COMMUNITY ECOLOGY An ecological community consists of all species inhabiting a specified area, which is often arbitrarily delineated. As mentioned before under our discussion of interspecies interactions, it is impossible to study the interactions among all species which share the same community - at least in terms of how each species influences the population growth of the other species in the community. The study of energy flow through ecosystems is an important aspect of community ecology because it is largely restricted to the living components of an ecosystem. Nevertheless, the source of biologically useful energy is abiotic - derived either from solar radiation or inorganic chemicals. This latter energy source drives those newly discovered ecosystems at the depths of the ocean which do not receive any sunlight. Life, being a process, depends upon energy and so the one attribute shared by all species in a community is their need for energy. Each species obtains its energy from other species (except for plants) and provides a source of energy to other species; in short, each species eats and in turn is eaten by another. This interdependence among species is illustrated by a simple food chain: grass - grasshopper - shrew - snake - hawk. Food chains, however, are generalizations of a more complex feeding structure known as a food web. Grasshoppers are not restricted to feeding on grass nor do shrews only consume grasshoppers. Food chains and food webs, however, are species-specific and therefore vary from community to community. A general pattern of community structure emerges when the components of these food relationships are abstracted in the form of trophic levels which are free of the specific details of individual communities. Trophic Structure of a Community Each species can be assigned to one of the trophic levels listed below based on its food supply in the community. Trophic levels are abstractions which group together all species in a food web that occupy the same position in the food chain. Species which can feed at different levels are classified in the level they occupy most frequently, e. g., omnivores would be considered herbivores or carnivores (but not both) depending upon their most frequent food preference. Producers: The first link in the food chain upon which the entire community depends for its energy. Producers fix energy from nonliving sources and make it available to all other species. For the most part producers are plants which tap the sun's energy to produce energy-rich organic molecules through the process of photosynthesis. Since these species do not depend on other species for food they are also called autotrophs. All other species ultimately obtain their energy from the autotrophs and they are collectively referred to as heterotrophs. Primary consumers (herbivores): These are animals which feed exclusively on plants. Herbivores, in turn, are eaten by carnivores (flesh-eating animals). Secondary consumers (primary carnivores): Animal species which feed exclusively on herbivores are called primary carni- vores. Note that a primary carnivore is a secondary consumer. Tertiary consumers (secondary carnivores): These animals feed on carnivores but are themselves carnivores. Quaternary consumers (tertiary carnivores): The final trophic level whose energy is not available to any other carnivore. The energy and nutrients they contain are used by decomposers, e. g. , bacteria which start a second type of food chain - the detrital or decomposer food chain. Note the following aspects of the trophic description of a community: 1. Trophic levels are abstractions of generalized food chains, which simplify the actual, more complex feeding relationships described in a food web. 2. In trophic analysis individual species lose their identity and are included in groups which have a common feeding pattern. 3. There are two types of food chains: grazing and decomposer. The grazing food chain starts with autotrophs; the decomposer food chain begins with heterotrophs. Furthermore, the distinction between producer and consumer (autotroph and heterotroph) is not the same as between producer and carnivore, since some heterotrophs (herbivores) are not carnivores. 4. There are only five different trophic levels in any community. The top carnivores (teritary carnivores) constitute the highest level and they are "consumed" by decomposers which comprise a separate food chain. The reason why there are only five levels in the grazing food chain will be described below. Trophic Dynamic Approach to Community Structure. Trophic level analysis provides an opportunity to compare different communities because this type of analysis is free of the specific differences in species composition and vegetation type, e. g., grassland vs. forest, which confound comparisons. Various attempts have been made to find a trophic pattern which is general enough to apply to all communities and which would shed some light on the most significant population processes that shape community structure. These attempts are described below. Comparison of trophic levels: pyramid of numbers One of the first attempts to analyze trophic relationships and arrive at a generalized picture of community structure involved relating the different trophic levels to the number of individuals found in each level. The study of fresh-water aquatic systems, e. g., lakes and ponds, whose producers are tiny plants called plankton, revealed a pyramid of numbers with the number of individuals per trophic level decreasing with each successive level. Although this relationship applied well to aquatic communities, it did not provide an accurate description of terrestrial communities. The reason why a forest ecosystem does not follow the pyramid of numbers at the producer level is that a single tree can support a large number of insect herbivores. Since the tree is much larger in size than the hervibores that feed on it, terrestrial ecologists suggested that number of individuals per trophic level is not the important parameter - rather size or mass is. Comparison of trophic levels: pyramid of biomass When a terrestrial, or more precisely, a forest ecosystem is analyzed in terms of the mass of its trophic levels (measured in grams per meter squared or g/m2), the anomalous position of the producer level is corrected. This measure of trophic structure, however, is not without its difficulty as was illustrated by samples taken from the English Channel in which the mass of the producers was less than that of the herbivores which grazed on them. In this marine ecosystem the producers are called phytoplankton and the herbivores are called zooplankton. How is it possible for a greater mass of zooplankton to be supported by a smaller mass of phytoplankton? The answer is that the samples were taken over a very short period of time and so only represented the standing crop of both trophic levels, i. e., the relative mass of both types of plankton at any given time. What the standing crop relationship does not consider is the reproductive differential between the two levels and this can only be determined by following both levels over a more extensive period of time, e. g., a year. Phytoplankton has a higher rate of reproduction than does zooplankton and this consideration gave rise to our final pyramid - one based on annual rate of production of organic matter. Comparison of trophic levels: pyramid of productivity Altough productivity could be measured in terms of biomass (g/m2), what the trophic relationship is really concerned with is the passage of energy from one level to another. Biomass is only a crude estimate of energy since some portions of an organism's mass cannot be converted into energy by another organism's digestive apparatus. Consider the difference between 10 grams of earthworms and 10 grams of clams. Since most of the earthworm is digestible, the mass of the clam's shell is not equivalent to an equal amount of worm tissue. Lest you think this example too gross, consider the shopper's dilemma when attempting to relate price per pound and quantity of meat obtained in comparing lobster meat and a whole lobster. The same principle applies. The appropriate unit of measure for energy is the calorie or (kilocalorie) which is the amount of heat needed to raise the temperature of a gram of water one degree centigrade (or in the case of a kcal the heat that must raise 1000 grams of water one degree centigrade). The calorie is used as a measure of energy because all energy can be converted to heat. Since productivity is the rate of producton of energy, the unit of measure is kcal/m2/yr. With this measure, the relationship between the phytoplankton and zooplankton is reversed with the phytoplankton having a larger base than the zooplankton. Consequently, the pyramid of productivity provides a general pattern common to all communities regardless of their species composition, whether they are aquatic or terrestrial, or whether they are fresh-water or marine. Why is this pattern so universal? To answer that question we will first have to examine what happens to the sun's energy as it is used to support the producers in a grazing food chain. Not all of the sunlight that strikes the planet is used by plants and only a small fraction of that which strikes the plants themselves is actually converted into biologically useful energy through photosynthesis. The plants use this energy for maintenance, growth and reproduction and the total amount of energy produced by plants is called their gross primary production. Obviously, the metabolic processes mentioned above require energy and so plants have to expend energy to maintain themselves, to grow and to reproduce. This energy is lost from the trophic level as heat in the form of respiration. Due to respiration, energy is not cycled through an ecosystem: once it is used, it is dissipated as heat and cannot be used again. For this reason energy is said to flow through the system. The part of the plant's energy production which is available to the next trophic level is that which remains of the gross primary production after respiration, i. e., the plant's net primary production. Even so, not all of this is passed on, otherwise the entire trophic level would cease to exist. A major portion of this net primary production goes into maintaining a standing crop and the rest is passed on to decomposers or herbivores. Ecological Efficiency The transfer of energy from one trophic level to the next can be measured in terms of its efficiency in the same way one might measure the efficiency of a machine. The efficiency of energy transfer is called ecological efficiency and it is measured through the following formula: % ecological efficiency = Output (or Yield)/Input x 100 Estimates of ecological efficiency usually range from 10% - 20% and so are quite low. This low efficiency of energy transfer between trophic levels accounts for the pyramid of productivity and also the pyramid of numbers. Since only about 10% (the value used as a generalization) of the net primary production of producers is passed on to herbivores, this trophic level has a much smaller energy base upon which to operate and so it can support fewer individuals. The same argument applies to all other trophic levels as well. The reason that there are only five links to a food chain and five trophic levels is that there simply would not be sufficient energy to support a single population of quaternary carnivores, especially if they had to be larger than the tertiary carnivores in order to overcome them. Thus, larger individual size and less energy available combine to limit the number of trophic levels to five. One of the concerns of community ecologists is to explain why there are so many kinds of living organisms. Why don't we have more species? Why not fewer? The trophic dynamic approach to community and ecosytem ecology has furnished us with at least a glimmer of an answer. The low transfer of energy between successive trophic levels places a severe constraint on the kinds of animals as regards their classification into trophic levels. Ecological efficiency will simply not allow the evolution of quaternary carnivores. Species Diversity and Community Structure Trophic analysis has nothing to say about what limits the kinds of animals and plants within a trophic level even though it can explain why there are just five different trophic levels. Some ecologists have suggested that within each trophic level there is a limit as to how far the resources which support that level can be partitioned. Hence, they argue that the kinds of species within trophic levels is determined by interspecific competition. This argument, however, ignores the possible role of predation in reducing the density of potential competitors so that they might coexist. As we mentioned before, attempts to determine the most significant interspecies interactions by analyzing species aggregates are short-cut methods which most likely won't be useful until we have more information on controls at the population level.