Consortium for Educational Communication Module on ENERGY FLOW IN AUTOTROPHIC AND DETRITUS BASED ECOSYSTEMS By Zahoor Ahmad Itoo M. Phil. Scholar Department of Botany Kashmir University Srinagar
TEXT 1.1. Physical laws govern energy flow in ecosystems The behaviour of energy in an ecosystem follows laws of thermodynamics. The first law of thermodynamics or the law of conservation of energy, states that energy may be transformed from one form into another but is neither created nor destroyed. Light for example, is a form of energy, it can be transformed into work, heat, or potential energy of food, depending on the situation, but none of it is destroyed. The first law of thermodynamics also states that energy cannot be created or destroyed but only transferred or transformed. Thus, we can potentially account for the transfer of energy through an ecosystem from its input as solar radiation to its release as heat from organisms. Plants and other photosynthetic organisms convert solar energy to chemical energy, but the total amount of energy does not change: the total amount of energy stored in organic molecules plus the amounts reflected and dissipated as heat must equal the total solar energy intercepted by the plant. The second law of thermodynamics may be stated in several ways including the following: (a) No process involving an energy transformation will spontaneously occur unless there is a degradation of energy from a concentrated form into a dispersed form, (b) because some energy is always dispersed into unavailable heat energy, no spontaneous transformation of energy (sunlight) into potential energy (protoplasm) is 100% efficient. Every ecosystem possess certain thermodynamic characteristics: can create and maintain a high state of internal order, or a condition of low entropy (a low amount of disorder). In the ecosystems order, a complex biomass structure is maintained by the total community respiration, which continuously pumps out disorder. Accordingly ecosystems are open, non-equilibrium thermodynamic systems that
continuously exchange energy and matter with the environment to decrease internal entropy but increase external entropy (thus conforming to the laws of thermodynamics). 1.2 ENERGY FLOW The behaviour of energy in an ecosystem is called energy flow. Energy flow is a fundamental property of ecosystems that links organisms with each other and to their environment. With respect to energy flow the ecosystems are open systems i.e., they are dependent on an external source of energy, which is the sun. Except for the deep sea hydro-thermal ecosystem, sun is the only source of energy for all ecosystems on earth. Of the incident solar radiation less than 50 per cent of it is photosynthetically active radiation (PAR). Only producers in an ecosystem have the ability to convert light energy into chemical energy and thus act as transducers or converters. Energy flows through an ecosystem in one direction, which is called the food chain. The producers contain the most energy; they are autotrophs and manufacture their own food. In a terrestrial ecosystem, major producers are herbaceous and woody plants. Likewise, primary producers in an aquatic ecosystem are various species like phytoplankton, algae and higher aquatic plants. All other organisms in an ecosystem depend on producers to meet their energy requirements, hence they are known as consumers. Consumers obtain energy by eating the producers, they are also known as heterotrophs. There may be several levels of consumers in an ecosystem, beginning with herbivores and then to the carnivores and omnivores. If they feed on the producers, (i. e., plants) they are called primary consumers, and if the animals eat other animals which in turn eat the plants they are called secondary consumers. Likewise, there are tertiary consumers also. Obviously the primary consumers
will be herbivores. Some common herbivores are insects, birds and mammals in terrestrial ecosystem and molluscs in aquatic ecosystem. The consumers that feed on these herbivores are carnivores, or more correctly primary carnivores (though secondary consumers). Those animals that depend on the primary carnivores for food are labeled secondary carnivores. Finally, the decomposers obtain energy from waste and dead organisms, e. g bacteria and fungi. So there is unidirectional flow of energy from the sun to producers and then to consumers, this is in accordance with the first law of Thermodynamics. Further, ecosystems are not exempt from the Second Law of thermodynamics. They need a constant supply of energy to synthesize the molecules they require, to counteract the universal tendency toward increasing disorderliness. Starting from the plants (or producers) food chains or rather webs are formed such that an animal feeds on a plant or on another animal and in turn is food for another. The chain or web is formed because of this interdependency. No energy that is trapped into an organism remains in it for ever. The energy trapped by the producer, hence, is either passed on to a consumer or to a decomposer when the organism dies. Energy transfer during these consumption events is not perfectly efficient. As no energy transfer occurs in an ecosystem unless there is loss of energy as heat. So in a food chain producers have maximum energy followed by primary consumers and secondary consumers and so on. Death of organism is the beginning of the detritus food chain. On average about 10 percent of net energy production at one trophic level is passed on to the next level. Processes that reduce the energy transferred between trophic levels include respiration, growth and reproduction, defecation, and non predatory death (organisms that die but are not eaten by consumers). The nutritional quality of material that is consumed
also influences how efficiently energy is transferred, because consumers can convert high-quality food sources into new living tissue more efficiently than low-quality food sources. The low rate of energy transfer between trophic levels makes decomposers generally more important than producers in terms of energy flow. Decomposers process large amounts of organic material and return nutrients to the ecosystem in inorganic forms, which are then taken up again by primary producers. Energy is not recycled during decomposition, but rather is released, mostly as heat. 1 Flow of energy in an ecosystem Fig. 1.2.1 Energy flow in an autotroph based ecosystems These ecosystems are characterized by a dependence on energy capture by photosynthetic autotrophs and secondarily by movement
of that captured energy through the system via herbivory and carnivory. Autotrophic ecosystems are directly dependant on an influx of solar radiation. The sun is the ultimate source of energy for these ecosystems. Living organisms can use energy in basically two forms: radiant or fixed. Radiant energy exists in the form of electromagnetic energy, such as light. Fixed energy is the potential chemical energy found in organic substances. This energy can be released through respiration. Producers utilize the radiant energy of sun which is transformed to chemical form, ATP during photosynthesis. These ecosystems depend on autotrophic energy capture and the movement of this captured energy to herbivores. Organisms that can take energy from inorganic sources and fix it into energy rich organic molecules are called autotrophs. Organisms that require fixed energy found in organic molecules for their survival are called heterotrophs. Heterotrophs who obtain their energy from living organisms are called consumers. Decomposers or detritivores are heterotrophs that obtain their energy either from dead organisms or from organic compounds dispersed in the environment. Energy flow pathway in Cedar Bog Lake The energy flow in this ecosystem was studied by Lindeman (1942) who reported that the total solar input was 118872 gcal/ cm 2 /year of which 118761 gcal/cm 2 /year remained unutilized. The autotrophs showed a gross production of 118872-118761= 111 gcal/cm 2 /year (i, e., 0.10%). From this energy 23gcal/cm 2 / year (21%) was consumed in respiration, 3.0 gcal/cm 2 /year in decomposition and about 70 gcal/cm 2 /year remained unutilized. The net primary production was therefore, 111 (23+ 0) = 88 gcal/cm 2 /year. Thus, the autotrophs transferred 17% of their net primary production to herbivores and accumulated about 79.5% of food energy. Out of 15 gcal/cm 2 /year the herbivores used 4.5
gcal/cm 2 /year (30%) in metabolic activities, 0.5 gcal/cm 2 /year in decomposition and 7% gcal/cm 2 /year remained unutilized. Only 3.0 gcal/cm 2 /year (28.6% of net production) was passed onto carnivores. Thus, carnivores used 60% (1.8 gcal/cm 2 /year) of energy in metabolic activities and 40% (1.2 gcal/cm 2 /year) remained unutilized. Thus, according to Lindman (1942) from gross primary production of 111 gcal/cm 2 /year by autotrophs, a total of 29.3 gcal/cm 2 /year was used in respiration, 3.5 gcal/ cm 2 /year in decomposition and 78.2 gcal/cm 2 /year remained unutilized. It may be noted that there was a progressive decrease in energy at each tropic level. Fig. 2 Energy flow diagram for Cedar Bog Lake, Minnesota (Energy in gcal/cm 2 /year) R. Lindeman 1942. From the energy flow diagram shown in (Fig. 2) two things become clear. Firstly, there is one way direction in which energy moves (unidirectional flow of energy). The energy that is captured by the autotrophs does not revert back to sun; that which passes to the herbivores does not pass back to the autotrophs. As it moves progressively through the various trophic levels it is no longer available to the previous level. Thus, due to one way flow of energy the system would collapse if the primary source, the sun, was cut off. Secondly, there occurs a progressive decrease in energy level at each trophic level. This is accounted largely by the energy dissipated as heat in metabolic activities and measured here as respiration coupled with unutilized energy. In Fig. 2 the boxes represent the trophic levels and the arrows depict the energy flow in and out at each level. Energy inflows balance outflows as is required by the first law of thermodynamics, and energy transfer is accompanied by dispersion of energy into unavailable heat (respiration) as required by the second law. Fig. 2 presents a very simplified energy flow model of three trophic levels, from
which it becomes evident that the energy flow is greatly reduced at each successive trophic level from producers to herbivores and then to carnivores. Thus, at each transfer of energy from one level to another, major part of energy is lost as heat or other form. There is a successive reduction in energy flow whether we consider it in terms of total flow (i, e., total energy input and total assimilation) or secondary production and respiration components. Thus, shorter the food chain greater would be the available food energy as with an increase in the length of food chain, there is a corresponding more loss of energy. 1.2.2 Energy flow in detritus-based ecosystems These ecosystems depend less on direct solar energy and more on the flux of dead organic material or detritus produced in this or other ecosystems. Indeed, some ecosystems, such as caves, are completely independent of direct solar energy and are instead completely energy dependant on the influx of detritus. Such ecosystems can be regarded as detritus based ecosystems. In other instances, sub-components of an ecosystem derive their energy entirely from that systems detritus through decomposition. Decomposition of organic material occurs in a variety of ways, among them leaching and fragmentation, but primarily by the activity of organisms that may, in turn, facilitate both leaching and fragmentation. The primary agents of the final stages of decomposition are microbes that act through the process of metabolism. Detritus can be broadly defined as any form of nonliving organic matter, including different types of plant tissue (e.g. leaf litter, dead wood, aquatic macrophytes, algae), animal tissue (carrion), dead microbes, faeces (manure, dung, faecal pellets, guano, frass), as well as products secreted, excreted or exuded from organisms (e.g. extra-cellular polymers, nectar, root exudates and leachates, dissolved organic matter, extra-cellular
matrix, mucilage). The relative importance of these forms of detritus, in terms of origin, size and chemical composition, varies across ecosystems (Moore et al. 2004). Detritus is a source of energy and nutrients to living organisms in most food webs. The amount of energy flowing through the detrital pathway can equal or exceed that of the grazing pathway (Heymans et al. 2002; Mulholland et al. 2002). Some food webs, such as those in caves, small streams in forested watersheds, and below-ground are based almost entirely on detritus pathway. For other food webs the detritus pathway can have strong influences on the structure and dynamics of the grazer pathway by providing energy that can sustain higher densities of consumers than would otherwise not be maintained if these consumers fed exclusively on energy derived from the grazer pathway (Moore et al. 2003). Energy flow in a temperate deciduous forest Gene Likens and F. Herbert Boreman have carried out extensive and long-term studies on the Hubbard Brook Experiment forest, a sugar maple, beech, and yellow birch forest in New Hampshire and the data obtained by them is as: Fig. 3 Fate of energy in c/m 2 /yr in the Hubbard Book Experimental forest (data from Gosz et al. 1978. Scientific American 238: 93-102) From above study it is clear that a substantial portion of energy (75%) from net primary production passes through detritus food chain and only 1% through grazing food chain. The sources of all the energy flowing through the detritus pathway is as follows: Leaves 83% Root death 12% Nonleaf litter fall 2% Organic matter via precipitation 2%
Fecal matter 0.9% Exuded by roots 0.1% Animal death traces Total 100% In this forest about 150 C (4%) of this detritus material was not consumed by detritus feeders (bacteria, fungi, many invertebrates) or transferred to carnivores (beetles, centipedes) or omnivores (salamanders, rodents, birds) and thus it accumulated annually on the forest floor.