5 Ecological Pyramid

Dr. Poonam Sharma

Learning Objectives

• Introduction

• Concept of Ecology

• Perspectives of Ecology

• Ecosystem Ecology

• Ecosystem Organization

• Energy Flow and Nutrient Cycle

• Ecosystem Productivity

• Ecological Efficiencies

• Ecological Pyramids

1. INTRODUCTION

Ecology is a multidisciplinary field encompassing the biological, physical and social sciences. It is the scientific study of relationships in the nature. It is the study of the interactions between organisms and their environment. It is concerned with relationships, distributions, abundance, scarcity, competition and cooperation among organisms in an environment. The environment comprises two distinct components i.e. the physical environment which includes water, wind, soil, temperature, alkalinity, salinity etc and biotic environment that includes the interdependence and interactions among organisms. Ecology includes material and processes ranging from the physiological genetics of small organism to carbon balance in the entire biosphere. There are numerous ways to study the material system of ecology at all scales.

    If the relationships between organisms and their physical environments are explored it is called physiological ecology; community ecology explains interactions between organisms of different species; it is population ecology when associations between organisms of the same species are studied; and ecosystem ecology explains interdependence between organisms and the movement of matter and energy through biological systems.

The word ecology was coined by Professor Ernst Haeckel in 1866 in the book General Morphology. The word ecology has been derived from Greek words oikos which means house and logos mean study. He defined ecology as: By ecology we mean the body of knowledge concerning the economy of nature- the investigation of the total relations of the animal both to its inorganic and to its organic environment; including above all, its friendly and inimical relation with those animals and plants with which it comes directly or indirectly into contact -in a word, ecology is the study of all the complex interrelations referred to by Darwin in as the conditions of struggle for existence.

The term ecology was coined in 1866 but it was not recognised and widely used for long until it became institutionalised with the formation of British Ecological Society in 1913 and Ecological Society of America in 1915. Many scholars have used this term with different perspectives to the original Haeckel’s definition; for example British Ecologist Charles Elton(1927) used it as scientific natural history; American plant ecologist Frederick Clement(1905) emphasised as science of community , another American animal ecologist Victor Shelford(1937) considered it as the branch of general physiology. German ecologist Karl Friederichs(1958) explained it as the science of environment, Eugene P. Odum(1959) American ecologist who had actually influenced the understanding of the concept, has defined ecology as the study of the structure and function of nature . Thus it was seen that Haeckel’s definition of ecology was applied by various scientists in a much diversified ways. This aspect can be further explained that how the study of ecology is approached in numerous ways. For example types of ecology as defined by concept or perspective include landscape, ecosystem, physiological, behavioural, community and many other kind of ecology. If it is defined by organism it comprises plant, animal, microbe, zooplankton, human and many other organisms. When ecology is explained in terms habitats the kind of ecology it includes are terrestrial, marine, freshwater, arctic, equatorial forest, urban and many more. There is another perspective to the definition of ecology that is with reference to application, the types of ecology which encompasses are theoretical, conservation, restoration, management academic, public policy and so on. Ecological studies are not restricted to natural systems rather understanding of both the human impacts on nature and the ecology of environment created by human. (Figure 1 and 2)

Figure 1: Ecology and its different perspectives

Figure 2: Ecology and its different perspectives

2. ECOSYSTEM ECOLOGY

Ecosystem ecology studies the interdependence and interactions between organisms and their environment as a complex system. The ecosystem approach is vital in understanding interactions that link biotic component with the physical systems on which they depend.

The term ecosystem was coined by British ecologist Arthur Tansley in 1935. He defined ecosystem as the whole complex of physical factors that is the environment of the habitat and organism complex. Tansley emphasised that it is not easy to study the interactions of organism separately from the physical and chemical features of their environment. He proposed that organisms and their physical-chemical environment be studied in an integrated way called ecosystem. Though the organisms are the essential concern but they cannot be separated from their unique environment with which they form the unified whole system. It is these unified systems so formed are the basic units of the nature on the face of the earth. The ecosystem dynamics conceptualisation evolved from Shelford (1918) where food chain has no link through detritus feeders; Storn(1928) elaborated the system with decomposers as central to the system and Lindemam in 1942 have developed the concept and introduced the unity of energy and nutrients. Odum in 1969 integrating Tansley and Lindeman conceptions in a contemporary light with humans as critical components gives it the grand view.

Ecosystem analysis seeks to understand the factors that direct the quantities and flows of materials and energy through ecological systems. These materials are found in abiotic pools which are physical chemical substances, inorganic elements and compounds which includes calcium ,oxygen, carbon dioxide ,carbonates and phosphates in soils, rocks, water, and the atmosphere and an array of organic compound which are by-products of organism activity and novel chemicals such as pesticides or radio nuclides that have been added to the environment. The aboitic factors also include physical factors as moisture, winds, heat, light, currents and tides. The biotic pools such as plants, animals, and soil micro organisms existing in that aboitic setting .There is great diversity in the types of ecosystems from small to large , terrestrial to aquatic, fresh water to salt water. The objective of ecosystem ecology is to understand the functioning of ecosystem that includes the flow of energy and matter through the organisms and their environment. Energy from the Sun is captured by plants which can be consumed by animals and further by next levels of organism and microbes. Thus, ecosystem ecology is concerned to understand the fundamentals of the production of organic energy, its transfer among other organism, nutrient cycling among organism and environment, problems of environmental management, impact of human activities on environment and many other interactions of people and nature.

2.1 LEVELS OF ORGANISATION

A system consists of constantly interacting and interdependent components forming a unified whole .Organisationally ecosystems are constituted hierarchically. The community and nonliving environment function together as an ecological system or ecosystem. Hierarchical theory provides a convenient framework for subdividing and examining the complex structures. Individuals which are at the base of the hierarchy compose a species, and population at following level; communities are at the next order, it is these communities existing in particular physicochemical environments that constitute an ecosystem. The aggregation of all ecosystems on the earth is referred as to as ecosphere, the ecosystem for the whole planet. In this pyramidal hierarchy there are many individual at the base ,a smaller number of species and individuals at the next level, fewer populations than species at further next and so on the top of the pyramid .

Figure 3: Level of Organisation

Different categories of organism fit together in the biotic structure and the feeding relationships between organisms constitute the trophic structure (trophe mean nourishment). All ecosystems have the same three basic categories of organisms that are connected in the same way. The major categories of organism are producers, consumers and decomposers. These groups together manufacture food, transfer along food chains and return the initial materials to the aboitic part of the environment.

Producers are also called autotrophs( self nourished) that imply the organisms which prepare their own organic material from the inorganic constituents of the environment. Producer performs the fundamental task, uses the radiant energy of the Sun and in the process of photosynthesis assimilates energy rich carbon compounds. They are mainly chlorophyll carrying green plants. Producer are of varied types ranging from microscopic photosynthetic bacteria, single celled algae through small sized plant as grass ,cacti to medium sized plant and massive trees. Chemosynthetic producers prepare organic material in anaerobic conditions of deep ocean floors in the absence of sunlight.

Consumers are the organism whose nutritional needs are met by feeding on to the other organisms. They are also called heterotrophs(other nourished). Primary consumer or herbivore is a heterotroph that obtains its nutrition from plants, followed by carnivore, the secondary consumer that feed on herbivore which may further provide nutrition to tertiary and higher order consumers and omnivore which obtain their energy from both plants and herbivore. This autotroph to heterotroph , producer to consumers and herbivore to carnivore connections are direction of energy transfer through the ecosystem which is one way flow and not a cyclic movement.

Decomposers are the heterotroph organisms which feed only on dead organic matter, mainly include bacteria, fungi and protozoa. They do not consume food in ingestive manner as herbivore or carnivores rather through the absorption. Chemicals or enzymes are produced within their bodies which are released on dead plant or animal. Terrestrial bacteria act on animal tissues and fungi on plant tissues. In the aquatic ecosystem decomposers mineralise the organic matter. Eventually the waste products of the final lines of decomposers are energy poor mineral nutrients that are reabsorbed and thus recycled by plants again into the system.

There is symbiosis in the interaction in the ecosystem which implies an interaction between individuals of different biological species. One of the organisms receive a benefit from the interaction, the other can either receive a benefit, be harmed, or not be affected in any way .Three main kinds of symbiotic relationships: commensalism, mutualism, and parasitism.

Table 1 : Ecological Symbiosis

2.2 ENERGY FLOW AND NUTRIENT CYCLING

The two vital ecological processes of energy flow and nutrient cycling is constantly working in the ecosystem is the central to the dynamics of ecosystem; the former has been explained as a unidirectional movement while later is cyclic. The flow of energy in ecosystem and the laws of thermodynamics says: as per the first law of thermodynamics energy is converted from one form into another and it is neither gained nor lost. The second law of thermodynamics states that every transformation results in loss of some of the usable energy. The transformation of energy from producers of the ecosystem to herbivore, through carnivore ,further trophic levels and to decomposers; in theses transformations energy flows in the unidirectional way and some energy is lost. The loss occurs largely in the form of heat as by product of metabolic reaction.

Within ecosystem matter (nutrient) is used and cycled within organisms, and among organisms and environment. In contrast of energy which flows from space through the Earth’s environment and then back to the space; matter is not lost to space but is recycled within the earth’s environmental system. The cycling of nutrients through the ecosystem is also called biogeochemical cycle because these pathways involve biological, geological and chemical processes. There are two types of nutrient cycles which include sedimentary and gaseous cycles. The sedimentary cycles begins from rock , different nutrient are released during variety of weathering process ,gets dissolved in water ,absorbed by plants ,animals ,transferred through the ecosystem and eventually end up as sedimentary residue at the end of the cycle. For example phosphorous, potassium, calcium cycles. These are very long cycles may be hundreds or millions of years. In gaseous cycles nutrients are exchanged between the biosphere and atmosphere without going into the lithosphere, this is a much faster cycle, may be hundred year .for example carbon, nitrogen and oxygen cycles.

3. ECOSYSTEM PRODUCTIVITY

The productivity of an ecosystem is of two types i.e. primary productivity and secondary productivity. The primary productivity of an ecosystem is the rate at which organic matter is produced during photosynthesis. There are two variations in the primary productivity; The gross primary productivity (GPP) is the total fixation of energy by photosynthesis which is expressed in Kcal/m2/yr. Part of this energy is used as plant matter and rest of the chemical energy is metabolised by the plant’s own respiration and released to the environment a heat. Subtracting this respiration(R) from GPP gives the Net Primary Productivity (NPP),it represents the actual rate of production of new biomass that is available for consumption by heterotrophic organisms. Biomass is the mass of an ecosystem component per unit area and per unit time.

Net Primary Productivity = Gross Primary Productivity – Plant Respiration

The productivity of an ecosystem is rate of generation of new biomass, it may be expressed either in energy per unit area or as weight. The total quantity of organic matter available in at any given time in an ecosystem is called biomass. The productivity depends on various factors such as sunlight, temperature, rainfall, availability of nutrients and also indirect impact of intervention of human activities. For comparing different ecosystems productivity is usually included over the year to compute annual production that means generation of new biomass per unit area in a year. A common ecological measure of efficiency is the trophic level efficiency, the rate of production at one trophic level to that of the next lower trophic level. The rate of production of new biomass by heterotrophs or consumers from the net primary production available to them is called secondary production. Though there are large variations from one ecosystem to another ecosystem, with reference to energy content, the transfer is only about 10 percent, and grossly for every 10kcal of plant obtainable to herbivore, about 1kcal is consumed and 0.1 kcal is absorbed in the form of body weight and in the similar way amount of energy transfer is dwindles with each transfer.

There are optimum conditions for an organism, low and high limiting factor creates a condition of tolerance threshold and may become acute if conditions are not corrected, where the survival is at stake. The disturbance from the external environment include the sudden events of flood, storms, draughts etc these situation disrupts the flow of the basic components of the photosynthesis for example extreme soil erosion will reduce availability of nutrient, or physical damage of plant foliage due storm. Ecosystems are in state of incessant adjustments with these sudden and progressive changes.

The ratio of net primary production that gushes along these pathways depends on transformation efficiencies in the way energy is used and transferred from one level to the next. Ecological efficiency is the ratio of the biomass integrated by consumer trophic level to the biomass from its lower trophic category. Some of the efficiencies are explained here. Photosynthetic efficiency (PE) is the percentage of received solar energy a plant utilises in complete photosynthesis process. It can be derived by dividing GPP by the solar input of energy per unit area per unit time. Consumption efficiency (CE) is the percentage of total productivity available at one trophic level which is consumed by a trophic category one level up. Assimilation efficiency (AE) is the percentage of food energy ingested by the consumers in a trophic category and the remainder is lost as faecal waste and enters the decomposers system. Production efficiency (PE) is the percentage of assimilated energy which is included into new biomass and remainder is entirely lost as heat. Exploitation Efficiency (EE) is the net productivity of each trophic level as a proportion of the net productivity of the previous level.

Table 2: Ecological Efficiencies

    4. ECOLOGICAL PYRAMID

British ecologist Charles Elton in 1920s for the first time, explained this feature about the energy flow in ecosystem, he established the principal that there is not enough energy to support more than a very few carnivore at the carnivore level.

The trophic relationships, the food chains, the ecosystem productivity, the flow of energy and organic matter among different organisms are visualized through flow model, statistical models and also as static pyramid. An ecological pyramid is used to graphically represent the structure of ecosystem depicting the new biomass at various trophic levels. The pyramid shows first trophic level or producers at the bottom that forms the base followed by the successive tiers of the trophic categories of consumers. By photosynthesis energy enters in the producers at the base of the trophic categories, energy flows through the food chain and moving up in the chain through various levels of consumers. Due to inefficient energy transfer from one level to the next, energy reaching to the higher levels is very low. It is very interesting to note that number and biomass of the organisms differ across various trophic levels which are influenced by the amount of energy available at the trophic level. The relationship between energy, biomass, and number is subjective to the growth form and size of organisms and ecological relationships occurring among trophic levels.

There are three types of ecological pyramids:

A  Energy Pyramid

B  Biomass Pyramid

C  Number Pyramid

4.1 ENERGY PYRAMID

An energy pyramid conveys the general trophic relations in the system and confirms that major principle of progressive decline of energy in the higher trophic levels. It is graphical method that shows the energy flow in ecosystem through a pyramid shape. Producers play vital role in the food chains and webs as they are capable to transform radiant energy of the Sun into large amount of organic energy through the process of photosynthesis. The organism at consumers’ level feed on plants .At this level the pyramid size is smaller than plants and energy is used for growth, reproduction and respiration. The secondary consumers depend on primary consumers and the pyramid size further reduces and in further movements to tertiary and higher order consumers the energy transfer becomes lesser and lesser thus the size of pyramid. It is estimated that 90 percent of energy is released in respiration and only 10 percent is available to the next level. This pyramid shows that energy is highest at lowest level and its minimal at highest category explains that there is successive loss of energy as it moves through the trophic levels. (Figure 4 and 5)

Figure 4 : Upright Energy Pyramid

Figure 5 : Energy Pyramid

4.2 BIOMASS PYRAMID

Biomass is mass of the organism per unit are of ground (or water). It is usually expressed per unit area in terrestrial ecosystem and for aquatic ecosystem it is expressed as per unit area or volume. The biomass pyramid graphically illustrates the quantity of biomass generated or available at each trophic level. Biomass decreases with each movement in the food chain. The autorophs or producers generates the highest biomass in the ecosystem , the quantity of biomass prepared by herbivores is lesser than producers and for carnivore and higher order consumers the biomass generation is very small. Biomass at trophic levels depends upon the reproductive potential and longevity of organisms. The biomass pyramids are of two types for different ecosystems. (Figure 6 and 7)

    a. Upright pyramid is observed in terrestrial ecosystems with broader base where producers generate larger net biomass to support the consumers with smaller weight.

b.Inverted pyramids are seen in aquatic ecosystems where smaller weights of producers provide growth support for consumers of larger weight.

Figure 6 : Upright Pyramid of terrestrial ecosystem

Figure 7: Inverted Pyramid of an aquatic ecosystem

4.3 NUMBER PYRAMID

This is another graphical way to represent the structure of ecosystem in terms of energy by carrying out a census of each of the major categories to produce the pyramid number. The number pyramid illustrates the trophic relationships in terms of the number of autotrophs and heterotrophs or the number of producers, different consumers i.e. herbivore, carnivore and further higher order consumers in the successive chain on trophic category. This pyramid explains that the number of individuals declines from the lower to higher levels of trophic category. This pyramid also varies as the energy and biomass pyramid from one ecosystem to another as follows: (Figure 8 ,9 and 10)

Figure 8 : Upright Number Pyramid

Figure 9 : Inverted Number Pyramid

Figure 10 : Spindle Shape Number Pyramid

A. Upright number pyramid is witnessed in forest ecosystem , as these ecosystems have huge number of small producers to support lesser primary consumers which further provide support even smaller secondary consumers and they give nutrition to just a few tertiary consumers. It is because of this type of transfer of organic matter the shape of the pyramid is upright.

B. Inverted number pyramid is observed in parasitic food chain where one primary producer sustains several parasites and which in turn support many more hyperparasites. Hyperparasites is an organism which is parasitic on or in another parasite.

C. Partly upright or Spindle number pyramid is seen in forest ecosystem where small number of producers maintain large number of primary consumers which further supports only a few secondary/tertiary consumers.

If the Energy, biomass and number pyramids of the same ecosystem are compared it is clear that base is broader in each type of presentation and it narrows down towards the top. (Figure 11)

Figure 11 : Comparison of Energy, Biomass and Number Pyramid

5. CRITIQUE

These graphical methods provide comprehensive and illustrious understanding to the structure and functioning of ecosystem through the pyramids of energy, biomass and numbers. But demerits are observed for example energy pyramid does not have provision to show the decomposers and to represent loss of energy. In biomass pyramid it is assumed that significance of unit are same for varied group of organism in a trophic level. In case of number pyramid besides these all limitations, the additional problem is that all organisms are considered same such as a bunch small grass at producer level is of same value as the carnivore in the secondary or tertiary trophic level. Therefore , the static pyramid model of an ecosystem are impressive graphical methods for understanding but is not a functional model to describe energetic of an ecosystem as flow models and mathematical formulation and computer replication.

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