Contents
One of the interesting Biology Topics is the study of animal behavior and how it is influenced by genetics and the environment.
Population Ecology in Plants, Animals, and Humans – The Population Ecology of Contemporary Adaptations
A group of individuals belonging to a species inhabiting a specific geographical area is known as a population. Therefore, all the individuals of a species residing in a habitat as a part of an ecosystem may be termed a population.
The total number of individuals of species inhabiting a geographical area at a particular time period is considered a population. Population is a dynamic entity related to the ecosystem. Several important properties of the population are density, natality, mortality, etc. Based on these properties human population of an area at a particular time is controlled. Presently, due to population pressure, environmental equilibrium has been distorted and non-renewable resources (coal, petroleum, etc.) are going to be exhausted. Besides, non-judicious exploitation of renewable resources like water, forest wealth, cereals, etc., are at the face of a destructed equilibrium. Therefore, to remove the pressure from the environment and to give man a protected future, a solution to the growth of the population is urgently necessary.
If population growth occurs at the rate in the present manner, the natural resources available will not be sufficient to meet human needs, and therefore environmental equilibrium will go into jeopardy. The term population has been derived from the Latin word populus which means a group of individuals of a species. However, population has a broader dimension. In ecology, the individuals of a species in a local area are considered as population. An increase in population or its reduction in growth is contrary to ecological equilibrium and results in the alternation of many properties of an ecosystem. Due to population growth, the pressure developed in human life is easy to realize.
Population Interactions
Different populations of a biotic community live together or influence each other’s life. In nature, no organism is free from the effects of other living beings. This leads to biotic interaction among different species. There are three types of interactions among different members of a biotic community – positive, negative, and neutral. In positive interactions, one or both the participating species are benefited. These include mutualism, scavenging, commensalism, and proto-cooperation. In negative interactions, one species is harmed and the other is benefited or remains unaffected. These include predation, parasitism, amensalism, and competition. In neutral interactions, no species is harmed or benefitted.
Mutualism
It is an interaction between two organisms of different species where both the partners are benefited. This mutualism can be differentiated into obligate and facultative mutualism.
I. Obligate Mutualism
In this relationship both organisms benefit by living in close association and the relationship is obligatory. The term symbiosis is sometimes used in the same sense as mutualism.
1. Lichen:
The classical example of mutualism is met with in lichens. The lichens are composed of a matrix formed by a fungus within which cells of alga are embedded.
The alga depends upon the fungus for water, minerals, and protection and the fungus receives carbohydrates prepared by the alga. A lichen is a composite organism consisting of a fungus (the mycobiont) and a photosynthetic partner (the photobiont or phycobiont) growing together in a symbiotic relationship. The photobiont is usually either a green alga (commonly Trebouxia) or cyanobacterium (commonly Nostoc). The morphology, physiology, and biochemistry of lichens are very different from those of the isolated fungus and alga in culture. The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations.
The algal or cyanobacterial cells are photosynthetic and as in plants they reduce atmospheric carbon dioxide into organic carbon sugars to feed both symbionts. Both partners gain water and mineral nutrients mainly from the atmosphere, through rain and dust. The fungal partner protects the alga by retaining water, serving as a larger capture area for mineral nutrients and, in some cases, providing minerals obtained from the substrate.
2. Rhizobium:
Another well-known example of mutualism is furnished by the bacteria of the genus Rhizobium. The bacteria form nodules on the roots of leguminous plants and obtain carbohydrates and other substances from the plants. In return, the bacteria fix the gaseous nitrogen and pass it on to the host.
3. Mycorrhiza:
The association between roots of higher plants and fungal hyphae is called mycorrhiza which is also a type of mutualism. The root provides food and shelter to the fungus. The fungus helps the plant in solubilization and absorption of minerals, and water uptake protection against pathogenic fungi.
4. Termites and Trichonympha:
Trichonympha campanula (A flagellate protozoan) lives in the hindgut of termites where it secretes cellulase and helps in the digestion of cellulose. The digested product is utilized by both Trichonympha and the termite. Newly hatched termites lick the anus of older termites for obtaining Trichonympha. The termites eat up their moulted skin to keep the Trichonympha association.
5. Ruminants and cellulose digestion:
The rumen part of the chambered stomach of ruminants contains a number of cellulose-digesting bacteria like Ruminococcus albus, R. flavefaciens, etc. Other herbivores also possess similar bacteria. The products of cellulose digestion are shared by the animals and bacteria.
6. Zoochlorellae and Animals:
Zoochlorellae are found inside the bodies of many sponges, coelenterates, mollusks, and worms. The alga manufactures food and liberates oxygen which are shared by the animals. In return it gets protection.
7. Myrmecophily:
Some mango trees are found to harbour ants which do not allow intruders to spoil the fruit. In South American Acacia sphaerocephala the plant develops hollow spines for shelter of ants and food substances at the tip of leaflets feeding the ants. The ants not only protect the plant from herbivores but also from other plants.
8. Zoophily and Zoochory:
The pollination of flowers by bees, moths, butterflies, and hummingbirds is another example of mutually beneficial symbiosis. The insects or birds derive food from the nectar, or other products of the plant and in return carry pollen from the anthers of one flower to the stigma of another flower thus ensuring cross-pollination. Fruit disseminator animals are served with juicy and nutritious fruit pulp.
Zoophily is a form of pollination whereby pollen is transferred by vertebrates. Here the plant-animal relationships are often mutually beneficial because of the food source provided in exchange for pollination. The dispersing agents for seed and fruits are indicated in such terms as zoochory which means dispersal by wind, water, and animals, respectively. Zoochory or seed dispersal is the movement or transport of seeds away from ‘the parent plant. Plants have limited mobility and consequently rely upon a variety of dispersal vectors to transport their propagules, including both abiotic and biotic vectors.
9. Fig and Fig wasp:
Fig is totally dependent upon wasp (Blastophaga) for pollination. The wasp depends upon figs for food and survival. In fig hypanthodium, female flowers are situated at the base and male flowers near the pore while gall flowers occur between the two. An impregnated female wasp visits a hypanthodium to lay eggs into the gall flowers. Simultaneously it brings pollens from another hypanthodium for pollinating female flowers. Grubs coming out of the eggs feed on gall flowers, grow, and come out of the hypanthodia laden with pollen grain from male flowers.
2. Facultative Mutualism
Facultative mutualism is also called protocooperation. In this relationship, both organisms benefit from living in close association but is not obligatory.
Competition
Competition may be defined as the active demand by two or more individuals of the same species or members of two or more species at the same trophic level for common resources, thereby contributing to the density and diversity of the population. Competition is of two types Intraspecific and Interspecific.
1. Intraspecific Competition:
Competition between individuals of the same species is called intraspecific competition. Intraspecific competition is more acute than interspecific one because all the members have similar requirements for resources, i.e., for food, space, and mates.
Cannibalism (eating members of the same species) reduces this competition (e.g., snakes, scorpions). The establishment of territories in animals occurs in such competition by pushing out the extra number and securing shelter, mates, and even food for the rest. Density-dependent reduction in natality, parental care, aggregations, and dominance-subordination are methods to reduce intraspecific competition and protect the interests of individuals.
2. Interspecific Competition:
Competition between individuals of different species is termed interspecific competition. It involves two or more populations belonging to the same tropic level or feeding habit, competing with one another for the natural resources. For example, carnivorous animals such as tigers, and leopards compete for similar types of prey. Trees, shrubs, and herbs in a forest struggle for sunlight water, nutrients, and also for pollinators and dispersal agents.
Gause’s Principle or Competitive Exclusion Principle:
Every type of organism has a particular niche. No two types of organisms can have the same niche. One of the two is eliminated. This phenomenon is called the Gause hypothesis or principle of competitive exclusion. Gause (1934) noticed that two closely related ciliates – Paramoecium caudatum and P.aurelia, in separate cultures, exhibited normal population growth and maintained a constant population level containing a fixed amount of food. When both of them were placed in the same culture, however, P. Caudatum could not face the competition and was eliminated.
In ecology competitive exclusion principle is referred to as Gause’s Law. According to Gause when two species exhibit normal population growth and maintain a constant population level containing a limited amount of food, cannot coexist at constant population values. When one species has more advantages over another, that species with an advantage will dominate and the weaker one will be eliminated or shifted to a different ecological niche.
Character displacement is a phenomenon in which two species that live together in the same environment tend to diverge in those characteristics that overlap. Competitor species can have evolutionary effects on each other that result in ecological character displacement i.e., divergence in resource-exploiting traits such as jaws and beaks. E.g., Galapagos finches. The finches with deeper, stronger beaks consume tough, large, seeds while finches with smaller beaks consume smaller, softer seeds.
Predation
It is an interaction between the members of two species in which members of one species capture, kill, and eat up members of other species. The former are called predators while the latter are called prey. Predators are mostly animals. Carnivorous animals eat up other animals. Herbivorous animals, while either eating parts or seeds of plants are in a way also predators because they remove individuals from the population. A few plants are also predators in nature. They are called carnivorous or insectivorous plants, e.g., Drosera, Utricularia, Nepenthes, and Dionaea.
Examples of simple predation include tigers and deer, frogs and insects, owls and rats, sea snakes, and eggs. The relation between land snakes and rats is more than that of predator-prey as the snake also uses rat holes as shelter. Some predators function as prey for others, e.g., frogs (feeding on insects) for snakes, snakes for eagles, and peacocks. Predators have special adaptations to capture and eat the prey. They include agility, strength, catching and tearing organs.
Some species of insects and frogs are cryptically coloured (camouflage) to avoid being detected easily by the predator. Some are poisonous and, therefore, avoided by the predators. The Monarch butterfly is highly distasteful to its predator (bird) because of a special chemical present in its body. The butterfly acquires this chemical during its caterpillar stage by feeding on a poisonous weed.
Herbivores like insects, deer, and Antilope are also considered to be predators as they catch, break, and feed on plants. Nearly 25% of insects are phytophagous. Plants have evolved a number of morphological and chemical defences against the herbivores. The important morphological means for defense are
(i) Thorns, spines, prickles, and bristles, e.g., Opuntia, Ziziphus, etc.
(ii) Stinging hair, e.g., Urtica dioica
(iii) Sticky, glandular hair, e.g., Cleome.
Chemical defences include
- latex
- alkaloids
- tannins
- bitter taste
- offensive smell
- irritating substances etc.
In nature, the population of predators is quite small as compared to that of the prey so the latter Js are never eliminated completely. The prey has high reproductive potential. If the population of prey is maintained for some period without predation, the prey individuals would go beyond the carrying capacity of the environment. The predator keeps the population of the prey under check so that an equilibrium is maintained. Predation has several functions
- It is a channel for energy transfer across the trophic levels.
- It maintains the species diversity of a biotic community by reducing competition amongst the prey populations.
- It keeps the prey population under control. It is called biological control.
Parasitism
It is a relationship between two living organisms of different species in which one organism, called a parasite obtains its food directly from another living organism, called the host. The parasite spends a part or whole of its life on or in the body of the host. Parasites may be of several categories
1. Ectoparasites and Endoparasites:
The parasites which live on the surface of the host’s body, are called ectoparasites. The parasites which live inside the host’s body, are called endoparasites. Examples of ectoparasites are head louse, bed bugs, plant aphids, etc., and endoparasites are Plasmodium, Trypanosoma, Ascaris, Taenia, etc.
Differences between Ectoparasites and Endoparasites:
| Ectoparasites | Endoparasites |
| 1. The parasites live on the surface of the host’s body. | 1. The parasites live inside the host’s body. |
| 2. Their respiration is aerobic. | 2. Their respiration is anaerobic. |
| 3. Ectoparasite may be temporary or permanent. | 3. Endoparasites are usually permanent. |
| 4. Example: Head louse, the bed bug, plant aphids, etc. | 4. Example: Plasmodium, Trypanosoma etc. |
2. Temporary and Permanent Parasite:
Temporary parasites live in contact with the host for only a part of their life or occasionally at the time of feeding (e.g., bed bug, Leech). The latter is often called intermittent parasites. Permanent parasites live in contact with hosts throughout their lives. They are transferred to a new host as an egg, cyst or directly, e.g., Ascaris, Entamoeba.
3. Holoparasite and Hemiparasite:
The parasites that are completely dependent on the host for all their requirements are called holoparasites, e.g., Cuscuta, Rafflesia, and animal parasites. The parasites that receive only a part of nourishment from the host, while the rest is manufactured by them, are called hemi or semiparasitic, e.g., Viscum, Loranthus, etc.
4. Pathogenic and Nonpathogenic Parasite:
The parasites that derive only food from the host and cause no disease to the latter, are called nonpathogenic parasites (Entamoeba coll), while the parasites, that not only derive food from the host but also cause certain diseases to the latter, are called pathogenic parasites, e.g., Plasmodium, Entamoeba histolytica.
5. Stem and Root Parasites:
They are parasitic on the plants and are in contact with the host plant either in the region of the stem (e.g., Cuscuta, Loranthus, Viscum, stem borer, aphids, lac insect) or root (e.g., Rafflesia, root nematodes.).
6. Hyperparasites:
It is the name of the parasite that lives on another parasite, e.g., some bacteriophages (parasitic over bacteria), Protist Nosema is parasitic over another protist Sphaerospora which in turn is parasitic on Toad Fish.
7. Brood Parasitism in Birds:
The parasitic bird lays its eggs in the nest of its host and the host incubates them, e.g., Cuckoo in a crow’s nest. The eggs of the brood parasite resemble the eggs of the host in size and colour so that the host bird is unable to detect the foreign eggs.
8. Facultative Parasite:
A living organism shows parasitism when it is associated with the host, otherwise, it is free and is known as a facultative parasite.
9. Obligate Parasite:
They lead only parasitic life known as obligate parasites, e.g., Ascaris etc.
Adaptations for Parasitic Life
- Anaerobic respiration in internal parasites.
- Loss of certain organs (e.g., lice, and bedbugs lack wings).
- Adhesive organs (Suckers in leeches, tapeworms).
- Excessive multiplication (parasites produce abundant offspring).
- Resistant cysts and eggs for safe transfer of their progeny to new hosts.
- Well-developed and complicated reproductive organs.
Amensalism
Amensalism is an interaction where an organism inflicts harm to another organism without any costs or benefits received by the actor. A clear case of amensalism is when sheep or cattle trample grass. Whilst the presence of the grass causes negligible detrimental effects to the animal’s hoof, the grass suffers from being crushed. Amensalism is often used to describe strongly asymmetrical competitive interactions, such as has been observed between the Spanish ibex and weevils of the genus Timarcha which feed upon the same type of shrub. Whilst the presence of the weevil has almost no influence on food availability, the presence of ibex has an enormously detrimental effect on weevil numbers, as they consume significant quantities of plant matter and incidentally ingest the weevils upon it. Roots of black walnuts produce a chemical that is toxic to tomatoes, apples, alfalfa, etc.
Commensalism
Commensalism benefits one organism and the other organism is neither benefited nor harmed. It occurs when one organism takes benefits by interacting with another organism by which the host organism is not affected. A good example is a remora living with a shark. Remoras eats leftover food from the shark. The shark is not affected in the process, as remoras eat only the leftover food of the shark, which does not deplete the shark’s resources. Commensalism is the relationship between two living individuals of different species in which one is benefited whereas the other is neither harmed nor benefited.
Another important example is the relationship between the Sea Anemone (Adamsia palliata) and Hermit Crab (Pagurus prideaux) is considered commensalism by some & mutua list by a few and proto-cooperation by others. The sea anemone is carried by the crab to fresh feeding sites and the crab in turn is said to be protected from its enemies by sea anemone. Another example is a barnacle (Balanus) that remains attached to a whale’s body and is transported from one place to another.
Protocooperation
Protocooperation is where two species interact with each other beneficially; they have no need to interact with each other. They interact purely for the gain that they receive from doing this. It is not at all necessary for protocooperation to occur; growth and survival are possible in the absence of the interaction. The interaction that occurs can be between different kingdoms. Protocooperation is a form of mutualism, but they do not depend on each other for survival. An example of protocooperation happens between soil bacteria or fungi, and the plants that occur growing in the soil. None of the species rely on the relationship for survival, but all of the fungi, bacteria, and higher plants take part in shaping soil composition and fertility. Soil bacteria and fungi interrelate with each other, forming nutrients essential to the plant’s survival. The plants obtain nutrients from root nodules and decomposing organic substances. Plants benefit by getting essential mineral nutrients and carbon dioxide.
Protocooperation can occur in birds. The Egyptian plover removes insect pests from the backs of buffalo, antelope, giraffes, and rhinos. The cattle egret in America as well does the same task of removing the unwanted insects and parasites. Another important example of protocooperation is between the red-billed oxpecker and the rhino. The bird removes all bugs and parasites on the animal’s skin by eating them. The rhino provides the bird with food and in return, the bugs are removed from the skin of the rhino.
Differences between Mutualism and Commensalism:
| Mutualism | Commensalism |
| 1. In this mode of association of two organisms, both are benefited. | 1. In this mode of association of two organisms, only one is benefited, and the second one is neither benefited nor harmed. |
| 2. In between two organisms, contact is obligatory. | 2. Here the contact between commensals is not obligate. |
Intraspecific Relationship
Territoriality: It is a type of intraspecific or interspecific competition that results from the behavioral exclusion of others from a specific space that is defended as territory. On the other way, it is a behaviour in defense of an area against another individual or individuals primarily of the same species.
Altruism
It is a characteristic form of behaviour among social animals by practicing concern for the welfare of others. For instance, in several species of vertebrates, invertebrates, and protists altruism is seen. For example, wolves and wild dogs bring meat back to members of the pack not present at the kill. An interesting example of altruism is found in the cellular slime moulds, such as Dictyostelium mucoroides. These protists live as individual amoebae until starved at which point they aggregate and form a multicellular fruiting body in which some cells sacrifice themselves to promote the survival of other cells in the fruiting body.
Dominance Hierarchy
Dominance hierarchy arises when members of a social group interact, often aggressively, to create a ranking system. In social living groups, members are likely to compete for access to limited resources and mating opportunities. The social order can be linear or not linear. In a linear ranking system (pecking order), every member of the gender is recognized as either dominant or submissive relative to every other member, creating a linear distribution of rank. On the other hand, in non linear ranking system, one member is considered dominant while the other members of the living group are equally submissive.
Dominance hierarchies can also be observed in many fish, birds, and mammals. For example, in the dragonet, males form hierarchies that are often exhibited during mating. They can act extremely aggressively towards another male if it intrudes upon courtship and pairing with a female, and fights can be very intense. Groups of spotted hyenas and brown hyenas both demonstrate linear dominance.
Scavenging
A scavenger is an organism that mostly consumes decaying biomass, such as meat or rotting plant material. Many scavengers are a type of carnivore, which is an organism that eats meat. For example, hyenas, and jackals eat upon the leftover flesh of animals preyed on by lion or tiger.
Interaction between Individuals and Species:
| Features | Interspecific Interaction | Intraspecific Interaction |
| 1. Active demand by two or more individuals for common resources. | Competition | Competition |
| 2. Capture, kill, and consumption of all or part of another species. | Cannibalism | Predation |
| 3. Mutual benefit between two organisms. | Mutualism or Altruism | Mutualism |
| 4. Individuals live is an association, at the cost of the host. | Parasitism | Parasitism |
Types of Interaction between Two Species Population [According to ‘Odum’ (1983)]:
| Types of Interaction | Species | Nature of Interaction | |
| A | B | ||
| 1. Neutralism | 0 | 0 | One species’ population is not influenced by the species. |
| 2. Competition | – | – | Due to competition, one species inhibits the growth of other species. |
| 3. Amensalism | – | 0 | Due to the presence of B-species, the growth of A-species is inhibited. |
| 4. Parasitism | + | – | Host Cell (B) being harmed due to the presence of parasites (A). Parasites take shelter and nutrition from the host cell & reproduce successively. |
| 5. Predation | + | – | Predator (A), large in volume and shape, and eating prey (B) due to the fulfillment of nutrition. |
| 6. Commensalism | + | 0 | A species takes benefit from B-species but B-species is not harmed. |
| 7. Proto-cooperation | + | + | A and B-species lived separately but they are mutually benefited. |
| 8. Symbiosis | + | + | A and B-species are mutually benefited by each other. |
Population Attributes
Population refers to a group of individuals of a species within a geographical area but with this, the total aspects of a population can not be realized. A population has a number of characteristics like density, natality, mortality, age distribution, growth forms, etc. Population density is also influenced by environmental resistance, carrying capacity, abiotic and biotic factors of the environment.
Characteristic Features of Population
1. Population Density:
Population density generally refers to the number of individuals or the population biomass (weight) of a species per unit area or volume of a geographical zone. For example, the numbers of tigers or deer/sq. km. in a forest, number of mango trees/acre of land. In the case of microorganisms, the biomass can be used for calculating the population density.
According to the patterns of dispersion of organisms in nature, density is of two types – Crude density and Ecological or Economic density. Crude density is the total number (or biomass) of an organism per unit of total space. Ecological or economic density is the total number (or biomass) of an organism per unit of habitat space i.e., an available area that can actually be colonized by the population.
2. Natality or Birth Rate:
It refers to the number of births during a given period in the population that are added to the initial density. Natality is the principal way to population growth. Natality thus may be defined as the number of individuals of a species, added to the population through reproduction at unit time. Natality may be categorized into Absolute or Maximum or Potential natality and Ecological or Realized natality. Absolute natality is the theoretical maximum production of new individuals under ideal conditions i.e., no ecological limiting factors, only physiological factors limit natality. On the other hand, ecological natality refers to the actual number of new members added to the population through reproduction at unit time. In an ecosystem, never all the conditions may be ideal in nature and therefore the realized natality is always less than absolute natality.
Natality may be expressed by the following formula:
N = \(\frac{nD}{T}\)
(When N = birth rate, nD = number of new bom, T = time)
3. Mortality or Death Rate:
It is the number of deaths in the population at unit time. As natality brings about population growth, mortality renders a reduction in population size. For this reason in the determination of the size and volume of a population, death rate is very important. Mortality is of two types – Minimum or Potential mortality and Realized or Ecological mortality. At ideal ecological conditions, the minimum number at which individuals die from a population is known as potential mortality. On the other hand, at unit time the number of individual actually dies from the population, is known as realized mortality.
Mortality may be expressed by the formula:
M = \(\frac{nD}{T}\)
(When M = Mortality or death rate, nD = total number of deaths, and T = time).
Natality and mortality are the two important properties in determining population characteristics. The opposing features of natality and mortality are pointed out in the following table.
Differences between Natality and Mortality:
| Natality | Mortality |
| 1. Number of individuals added to a population at unit time. | 1. The number of individuals gets reduced from the population at unit time. |
| 2. It increases the size and volume of the population. | 2. It decreases the size and volume of the population. |
| 3. The density of population increases. | 3. The density of population decreases. |
| 4. Biomass is increased. | 4. Biomass is decreased. |
| 5. Natality is meant for the continuity of a species. | 5. Mortality is meant for the elimination of the aged and infirm. |
| 6. Natality is high when the population size is small and low when the population size is large. | 6. Mortality is low when the population size is small and high when the population size is large. |
4. Immigration and Emigration:
When some species come into a habitat from the neighbouring population is called immigration.
When an individual is moving out from one population to another is called emigration.
5. Sex Ratio:
It is the number of males and females per thousand individuals.
6. Age Distribution:
A population has three ecological age groups – Pre-reproductive, Reproductive, and Post-reproductive. Pre-reproductive individuals are young individuals who will enter the reproductive age after some time. A population having a larger number of pre-reproductive individuals will show a rapid increase (positive growth). Reproductive individuals are actually adding new members to the population. Their number is almost equal to the pre-reproductive individuals in a stable population. Here population growth is zero. Post-reproductive individuals are older individuals who no longer take part in reproduction. Their small number indicates a growing population while a large number indicates a stable or declining population.
Age Pyramid:
It is the graphic representation of the proportion of different age groups in a population with pre-reproductive groups at the base, reproductive ones in the middle, and post-reproductive at the top. Age pyramids are of three types-
- Triangular Age Pyramid: It represents a larger number of pre-reproductive individuals, a moderate number of reproductive individuals, and fewer post-reproductive individuals. Because of the very large number of pre-repoductive individuals, more and more of them enter the reproductive phase and rapidly increase the size of the population.
- Bell-Shaped Age Pyramid: It represents an almost equal number of pre-reproductive and reproductive individuals. Post-reproductive individuals are comparatively fewer. The population size remains stable, neither growing nor diminishing.
- Urn-shaped Age Pyramid: It represents a greater number of individuals of higher reproductive age group than the individuals of pre-reproductive age group. The number of post-reproductive individuals is also sizeable. It is a declining or diminishing population with negative growth.
7. Dispersal:
When the number of individuals of a species increases, due to a genuine reason for obtaining amenities, the members spread out beyond their own geographical boundary. This is known as population dispersal. Along with the dispersal of the population, other features of it are also influenced.
8. Bioenergetics:
The amount of biomass present in a population is the statistical form of solar energy. Therefore, the amount of solar energy stored in a population may be realized through its population density.
9. Interaction:
Interaction between the members of a species and between different species appears to be a common phenomenon in a geographical boundary. However, sometimes the members of a species may remain neutral. Altogether two types of interaction may be observed, namely, negative interaction and positive interaction. Competition, parasitism, prey-predator relationship, and allelopathy represent negative interactions. On the other hand, positive interactions include commensalism, symbiosis, and protocooperation. Population growth or reduction as well as its dispersal depend upon the interaction.
10. Isolation and Territoriality:
If in a geographical area, more than one species population exists, the members of the species try to remain in isolation to avoid competition. For this purpose, the members of a species population form a territory to reside within. Therefore, isolation and territoriality are interrelated phenomena.
11. Population Fluctuation:
The permanent existence of a population is almost an impossible event. At times population may exhibit growth or reduction. This is known as population fluctuation. This type of fluctuation may occur due to changed conditions in the environment. There are two types – Seasonal fluctuation and annual Fluctuation. There are two types of species of Paramoecium are show the population growth curve graphically depending on time.
12. Distribution:
Remaining within the same geographical boundary, members of a population may exhibit different types of distribution. Altogether three types of distribution may be observed namely uniform distribution, random distribution, and clumped distribution. Three distributions may be represented through a diagram. Therefore in the determination of the population feature, distribution in the ecological zone of the population members may be a criterion.
Difference between Population and Community:
| Population | Community |
| 1. In population, individuals can interbreed freely. | 1. In the community, interbreeding is absent amongst different members. |
| 2. Population is a small unit of an organization. | 2. Community is a larger unit of organization. |
| 3. Here the individuals are morphologically and behaviourally similar. | 3. Here the members are morphologically & behaviourally dissimilar. |
Population Growth
The growth of a population is another important character. Due to growth, the structure of a population cannot remain static. Due to such altering properties, the population becomes dynamic. The population growth and its alteration may be expressed by the formula: X = (r + p) – (q + s), when X = change of population due to growth, r = birth rate, p = immigration, q = death rate, s = emigration. Therefore the growth of a population depends upon four limiting factors. These are birth, death, emigration, and immigration. Birth and immigration promote the addition of members to the population but death and emigration promote a reduction in population size. Therefore, in order to determine the effective growth of a population, the loss due to death and emigration should be deducted from the growth due to birth and immigration at a specific time period.
- Immigration: In some species, immigration is the number of individuals that have come into the habitat from elsewhere during the time period.
- Emigration: Emigration is the number of individuals if the population who left their habitat and gone elsewhere during the time period.
There is a relationship between Immigration, Emigration, Population density, Natality, and Morality.
So, if N is the population density at the time ‘t’, then its density at time t + 1 is Nt+1 = Nt + [(B + I) – (D + E)]
From the above equation, we can see that the population density will increase if the number of births plus the number of immigrants (B + I) is more than the number of deaths plus the number of emigrants (D + E), otherwise, it will decrease.
During normal conditions, births and deaths are the important factors that influence population density, and the other two factors assume importance under special conditions. For instance, immigration may contribute more significantly or specifically to population growth than birth rates. The growth of the population is always changing depending on various factors like food availability, predation pressure, adverse weather, etc., and the population becomes dynamic.
Different populations may exhibit differential growth potential. The maximum growth that a population may exhibit, is known as biotic potential. Whatever the biotic potential of a population, it can not exhibit growth in an unlimited fashion. The sum of environmental factors that limit the population size and keep a check on the biotic potential, is called environmental resistance. It consists of both abiotic (light, water, space, minerals) and biotic factors (food, competition, pathogens). The value of environmental resistance is inversely proportional to the difference between carrying capacity K and number of individuals in the existing population N. Carrying capacity (K) is defined as the maximum number of individuals of a population that can be provided with optimum resources for their healthy living. It is the limit beyond which no major increase can occur. When the population reaches the carrying capacity of its environment, the population has a zero growth rate.
The intrinsic rate of natural increase or per capita increase in population (r) is optimum with environmental conditions favouring maximum natality and minimum mortality. The influence of environmental resistance over the intrinsic rate of natural increase is denoted by
rN\(\frac{(\mathrm{K}-\mathrm{N})}{\mathrm{K}}\), where r = intrinsic rate of natural increase, N = population size and K = carrying capacity of the area.
\(\frac{(\mathrm{K}-\mathrm{N})}{\mathrm{K}}\) = environmental resistance.
Reproductive Fitness:
It can be defined either with respect to a genotype or to a phenotype in a given environment. In either case, it describes individual reproductive success and is equal to the average contribution to the gene pool of the next generation that is made by individuals of the specified genotype or phenotype. In sexual reproduction, genotypes are scrambled every generation. In this case, fitness values can be assigned to alleles by averaging over possible genetic backgrounds. Natural selection tends to make alleles with higher fitness more common over time, resulting in Darwinian evolution.
Growth Patterns
The growth of a population with time shows a specific pattern. There are two types of population growth patterns – S-shaped or logistic and J-shaped or exponential.
1. S-Shaped Growth Patterns:
Sometimes in a new environment, if a new population is established with a few members of a species in the presence of adequate amount of food resources, the population exhibits growth following certain principles. The growth of this type is plotted on graph paper taking time along the X-axis and population density along the Y-axis. The population growth with respect to time exhibits an S-shaped or sigmoid curve. In such cases of population growth, initially, the growth occurs at a slow rate, but after a certain time period growth becomes rapid. However, after achieving a certain level of growth, the growth becomes static or occurs at a slow rate, when the curve becomes horizontal. The first phase of growth in this curve becomes horizontal. The first phase of growth in this case, is known as the positive acceleration phase, the second phase with rapid growth, is known as the rapid growth phase or logarithmic phase (log phase), and the third phase is known as the slowing down phase (stationary phase). It is also known as carrying capacity (K).
This type of population growth is called Verhulst-Pearl Logistic Growth as explained by the following equation:
\(\frac{\mathrm{dN}}{\mathrm{dt}}=\mathrm{rN}\left(\frac{\mathrm{K}-\mathrm{N}}{\mathrm{K}}\right)\)
where \(\frac{\mathrm{dN}}{\mathrm{dt}}\) = rate of change in population size, r = intrinsic rate of natural increase, N = population size, \(\left(\frac{K-N}{K}\right)\) = environmental resistance.
2. J-Shaped Growth Pattern:
In this sort of growth form, there are only two phases – the lag phase and the exponential phase.
In the lag phase, there is no significant increase in population for some time in which individuals adapt to their environment. In the exponential phase, the individuals start reproducing rapidly. The increase in population size is so rapid that soon it reaches beyond the carrying capacity (K) of the environment. A point is reached when the population declines suddenly due to mass-scale deaths. It is called the crash phase. This type of curve is observed in the case of a small population of reindeer experimentally reared in a natural environment with plenty of food but no predators. Lemmings of tundra, some insects, algal blooms, and annual plants also show J-shaped curves.
The J-shaped growth form is represented by the following equation:
\(\frac{\mathrm{dN}}{\mathrm{dt}}\) = rN
where \(\frac{\mathrm{dN}}{\mathrm{dt}}\) = rate of change in population size, r = intrinsic rate of natural increase, N = population size
Example:
The r of the rat population in Norway is 0.015; whereas in 1981, the rat r of the human population in India is 0.0205. The r is calculated by the mortality and natality rate.
With the knowledge of basic calculus, the integral form of the exponential growth equation can be derived as Nt = Noert
Where Nt = population density after time t; N0 = population density at time zero; r = intrinsic rate of natural increase; e = the base of natural logarithms (2.71828).
In the population growth curve k is carrying capacity, a is an exponential curve and b is a logistic curve.
Differences between S-shaped and J-shaped growth forms of Population:
| S-shaped Growth Form | J-shaped Growth Form |
| 1. Found in stable type population. | 1. Found in the fast-growing type of population. |
| 2. It has three main phases – positive acceleration phase, logarithmic phase, and slowing down phase. | 2. It has two main phases – a slow phase of growth and a rapid phase of growth. |
| 3. A stage of equilibrium comes during growth. | 3. An equilibrium stage is never reached in population size. |
| 4. Population rarely grows beyond the carrying capacity of the area. | 4. Population reached beyond the carrying capacity of the area. |
| 5. Environmental resistance plays an important role in slowing down the exponential phase. | 5. Environmental resistance has no role in slowing down the exponential phase. |
| 6. A crash phase does not occur. | 6. There occurs a crash phase at the end of the J-shaped growth curve. |
Factors Resisting in Population Growth:
| Growth Aiding Factors | Growth Inhibiting Factors |
| Non-Living | |
| 1. Optimal light. | 1. High or low light. |
| 2. Favourable temperature. | 2. High or low temperature. |
| 3. Favourable chemical environment. | 3. Unfavourable chemical environment. |
| Living | |
| 1. High natality rate. | 1. Low natality rate. |
| 2. Common niche. | 2. Special niche. |
| 3. Proper food distribution. | 3. Improper food distribution. |
| 4. Favourable habitat. | 4. Unfavourable habitat. |
| 5. Ability to survive from a predator. | 5. Disability for survival from predators. |
| 6. Ability to compete for food. | 6. Disability for competition for food due to many competitors. |
| 7. Ability to survive in the changed environment. | 7. Disability for surviving in the changed environment. |