with few trophic levels appear to be adapted to certain types of frequent disturbances Ecologists realize that a single explanation or mechanism is unlikely to account for all the various complexities that exist in determining how ecosystems are organized in terms of energy flow However, comparisons among well-studied ecosystems and their numbers of trophic levels (food chain length) can provide a useful basis for predicting vulnerability of food webs to major disturbances and movement of toxins such as mercury and other heavy metals