ECOLOGICAL PYRAMIDS

            Graphic representation of trophic structure and function of an ecosystem, starting with producers at the base and successive trophic levels forming the apex is known as ecological pyramids.

            Ecological pyramid may be of three types – pyramid of numbers, pyramid of biomass and pyramid of energy.

            1. Pyramid of Numbers:- It represents the number of individual organisms at each trophic level. We may have upright or inverted pyramid of numbers depending upon the type of ecosystem and food chain.

            As for example – Grassland and a pond ecosystem show an upright pyramid of numbers. The producers in grassland are grasses which are very large in numbers. So the producers form a broad base. The herbivores in the grassland are insects while tertiary carnivores are hawks and other birds which are gradually less and less in number and hence the apex of the pyramid becomes gradually narrower.

            Pyramid of numbers may be inverted (e.g., parasitic food chain). The producers like a few big trees harbor fruit eating birds acting like herbivores which are larger in number. A much higher number of lice, bugs, etc., grow as parasites on these birds while a still greater number of hyperparasites like bugs, fleas and microbes feed upon them, thus making an inverted pyramid.

            2. Pyramid of Biomass:- It represents the total biomass (dry matter) at each trophic level in the food chain. The pyramid of biomass can also be upright or inverted.

            As for example – The pyramid of biomass in forest ecosystem is upright. This is because the producers (trees) accumulate a huge biomass while the biomass of consumers feeding on them declines as it moves to higher trophic levels, resulting in broad base and narrowing tip.

            The pyramid of biomass may be inverted (e.g., pond ecosystem). Here the total biomass of producers (phytoplanktons) is much less as compared to herbivores (zooplanktons, insects), carnivores (small fish) and tertiary carnivores (big fish). Thus, the pyramid takes an inverted shape with narrow base and broad apex.

            3. Pyramid of Energy:- It represents the total amount of energy present at each trophic level. Pyramid of energy gives the best representation of the trophic relationships and it is always upright.

            At every successive trophic level, there is a huge loss of energy (about 90%) in the form of heat, respiration, etc. Thus, at each next higher level only 10% of energy passes on. Hence, there is a sharp decline in every level of each successive trophic level as it moves from producers to top carnivores. Therefore, the pyramid of energy is always upright.

EVOLUTION OF SPOROPHYTES IN BRYOPHYTES

            According to the complexity of structure, the sporophytes of bryophytes may be arranged in a series between the simples and the most elaborate. The series starts with the simple sporophyte of Riccia, runs through that of Sphaerocarpos, Targionia, Marchantia, Pellia, Anthoceros and finally ends in a highly complex sporophyte of Funaria and Pogonatum. However, the evolution of sporophytes has been explained with the help of two theories put forward by botanist – (a) Theory of Sterilization and (b) Reduction Theory.

            1. Theory of Sterilization:

            This theory was put forward by Bower and was supported by Cavers, Campbell and Smith. This theory illustrates that a natural advance in the progressive elaboration and complexity of the sporophyte. The fundamental principle upon which he formulated his argument is “the progressive sterilization of the potentially fertile cells (sporogenous tissue)”. Instead of forming spores and serving a propagative function they remain sterile. These sterile cells are put to other uses such as nutrition, support, dehiscence, dispersal, etc. This hypothesis of Bower is called theory of sterilization.

            The detailed process of sterilization of some of the important genera are discussed as follows –

            (a) Riccia Sporophyte. In Riccia, the zygote divides and redivides to form a mass of spherical mass of 20-30 undifferentiated cells. Periclinal segmentation forms an inner mass of cells called endothecium and outer single layer amphithecium. The amphithecium forms the single layered capsule wall. The endothecium forms the central mass of sporogenous tissue. Practically, all the sporogenous cells are fertile and develop into spores. However, few of them undergo degeneration to form the nurse cells.

            The sporophyte of Riccia is the simplest among all the bryophytes and has the least amount of sterile cells. The entire embryo forms the spore producing capsule. There is no foot and seta. It is just a spore producing organ without any distributing function.

            (b) Marchantia Sporophyte. Sterilization of the fertile cells is more advanced in this genus. Half of the embryo derived from the hypobasal region remains sterile. It forms the foot and the seta. The upper epibasal half is fertile and forms the spore producing capsule. The sterile cells elongate, develop spirally thickened walls and become the elaters. A few of the cells of sporogenous cells at the top may differentiate into sterile, apical cap.

            The capsule of Marchantia has both spore producing and spore distributing body. It illustrates a step further in the progressive sterilization of the sporogenous tissue.

            (c) Anthoceros Sporophyte. It illustrates a step further than Riccia and Marchantia in the progressive sterilization of the potentiality of fertile tissue. The endothecium cells become completely sterile and forms a group of cells known as columella. The sporogenous cells arise from the innermost layer of the amphithecium. It surrounds the columella. The sporogenous cells become differentiated into spore mother cells and pseudo-elaters. The archesporium of Anthoceros is extremely reduced. The outer amphithecium develops into several cells layer thick capsule wall. The capsule wall develops a well ventilated photosynthetic tissue protected by the epidermis.

             (d) Funaria Sporophyte. In Funaria, major portion of the sporophyte remains sterile to form the foot and the seta. The capsule is differentiated into central column of endothecium surrounded by many layered amphithecium. The inner layer of the endothecium forms the sterile columella and the superficial cells forms the sporogenous tissue. Thus the archesporium arises from the outermost layer of cells of the endothecium. It is thus extremely reduced and consists of single layer of fertile tissue. The amphithecium becomes differentiated into the epidermis, the photosynthetic tissue of the capsule wall and the outer spore sac.

            Thus Bower’s theory of sterilization gives a clear explanation of the evolution of the sporophyte into upward direction. This theory is more convincing and reliable.

            2. ReductionTheory:

            This theory was put forward by Kashyap, Church, Goebel and Evans. They hold that the evolution of sporophyte has been in downward direction. They hold the fact that the evolution of sporophyte is retrogressive evolution. They mainly based their theory on the reduction of different organs which results in the simplification of the structure of the sporophyte. On the basis of this view the simplest type of sporophyte of Riccia is considered as the most advanced one.

            The significant steps in the reduction series are –

            (a) Simplification of the dehiscence apparatus.

            (b) Reduction of the green photosynthetic tissue in the capsule wall.

            (c) Disappearance of stomata and intercellular spaces.

            (d) Increase in the thickness of capsule wall.

            (e) The gradual elimination of seta and foot.

            (d) All these changes are accompanied by the progressive increase in the fertility of the sporogenous cells. The change eliminates the presence of sterile cells and elaters in the capsule.

            Evidence from comparative morphology and experimental genetics support the view that the simple sporophyte of Riccia is an advanced but a reduced structure.

FOOD CHAIN AND FOOD WEB

                                                                            FOOD CHAIN

            The transfer of food energy from the producers through a series of organisms (herbivores to carnivores and then to decomposers) with repeated eating and being eaten, is known as food chain.

            The solar energy is trapped in the ecosystem by the green plants and produce energy rich carbohydrates. These green plants are known as primary producers. The green plants are eaten by plant eaters (herbivores) also called primary consumers. Herbivores are in turn eaten by meat eaters (carnivores) also called secondary consumers. Secondary consumers in turn may be still eaten by other carnivores which are known as tertiary consumers. After decay of both plants and animals simpler compounds are being released into the atmosphere due to the activity of decomposers which are re-utilized by green plants.

            In nature, we generally distinguish two types of food chain – Gazing food chain and Detritus food chain.

            Example – A food chain in grassland ecosystem starts with grasses and goes through grasshoppers, the frogs, the snake and finally the hawks.

            Grasses ---------------- Frogs --------------- Snakes --------------- Hawks

            A detritus food chain starts with the dead plants and animals and goes through (fungi, bacteria, protozoa), (insect larvae, fishes, molluscs), (small fishes), (large fishes) and finally the )large fishes and birds).

            Fungi                           Insect larvae                                                     Large fishes

            Bacteria ---------------- Fishes ---------------- Small fishes ------------- Birds

            Protozoa                       Molluscs

FOOD WEB

            Food chains in ecosystems are found to be interconnected and usually form a complex network with several linkages and are known as food webs. Thus, “food web is a network of food chains where different types of organisms are connected at different trophic levels, so that there are a number of options of eating and being eaten at each trophic level.”

            For example – In an Antarctic ecosystem resemble the total ecosystem including the Antarctic sea and the continental land. The land does not show any higher life forms of plants. The only species are that of some algae, lichens and mosses. The animals include penguins and snow petrel which depend upon the Antarctic chain for their food energy.

            Food web gives greater stability of the ecosystem. In a linear food chain, if one species becomes extinct or one species suffers then the species in the subsequent trophic levels are also affected. In a food web, on the other hand, there are a number of options available at each trophic level. So if one species is affected, it does not affect other trophic levels so seriously.

            Consider the simple food chains of Arctic tundra ecosystem.

            Cladonia ------------ Reindeer ------------- Man

            Grass ---------------- Caribou ---------------Wolf

            If due to some stress, the population of reindeer of Caribou falls, it will leave little option for man or wolf to eat from the ecosystem. Had there been more biodiversity, it would have led to complex food web giving the ecosystem more stability.

            SIGNIFICANCE OF FOOD CHAINS AND WEBS

            1. Food chains and food webs play a very significant role in the ecosystem because the two most important functions of energy flow and nutrient cycling take place through them.

            2. The food chains also help in maintaining and regulating the population size of different animals and thus, help maintain the ecological balance.

            3. Food chains show unique property of biological magnification of some chemicals. There are several pesticides, heavy metals and other chemicals which are non-biodegradable in nature. Such chemicals are not decomposed by micro-organisms and they keep on passing from one trophic level to another. And, at each successive trophic level, they keep on increasing in concentration. This phenomenon is known as biomagnifications or biological magnification.

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ECOSYSTEM - COMPONENTS OF AN ECOSYSTEM, BIOTIC AND ABIOTIC COMPONENTS

            Ecosystem may be defined as a natural, functional, ecological unit comprising living organisms and their non living environment that interact to form stable, self supporting system.

            COMPONENTS OF AN ECOSYSTEM

            Components of an ecosystem can be classified into two major divisions – Biotic and Abiotic components.

            1. Biotic Components:- All living organisms present in the environment are included within the biotic component. The biotic components can be sub divided into three major groups –

            a) Producers – These includes all the autotrophic green plants which can make their own food with the help of sunlight. Besides these some organisms like Cyano-bacteria and some Chemo-systhetic bacteria can produce their own food, which are also included in producers.

            b) Consumers – These includes all heterotrophic organisms which depend on plants and other organisms for food and nutrition. Based on the food they eat, consumers are again sub-divided into three groups – Herbivores (Plant eaters; e.g., – Rabbit, deer, cow, goat, etc.), Carnivores (Meat eaters; e.g., – Tiger, lion, fox, etc.) and Omnivores (Both plant and meat eaters; e.g., – Man, bear, etc.)

            c) Decomposers – These are mainly bacteria and fungi that obtain their food form organic materials of dead plants and animals and decompose the complex organic molecules into simpler ones which can be readily used up by green plants again.

            2. Abiotic Components:- The non-living matters constitute the abiotic components of an ecosystem. It includes physical and chemical components like climatic factors, edaphic (soil) factors, geographical factors, energy, nutrients and toxic substances.

            a) Physical factors:- The sunlight and shade, intensity of solar flux, duration of light hours, average temperature, annual rainfall, wind, latitude and altitude, soil type, water availability, water currents, etc., are some of the important physical features which have a strong influence on the ecosystem.

            b) Chemical factors:- Availability of major nutrients like carbon, nitrogen, phosphorous, potassium, hydrogen, oxygen and sulphur, level of toxic substances, salts causing salinity and various organic substances present in the soil or water largely influence the functioning of the ecosystem.

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ENVIRONMENTAL STUDIES AS MULTIDISCIPLINARY SUBJECT

            The science of environment studies is a multidisciplinary science because it depends on various disciplines like chemistry, physics, medical science, etc. It is the science of physical phenomena in the environment. It is inherently a multidisciplinary field that draws upon not only its core scientific areas, but also applies knowledge from other non-scientific studies such as economic, law and social science.

            1. In the field of physical science it helps to understand the flux of material and energy interaction. It also helps to construct mathematical models of environment.

            2. Environmental study is related with chemical science in the sense that it helps in understanding the molecular interactions in the system.

            3. In the field of biological science it describes the effects within the plant and animal kingdom and their diversity. 

            4. In the field of atmospheric science, environmental studies deals with the examination of the phenomenology of the Earth's gaseous outer layer with emphasis upon interrelation to other systems. It comprises meteorological studies, greenhouse gas phenomena, airborne contaminants, sound propagation phenomena related to noise pollution, and even light pollution. 

            5. Environmental studies is very much related to ecological studies. It helps to analyse the dynamics among an interrelated set of populations, or a population and some aspects of its environment. The study of endangered species, predator interactions, effects upon populations by environmental contaminants, or impact analysis of proposed land development upon species viability, etc.

            6. In environmental chemistry environmental studies deals with the study of chemical alterations in the environment; principal areas of study include soil contamination and water pollution; the topics of analysis involve chemical degradation in the environment, multi-phase transport of chemicals and chemical effects upon biota.

            7. The relation of environmental studies with Geo-science includes environmental geology, environmental soil science, volcanic phenomena and evolution of the earth's crust. In some classification systems, it can also embrace hydrology including oceanography. 

            8. In the field of mathematics and computer Science it will help in environmental modeling and analysis of environment related data.

            9. Environmental studies is also related with Economics. It deals with economical aspects of various components of environment.

            10. In Law it helps in framing of environment related laws, Acts, rules and their monitoring.

            11. In the field of Social Science it helps in dealing with population and health related issues. 

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ALTERNATION OF GENERATIONS IN PTERIDOPHYTES

In the life cycle of Pteridophytes, two distinct individuals can be observed i.e., there is a typical heteromorphic alternation of sporophytic and gametophytic generations. These two generations alternate with each other in regular succession one after the other, i.e. from sporophyte to gametophyte and from gametophyte to sporophyte. The sporophytic or asexual generation is diploid (2n), while the gametophytic or sexual generation is haploid (n).

The actual phenomenon responsible for bringing about alternation in generation is the “Periodic Reduction of Chromosomes” as a result of meiosis. Due to meiosis a reduction in the chromosome number takes place which leads to the formation of haploid individuals. The haploid individuals in turn by produce haploid gametes which proceed the process of fertilization to produce diploid individuals.

1. Gametophytic Generation:- The haploid individual bears sex organs, antheridia and archegonia. Antheridia produce male gamete i.e., antherozoids or sperms and archegonia produce female gamete oosphere or egg, which is concerned with the sexual reproduction. These individuals are named as gametophyte or prothallus and represents gametophytic generation. Both the haploid gametes (male and female) come together and unite (i.e., fertilization or syngamy) to produce a diploid (2n) zygote. Zygote is the pioneer structure of the sporophytic generation. It germinates to form embryo which develops into new sporophytic individual.

The gametophyte in the homosporous forms is short lived, independent and may be surface living and green (autophytic) in nature. They are always exosporic and thus are not enclosed by a spore wall, e.g., Lycopodium. In heterosporous forms, they have separate male and female prothalli. The male prothallus is extremely reduced and is represented only by a single prothallial cell. The female prothallus is well developed and larger in size. Both male and female prothalli are endosporic and are enclosed inside a spore wall, e.g., Selaginella.

2. Sporophytic Generation:- Zygote is the pioneer structure of the sporophyte. It develops into embryo which in turn develops into diploid individual. This diploid individual is known as sporophyte and the generation is termed as sporophytic generation. The sporophyte bears a specialized spore bearing structure known as strobilus. Inside the strobilus numerous haploid spores are formed after meiosis which are known as meiospores. There are two types of sporophytic individuals – homosporous and heterosporous. Homosporous individual produce only one type of spores, whereas heterosporous individuals produce two types of morphologically distinct spores i.e., microspores and megaspores. The spores are the pioneer structure of gametophytic generation. It germinates to form a new haploid gametophytic individual.

In pteridophytes the sporophytic individual is complicated, independent and dominant generation.  It is no doubt independent, but it has to depend upon the gametophyte during earlier stages of development. It achieves its complexity after establishing its independence.

Both the homosporous and heterosporous pteridophytes exhibit heteromorphic type of alternation of generations, because the sporophyte and gametophyte individual present marked morphological and anatomical differences.

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HETEROSPORY AND SEED HABITAT IN PTERIDOPHYTES WITH RESPECT TO SELAGINELLA

Heterospory is the condition that interprets the production of spores of two different sizes and two different development patterns. The two different sizes are smaller spores also known as microspores and the larger spores also known as megaspores. Hetrrospory can be observed in some of the pteridophytes such as Selaginella, Marsilea, etc.

They have differential developmental patterns because the microspores germinate to produce male gametophytes or microgametophytes that bear male sex organs called antheridia, and the megaspores germinate to produce female gametophytes or megagametophytes that bear female sex organs called archegonia.

The two kinds of spores are produced in two kinds of sporangia. The microspores are produced in microsporangia and the megaspores in megasporangia. The microspores are produced in large numbers and are comparatively smaller than megasporengia which are produced in lesser numbers and larger in size.

Importance of Heterospory

(a) The most important aspects of heterospory is that it is an expression of sex determining process of the plant. It has brought about along with its onset, the sex determining capacity from the gametophyte to sporophyte. In all the homosporous individuals, sex can be determined in their gametophytic phase, during the formation of antheridia and archegonia. But in the heterosporous individuals sex can be determined in their sporophytic phase during sporogenesis i.e., during the formation of microspores and megaspores.

(b) Heterospory is the most important evolutionary development in pteridophytes because it has ultimately led to seed developments. It is rather a pre-requisite to seed habit. Heterospory has brought about a number of changes in the characteristic of spore development which is the pioneer characters of seed habits in higher plants.

Heterospory in Selaginella

Selaginella, no doubt, illustrates an example of heterosporous pteridophyte that approach seed habit because of the following notable characteristics –

(a) It is heterosporous.

(b) The megaspore starts germination within the megasporangia and their time of release from the megasporangia varies with species.

(c) The number of megaspores in S.rupestris and S. monospora is reduced to one.

(d) In S. rupestris the megaspore is never shed and fertilization and development of the embryo takes place while the megaspore is still within the megasporangium, which retains its connection with the parent plant. This condition can be linked with the vivipary in some angiosperms.

Considering the above points we can reach to the conclusion that had reached to the level of seed habit but fail to develop seeds because of the following shortcomings –

(a) They have no protective structures like the integuments surrounding their megasporangia.

(b) The permanent retention of megaspores within the megasporangia is not established.

(c) Histological union between the megaspore and the megasporangium is absent.

            (d) Lack of resting period after the development of embryo.

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