Chapter9

COMMUNITY ECOLOGY

Community ecology deals with the groups of different kinds of populationsin the .area. A community is defined as an assemblage of different plants,animals, and microbes living in a defined area of habitat. Thus, the forest withvarious kinds of trees, shrubs, herbs, animals (such as, mammals, birds, reptiles,insects, etc) fungi and other microorganisms is recognized as a community.Similarly, a grassland, a swamp, a desert, an ocean, a lake and many other maybe regarded as natural biotic communities. Within a large community, such asforest, there may be many small communities e.g. a decaying log of wood maysupport a community of algae, protozoa, small insects, millipedes and bacteria ora tiny pool of water may hold a variety of f?hytoplankton and animals. Bydefinition, a community must include only living entities of the environment. Ifnon-living factors (abiotic factors) .were considered together with the livingorganisms, then we would be dealing with ecosystem, rather than a community.The community, thus, is essentially a biotic community where different species ofplants, animals and microbes live together inter~cting mutually, and sharing thesame habitat.

Communities may be large or small. Some may cover thousands ofsquare kilometers such as forests or an ocean. Other such as a swamp, lake orstream may have dimensions in hundreds of kilometers. The communities ofponds, tide-flats, mountain meadow occupy a more restricted area. The plant andanimals living on an isolated stone or rotten log of wood constitute still smaller.comm'lJnities.

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The number of species and the population Elbundance also vary greatly.Furthermore, all the species in the community are not equally important. Out ofthe hundred or thousands of kinds of organisms that might be present in acommunity, a relatively few species or speGies groups generally exert the majorcontrolling influence by virtue of their num rs, size or activities. To study thedetails of these aspects of any r.n:""lnlunity there are taken into consideration anumber of character (parameters). These then are used to express thecharacteristics of a community. For instance, if we say this particular species isdominant in the community we must have some basis to draw this conclusion.

These various characters used for such purpose are broadly divided intomajor categories: analytical and synthetic. The analytical characters are those,which are directly observed in the field while the synthetic characters are basedupon the data obtained for the analytical characters. The analytical charactersare either qualitative or quantitative depending upon whefher they can beexpressed and measured in quantitative terms.

These are of two types: 1) QuanHtative, which are expressed inquantitative terms, and 2) Qualitative, which are expressed only in qualitativeterms.

1. Quantitative Characters: The major quantitative characteristi,:;s areas follows:a. Frequency: Frequency is the number of sampling units (quadrate,

transect, point, .etc) in percentage, in which given species occursIt IS determined by the following formula:

Number of sampling units in which species occurred0/0 Freq uency = --------------- --------------------- ---- -------------- ----------

Total number of sampling united studied

Thus, the estimate of frequency depends upon the size of thesampling unit. A very small sampling unit underestimates thefrequency of widely spaced individuals while a large sampling unitoverestimates the frequency.

After determining the frequency percentage of each species,various species are distributed among Raunkiare's five frequencyclasses, depending upon their frequency value as follows:

Frequency % Frequency Class'0-20 A21 - 40 B41 - 60 C61 - 80 D81 - 100 E

b. Density: Density represents the numerical strength of the speciesin the community. it is calculated as follows:

Total number of individuals of the species in all unitDen sity = -. --------~-------------------- ---------------- --------- ------ ---

Total number of sampling units studied

Density is expressed as number of in~ividuals per unitarea, as for example, per square meter.

Density gives an idea of competition.,lf density is more, itmeans there is more degree of competition between theindividuals of the species for food, space, etc.

c. Abundance: This is the number of individual of the species persampling unit of occurrence. This is calculated by the followingmethod:

Total number of individuals of the species in all unitsAbu ndance = -.. -.. ----- .... --- .---- .-.-.----- .--.-.---.---.-- .--- ------------- ----

Number of sampling units in which species has occurred

Abundance is related to density but is a qualitative estimate. Largenumber of individuals occur'ring at one place in community will notbe referred to as abundant while the same number of individualsspread throughout the community may appear to be abundant.The abundance is expressed in five arbitrary groLlps as Very rare,Rare, Common, Frequent and Very frequent.

Q. What is fog?

d. Cover and Basal area: Cover is the area of ground occupied bythe above ground parts of plants such as, leaves, stems,inflorescence as vie~ed from the above (or in other words, tiletotal land area under the canopy of a plant). Basal area can onlybe a small fraction of the total land area in a community but thecanopy cover of a single species may be several times the totalland area because of overlapping canopies (Fig 9.1).

basal

£,--~>..,,> Icanopy cover I

.•....Jcanopy cover'

Fig. 9.1: Relationship between Canopy cover and Basal cover

e. Dominance: It describes the influence exerted by a species on acommunity. The. common parameter of dominance is cover,biomass, and productivity. In fact, the number of organismssometimes may not give correct idea of the species. For example,a few large cows have a much greater effect on a grassland thanmillions of small herbivorous insects. In this instance, the cows willbe the dominant species. Similarly, in a forest community, bigtrees are dominant though less numerous as compared to shrubsand herbs, because they monopolize most of the nutrients in thesoil and decrease light intensity, increase the relative humidity,intercept the precipitation and furnish shelter and food for theanimals.

I Ails: Aerosol consisting of water droplets.

2. Qualitative Characteristics: The major qualitative characteristics areas under:

a. Physiognomy: It is the external appearance of vegetation, whichmay be determined on the basis of dominant plants, density,height, etc. For example, a simple look to such a community ofplants where larger trees are dominant with some shrubs andherbs would tell us that it is a forest. Similarly on the basis ofappearance, the terms like grassland, desert, savanna, bogs, etc,describe the physiognomy. of various communities. Thephysiognomy does not emphasize any particular species orindividuals.

b. Phenology: Phenology is the scientific study of life cycle phasesof animals or activities of plants in relation to their climate .

. Different species of plants, for example, have different periods ofseed germination, vegetation growth, leaf fall, seed dispersal,flowering, fruiting, etc. A study of time and date of these events iscalled phenology. In other words, phenology is the calendarevents in the life history of plants and is sh.own by phenograms(using diagrams and symbols).

c. Stratification: In ecological terminology, the term stratificationrefers to separation between organisms in space or time. Anecosystem can be stratified in space either vertically (in layers) orhorizontally (in concentric circles). Organisms within ecosystemcan also display daily, lunar, or seasonal patterns that separatethem in time (i.e. temporal stratification). It should not, however,be confused with zonation, which more properly refers to thehorizontal or vertical arrangements of biotic or abiotic factors.

Vertical stratification is usually the result of inter-specific competition forght and water in plants and food in animal. Two layers of vertical stratificationire commonly recogni~ed: an upper stratum of light penetration and loweregenerating stratum where organic matter accumulates.l. Define smog.

The upper stratum of light penetration is dominated by autotrophicorganisms. In a terrestrial ecosystem the upper stratum is often forest vegetation;in an aquatic ecosystem it consists of the entire water layer of the sea, lake orpond. Decomposers dominate the lower regenerating stratum wherpmatter accumulates. In terrestrial ecosystem this stratu is soil; in aquaticecosystem it is the sediment. Both the upper and low ; stratum can further .bedivided into substrata.

Vertical distribution (stratification) ina terrestrial ecosystem is bestillustrated by reference to a typical forest. The Lipper stratum has four substrata:

1. The tallest tree (Overstory), about 15 meter or more in height, whichmake up the canopy and receive full sunlight. The foliage of thesetrees may absorb and scatter more than half of the available light.

2. The shorter trees (Understory), about 6 meter in height, which containsome of the younger individuals of the canopy species as well otherspecies of tall shrubs. These prefer some shade.

3. The shrubs, which receive only.10% of the sunlight after it has filtered'down through the overstory and understory.

4. The herbs, ferns, mosses (ground layer) which oed very little light(usually 1% sunlight) to exist. In a enC;E:" 'I"" .. t, iC:;S tn;:l': 1 ~':. uf hesunlight re' ch the grod,~d flOur

The lower regenerating stratum of forest floor is the soil which alsoexhibits stratification. The roots may be spread nearer the ground or may bedeep penetrating. This spacing among the roots permit the plant to draw theirwater and nutrient requirements from the different layers of the soil withoutaffecting each other. The stratification in aerial and underground stratum isshown in Fig 9.2.

A similar stratification is observed in many animal communitiesassociated with the plant communities. For example, different groups of birdspecies may be found feeding or nesting near the ground; in shrubs beneath thecanopy and in the canopy itself.

I Ans: A popular term to describe a mixture of smoke and fog.

subterranean (rich humus withroots of plants, soil, fungi, bacteria,and protozoans)

Fig. 9.2: Diagrammatic representation of a forestcommunity showing stratification (after Sharma, 1987).

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herbs

forest floor (dead organic matter,litter, fungi, bacteria, animals likemites, ants, etc .

In aquatic ecosystem, 'such as oceans, lakes, ponds, etc. the stratificationis determined by light penetration, temperature and oxygen profiles. In thesummer, well stratified lakes has a layer of freely circulating surface water, theEpilimnion; a second layer, the Metalimnion which is characterized by athermocline (i.e. a very steep and rapid decline in temperature); the Hypolimnion,a deep cold layer of dense water about 4 - 5° C, often low in oxygen; and a layerof bottom mud.

Horizontal stratification or Dispersion relates to the distribution oforganism, principally plants, in the horizontal space (i.e. on the ground) or acrossthe canopy. Like vertical stratification, it can influence the presence and absenceof certain forms of animal life. The following three distribution patterns areobserved for the organisms:

1. Random distribution when each organism appear to be placedwithout any regard to where another organism occur; there is nonegative or positive interaction between individuals.

Q. What is haze?

2. Clumped or Contagious distribution, when several individuals ,-tll~

clumped together in various spots throughout the area. This is themost common pattern of dispersion found in animals and plants. Forexample, ants, bees, termites are grouped in colonies, fish into shoalsand humans are clumped together because of social behaviour,economies and geography.

3. _Regular or Uniform distribution occur where competition betweenindividual is severe and thus individuals are more or less evenlyspaced.

In animals, such distribution occurs due to some form of intra-specificcompetition (e.g. birds during breeding season establish and defend territoriesfrom other individuals, thus parceling spaces in a regular way). Uniformdistribution happens among plants when there is severe competition for crownand root space, as among forest trees, or for moisture, as among desert plants.

Temporal stratification refers to the more or less rhythmic changes inactivities or movements of organisms, which produce regular recurring changesin the complexion of the community a~ a whole. They are synchronized withphysical influences such as length of day and night (photoperiodicity), lightintensity, humidity, temperature, tides, etc. The solar day, lunar day, tidalrhythms, monthly and annual rhythms are comm6n among animal and plants.

The term diel periodicity refers to the events, which occur at intervals of24 hours or less. For example, photosynthesis is a good example of dailyperiodicity. In order to manufacture food, a plant must have sunlight. Therefore,the dawn signals the beginning of the food manufacturing process while the dusk

. ends the process of photosynthesis. Many animals are active only during theperiod of darkness (nocturnal), others during the day a(diurnal) and still othersduring twilight periods (crepuscular). A striking example of periodicity in aquatichabitat is found in vertical migration of plankton. Thus, in a freshwater lake, thezooplankton migrate upwards to the surface at night, and then migratedownwards when the day arrives. Some of the phytoplankton (diatoms) have areverse procedure. They migrate to the surface during the day and carryonphotosynthesis and move downward during the night (Fig. 9.3).

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!/- ----. sun.............

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.;, rhythmic rise and fall of oceanic water twice a day, exerts a strongI marine biota living in shallow waters. Lunar cycles are closelyreproductive cycle in some marine forms. For example, the gruniontenius)· a small fish of the Pacific coast, 'spawns three or four daysmoon, corresponding to the highest tide each month between Aprilthe sandy shores of California ..The marine alga, Oictyota produces

t the time of full moon springtime.

seasonal activities of plants and animals are sometimes due toemperature and sometimes controlled by the cycle of dry rainylight may also influence seasonal periodicities. Seasonal activitiesJrowing season of plants, mating and migration of animals, floweringy of plants. and hibernation of animals.

d. Life Form: The life form represents the morphological adaptationof organisms to their environment. A number of plant ecologistshave attempted to classify land plants according to their form(growth forms as tree, shrubs, herbs, forbs, etc.), habitat or someother characteristics. Of the various classifications, the oneproposed by Raunkiaer (1934) has gained wide recognition andhas been extensively followed. Raunkiaer classified terrestrialplants into following five categories, on the basis of the position ofperennating buds on plant and the degree of prot6ction duringadverse conditions (Fig. 9.4).

1. Phanerophytes (visible plants): These have their buds morethan 50 cm above from the soil and are usually not protectedexcept that sometimes scales present. They include woody shrubsand trees.

2. Chamaephytes (dwarf plants): These are small shrubs, whichhave their buds close to the ground surface (less than 25 cm inheight).

3. Hemicryptophytes (half-hidden plants): These are plants likeherbs and grasses in which buds are just below the soil surface.

4. Cryptophyes (hidden plants): The perennating organs of theseplants are well protected under water or soil. This type includesthe Geophytes (terrestrial plants with underground tubers. bulh:::.etc); Helophiytes (marsh plants with rhizomes Illl, \';:'.rthe soil); anelHydrophytes (aquatic plants with buds remaining under water).

5. Therophytes (fine seasonal plants): These plants do not havespecial perennating organs and pass unfavourable period in theform of seeds. They are mostly annual herbs common ingrassland and deserts.

Attempts have been made to classify animals also on the basis of Illeform, but no definite system has resulted.

The major life forms often agree with their taxonomy than do plants. Eachmajor type may again be divided into narrower structural or behavioral types. Forexample, Obsurn et al (1903) classified four-footed mammalian forms into tilefollowing types:

1. Aquatic (swimming)2. Fossorial (burrowing)3. Cursorial (running)4. Saltatorial (leaping)5. Scansorial (climbing)6. Aerial (flying)

Seal, Whale, Warlus.Mole, Shrews.Deer, Horse, Zebra.Kangaroo, Jumping mice.Monkey, SquirrelBat.

Fig. 9.4: Raunikiare's life formsThe barred region inphanerophytes and solid regions inthe rest of life forms, survive duringadverse periods of the growingseason. Note the persistent partwith buds (shaded dark).

IVitality: Vitality means the condition of a plant and its capacity tocomplete the life cycle. Some species are weak and their seedsfail to germinate. The health of a species is determined by theweight of a plant, stem height, root length, leaf area, leaf numberand number of flowers, fruits, seeds, etc. On this basis, plants aregrouped into five vitality groups:

I Q. What are the most productive ecosystems?

V1 = plants which germinate but die soon without reproducing.

V2 = plants which linger after germination but can not reproduce

V3 = plants reproducing but only vegetation.

V4 = plants reproducing sexually but rather feebly.

V5 = plants reproducing well sexually and grow regularly.

II. SYNTHETIC CHARACTERISTICS:Tilese are determined after computing the data on the quantitative and

qualitative characters of the community. For comparing the vegetation of differentareas, community comparison needs the calculation of their synthetic characters.These are determined in terms of:

a. Presence and Constancy: Constancy (C) is the ratio piP expressedas a percentage, where "p" is the number of units containing aparticular species out of a total sample of "P" unit. It expresses theextent of occurrence of the individuals of a particular species in tilecommunity i.e. how uniformly a species occurs in a number of standsof the same type of in the community. The species on the basis of itspercentage frequency may belong to any of the followingclasses: .

1. Rare2. Seldom3. Often4. Mostly5. Constantly

present in 1 - 20% of the sampling units.present in 21 - 40$ of the sampling units.present in 40 - 60% of the sampling units.present in 61 - 80% of the sampling units.present in 81 - 100% of the sampling units.

b. Fidelity: This is the degree with which a species is restricted indistribution to one kind of commLinity. such species are sometimesknown as indicators. The species have grouped into five fidelityclasses as follows:

1. Strangers: Rarely occurring species, which have arrived fromother communities or are relics from earlier stages of succession

I Ans: Tropical forests, estuaries, croplands and swamps.

3. Preferential: Species, which are present in several neighbouringcommunities but are predominant in one.

4. Selective: Species occurring frequently in one kind of communityand sometimes also in others.

5. Dominance: The relative importance of a species in thecommunity is expressed by dominance. The characters' like

, frequency,. density, cover or biomass cannot be used singly toshow the relative position of a species: A single tree with a largecover and biomass is of little importance in a grassland communityand likewise, grasses although numerous have little importance ina forest community in comparison to a big tree. Curtis andMcintosh (1950) proposed an Importance Value Index (IVI),which is the sum of the relative values of three quantitativecharacteristics. The relative values are expressed as percentageof the total values for all species in a community, as given below:

Now for lVI, three values are added. IVI values of different species arethan arranged in a decreasing order:

2.'0The methods to study the community structure depend upon the type of

community, the habitat and physiography of the area. Accorciingly different'methods have been followed for terrestrial, marine, or freshwater com unities,for forests, grasslands or hilly ·vegetation. These include Floristic, Physiognomicand Phytosociological methods (such as, quadrate, transect. point methoc. etc,)However, .it is beyond the scope of the text of the book to go into details of suchmethods.

In nature, horizontal changes in major communities do not begin and endabruptly. But usually communities blend into one another so that it is difficult todetermine where one tarts and other enqs. Transition areas in which two largebiotic communities meet and blend are called Ecotones or Ten'sion Zones, Forexample, a seashore, is the ecotone between the terrestrial and marinecommunities; the border between forest and a grassland; the bank of a streamrunning through a meadow; the marshy belt between a lake and terrestrial land,

A general characteristic feature of the ecotone is that it has sufficientlygreater number of species and more9ver, the density of most of the species ishigher than that in the nEi,ighbouring communities. The reason for the greaternumber of species and diversity found in an ecotone is that there is anintermingling of the species from both the bordering communities as well asspecies specifically adopted to the ecotone. This species diversity in an ecotoneis sometimes called Edge Effect. For instance, birds are often present in greatervariety and density at the edge of a forest than in either the forest or the fieldproper.

In the ecotone area the conditions of temperature, moisture, light, wind,and other physical influences are different, from, and usually, intermediatebetween those existing well within either of the bordering communities, These oroth-er conditions, such as food or shelter may. be superior in the region of theecotone for certain species, For example, at the margin of a pond willows andcattails thrive in the transition zone between,land and water, and here are foundsuch animals as turtles, frogs, herons, muskrats and a number of insects andinvertebrates that are entirely absent or much less abundant in the centre of thepond or in terrain far removed from the water.

I Ans: NU~lear fission,