POPULATION ECOLOGY

The word ‘population’ is derived from the Latin word ‘populus’ which means people.

Organisms in nature rarely grow as separate from each other. Each species in an ecosystem exist

as a population. Ecologically, a Population is an assemblage of individuals of a species which

potentially interbreed and occupying a particular space at the same time. For example,

grasshopper in the field, deer population in the forest, pine trees in a forest etc. Sometimes

individuals of some species live solitary but they interact with other members of the same

species or others at different times in their lives.

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.

There are two types of populations:

a) Unitary Populations are those in which 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.

b) 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.

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 indeterminate

and thus unpredictable.

Clarke (1954) distinguished the population into two types.

1. Monospecific population in which group of individuals belong to the same species.

2. Polyspecific population or Mixed population in which groups of individual belongs to

different species. The term community is often used for the polyspecific population.

The populations of species have numerous local populations or subpopulations known as Demes.

It can be defined as a smallest collective unit of animal population which is able to interbreed.

These individuals share a common gene pool. The gene flow between populations occurs

through emigration and immigration. The natural selection acts on individuals of a population

leading to its evolution due to change in gene frequency over a period of generations. It may

change the physical expression of organisms in the population.

  

 

Biologists also use the term Genet to the organism, which arises from a zygote, whereas others

arising asexually are known as Ramet. Individuals of ramets are genetically alike and replica of

parent plant. A group of ramets developing from a genet is known as Clone since all organisms

are genetically alike.

The subject of population ecology can be divided into 3 sections for the sake of

convenience:

A) Population characteristics B) Population dynamics C) Population regulation

A) POPULATION CHARACTERISTICS

The population ecologist ‘Thomas Park’ expressed that a population has several characteristics

or biological attributes which can be best expressed as statistical functions. These functions are

unique to the group and cannot be applied to the individuals in the group. Some of these

properties are density, natality, mortality, age distribution, biotic potential, dispersion,

dispersal and growth forms.

A brief description of the characteristics of the population is given below.

POPULATION DENSITY

It is defined as number of individuals in relation to a definite unit of space. It is generally

expressed as number of individuals or biomass per unit area (if on land) or volume (if in water)

e.g. 100 people/square km or 4000 crustaceans/cu.mm of water.

The density of the organisms in the given area can vary with the food supply, seasons and

climatic conditions but always has definite upper and lower limits. The upper limit to density is

imposed by the size of the organism and its trophic level. If the animal is smaller, greater is its

abundance per unit area. For example, a forest is able to support more wood lice than deer

population. The lower limit of density is not well defined but homeostatic mechanisms help to

keep the density within limit. The density is one of the important characteristic of population

which is dependent upon a number of factors like resource availability, productivity of the

population, energy flow, utilization, physiological stress and dispersal.

The important factors responsible for change in the population density are shown in figure 1:

  

 

Population density is categorized into two types:

1. Crude Density: It is defined as the total number of individuals or biomass per unit of the total

space e.g. the number of Rhinoceros living in the Kaziranga National Park. It takes in account all

area of land or aquatic ecosystems under consideration

2. Specific or Ecological or Economic Density: It is defined as the number of individuals or

biomass per unit of the habitat space. It is the available area or volume which is actually

colonized by the population. For example Rhinoceros do not occupy all area of Kaziranga

national park but it may avoid some of the area due to the lack of food, shelter and human

habitations. Therefore the area inhabited by the Rhinoceros actually will be its ecological

density.

The difference in the two types of densities becomes more apparent when the species are

clumped together in a small area. However, crude density is studied frequently more than the

ecological density because it is very difficult to determine the ‘actual area’ of inhabitation of a

species. Further, the area of inhabitation may vary with developmental stages of species.

The difference between crude and ecological density can be explained by taking the example of a

fish density in pond. The crude density of the fish in the pond goes down as the water level drops

during the summer season but the ecological density in the contracting pond increases as the fish

are crowded into smaller water area. It becomes very easy for the predatory bird to catch fish at

this time of the year as the ecological density of the fish is at its peak.

  

 

Methods of Measuring Population Density

Population density of organisms can be measured by the following methods:

1. Total counts: It is the total counts of all the organisms in a given area especially with large or

conspicuous organisms. eg. conducting census of human population or the no. of whales in an

area of sea etc.

2. Sampling methods: In this method, a small proportion of the population is counted which is

used to estimate the total population. Sampling can be done in two ways.

a) Use of quadrat: In this method, count all the individuals on several quadrats of known size

and extrapolate the average to the whole area. The shape and size of quadrat vary with type of

organisms studied. It can be circular, triangular or square type (figure 3). The method is

extensively used for plant populations and many invertebrates.

b) Capture- recapture method: The simple type of population method is known as Peterson

Method as it was developed by scientist C.G.J. Peterson in 1898. This method is used to count

  

 

larger animals like birds, fish etc. The animals are captured, marked and released in the

environment. On subsequent day animals are recaptured to calculate the population density. The

population at any time will have marked and unmarked individuals. The proportion of marked

individuals in a later sample is used to determine the total population. For e.g. suppose 100 fishes

are captured, marked and released again into the pond. After some time again 120 fishes were

captured this includes 30 marked fishes. The size of the population is counted by the formula:

Marked animal in second sample = Marked animal in first sample

Total caught in second sample Total population size

30 = 100

120 N

Thus total population size estimated is 400.

Sometime estimation of absolute density of a population is very difficult. So, it is simple to

determine the relative density.

The Relative Density never provides an estimate of density but it is rather an index of

abundance that is more or less accurate. Relative density can be measured by the following

methods:

a) Trap: It is generally used in capture- recapture methods but also used to determine

relative density as the number of individuals’ caught per day per trap.

b) Number of fecal pellets: By counting the number of fecal pellets in an area, index of

population size can be determined. For e.g. population of rabbits, deer etc.

c) Vocalization frequency: By analyzing the calls of animals in particular time duration,

index of population size can be determined. The number of pheasant calls heard in 15

minutes in the early morning has been used as an index of the size of the pheasant

population.

d) Feeding capacity: Population density can be counted by calculating the amount of food

consumed by animals before and after treatment. For eg. the amount of bait consumed by

rat/mice before and after poisoning can be used to determine an index of change in their

density.

NATALITY

It is defined as the birthrate of 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).

Crude Natality or Birth Rate is expressed in terms of population size e.g. 40 births per 1000

individuals of the population. The specific birth rate will be 4%.

Natality Rate is expressed as the number of offsprings produced per female per unit time. The

magnitude of natality rate is dependent on the type of organisms being studied. Some species

breed once in a year or some throughout year; some produce few offsprings or some produces

enormous. For e.g. Fish commonly lays few thousand eggs, frog produces few hundred, birds

lays 1 to 20 eggs and mammals produces only few offsprings (between 1to10).

The birth rate is usually inversely related to parental care.

  

 

Types of Natality: Natality can be categorized into two types:

1. Absolute or Physiological natality / Maximum natality / Fecundity rate: It is defined

as the physiological ability of organisms to produce maximum number of new individuals

under optimum conditions. This value is theoretical (since the environmental conditions

are never static and keep on changing) and constant for a given population. Thus,

absolute natality is very difficult to be achieved in the wild populations but it can be

observed only under favorable conditions or when major limiting factors are temporarily

non-operative, e.g. If a small number of Paramecia are placed in the culture medium,

under favorable conditions they achieve maximum natality by multiplying at maximum

reproductive rate.

It is beneficial for two reasons.

a) It provides a standard for comparison with realized natality.

b) It is useful in setting up equations to calculate the rate of increase in a given

population.

2. Ecological or Realized Natality / Fertility rate: It is defined as an increase in the

population size under the prevailing ecological or actual environmental conditions. It is

never constant for a population but varies with the age, composition, size and physical

environmental conditions of the population.

Natality can be expressed as:

Δ Nn/ Δt = Absolute or crude natality or birth rate i.e. the average rate of change in the

number of organisms per unit time.

Δ Nn/ N Δt = specific natality or specific birth rate (=Natality per unit population)

i.e. the number of new individuals per unit time per unit population

where N = the reproductive part of the Population

ΔNn = New individuals added into the population

Δt = time lapsed during change in population

Since increase in population depends upon the number of females in a population, the age

specific birth schedule counts only females giving rise to females. The age specific schedule is

obtained by determining the mean number of females borne in each group of females.

MORTALITY

It is defined as the rate of death of individuals in a population.

Types of mortality: Two types of mortality can be recognized.

1. Theoretical or Minimum mortality / Physiological Longevity: It is the number of

death of individuals under ideal conditions with minimum limiting factors. Therefore,

even if the conditions are ideal for individuals, they would die because of old age

determined by their physiological longevity. This value is a theoretical value and constant

for a given population.

  

 

2. Ecological or Realized mortality: It is the actual loss of individuals under a given

environmental conditions. It is not constant for a population but varies with the

environment conditions and population such as predator, disease, etc.

Mortality is the number of deaths of individuals in a population per thousand in a given period

of time. Generally mortality is expressed as either the probability of dying or as death rate.

Death rate can be defined as the number of deaths during a given interval of time divided by the

average population. For example, a population with initial size of 500 individuals in which

individuals survived at the end of the period is 400. The average size of the population for the

period will be 900/2 = 450. The total number of individuals died is 100, so the death rate is

100/450 = 0.22.

The probability of death of individuals is the number that died during a given time interval

divided by the number of individuals at the beginning of the period, i.e.100/500=0.2.

The complement of probability of dying is the probability of surviving i.e. the number of

survivors divided by the number alive at the beginning of the period. As we are more interested

in the number of survivors, so mortality can be expressed in terms of life expectancy.

Life Expectancy is the average number of years to be lived in future by the members of a given

age in the population.

AGE STRUCTURE and AGE DISTRIBUTION

In most populations, individuals are of different ages. The proportion of individuals in each age

group is called Age Structure of that population. The nature of a population is influenced by its

age structure. The age structure also determines the future population projections.

Age Distribution is the total number or percentage of individuals in a given population in

different age groups. Age distribution influences natality, mortality and the ratio of various age

groups in a population which indicates its reproductive status.

Age is generally expressed in days, months or years but it is also considered in other categories

such as pre-reproductive, reproductive and post-reproductive.

The age distribution in a population can be represented in the form of polygons or geometrical

models called Age Pyramids where the relative width of the successive horizontal bars

represents the number of individuals or the percentage in different age classes.

There are three types of pyramids recognized:

1. Bell shaped pyramid: represent an expanding or progressive population - with high

reproductive phase individuals than post reproductive phase. Pyramid of such a

population would have a broader base, thus a Bell shape. However, some of the plants

and animal including insects and fishes etc. do not have post reproductive phase. For

example, the catadromous fish European eel (Anguilla) die immediately after spawning.

2. Dome shaped pyramid: is shown by a stable or stationary population i.e. a population

having more or less even distribution of all major three reproductive stages. This type of

population forms a Dome-shaped age pyramid.

  

 

3. Urn shaped pyramid symbolize a declining or retrogressive population in which prereproductive

individuals are less in number and older or post reproductive individuals are

in large proportion.

Usually a population tries to maintain a stable age distribution. The ratio of various age groups in

a population indicates its current reproductive status. Usually the ratio of young to adult in a

relatively stable population of most animals is approximately 2:1. Once a stable age distribution

is reached, changes in natality and mortality affect population stability temporarily and it tries to

return to the stable condition at the earliest.

Age distribution has practical value in the wild life management. A low ratio of pre-reproductive

to reproductive phase indicates a poor reproductive season. However, changes in age distribution

alone do not imply changes in survivorship or fecundity and it alone should not be used to

predict population trends.

SEX RATIO

For rapid increase in size, populations may increase asexually. But sexual reproduction is a must

for maintaining the genetic variability, which is achieved by crossing over and recombination

processes. The eukaryotic organisms generally reproduce through sexual reproduction. There are

some organisms which have the capacity to reproduce asexually but still they have provisions for

sexual reproduction to maintain genetic variability in the population.

Sex ratio can be defined as the percentage of male and female individuals in a sexually

reproducing population. The sex ratio can be categorized into:

i) Primary sex ratio: It is the ratio of male and female at the time of conception and it

tends to be 1:1

ii) Secondary sex ratio: It is the ratio of male and female at the time of birth. This ratio

often favors males in younger age but in the older age it shifted towards females.

iii) Tertiary sex ratio or adult sex ratio — It is the ratio in sexually mature organisms. It is

calculated as the proportion of adults in a population that are male.

iv) Quaternary sex ratio — ratio in post-reproductive organisms

In the case of humans also, the proportion of males is more than females at birth (52%) but as

age increases, the proportion of females goes on increasing. For example: at age group 1-4, the

proportion of male:female is 104:100, at 40-44, the proportion shifts towards 1:1, at 60-64, it is

88:100 and at age 80-84, it is 54:100.

Similarly in birds, cattle and rabbits, the proportion of males is higher than females but in

domestic chicken, sheep and horse, males constitute 49% of the population.

  

 

However, the secondary sex ratios of vertebrates show greater variation. For example, in Alaskan

fur seal, one male always dominate a breeding group of 30 females and the male:female sex ratio

exceeds 3%. The other male adults live at the periphery of the breeding groups and suffer high

mortality.

In some birds, mortality of female increases then males particularly in nesting season.

Factors that control the sex ratio are:

• Genetic variation

• Availability of resource

• Access to mate

• Parental care

In sexually mature animals, there is a selection process of choosing mate. This process is called

MATING SYSTEM. The mating system is categorized as MONOGAMY, involves the pairing

of one male and one female and the relation is maintained for longer duration of time. Other

mating system is PROMISCUITY where males and females mate with more than one of the

opposite sex but does not form any relation between them. Monogamy is most commonly found

in birds and rarely among mammals.

In monogamy, both parents show cooperative behavior to successively grow their young ones.

Most species of birds are monogamous during the breeding season, because their young one are

helpless and need food, warmth, and protection. Among mammals, females produce milk for

providing food to the young. Males often can contribute little or nothing to the survival of the

young, so it is to their advantage to mate with as many females as possible. Among many species

of monogamous birds, such as bluebirds (Sialia sialis), the female or male may “cheat” by

mating with others while maintaining the reproductive relationship with the primary mate and

caring for the young to increase their chances in contributing their gene into gene pool.

POLYGAMY is the acquisition by an individual of two or more mates. It can involve one male

and several females or one female and several males. A pair bond exists between the individual

and each mate. The individual having multiple mates is generally not involved in caring for the

young. Environmental and behavioral conditions result in various types of polygamy.

In Polygyny, one male pairs with more than one female.

In Polyandry, one female pairs with more than one male. Polyandry is interesting because it is

the exception rather than the rule. This system is best developed in three groups of birds, the

jacanas, phalaropes and some sandpipers. The female competes for and defends resources for

the male. The female produces multiple clutches of eggs, each with a different male. The male

begins incubation and becomes sexually inactive. The male African jacana also shows polyandry

after the female lays the eggs, the male incubates the eggs and cares for the young while the

female seeks additional mates.

  

 

DISPERSION

Dispersion refers to the manner in which individuals of a population are distributed in space and

time. Accordingly, dispersion is called spatial (varying with respect to space) or temporal

(varying with time). In the temporal dispersion, example of migratory birds is well- known. In

case of spatial pattern, broadly three types of dispersion patterns are recognized

a) Regular or Uniform dispersion: In this type, the individuals of a species occur

uniformly i.e. with even spacing among the individuals. It occurs when intra-specific

competition (between individuals of same species) is severe. The competition involves

moisture requirement in desert bushes, severe competition for light among forest plants,

competition for territory maintenance in carnivore birds. Autotoxicity among plants

(production of substances toxic to seedlings of the same species) of arid region also

results in uniform distribution of plants in these areas. This type of dispersion is rare in

natural ecosystems but common in manmade ecosystems like agro-ecosystems or tree

plantations.

b) Random dispersion: In this type of distribution, the individuals are uniformly and

randomly distributed in the area. It emphasizes that every individual has equal chance to

occur at a place. In random dispersion, the 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 as it occurs in the environment which is uniform and where resources are equally

available throughout the year or there is positive antagonism. There is no tendency of

aggregation. Spiders in the forest floor and the clam (Mulinia lateralis) of the intertidal

mudflats of the northeastern coast of North America show random distribution.

c) Clumped dispersion: It is also known as clustered or aggregated or contagious

distribution. It is the most common pattern of distribution in which the individuals of a

species are clumped together in space in the form of patches, with some scattered

individuals outside the group. In this, the presence of one individual means there is a high

probability of finding other individuals of the same species in immediate neighbourhood.

Since, environment is never uniform in space due to differences in the habitat, climatic

conditions, social behaviour and reproductive pattern. As a result population frequently

produces attraction and avoidance and results in clumped distribution pattern. Individuals

of a population occur together because of food availability or better survival rate as in

animal populations.

Examples of this kind can also be seen in the social aggregations that are formed in

response to some environmental suitability.

  

 

Human populations show clumped distribution due to their economic condition,

geographic factors and social behaviour.

In Plants, the clumped distribution is very common, and attributed to nutrient

availability, specific habitat preference or better environmental conditions.

e.g. The large aggregation of ferns occurs due to vegetative propagation. Some birds

eject number of undigested seeds in their fecal matter (e.g English robin may eject 20

raspberry seeds in a pellet) which germinates in a cluster of seedlings.

Varying degrees of clumping are characteristic of the internal structure of most

populations at one time or another. Such clumping is a result of individuals aggregating

(1) in response to local habitat or landscape differences

(2) in response to daily and seasonal weather changes

(3) because of reproductive processes

(4) because of social attractions (in higher animals).

Aggregations may increase competition between individuals for resources such as

nutrients, food or space, but this is often more than counterbalanced by the increased

survival of the group because of its ability to defend itself, to find resources, or to modify

microclimate or microhabitat conditions.

The degree of aggregation and density that result in optimum population growth

and survival varies with the species and conditions. Therefore undercrowding (lack of

aggregation) and overcrowding may be limiting to the growth and survival of a

population. This is known as Allee’s Principle of aggregation after the well known

behaviour ecologist W.C.Allee. Allee’s prinicple is a relationship between population density

and survival of animals.

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. Survival chances and fitness of a 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.

Allee observed that fishes who lived in groups could survive better to a particular

dose of poison in water than isolated individuals. The mucous and other secretions aided

in counteracting the poison, indicating some group action type of mechanism. Similarly,

a group of bees in a hive can generate and retain enough heat for the survival of all the

individuals when the temperature is low enough to kill all the bees if each were isolated.

Allee concluded that these types of primitive cooperation initiate process of social

organization, which shows varying degree of development in the animal kingdom

culminating in the group behaviour of human beings. Aggregation of humans into cities

and urban dwellings is beneficial up to a certain point. Since the optimum size of the

cities has not yet been objectively determined, the cities should reduce their population

when the costs exceed their benefits. Ecologically it is a mistake to maintain a city that is

too large for its support.

  

 

d) Refuging: It is another special type of aggregation of a large, socially organized group of

animals, which establish themselves in a favourable, central place (REFUGE) from

which they disperse and to which they return regularly to satisfy their needs of food and

material comforts. Human populations follow refuging type of aggregation. It is a

common phenomenon in developing and under developed countries.

e) Temporal dispersion: It is related to daily change in light and dark (circadian cycle) as

observed in the activities of some nectar feeding insects and oceanic planktons etc. It is

related to lunar cycles, seasons and tidal cycles e.g. certain activities of marine animals

are regulated by periods of moon and intensity of tides whereas in terrestrial animals

photoperiodism play an important role in regulating their activities.

DISPERSAL

Some organisms are not able to occupy their potential range due to their failure to reach

that area because of several physical or biological factors acting as barriers to the

dispersal power of the organism. When the organisms are transported outside their

normal range, they survive, reproduce and may form new species in response to changed

environmental conditions. Thus dispersal is an ecological process affecting distribution

and a genetic process in geographic differentiation.

Dispersal is the movement of individuals or their reproductive products (seeds,

spores and larvae, etc.) into or out of the population while leaving behind some

individuals in the original area. It leads to gene flow. Departure, transfer and

settlement are the three phases of dispersal.

Some of the organisms disperse at particular stage of their life cycle to increase their

chances of survival. Many factors like shortage of food, shelter and increased breeding

may force the organisms to disperse.

In addition to natality and mortality, dispersal also helps population in shaping their

growth forms. Small changes in the population due to continuous entering or leaving of

individuals or their reproductive parts has little effect on the population, however, larger

changes will affect the population.

Dispersal is influenced by barriers and by the inherent power of movement of individuals.

Dispersal acts as an important agent in the process of colonization of new areas and

  

 

establishing equilibrium diversity. It is also an important component of gene flow which

leads to the formation of new species.

Mac Arthur and Wilson, 1967 has described three types of dispersal in animals:

a) Dispersal of small organisms and propagules takes an exponential form as density

decreases by a constant amount of equal multiples of distance from the source.

b) Normally distributed pattern observed in large active animals.

c) Uniform pattern also called as Set Distance Dispersal e.g. birds flying from one

island to another island. Honeybees may avoid food near the hive in favour of food at

greater distances.

In animal population, dispersal can be divided into following different forms:

1) Emigration: It involves one-way outward movement of organisms from a

population. Emigration can occur due to shortage of resources, overcrowding or some

unfavourable social or physical factors. For example, in a bird population, when some

birds move away from the original population to another place, they are said to

undergo emigration. Many individuals in human population leave their place for

search of better job opportunities and other resources and do not return to their

original place.

2) Immigration: It involves one-way inward movement of organisms from a

population into an area, which is useful for the survival and reproduction of the

organisms. For example, if the birds move away from original population and enter

into new population, they are said to undergo immigration. People leave India for

better job prospects and living condition in U.S.A. It is an example where people are

emigrating from India and immigrating to U.S.A.

3) Migration: It is a two-way periodic or seasonal departure and return of organisms

from and to the population. Migrations are particularly seen in mammals, birds, fishes

and some insects. Migratory movements of animals are categorized into three types.

a) Migratory movements may be daily.

‒ As observed in oceanic zooplankton: the crustacean (cladocera and

copepods) undergo vertical migration in response to changes in light

  

 

intensity. During day time, light causes heating of surface water which

make the animals to move deeper into water and as the temperature

decreases during night, they come back to the surface of water.

‒ Bats migrate to feeding grounds at night and return to their roosting

places at dawn.

b) Migration may be seasonal: Grazing animals of mountains move up in the

summer and down in the valleys in winter. This movement is reflected in

traditional livestock management in mountain regions.

An earthworm shows seasonal migration in which they undergo vertically

down into soil to spend winter and appears on the surface of soil in spring and

summer.

The Kashmir Stag also undergo vertical migration in which it comes to the

valley in winter and ascends to the higher elevations of the mountainous

region in the summer.

Migratory birds usually breed in colder part of their range. Thus in the

Northern Hemisphere, birds move north in spring and towards south in

autumn. During summer, swallow Hirundo rustica is present in England,

western Asia and north-western Africa. During winter they migrate to central

and south Africa and India.

Siberian Crane makes annual visits to Indian subcontinent in the months of

November and December, particularly to Keola Deo National Park, Bharatpur.

Amphibians move from an aquatic breeding area to a land feeding area. Each

year they return to the same breeding ground and the pattern is repeated.

Baleen whales feed in summer in the Antarctic, then breed in winter in

tropical seas to the north.

c) Third type of migration is an unusual type of migration characterized by the

migratory fishes and monarch butterfly. Some fishes like salmon and eel take

long journey while other fishes take short journey. The eel, Anguila vulguris

moves from river to sea (catadromous fish) for reproduction. After laying

eggs the adult die. The young one hatch from eggs & start their journey from

sea to river and after maturity return to same breeding grounds. In some

species of Pacific salmon (anadromous), the young ones hatch and grow in

coastal streams and rivers. The young fish move downstream into the open

sea, where they reach sexual maturity. After attaining maturity they return to

their home streams (sea) to spawn and then die. Similarly, the adult Monarch

butterfly migrants never return to their home grounds of north in summer

from the wintering grounds of south. But their offspring return to their home

grounds. Some leafhoppers, the harlequin bug and the milkweed bug

undertake similar but less extensive migrations.

4) Nomadism: It is defined as the random movement of individuals in search of food

and shelter without definitely returning to the place of origin. Some tribes in

Rajasthan and Andhra Pradesh of India are nomads. e.g. Gadia Lohars, the nomadic

  

 

blacksmiths on carts, practicing architecture in Erragadda, Uppal, and other areas in

Hyderabad.

The effect of dispersal on a population depends upon its growth form and its rate of dispersal. A

population balanced with respect to limiting environmental factors is not affected by emigration

and immigration, as it is compensated by natality and mortality. However, if the population is

below or above the carrying capacity level, population is affected. Immigration accelerates the

population growth or in case of extreme reduction prevents extinction. Mass dispersal can

change the population in other ways. Bluegill fingerlings when introduced into a pond, where its

population has reached carrying capacity, results in decreased growth and small average size of

the fish. In this case the biomass remains unchanged but the individual size is so reduced that fishing

 becomes poor. The dispersal is frequent in motile animals but dispersal in sessile organisms depends on 

other agencies. They disperse by wind, water, coats of animals and feathers of birds. Some 

organisms are also dispersed through the digestive system of the animals. Wind helps in the dispersal 

of spiders, seeds of some plants, larvae of insects and cysts of brine shrimp. Running water of 

streams carries the larval forms and other organisms to their suitable microhabitat and help in 

their dispersal too far off places.