Study with the several resources on Docsity
Earn points by helping other students or get them with a premium plan
Prepare for your exams
Study with the several resources on Docsity
Earn points to download
Earn points by helping other students or get them with a premium plan
Community
Ask the community for help and clear up your study doubts
Discover the best universities in your country according to Docsity users
Free resources
Download our free guides on studying techniques, anxiety management strategies, and thesis advice from Docsity tutors
population ecology on different environments
Typology: Study notes
1 / 25
This page cannot be seen from the preview
Don't miss anything!
Centre for Environment and Vocational Studies, Panjab University, Chandigarh Department of Botany, Panjab University, Chandigarh
Significant Key words
Population ecology: Population Characteristics, Dynamics and Regulation, Growth curves; Community ecology: Community characteristics, methods of studying, frequency, density, cover, life forms, biological spectrum; ecotypes; ecads. Ecological succession
A population refers to a group of individuals of one kind with no barriers to exchange of genetic material in a given area at a given time. For example, population of human beings in a city, or population of squirrels or of lions in a forest, or pine trees in a given land (Photo 1). The study dealing with structure and dynamics of individuals in a population and their interactions with environment is known as Population Ecology. It has almost the same meaning as that of conventional term Autecology (the study of ecology of individual species or its population), which is less in use now. Population ecology is a significant branch of ecology that plays an important role in protecting and managing populations, especially those of rare species, through various means including PVA (Population Viability Analysis). PVA helps to determine whether a population would survive or face the risk of extinction (complete disappearance of a species from the biosphere) under a given set of environmental conditions. Further, each population has a minimum viable size - the size at which it can avoid the extinction due to various biotic and abiotic factors.
Photo 1. A population of Pine trees
There are two types of populations:
a) Unitary Populations b) Modular Populations
In unitary populations, each individual is derived from zygote (the product of fertilization of male and female gamete) and the growth of such individuals is determinate and predictable. Examples include mammals (including humans), birds, amphibians and insects. Each cow has four legs, two eyes, and a tail., i.e., each individual shows a definite shape and size (Photo 2a).
In contrast, modular populations are those where an organism develops from a zygote and serves as a unit module and several other modules are produced from it, forming a branching pattern (Photo 2b). Examples of modular organisms are plants, sponges, hydroids, fungi, bacteria and corals. Some modular organisms such as trees may grow vertically while others like grasses spread horizontally on the substratum. The structure and pattern of modular organisms is not determinate and thus unpredictable.
Photo 2a. Unitary Population of Cow Photo 2b. A Grass showing different Ramets
Regular or Uniform dispersion: In this type, the individuals of a species occur uniformly which is observed in terms of almost equal distances between individuals (Figure 1). This type of dispersion is rare in natural ecosystems but common in manmade ecosystems like agro-ecosystems or tree plantations.
Random dispersion : In random dispersion, position of an individual in a population is unrelated to the positions of other individuals (Figure 1). In other words, individuals do not show any systematic pattern of dispersion. This type of dispersion is also rare in nature.
Clumped dispersion : In this type of pattern, the individuals of a species are clumped together in space in the form of patches (Figure 1). This type of patchy distribution is quite common in nature as individuals of a population occur together because of food availability or better survival rate as in animal populations. In plants, the clumped distribution is very common, and attributed to nutrient availability, specific habitat preference or better environmental conditions. Example of this kind can also be seen in the social aggregations that are formed in response to some environmental suitability.
Regular Random Clumped
Figure 1. Different types of dispersion of organisms in a population
A population is comprised of individuals of different age groups that constitute its age structure. Age structure of a population thus derives from the proportion of individuals in different age groups. For the sake of convenience, the age categories have been divided into three major stages, Pre-reproductive , Reproductive and Post- reproductive. The proportion of different stages in a population is presented graphically in the form of age pyramid. An age pyramid is thus a geometrical model showing the proportions of different age groups of a population. Populations with equal proportion of major three stages are said to be stationary populations (Figure 2a). A population with high number of young individuals as compared to the older organisms is increasing or progressive type and the pyramid of such a population would have a broader base (Figure 2b). On the other hand, if the number of older organisms is more than the younger ones, the population is said to be retrogressive or declining type. The base of the pyramid of such population would be narrow (Figure 2c).
Stationary Progressive Declining
(a) (b) (c)
Figure 2. Different types of Age Pyramids ( Pre-reproductive; Reproductive; Post-reproductive).
Natality means production of new individuals (offspring) of an organism in a population. The new individuals can be formed through birth (as in human beings), hatching (for example, in chicken eggs), germination (in plants), or cell division (lower organisms). The number of offspring produced per female per unit time is known as rate of natality. Natality can be of two types:
Maximum or Absolute natality Ecological natality
Maximum or Absolute natality also known as Fecundity rate means maximum offspring produced under most suitable environmental conditions. This value is theoretical (since the environmental conditions are never static and keep on changing) and constant for a given population.
Ecological natality also known as Fertility rate , on the other hand, refers to number of offspring produced under prevailing environmental conditions.
Mortality refers to death of individuals in a population. Rate of death of individuals referred to as Mortality rate is of two types:
Minimum mortality rate Ecological mortality rate
Minimum mortality rate , or also known as Physiological longevity , refers to the theoretical minimum death rate which occurs under ideal conditions of environment with minimum limiting factors. This value is a theoretical value and constant for a given population. Under actual environmental conditions, the death rate may be more and this actual death rate is referred to as Ecological mortality.
The other way of expressing mortality is vital index, which is ratio of birth to death rate and expressed as percentage.
Vital Index = -- Number of births- × 100
Number of deaths
The most popular way to express mortality in a population is to prepare a survivorship curve. A survivorship curve for a given population is a graph drawn between numbers of survivors (on a log scale) on Y-axis against age on the X-axis. In general, there are three patterns of survivorship curves (Figure 3).
Type 1 - It is also known as highly convex curve. It reflects higher rate of survival or low rate of mortality of younger individuals as compared to the older ones. This type of curve is found in human beings.
Type 2 - This curve shows a steady death of individuals per unit time throughout the life, and is found in some reptiles, corals, honeybees and rodents. This shows a straight-line relationship between age and number of survivors.
Type 3 - This is also known as highly concave curve. It shows higher mortality of individuals at young stage as compared to old stage. It is found in plants, sea urchins and fish species.
It is the inherent power of a population to grow and reproduce when environmental conditions are favorable and resources are unlimited. Biotic potential is represented by r.
Populations are never static and keep changing in time and space. These changes in population size over time show varied trends. When environment is unlimited (adequate space and food supply) the specific growth rate (population growth rate per individual) of populations becomes maximum and constant under a set of environmental conditions. On the other hand, if the food supply or other resources are limited, the growth rate is typically sigmoid, i.e. increases slowly in the beginning followed by rapid increase and then becomes constant as it approaches the upper limit. To address these growth patterns, there are two types of growth models These are:
a. J-shaped or Exponential Growth Model b. S-shaped or Sigmoid or Logistic Growth Model
In exponential growth type population increases geometrically or exponentially until there is resource limitation or population growth is limited by other factors. Growth then declines rapidly until favorable period is restored. Mathematically this growth model can be expressed as rate of population increase with time t, i.e.
dN/dt = rN Where N = population size, t = time, and r = intrinsic rate of natural increase. The value of r is the maximum when resources are not limiting. Since the curve drawn between population size (Y-axis) and time (X-axis) is J-shaped , it is also known as J curve or J-shaped growth model (Figure 5a).
When population growth occurs at a place where resources are limited, it attains a sigmoid or S -shaped curve
showing minimum death during early stages. The population increases in size until it reaches an upper limit. This
upper limit is known as the Carrying capacity , which is denoted by ‘K’. Carrying capacity thus may be defined as
capacity of an ecosystem to support maximum number of individuals of a species. As the population size increases,
population growth rate declines as it approaches carrying capacity. Sigmoid growth is thus density dependent and
can be expressed by the following equation:
dN/dt = rN (K-N/K)
Where N = population size, t = time, r = intrinsic rate of natural increase, and K = carrying capacity. When N equals K, the growth rate becomes zero and the population reaches equilibrium.
dN/dt = rN
Number of Individuals
Time
Time
Number of Individuals
Figure 5. Population Growth Models; a: J -shaped or Exponential; b: Sigmoidal or S -shaped. Figure 5
W.C. Allee, an ecologist known for his extensive research on social behaviour of animals, gave a concept known as Allee’s principle. Allee’s prinicple is a relationship between population density and survival of animals. According to Allee, both under-crowding (low population density) and over-crowding (high population density) limit growth and survival of a population. There are a number of examples (in both plants and animals) where Allee’s principle holds good. A number of plant species occur in groups, which may be in response to habitat preference or suitable climatic or environmental conditions or due to reproductive strategies. Within a group, the survival rates of species increase in response to the adverse environmental conditions. For example, species of Polygonum pleibium prefers to grow in clayey soil and often form groups or patches. Likewise, populations of Stellaria media or Anagallis arvensis form patches owing to their preference for better moisture conditions. Some species form groups or patches due to vegetative reproduction or due to lack of effective seed dispersal mechanism. Survival chances and fitness of such species is best at moderate populations. As the population density increases beyond limit, there is competition for resources, and it is detrimental to growth and survival of such species. The Allee’s Principle is also valid in animal populations. There are a number of social insects, termites, ants, which survive and grow best at moderate densities and are able to overcome harsh conditions. Bees and colonial bird are the best examples of group survival. Allee’s principle is also very relevant to human beings who form social aggregations particularly in the urban environment.
Mean Weight (g)
Number of Individuals m -
Figure 7. Relationship between plant density and biomass indicating Yoda’s − 3/2 law.
Community ecology has almost similar meaning as that of Synecology.
The concept of community is very old and traced back to the times of Theophrastus (370-250 BC). A community, also known as biotic community or ecological community or biocoenosis, refers to a group of co-existing and interacting populations in a given space and time (Photo 3). For example, a forest community is reflection of co- existence and interactions of a variety of populations – the trees, shrubs, herbs, grasses, animals, and microorganisms. In other words, it is the biological part of the ecosystem distinct from the abiotic part. Earlier, a community was interpreted as a superorganism because it was thought to behave as a single entity. In contrast to this, another view perceived community as a collection of species where each individual species has its own identity.
Each community has spatial limits or boundaries. The boundaries between communities may be very sharp such as boundary between a forest and a lake or less sharp, e.g. boundary between two types of forests or a forest and a grassland community. Often there is some transitional area between two communities that is knows as Ecotone where species of both adjacent communities are found. The ecotonal communities are rich in species diversity because of the edge effect (contrasting environmental conditions at the boundaries or the edges supporting a high species richness). For instance, a patch of land between two forest communities will have animals and plants common to both the forest communities.
There are various characteristics of communities such as species diversity, structure and composition, dominance, succession (or developmental history) and trophic structure. Each one of these is discussed as under:
Species diversity : Each community is composed of taxonomically different species. Species diversity refers to number of different species in the community including both abundant and rare species. Species diversity is very high in natural communities like tropical rain forests or coral reefs in oceans, whereas it is very low in physically or human controlled communities. Species diversity has two components: species richness and species evenness. In simple words, species richness refers to different types of species and their numerical strength. Technically, it refers to ratio between different species (S) and total number of species (N). Species evenness refers to a measure which qualifies as to how even species are in terms of their number. In a community, it refers to the apportionment of each species. For example, a community is quite even if there are 10 species with 10 or 9 individuals of each species; whereas a community is uneven if there are 10 species of which one species has 90 individuals and the rest 9 species have only 10 individuals.
Species diversity can be measured by using various diversity indices – the mathematical expressions based on species abundance data. Species diversity can be measured separately either as species richness or evenness or diversity as a whole. Species richness is measured by Index of richness (denoted by R in the formulae given in Box 1) given by Margalef (1958). Species evenness can be measured with evenness index (denoted by E) given by Hill (1973). Diversity of the species can also be calculated directly with a variety of indices, of which two
commonly used are Shannon-Weiner Index or simply the Index of diversity or Shannon’s index (denoted by H´; as given by Shannon and Weaver, 1963) and Index of dominance (or λ ) or Simpson’s index given by Simpson (1949) (See Box 1). Shannon‘s index has a direct relationship with the species diversity, whereas index of dominance has an inverse relationship. The formulae for calculating various species diversity indices are given in the Box 1.
Box 1. Formulae for calculating various species diversity indices.
(^2) P as per Simpson (1949)
H´P
Where, S = total number of species, N = total number of individuals of all the species in a given area, n i = number of individuals of the i th species of the area_._
For a community to be stable, it requires two components - Resilience and Resistance. Resilience is the ability of a community to recover after facing a disturbance or displacement. Resistance , on the other hand, is the ability of a community to avoid disturbance (any event that alters structure of a community) or displacement (shifting of the community to some other place). One of the reasons for the species rich complex communities acquiring stability is that any change in one or a few species would be compensated by the other species. Some other studies, however, have indicated that greater complexity in a community leads to instability. Thus, it remains controversial whether complexity of a community leads to stability or unstable conditions. However, ecologists have successfully found a relationship between competition and diversity or stability. If the competition is severe, there is low diversity because only those species survive what are able to withstand harsh conditions by suitably adapting themselves. On the other hand, if the competition is weak and the requirements of species do not overlap, the species will not fight for resources and thus more and more species can coexist. The competition becomes intense if the resources for the life support system – food, air, water, space, sunlight (in case of plants, especially) are scarce and the requirements of the species overlap.
Each community has its own structure and composition. For instance, the community of rain forests in silent valley will be different from that of rain forests in Arunachal Pradesh. Community structure is often expressed in terms of its major growth form such as trees in forests or grasses in grasslands. The arrangement of different growth forms determines the structural pattern of the community. In a community, spatial arrangement of the components is also very important. For example, in a forest, some plants may be shade loving and confined to understorey while others are adapted to intense sunlight like emergent trees.
compared to the lower layers. As a result, the water circulates only in the top warmer layers and does not mix with the lower colder water layers. This creates a sharp temperature gradient separating upper circulating warmer layers known as Epilimnion from lower non-circulating colder layers known as Hypolimnion. In between Epilimnion and Hypolimnion is Thermocline - a zone differentiating the two layers of water based on temperature difference.
Thermocline
Epilimnion^ Hypolimnion
Planktons
Figure 9. Different layers (communities) in a pond formed in response to temperature.
The forest communities are highly stratified (forming distinct vertical storeys). In a typical forest, there are five different vertical storey viz. subterranean part (deep in soil), forest floor, herbaceous vegetation, shrubs, and trees. In contrast, grasslands show poor vertical stratification. It has only two layers - a subterranean part with roots and rhizomes and herbaceous part consisting of grasses, herbs and weeds.
Dominance: A community is a heterogeneous assemblage of species. Not all species present in it are equally important and thus only a few of them have a major controlling influence based on their number, size or productivity. Such groups of species are not taxonomically related and influence the energy flow and affect the environment of other species. These are known as Ecological Dominants. In land communities, some plants have a major influence over the others by virtue of their greater number (Numerical dominance). These protect and provide shelter to the organisms and are capable of influencing physical environment.
Trophic Structure : In addition to above, each community has its own trophic structure or organisms grouped based on feeding habits. Trophic structure of a pond consists of a variety of organisms as producers (which can prepare their own food through photosynthesis), consumers (heterotrophs which can not prepare their own food but are dependent on producers for nutrition directly or indirectly) and decomposers (which decompose the dead and decaying matter and in this way release nutrients). Rooted or free-floating green plants (macrophytes), free- floating minute organisms (phytoplanktons – green algae and diatoms) constitute producers or autotrophs of a pond community. The consumers may be primary (herbivores that directly feed on green plants or algae), secondary (carnivores that feed on herbivores) or tertiary (feeding on other carnivores). Zooplankton or floating minute animals like rotifers, crustaceans and protozoans, which feed on phytoplanktons, constitute herbivores in the pond community. In addition, there are several animal species associated with the green plants and feed on them. Some herbivores are also present at the bottom of the pond and feed on dead decaying plant parts. These may be beetles, mollusks or even crustaceans. Some birds and domesticated animals such as cow, goat and buffaloes also feed on green plants found in the pond especially on the margins in the littoral zone. Fishes constitute the secondary consumer of the pond feeding largely on herbivores. Some insects are also included in this category. In the pond, some larger fish or the game fish that feed on smaller fish constitute tertiary consumers. Besides there are varieties of decomposers (microconsumers – since they take a fraction of food) in the pond and these decompose complex, dead and decaying matter into simpler forms like nutrients, which are absorbed by the plants for their growth and development.
Trophic structure of a forest community also has a same pattern but with different species composition. In a forest, the autotrophs or the producers are the trees, which are also the dominant species. Additionally, the forests also have shrubs, herbs and grasses that are autotrophic and form a distinct understorey, but their role is lesser than that of trees. The type of the trees in the forests varies from place to place depending upon environmental conditions.
For example, a typical tropical moist deciduous forest is composed of tree species like teak ( Tectona grandis ), sal ( Shorea robusta ) or Queen’s myrtle ( Lagerstoemia parviflora ); whereas a temperate deciduous forest has trees like oak ( Quercus ), maple ( Acer sp.), birch ( Betula sp.) and spruce ( Picea sp.). The primary consumers in the forests include ants, beetles, leafhoppers, spider and bugs that feed on tree leaves. Besides, there are larger animals like elephants, nilgai, squirrels, rabbits, flying foxes and birds, which feed on shoots or fruits of the trees Birds, snakes, lizards and foxes constitute secondary consumers whereas lions and tigers constitute tertiary consumers., The decomposers include several types of fungi, bacteria and actinomycetes.
Succession: Succession is the orderly process of community development and refers to the continuous, unidirectional and sequential change in the species composition of a given community over time. It involves various stages during which a specific set of species occupy the area and replaced by the next group of species. All these stages of succession are known as seral stages. The first stage of succession when the bare area is colonized for the first time is known as Pioneer stage and such species are referred to as Pioneer species. The final, mature, stable and long lasting community is known as Climax community.
Plant communities can be studied by different methods such as floristic (by simply studying various genera and species) and physiognomic (based on Raunkiaer’s life forms) and phyto-social methods. Of these, phyto-social methods are preferred. In these, the data on the vegetation is collected in terms of types of species present and individual number of each type in an area. As the areas are very large, it is not possible to count every plant, thus the area is divided into smaller units known as sampling units. Three types of sampling units are generally considered for studying various plant communities. These are: a) Area, b) Line, and c) Point. Area and line both are based on definite size of the sampling unit while point is used in those situations where it is difficult to determine area e.g. thick forest. Sampling unit where definite area is selected is known as Quadrat. Quadrat is thus a sampling unit of definite area that is usually a square but it can also be a rectangle or circle. Size and number of quadrats are determined based on the objective and features of area under consideration. Depending upon the purpose of study, the quadrat may be list quadrat (where species present in the area are listed), list-count quadrat (where species are listed as well as their numbers counted), chart quadrat (where all details like distribution of species, their number are recorded on a graph paper periodically using an instrument pantograph) and permanent quadrat (used in the experimental studies where vegetation is recorded for a long time to find out changes). Transect is the term used in cases where sampling unit is a strip of definite area. Transect may be a line or belt depending upon the study area. In a line transect, sampling is usually done across a line. In belt transect, an area (belt) of suitable size is selected where the sampling is done. Belt transects are particularly used in forests and can be further divided into segments for convenience.
For determining quadrat size, species-area curve method is used. Sampling unit size is increased gradually (starting from a minimum) and the number of types of species counted in each sampling unit. It is continued until number of species become constant for three consecutive times. Then, a graph is drawn between area (X-axis) and number of species (Y-axis) and from the curve so obtained, optimum size of quadrat (where the number of species becomes constant) is determined.
Area (cm 2 )
Number of species
Minimum quadrat size
Figure 10. A typical species-area curve for calculating the minimum quadrat size.
Q means quadrat; 5 quadrats of 1m^2 were laid.
Individual density of a species Relative density of a species = ------------------------------------------------------ Total density of all species encountered
Density can also be presented as relative density. Relative density is calculated based on following formula:
Note: Since the relative value of density is less than 1.0. It can thus be converted into percent by multiplying with
From the above example (Table 1), the relative density for species 1 is calculated by taking its individual density i.e. 30 and dividing it by 137, i.e. total densities of all species. To convert it into percent relative density, multiply it with 100. It comes out to be 21.80%. Likewise, the relative density of other species can be calculated.
Abundance is also calculated like density but in this case, only those quadrats are considered for calculation where a species actually occurs. For example, if a species has occurred in only 3 quadrats out of total 5 studied, then the total number of individuals of the species is divided by 3 (instead of 5, as in case of density). The difference between density and abundance thus becomes clear from the example given in Table 1. The formula for calculation of species abundance is:
Total number of plants in all the Quadrats Abundance of a species = -------------------------------------------------------------------- Number of Quadrats of occurrence of plant species
Abundance is also presented on the basis of unit area, i.e. 1m^2 especially in smaller areas or grasslands. However, it is not much used as compared to density in ecological studies. It can also be multiplied by 100 to get percent abundance.
Individual abundance of the species Relative abundance of a species = ---------------------------------------------------------- Total abundance of all species encountered
Frequency : Frequency is another important parameter of vegetation analysis, which reflects the spread, distribution or dispersion of a species in a given area, and given in percent. For example, a species is distributed uniformly in an area there is greater probability of its occurrence in all quadrats and it would have maximum frequency. In another case, a species may be clustered or present only in a part of the area. In this case, it will occur only in few quadrats and hence it would have lesser frequency. The frequency of a species in a given area is studied by either quadrat method or transects and is calculated by the following formula:
Number of Quadrats in which a species occurs Frequency = ---------------------------------------------------------------- × 100 Total number of Quadrats studied
Thus, if a species occurs in 5 out of total 10 quadrats studied, its frequency would be 50%. If a species occurs in all the quadrats studied, its frequency would be 100%. The frequency determination also becomes clear from Table 1.
Frequency is a very important quantitative parameter. Raunkiaer (1934) made an elaborative study on the frequency of species in about 8000 quadrats and based on his data, he divided species into 5 classes viz. A, B, C, D, E. The distribution of frequency in 5 classes is given hereunder in Table 2.
Table 2. Raunkiaer’s frequency classes.
Frequency Class Frequency Range A 1-20% B 21-40% C 41-60% D 61-80% E 81-100%
Further, Raunkiaer suggested Law of frequency and Normal Frequency Diagram based on the data from his studies in all the natural ecosystems. According to law of frequency, species poorly distributed or dispersed in an area are likely to be presented more compared to those that have better or more dispersion in an area. In other words, A>B>C> = <D<E, i.e. A is greater than B, which is greater than C, and C may be greater or equal or lesser than D, which in turn is lesser than E. Raunkiaer’s normal frequency diagram was a histogram made on the basis of the average frequency data in which value of class A was 53%, that of B 14%, C 9%, D 8% and E 16%. Raunkiaer also prepared a normal frequency diagram, a J -shaped curve, which represents homo- or hetero-geneity of a community. It is a J -shaped curve. After the preposition of Law of frequency, a number of studies were undertaken in various parts of the world and similar observations were obtained especially in the natural and undisturbed ecosystems. In disturbed ecosystems, however, the frequency distribution varies from that of normal as proposed by Raunkiaer.
5
6
1
3
4
2
1 Germination 2 Vegetative Phase 3 Flowering 4 Fruit formation 5 Seed maturation 6 Death
Different Stages of Life Cycle
Figure 11. A typical phenogram showing different stages in life cycle of plants.
Physiognomy: Physiognomy refers the outer appearance of a community and is an important parameter that tells us about the structure of community. It is based on the growth form of its dominant species. For example, a grassland community is dominated by grasses, forests by trees and chaparral community by shrubs.
by a Danish botanist Christen C. Raunkiaer. According to Raunkiaer, in a community it is very important to know how a plant survives during unfavourable conditions. He took the criterion of protection of perennating buds during adverse conditions as an adaptation of plant to climate. Accordingly he proposed a system known as Raunkiaer’s system in which plants were categorized into various life forms based on the position of their buds during seasons of unfavourable conditions (too much cold or too much hot). Raunkiaer considered five major types of life forms viz. Phanerophytes, Chamaephytes, Hemicryptophytes, Cryptophytes, and Therophytes.
whose buds are situated high up on the plant on the top of the shoots. These are either naked or covered with scales. Phanerophytes are very common in tropical areas and their number decreases towards temperate and polar areas. Based on the height of trees, phanerophytes are further divided into 4 categories:
a) Mega-phanerophytes – trees taller than 30 m
b) Meso-phanerophytes – trees between 8 – 30 m
c) Micro-phanerophytes – trees between 2 – 8 m height
d) Nano-phanerophytes – shrubs shorter than 2 m but more than 25 cm
plants are found in colder regions at high altitudes or latitudes, e.g. Temperate America. During the growing season, sometimes the aerial parts of chamaephytes die and cover the buds. Fresh growth occurs during the onset of favourable season.
(but not deep-seated), and are protected. These include annual (plants which complete their life cycle in one year or one season) or biennial (which complete their life cycle in 2 years or 2 seasons) herbs.
bulbs and rhizomes. Such plants are mostly found in the arid regions of the world.
favourable conditions and during the rest of the unfavourable conditions remain in the form of seeds.
Besides these five major categories, Raunkiaer also identified epiphytes (plant growing on or attached to other plants) as a separate category of life forms. Additionally, he also divided cryprophytes into three subtypes: geophytes (plants buried in soil with subterranean or perennating buds), hydrophytes (plants submerged or floating in aquatic systems with perennating buds inside water), and halophytes (plants in marshy swampy areas with high salt concentrations).
community is and is also known as Phyto-climatic Spectrum. Thus for preparing a biological spectrum percentage of each of the 5 life forms is calculated. Raunkiaer (1934) also prepared a normal biological spectrum of flora of the world based on his elaborative and extensive ecological studies. For the normal biological spectrum, the percent values of different life form are given (Table 3).
Table 3. Percent of different life forms in a community forming a normal biological spectrum.
Life Form Relative % of species Phanerophytes 46 Chamaephytes 9 Hemicryptophytes 26 Cryptophytes 6 Therophytes 13
The biological spectrum obtained for any area is compared with normal biological spectrum that reflects the variations or deviations from the normal. It is generally thought that biological spectrum of a region reflects its environmental or the climatic conditions. For example, higher ratio of phanerophytes in an area indicates tropical conditions and those of chamaephytes reflect extreme cold conditions. Thus, it has been suggested as an indicator of climatic condition of an area. However, its utility is limited since biological spectrum is disturbed when the environmental conditions fluctuate. Further, biotic stress also affects the biological spectrum of an area that too limits its use.
In addition to various qualitative and quantitative features, communities are identified by various synthetic features such as Importance value index (IVI) and various ecological indices like index of diversity, index of dominance, index of evenness, and index of richness (see Box 1). IVI is calculated by adding relative density, relative dominance and relative frequency (the method of their calculation is already stated). IVI is an important parameter, which indicates the overall ecological importance or status of species such as its numerical strength, its degree of dispersion and area of ground covered by it. Various indices such as index of diversity, index of dominance, index of evenness, index of richness can be determined by standard formulae as indicated in Box 1. For this, computer based software are also available, of which the most widely used is the one given by Ludwig and Reynolds (1988) pertains to statistical ecology.
Environmental conditions exhibit great fluctuations. Plant species have to tolerate these variations in the environmental conditions, which may be reflected in terms of different climatic conditions, habitats, edaphic conditions or even different geographic areas. The survival of species in such conditions is dependent upon its ecological amplitude – the extent to which a species may tolerate environmental variations. Species with a wider ecological amplitude have better adaptability and vice versa. The response of a species to a particular environmental condition may be reflected through several morphological variations (changes in the external appearance, i.e. in terms of height, number and size of leaves, number of branches, number of flowers produced, size of flowers and seed output).
