Paper No. : 12 Module : 29 Ecosystem: Ecosystem Processes-I (Part-I) Development Team Principal Investigator: Co-Principal Investigator: Paper Coordinator: Content Writer: Content Reviewer: Prof. Neeta Sehgal Head, Department of Zoology, University of Delhi Prof. D.K. Singh Department of Zoology, University of Delhi Prof. D.K. Singh Department of Zoology, University of Delhi Dr. Kapinder Kirori Mal College, University of Delhi Prof. K.S. Rao Department of Botany, University of Delhi 1
Description of Module Subject Name Paper Name Module Name/Title Module Id Keywords Ecosystem 29: Ecosystem, natural ecosystem, Artificial ecosystem, Food chain, food web, Ecological pyramids. Contents 1. Learning Outcomes 2. Introduction 3. Types of ecosystem 3.1. Natural ecosystem 3.1.1. Terrestrial ecosystem 3.1.2. Aquatic ecosystem 3.2. Artificial ecosystem 3.3. Microecosystem 4. Food chain 4.1. Types of food chain 4.1.1. Grazing food chain 4.1.2. Detritus food chain 5. Food web 6. Significance of food chain and food web 7. Ecological pyramids 7.1. Pyramids of number 7.2. Pyramids of biomass 7.3. Pyramids of energy 8. Summary 2
1. Learning Outcomes After studying this module, you shall be able to Understand the concept of Ecosystem. List the major types of ecosystem, their structure and function. Understand various types of food chains present in nature. Understand structure and function of food web. Describe the ecological pyramids and their types. 2. Introduction An organism in the nature always associated with individuals of its own species and of different species (community) along with their physical and chemical environment (habitat). Thus, there is an uninterrupted interaction among organisms and between organisms along with their habitat resulting into an integrated and functional unit. These organisms interact with the natural environment within 20 km range over earth surface of earth to make an ecosystem. In ecosystem, all biotic and abiotic component of the environment connected through energy flow and nutrient cycle. The term ecosystem was coined by Sir Arthur G. Tansley (1935). He defined ecosystem as a system resulting from the integration of all living and non living factors of the environment. It involves energy and cycling of nutrients between biotic and abiotic components. In 1935, E. P. Odum explains the ecosystem as a basic functional unit which consists of living and non living factors integrating each other and is necessary for the development and maintenance of the system. In 1975, he further elaborate the term ecosystem as Any unit which includes all living organisms in the given area interacting with abiotic component to maintain the flow of energy that leads to clearly defined trophic structure, biotic diversity and exchange of materials in living and non living component within the system. In the simplest term, ecosystem is the assemblages of organisms and their associated environment which collectively functions as a system to exchange material and energy (Reichle, 1975). Some European scientists use the alternative term of ecosystem known as biogeocoenosis. The ecosystem in the ecological hierarchy is the first unit that is complete. It bears all biological and physical components that are essential for the survival. Thus, the Ecosystem is 3
inimitable to ecology as sustained life on earth is a characteristic of ecosystem not for the individual or population. 3. Types of ecosystem An ecosystem can be natural or artificial, temporary or permanent and large or tiny. Thus, ecosystems can be categorized into following categories: 3.1. Natural ecosystems These ecosystems operate and develop in natural ways with no major interference by humans. They are further classified on the basis of kind of habitats as: 3.1.1. Terrestrial ecosystems It consists of Grassland ecosystem (dominated by grasses and cover about 20% of land surface), Savana Ecosystem (grassland with low frequency of trees), Taiga ecosystem (sub alpine ecosystem with low or medium sub rainfall having acidic soil), Tundra ecosystem (occur in alpine zone between taiga and polar ice zone, dominated by lichens, mosses, herbs, grasses and low shrubs), Desert ecosystem (ecosystem with very low rain fall, in which warm desert occurs in tropical region and cold desert occurs in temperate zone) and forests ecosystem (dominated by trees and found in tropical and temperate zone) (figure 1). Figure.1. Terrestrial forest ecosystem 4
3.1.2. Aquatic ecosystems It is further divided into two types: (a) Fresh water ecosystems: This type of ecosystem is further categorized as lotic (ecosystem of running water as stream or river) ecosystem or lentic (ecosystem of still water such as lake or pond) ecosystem (figure 2). Figure.2: Freshwater ecosystem of still water such as lake ecosystem. (b) Marine ecosystems. These ecosystems include shallow water ecosystem (ecosystem along coastal line tidal zone), estuarine ecosystem (ecosystem of river mouth) and deep sea ecosystem (ecosystem beyond coast line) (figure 3). Figure.3: Marine ecosystem 5
3.2. Artificial ecosystems These are also known as man-made ecosystems (figure 4). In 1959, Odum classified artificial ecosystem on the basis of photosynthesis respiration ratio (P/R) into three types. a) Stabilized ecosystem: This type of ecosystem is formed when photosynthesis /Respiration ratio is almost one. b) Autotrophic ecosystem: This type of ecosystem is formed when photosynthesis /Respiration ratio is more than one. c) Heterotrophic ecosystem: This type of ecosystem is formed when photosynthesis /Respiration ratio is less than one. Figure.4: Artificial ecosystem (Crop land) These ecosystems are regularly manipulated by humans in which addition of energy and natural balance is disturbed at particular period of time, e.g., croplands such as rice fields, sugarcane, wheat fields, maize, orchards, gardens, cities, villages, aquarium and many more. 3.3. Microecosystems The natural ecosystems are generally larger in size in which large number of factors operates at particular time leading to great complexity. So, it is not very easy to study them with the normal scientific methods. Therefore, ecologists are continuously working on different methods to reduce the number of variables and to work in a system with a discrete boundary. 6
Ecologists are trying to simulate microecosystems in the laboratory conditions that can be replicated and manipulated when they want. These microecosystems can be build by taking one or few species, at a time, from axenic cultures (An axenic culture is a bacterial culture which consists of single species) and then studying in different desired combinations. Recently, the concept of polyaxenic cultures has been developed. Odum (1971) and others have elucidated the microecosystem concept. The natural ecosystems do not have separate boundaries. An ecosystem progressively combines with the adjacent one through a transitional zone called as ecotone (figure 5). Forest Ecosystem Ecotone Grassland Ecosystem Figure.5A: Example of ecotone where forest ecosystem progressively combines with the adjacent grassland ecosystem. The ecotone consists of species present in two adjacent ecosystems. Generally, the ecotone has other species which is not present in both ecosystems. The ecotone has more population density of certain species as compare to either of the adjoining communities. This phenomenon is known as edge effect (figure 5B). 7
Figure 5B: Ecotone and Edge effect. Edge effect can observe in the coastal zone where the land and the ocean meet. Due to the flow of tides, the coastal zone does not follow the fixed boundaries on the seaward and landward directions. As a result, it provides the place for those organisms who can survive on land as well as in the water and also increases the biodiversity. It also contains specific plants like mangroves that flourish in salt water. The most common example of ecotone is estuary, where river meets the sea. In estuary, there is a regular mixing of fresh water and sea water which results in higher biodiversity. Estuary present in Chennai, where the river Adyar meets the sea. It accommodates large number of organisms including many species of bird. 4. Food chain In an ecosystem, flow of energy occurs through a series of organisms by eating and being eaten by another organism comprises a food chain. A food chain is called simple when it has only one trophic level besides the decomposers e.g., Eichhornia in Eutrophic pond. A food chain is called complex food chain when it bears both producer and consumer levels. At each level of energy transfer there is loss of 80-90% of the potential energy in the form of heat energy. All living or dead organisms serve as food for the other organisms which results in zero wastage of energy in the ecosystem. The number of trophic levels in the food chain is 8
always restricted to four or five, as the available energy decreases with each level. Many direct or indirect methods are used to study the relationships of food chain in the nature including gut content analysis, use of radioactive isotopes, precipitin tests etc. 4.1. Types of food chain In nature, basically two types of food chains are recognized: 4.1.1. Grazing food chain It always begins with producers (i.e. green plants) which produces their food by the process of photosynthesis and then moves through herbivores to carnivores. This type of food chain is directly influenced by influx of the radiant energy from sun. Thus, this grazing food chain depends on the captured energy of autotrophs and then the movements of this energy to the herbivores. Example of food chain is presented in the Figure 6. Figure.6: Example of grazing food chain. 4.1.2. Detritus food chain This type of food chain begins with the dead organic wastes and matters which are derived from the grazing food chain are known as detritus. The energy contained in the detritus is utilized by a group of organisms called detritivores that are separate from the grazing food chain. Detritus food chain starts with dead and decaying organic matter and moves through 9
micro-organisms (like fungus etc.) to detritus feeding organisms (detritivores) and then their predators (figure 7). It is not directly dependent on the solar radiant energy. Detritus Primary Detritivore Secondary Detritivore Figure.7: Example of Detritus food chain. The detritus food chain characterizes an extremely important component of energy flow in the ecosystem. The organisms of the detritus food chain includes bacteria, protozoa, slime molds, fungus, insects, crustaceans, rotifers, centipedes, mollusks, nematodes, annelids, and also some vertebrates. Detritus food chain can be easily explained by the example of Mangrove leaves which was studied by Heald (1969) and W.E. Odum (1970) in brackish zone of southern Florida. The leaves of red mangrove, Rhizophore mangle, when fall into warm shallow water, only 5% of leaves were consumed by phytophagous insects before it falls. The fragments of fallen leaves are consumed by number of smaller animals which includes crabs, copepods, insect larvae, nematodes etc. i.e. detritivores. These organisms are then consumed as a food by small game fishes and minnows etc. i.e., carnivores which are further eaten by larger game fishes and fish eating birds i.e. top carnivores. 5. Food web In nature, food chains are not isolated sequences but are interconnected with each other and make an interlocking pattern. The same organism may operate in the ecosystem at more than one trophic level i.e. it may derive its food from more than one source (figure 8 & 9). Even the same organism may be eaten by several organisms of higher trophic level or an organism may feed upon several different organisms of a lower trophic level. Thus in a given ecosystem various food chains are linked together and interrelated with each other to form a complex network called food web. 10
Figure.8: Example of Food web. Hawk Snake Fox Frog Rat Rabbit Insect Grass Figure.9: Diagrammatic representation of complex food web. 11
In complex communities, an organism whose food is obtained from the plants by same number of steps belongs to the same trophic level. The producers (green plants) occupy the first trophic level, herbivores (plant eating animals) occupy second trophic level, carnivores (herbivores eating animals) third trophic level and secondary carnivores (carnivores eating animals) occupy fourth trophic level. This trophic level classification is one of the function and not of species. The number of species of predators in a food web typically exceeds the number of species of prey by an average of 1.3 predator species per prey species. The energy flow through trophic level equals to total assimilation at that level which is equal to the production of the biomass and respiration. 6. Significance of food chain and food web Both food chain and food web play important role in the functioning of ecosystem. The significance of food chain and food web in the ecosystem are: 1) Both food chain and food web maintains the nutrient cycle and energy flow through them. 2) Food chain play important role to understand the feeding relationship as well as the interaction between organisms in the ecosystem. 3) Food chain helps in maintaining the ecological balance by sustaining the size of populations 4) Food Chain also helps to understand the movements of toxic substances in the ecosystem and the problem of biological magnification. 7. Ecological pyramids The producers in the ecological pyramids form the base and the successive trophic levels make up the peak. Generally, the terrestrial and shallow water ecosystems show gradual leaning of pyramids as the producers remain large and characterized by buildup of organic matter. However, this trend is not always true for all ecosystems. In aquatic ecosystems such as ocean and lake, the main producers includes algae and phytoplanktons which have shortcycle, high reproductive rate and accumulate of small amount of organic matter. These 12
producers are heavily exploited by herbivore zooplanktons. As a result, they forms inverted pyramid of biomass i.e. the base is much smaller (Producers) than the structure it supports (Herbivores). In consecutive steps of grazing food chain the number and mass of the organisms in each step (Trophic level) is limited by the amount of energy available. As some energy is lost in the form of heat, the successive steps become progressively smaller in each transformation. This relationship is known as ecological pyramids. The ecological pyramids represent both the trophic structure as well as the trophic function of ecosystem. The ecological pyramids can be divided into three types: 7.1. Pyramid of number It represents the total number of individuals at different trophic levels in the food chain. The pyramid of number was advanced by Charles Elton (1927) who explained that there is difference in the number of organisms present in each step of the food chain. The organisms present at the base of pyramid are the most plentiful. The successive trophic level of the pyramid decrease rapidly in number until there are few carnivores present at the top (figure 10A). The pyramid of number overlooks the biomass of organisms and also does not specify the energy transferred by the organisms involved. The lake ecosystem provides an example for pyramids of number. In lake ecosystem, base of the pyramid is occupied by Producers (algae, diatoms etc). These are more in numbers as compared to herbivores. The second trophic level is represented by primary consumers (Herbivores) (includes zooplanktons) which are less in number. The third trophic level is occupied by secondary consumers (primary carnivores) (includes small and medium sized fishes) which further reduced in number. The top of the pyramid is occupied by tertiary consumer (secondary carnivore) (includes large fishes) which are minimum in numbers. Thus in the pyramids of number, there is a gradual loss in the number of organisms and there is increase in the size of the body (figure 10A). 13
C 3 A C 2 B C 1 C 1 P P Figure 10: Pyramid of number. A) Upright B) Inverted. (P- Producers; C 1 -Primary consumers; C 2 -Secondary consumers; C 3 - Tertiary consumers). In the parasite food chain, the pyramid of number is reversed for the successive steps (trophic level) (figure 10B). Thus, the parasites are more in number as compare to their hosts. For example, a single tree can support large number of fruit eating birds (herbivores). These birds can support still more number of parasites such as bugs, ticks and mites. 7.2. Pyramid of biomass The living weight of the organism present at any time in each trophic level forms the pyramid of biomass. Pyramid of biomass indicate gradual decrease of biomass in successive trophic level from base to top of the pyramids (figure 11A). For example, the total biomass of the producers ingested by herbivores is more than the total biomass of the herbivores in an ecosystem. Similarly, the total biomass of the primary carnivores (or secondary consumers) will be less than the herbivore and so on (figure 11A). As some amount of energy or material is lost from lower trophic level to upper trophic level, the total biomass supported at each trophic level is restricted by the rate at which energy is being stored at the next-lower level. In other words, the biomass of producers should be larger to support the herbivores. Similarly, the biomass of herbivores should be more than that of carnivores. This successive decrease of biomass from lower to upper trophic level causes narrowing of pyramid in most of the ecosystems. 14
C 3 A C 2 C 1 P B C 1 P Figure 11: Pyramid of biomass. A) Upright B) Inverted. (P- Producers; C 1 -Primary consumers; C 2 -Secondary consumers; C 3 - Tertiary consumers). But this is not true for all ecosystems. Ecosystems such as lakes and ocean, primary productivity are restricted to phytoplanktons. They have short life cycle and fast reproduction. These phytoplanktons are heavily grazed by herbivorous (zooplanktons) which are larger in size and have longer life span. Thus, despite the high productivity of producers, they have low biomass as compared to herbivores (zooplankton). It results in an inverted pyramid, having low standing crop biomass of producers (phytoplankton) than herbivores (zooplankton). Similarly, In case of parasite like pyramid of number, the pyramid of biomass is inverted (figure 11B). 7.3. Pyramid of energy It indicates the total amount of energy at each trophic level of the food chain. It also exhibits that at each trophic level loss of energy occurs due to the process of assimilation and growth (figure 12). Thus, at the producer level, total energy is more than at higher trophic level. When productivity is measured in term of energy, the pyramids specify not only the energy flow at each level, but also the actual role played by various organisms in the transfer of energy. Some organisms have smaller biomass but they assimilate and transfer more energy than that of organisms having much larger biomass. 15
C 3 C 2 C 1 P Figure 12: Pyramid of energy. The pyramids of energy always show slopping because there is always declining of transferred energy from one trophic level to other (figure 12). In the open water communities, the producers have less biomass than consumers but they store and transfer more energy than that of the next level. The high energy flow is maintained by a rapid turnover of individual plankton, instead of increase of total mass. 8. Summary An organism in the nature is always connected with individuals of its own species and of different species along with their physical and chemical environment. In ecosystem, all biotic and abiotic component of the environment are connected through energy flow and nutrient cycle. It bears all biological and physical components that are essential for the survival. An ecosystem can be natural or artificial, temporary or permanent and large or small. The natural ecosystems do not have separate boundaries. An ecosystem progressively combines with the adjacent one through a transitional zone called as ecotone. It has more population density of certain species than either of the adjoining communities. This phenomenon is known as edge effect. In an ecosystem, flow of energy occurs through a series of organisms by eating and being eaten by another organism comprises a food chain. The Food chain is simple when it has only one trophic level besides the decomposers like Eichhornia in Eutrophic pond. It became complex when it bears both producer and consumer levels. Food chain can be categorized as 16
grazing food chain and detritus food chain. The grazing food chain begins with producers which produces their food by the process of photosynthesis and then moves through herbivores to carnivores. It is directly influenced by influx of the radiant energy from sun. The detritus food chain begins with the dead organic wastes and matters which are derived from the grazing food chain are known as detritus. The energy contained in the detritus is utilized by a group of organisms called detritivores that are separate from the grazing food chain. In nature, food chains are not isolated sequences but are interconnected with each other and make an interlocking pattern. The same organism may operate in the ecosystem at more than one trophic level. Thus in a given ecosystem various food chains are linked together and interrelated with each other to form a complex network called food web. In consecutive steps of grazing food chain the number and mass of the organisms in each trophic level is limited by the amount of energy available. As some energy is lost in the form of heat, the successive steps become progressively smaller in each transformation. This relationship is known as ecological pyramids. The pyramid of number represents the total number of individuals at different trophic levels in the food chain. The organisms present at the base of pyramid are the most plentiful. The successive trophic level of the pyramid decrease rapidly in number. In the parasite food chain, the pyramid of number is reversed for the successive steps. Pyramid of biomass represents the living weight of the organism present at any time in each trophic level. Pyramid of biomass indicates gradual decrease of biomass in successive trophic levels. In case of parasite like pyramid of number, the pyramid of biomass is inverted. Pyramid of energy indicates the total amount of energy at each trophic level of the food chain. The pyramids of energy always show slopping because there is always declining of transferred energy from one trophic level to other. 17