The major qualitative characters of the plant community (and also of the animal community) are as follows:
Physiognomy is the external appearance of the plant community which may be described on the basis of dominant plants, density, height, colour, etc., of the plants. It does not emphasize any particular species or individuals.
In fact, Cain and Castro (1959) have defined physiognomy as the form and structure of the vegetation, the appearance that results from the life-form of the predominant plants. (Life-form is a general term referring to the shape or appearance of an organism irrespective of how formed, Daubenmire 1947). The terms like forests, grasslands, savannahs, scrub, bog, etc., describe the physiognomy of various communities.
2. Growth-forms and life-forms:
Many plant ecologists have attempted to classify plants according to their form (i.e., growth form and life form), habitat, or some other characteristics. Thus, on the basis of growth forms, Pound and Clements (1900) have classified plants into shrubs, trees and herbs and further subdivided these categories into needle-leaf evergreens, broad-leaf evergreens, evergreen selerophyll (small, tough, evergreen leaves, as in chamise), broadleaf deciduous, thorn trees and shrubs, dwarf shrubs, ferns, grasses, forbs, mosses, liverworts, lichens, fungi, algae and so on.
Of the other various classifications, the one proposed by the Danish botanist Christen Raunkiaer (1903, 1934) has gained wide recognition and has been extensively followed. Instead of considering the plant’s growth form, Raunchier classified plant life by the relation of the embryonic or meristemic tissues that remain inactive over winter or a dry period (perennating tissue) to their height above ground. Such perennating tissue includes buds, bulbs, tubers, roots, and seeds.
All the species in a community can be grouped into the five classes and the ratio between them expressed as a percentage, providing a life-form spectrum of the area that reflects the plant’s adaptations to the environment, particularly climates.
For example, community with a high percentage of perennating tissue well above ground (phanerophytes) would be characteristic of warm climates. A community with most of its plants chamaephytes and hemi cryptophytes, and a community dominated by therophytes would be characteristic of deserts.
Life-forms of animals:
There have been several attempts to classify the life-forms of animals, but no definite system has resulted (Remane, 1952; Krivolutskii, 1972). The major life-forms more often agree with their taxonomy than do plants, but some life-forms include representatives from several different taxonomic groups.
These can be recognized encrusting forms, such as fresh-water bryozoans Plumatella and some sponges; coral forms, including grass, leaf, or shrub forms: radiate forms, such as coelenterates and echinoderms; bivalve forms; snail forms; slug forms; worm forms; crustacean forms; insect forms; fish, snake, hard and four-footed forms. Each of these major types may be divided into narrower behavioural types, for example, Osburn et al., (1903) subdivided four-footed mammalian forms into the following types:
1. Aquatic (swimming): Seal, whale, walrus.
2. Fossorial (burrowing): Mole, shrew, pocket gopher.
3. Cursorial (running): Deer, antelope, zebra.
4. Saltatorial (leaping): Rabbit, kangaroo, jumping mouse.
5. Scansorial (climbing): Squirrel, opossum, monkey.
6. Aerial (flying): Bat.
1. Phanerophytes (Gr., phaeros, visible). Perennial buds carried well up in the air and exposed to varying climatic conditions. Trees and shrubs over 25 cm; typical of moist, warm environments.
The woody lianas, epiphytes and stem succulents (eg, cacti and euphorbias) are recognised as sub-types of phanerophytes.
2. Chamaephvtes (Gr., charr.ai, on the ground). Perennial shoots or buds on the surface of the ground to about 25 cm above the surface. Buds receive protection from fallen leaves and snow cover. Plants typical of cool, dry climate, e.g., Thymus, Silene, Trifolium, Rhanmus, Druba, Grewia, etc.
3. Hemicryptophytes (Gr., krypos, hidden). Perennial buds at the surface of the ground where they are protected by soil and leave many plants are characterized by rosette leaves and are characteristic of cold, moist climate, e.g., grasses and herbs like Thalicirum, Frag.nia, Primula, etc.
4. Cryptophytes. Perennial buds buried in the ground on a bulb or rhizome where they are protected from freezing and drying. This type includes the hydrophytes (buds remaining under water), helophytes (marsh plants with rhizomes under the soil) and geophytes (terrestrial plants with underground rhizomes, tubers), etc.
5. Therophytes (Gr., thews, summer). Annuals with complete life cycle from seed to seed in one season. Plants (e.g., annual herbs) survive unfavourable periods as seeds and are typical of deserts and grasslands.
3. Phenology, periodicity or aspection:
The form of the plant changes with the age, so the same species in different stages of their life cycle provide a different structure to the community. As already discussed in section 11-52, periodicity refers to the regular seasonal occurrence of various processes and their manifestations (as formation of leaves, flowering, seed shedding).
Aspection refers to the appearance of the community as a whole at different seasons as its appearance in rainy season, summer spring or winter seasons. Phenology has been defined by Lieth (1970, 1974) as the art of observing life-cycle phases or activities of plants and animals in their temporal occurrence throughout the year. Each phase in the life-cycle is called phenophase. The phenophases have been described by various methods using diagrams and symbols.
Organisms are distributed unevenly throughout the biotic community. Spatial distribution may be considered in both vertical and horizontal planes. Some also add another kind of stratification to these, the temporary stratification.
1. Vertical stratification:
Stratification refers to vertical layering of organisms or environmental conditions within a biotic community. The stratification of a community is determined largely by the life-form of plants—their size, branching and leaves—which, in turn influences and is influenced by the vertical gradient of light.
The vertical structure of the plant community provides the physical structure in which many forms of animal life are adapted to live. Vertical stratification has been observed in many communities and some distinct examples of this phenomenon are the following.
(i) Forest community:
A well-developed forest ecosystem has several layers of vegetation. From top to bottom, they are over- story stratum or canopy; understory stratum; transgressive stratum or the shrub; ground layer, seedling stratum or herb, and the forest floor or subterranean stratum.
And one can continue down into the root layer and soil strata. For example, roots may be spread nearer the ground or may be deep penetrating. This spacing among the roots permits the plants to draw their water and nutrient requirements from different layers of the soil without affecting each other.
The canopy, which is the major site of energy fixation, has a major influence on the rest of the forest. If it is fairly open (i.e., forest of dry climate), considerable sunlight will reach the lower layers and the understory tree strata and the shrub will be well developed.
If the canopy is closed (i.e., in rain-forest), the shrub and the understory trees and even the herbaceous layers will be poorly developed. Further, by reaching the canopy, the forest trees may gain advantage, where abundant sunlight supports photosynthesis, but the tree must spend much of the energy of photosynthesis in the growth of woody tissue of stem and branches to support the foliage in the canopy.
There may be apparent disadvantages in the low light intensities in which the forest herbs must live, but the herbs need not spend its more modest photosynthetic profit on woody supporting tissue. Forest structure, thus, involves a gradient of growth-forms-upper and lower trees, upper and lower shrubs, upper and lower herbs, and soil-surface mosses—in adaptation to the gradient of light intensity.
Along the gradient growth- form designs change from one extreme (the upper tree with foliage in full sunlight, massive supporting stem and branch structure, and a root system smaller in mass than the above ground structure) to herbs with adaptations at the other extreme (photosynthesis at low levels of light intensity, small investment in above ground supporting structure, and accumulation of reserve food in a root system more massive than the above ground structure).
The understory stratum consists of tall shrubs, understory trees and younger trees; some are the same as those the crown, others are of different species. Species that is unable to tolerate shade and competition will die; others eventually reach the canopy after some of the older trees die or are harvested.
The shrub layer varies from forest to forest. The nature of the herb layer depends on the soil moisture conditions, slope position, the density of over story and slope attributes, all of which vary from place to place through the forest.
Beneath the herbs, small prostate plants, liverworts and mosses on the ground may form still another vegetation layer. The lichens and epiphytes grow at different strata on the tree trunk and branches. The final layer, the forest floor, is the site where the important process of decomposition of the forest litter takes place and where nutrients are released to the nutrient cycle.
Moreover, the variety of life in the forest is directly related to the number and development of the layers of the forest. For example, in the rain forests with greater species diversity, the stratification is more.
While, in an agricultural field where weeds have been removed all the plants belong to the same strata. However, if certain layers are absent, then the animals they normally shelter and support are also missing. Thus a well-developed forest supports a rich diversity of life.
Vertical stratification in animals:
As different plant species are adapted to different positions in this vertical gradient, so different animal species also occupy different levels in the forest different groups of bird species, for example, may be found feeding and nesting near the ground, in the shrub and small tree foliage beneath the canopy and in the canopy itself.
Different arthropod species occur at different levels from the canopy downward to the herb stratum and to and below the ground surface. A group of animals—mites and spring tails, millipedes and centipedes, ground beetles, and so on —occur primarily in the leaf litter on the soil surface ; these animals, which are seldom seen on the surface by daylight are called cryptozoans. Other animals occur at different depths in the soil.
(ii) Grassland Community:
In grassland community, only three strata, namely the herbaceous layer, the ground or mulch layer, and the root layer, can easily be recognized. The root layer is more pronounced in grasslands than in any other ecosystem and provides permanent residence to soil bacteria, fungi; protozoan’s, nematodes, earthworms, spiders, insects and other invertebrates.
(iii) Aquatic communities:
Aquatic ecosystems such as ponds, lakes and oceans have strata determined by light penetration, temperature profiles and oxygen profiles. In the summer, well-stratified lakes have a layer of freely circulating surface water, the Epilimnion ; a second layer, the metalimnion, which is characterized by a thermo cline (i.e., a very steep and rapid decline in temperature) ; the hypolimnion, a deep, cold layer of dense water about 40°C, often low in oxygen ; and a layer of bottom mud.
In addition two other structural layers are recognized, based on light penetration: an upper zone roughly corresponding to the Epilimnion, which is dominated by phytoplankton and is the site of photosynthesis and a lower layer, in which decomposition is most active. The lower layer roughly corresponds to the hypolimnion and the bottom mud.
All communities, whether terrestrial or aquatic have similar biological or tropic structures. They possess an autotrophic layer, concentrated where light is mostly available, which fixes the energy of the sun and manufactures food from inorganic substances.
In forests this layer is concentrated in the canopy; in grasslands, in the herbaceous layer; and in lakes and seas, in the upper layer of water (i.e., in phytoplankton’s). The biotic communities also possess a heterotrophic layer that utilizes food stored by autotrophs, transfers energy, and circulates matter by means of herbivore, predation in the broadest sense, and decomposition.
2. Horizontal stratification:
Horizontal stratification or dispersion relates to the distribution of organisms, principally plants, in the horizontal space (i.e., on the ground) or across the canopy. Like vertical stratification, it can influence the presence and absence of certain forms of animal life. The following three distribution patterns are observed for the organisms:
(i) Random distribution when each organism appears to be placed without any regard to where another organism occurs; the is no negative or positive interaction between individuals, (ii) Clumped or contagions distribution, when several individuals are clustered together in various spots throughout the area. Ants are grouped into colonies, fish into schools and humans into cities rather than a helter-skelter arrangement, (iii) Regular or uniform distribution which is produced due to negative effect of one organism on another; each organism is spaced at certain intervals from others in the same species.
Such distribution pattern is seen in certain desert plants, such as the creosote bush (Larrea), which are rather uniformly spaced, it is also seen in certain breeding birds that establish and defend territories from other individuals, thus parceling out the space in a regular way.
The plant distribution is mostly clumped to varying degrees. The horizontal distribution of plants and also of some animals is influenced by the following factors:
(i) Seed distribution and vegetative reproduction:
Plants with airborne seeds may be distributed widely, while plants with heavy seeds or with pronounced vegetative reproduction by runners or rhizomes will be clumped near the parent plant. A similar dispersal process also explains clustering of certain sessile animals which release larvae that settle near the reproducing adult.
(ii) Differences in environment:
Herbaceous plants of the forest may be clumped where pools of light reach the forest floor through the canopy. Further, there may be other differences, in the environment. Since the area is not homogeneous, some areas are more suitable for growth, reproduction and survival than others.
The mosaic pattern of the environment leads to a patchy distribution of individuals within it. Clusters of individuals may occur not because of any attraction between them but because they are independently influenced by the same environment.
A pronounced form of horizontal stratification is donation, which is caused primarily by differences in climatic or edaphic conditions that retard or inhibit rooted vegetation. This type of stratification is most conspicuous around ponds and bogs.
(iii) Species interrelations:
There may be negative or positive interactions of species. Positive interactions lead to clustering of individuals and negative interactions lead to an even spacing between them.
3. Temporal stratification:
Besides the differentiation of communities in space, there occurs a differentiation of communities in time also. Environments of biotic communities are rhythmic: in most environments light, temperature and other environmental factors go through daily and yearly cycles.
In some communities, especially those of ocean shores, there are also complex rhythms set by the rise and fall of tides. In general, there occur rhythms of function in biotic communities in adaptation to rhythms of environment.
Plankton population fluctuates rapidly in many water bodies, with species replacing one another in periods of days and weeks. A number of major species may occur as plankton dominants in the course of the year. Thus in freshwater lake species of diatoms and other yellow-green algae may predominate in the winter plankton As water temperatures warm in later spring and early summer, these are replaced by desmids and other green algae.
At peak summer temperatures blue-green algae may predominate or share dominance with the green algae; as temperatures cool dominance shifts back toward the green and yellow-green algae. Each species has an inactive stage in which it survives the season unfavourable for its activity. Each species has its own place in the annual pattern, determined by its own response to the fluctuations of temperature and other environmental factors.
The plankton community shows s differentiation in time: different groups of species occur at different times during the seasonal cycle. The year-round total number of plankton species is much larger than the number present at a given time.
Seasonal and daily differentiation (stratification) occurs also in forests. One group of insects is active in the day time another group at night, and a third group may be active in the twilight transitions of morning and evening. In terrestrial communities flycatchers, warblers, and other insectivorous birds are active by day, bat at night and nighthawks in the dusk.
Progress of the seasons is marked by the appearance of different groups of plants in flower and different groups of insects visiting these flowers. In deciduous forests spring beauties, dogtooth violets, and other herbs develop their foliage and flower early, before the trees are in leaf.
5. Species abundance and species diversity:
Among the array of species that make up the community, relatively few are abundant, and most of them are rare. Abundance or species richness of a species in the community represents its relative distribution in it. It is related to density but is a qualitative estimate.
Large number of individuals occurring at one place in the community will not be referred to as abundant while the same number of individuals spread throughout the community. May appear to be abundant. The species richness or abundance is expressed in five degrees on an arbitrary scale such as follows: (i) rare (r) or very sparse; (ii) occasional (o) or sparse; (iii) frequent (?) or not numerous; (iv) abundant (a) or numerous; and (v) very abundant (va) or very numerous.
Two parameters of species distribution within the community, species richness and species evenness, are useful in measuring an attribute of the community species diversity. A community that contains a few individuals of many species will have a higher diversity than will a community containing the same number of individuals but with most of the individuals confined to a few species.
In order to quantify species diversity for the purpose of comparison, the Shannon wiener index is used. This index measures diversity by ‘the following formula:
Where, H = the diversity of species
S = the number of species
Pi = proportion of individuals of the total sample belonging to the ith species.
This index takes into consideration the number as well as the relative abundance of species.
Besides being a measure of comparison of similar communities within a given region, species diversity is also useful in examining global ecosystems. If one were to leave the tropics and travel north through the Temperate zone to the Arctic zone, one would find the numbers of plants and animals decreasing on a latitudinal gradient.
For example, Fisher (1960) found that species of nesting bird’s approach 1395 in Colombia, drop to 1100 in Panama, to 143 in Florida, to 118 in Newfoundland, and to 56 in Greenland. One can follow the same pattern in mammals (Simpson, 1964), lizards (Pinaka, 1967), trees (Monk, 1967), and fish (Lowe-McConnell, 1969). Species diversity increases from cold to warm climates.
However, in the oceans species diversity increases from the continental shelf, where food is abundant but the environment is more changeable, to the deep, cold water where food is less abundant but the environment is stable.
Likewise mountain areas in general support more species than do flatlands, and peninsulas have fewer species than do adjoining continental areas. Small or remote islands have fewer species than do large islands and those nearer continents (Smith, 1977).
The following hypotheses have been proposed to explain why the tropics should hold a greater abundance of species than the temperate region or why one island should hold more than another:
1. Time theory:
Fischer (1960) and Simpson (1964) have related species diversity to evolutionary time. It is found that older communities hold a greater diversity than young communities. Communities in the tropics evolve and diversify faster than temperate or arctic communities, in part because the environment is more constant and climatic catastrophes less likely. Some paleontological evidences support this theory.
2. Spatial heterogenity theory:
Simpson (1964) also believed that more complex and heterogeneous the physical environment, the more complex and diverse would be its flora and fauna. The greater variation in topographic relief, the more complex the vertical structure of the vegetation, and the more types of microhabitats the community contains, the more kinds of species it will hold. For Sample, the more complex the vertical stratification of a community, the more species of birds it holds (MacArthur, 1972).
3. Climatic stability theory:
Fischer (1960) proposed that the more stable the environment, the more species would be present. Through evolutionary time, the tropics, of all regions of the earth, has probably remained the most constant and has been relatively free from severe environmental conditions that could have effects on a population. Under tropical conditions, natural selection is strongly influenced by the competition of individuals against members of other species.
4. Productivity theory:
Connell and Orias (1964) held that the level of diversity of a community is determined by the amount of energy flowing through the food web. The rate of energy flow is influenced by the limitation of the ecosystem and by the degree of stability of the environment.
5. Competition theory:
Dobzhansky (1951) and Williams (1964) proposed that in mild climatic regions like tropics biological competition becomes more important in the evolution of species and in the specialization of niches than in environments of high physical stress, such as Arctic.
6. Predation theory:
Paine (1966) proposed that the more diverse communities such as tropics support more predators and these predators regulate the abundance of prey species, so that competition in prey species is greatly reduced.
7. Stability-time hypotheses:
Sanders (1968) have suggested that two contrasting types of communities exist, the physically controlled and the biologically controlled. The species numbers diminish continuously along a stress gradient. The greatest diversity occurs among the predominantly biologically accommodated com munities.
Fig. 13.3. The bar-graph representation of the stability time hyposthesis (after Smith. 1974).
The sociability expresses the relation of individuals to each other and indicates the closeness between individuals, within the community. Braun Blanquet (1932) have recognised the following five arbitrary types of sociability : S1, plants growing singly; S2, plants growing in small groups; S3, plants occurring in small patches; S4, plants forming large patches; and Ss, plants occurring in essentially continous populations.
Whitford (1949) expressed sociability of plants in quantitative terms of the following equation:
Sociability = Density ? 100/Frequency
Vitality refers to normal growth and reproductive ability of the plant species which help it in maintaining its position in the community. Plant ecologists have often recognised the following five categories of vitality : V1, plants which geiminate but die soon without reproducing; V2, plants which linger after germination but cannot reproduce; V3, plants reproducing but only vegetatively; V4, plants reproducing sexually but rather feebly; and V5, plants reproducing very well sexually.
8. Disseminule type:
In recent years the type of the dispersal organ or disseminule produced by a plant species has also been used as a community characteristic. Dansereau and Lems (1957) have recognised the following disseminule types of plants (Table 13 2):
Table 13-2. Disseminule types of plants:
Disseminule type according to agentDisseminule type according to morphologyExamples
1. AutochoreExpulsiveVinca, Ruellia
2. AnemochorePu ImoseTridax, Taraxacum
(wind dispersed)MinuteOrchids, ferns
(animal dispersed)NutlikeFicus, Psidium
4. Hydroc oreBuoyantEichhornia, Hydrilla
(water dispersed)Splash cupMarchantia