NATALITY AND MORTALITY IN PLANT POPULATIONS
Populations are dynamic entities under which all organisms belonging to
the same species living in the same area at the same time and they are
cytologically interbred ones. The growth is controlled by the rate at which new
individuals are added to the population through the process of birth and
immigration and the rate at which individuals leave the population through the
process of death or emigration. The population in which immigration or
emigration occur is known as open populations. Those in which movement into
and out of the population does not occur is referred to as closed population.
Processes like natality, mortality immigration and emigration are direct causes
for the changes in the population's dimensions, composition and dynamics.
Studying changes in populations’ dimensions helps in predicting future changes
in population sizes and growth rates.
Natality (birth rate)
Birth rate is the rate at which new individuals are added to a particular
population by reproduction. It is simply a broader term covering the production
of new individuals by birth, hatching, by fission, etc. The natality rate may be
expressed as the number of organisms born per female per unit time. In human
population, the natality rate is equivalent to the birth-rate. Birth rate or Natality
(B) = Number of births per unit time/Average population. It is generally
expressed as number of births per 1000 individuals of a population per year.
Birth rates are affected by factors such as nutrition, fertility and abiotic
conditions. Natality varies from organism to organism. It depends upon the
population density and environmental factors. It is a general rule that if the
population density is usually low, the birth rate is also low. This is so because
the chances of mating between males and females are low. If population density
is unusually high, the birth rate may also be low due to poor nutrition or
physiological or psychological problems related to crowding. There are two
types of natality.
(a) Maximum natality:
Also called as absolute or potential or physiological natality, it is the
theoretical maximum production of new individuals under ideal conditions
which means that there are no ecological limiting factors and that reproduction
is limited only by physiological factors. It is a constant for a given population.
This is also called fecundity rate.
(b) Ecological natality:
Also called realized natality or simply natality, it is the population
increase under an actual, existing specific condition. Thus it takes into account
all possible existing environmental conditions. Natality usually increase with
the period of maturity and then falls again as the organism gets older.
Mortality
Mortality refers to the number of deaths in population per unit time.
Death rate is the rate at which the individuals die or get killed. It is the opposite
of natality rate. Death rate is generally expressed as number of deaths per 1000
individuals of a population per year. Death rate in plants is affected by disease,
nutritional deficiency and different natural and/or man made habitat destruction,
whereas, in animals it is affected by disease, war, medical technology, improved
health care, transportation development and nutrition. The lowest death rate for
a given species under most favourable conditions is called potential mortality.
The actual death rate is observed under existing conditions is called realized
mortality. The realized mortality decreases with population size and population
density.
The percentage ratio of birth rate per death rate expressed in percentage is
called Vital Index. It determines the normal rate of growth of a population.
Mortality rate = D/t where D is the number of deaths in the time t.
There are two types of mortality:
(i) Minimum or Specific or Potential Mortality:
Also called specific or potential mortality, it represents the theoretical
minimu loss of individuals under ideal or non-limiting condition. Thus, even
under the best conditions individuals of a population would die of old age
determined by their physiological longevity. So it is constant for a population.
(ii) Ecological or Realized Mortality:
It is the actual loss of individuals under a given environmental condition.
It refers to the death of individuals of a population under existing environmental
conditions. Since it varies with environmental conditions, it is never constant.
The maximum mortality occurs at the egg, larval, seedling and old age.
Mortality is affected by a number of factors, such as, density, competition,
disease, predation and environment. Death rates vary among the species and are
correlated with birth rates. When the rate of natality is equal to the rate of
mortality the population is stationary.
A birth death ratio (Births/death x 100) is called vital index. For a
population, the survival of individuals is more important than the death. The
number of births in relation to the carrying capacity of the habitat is a
fundamental factor influencing the mortality rate. When more young’s are born
than the habitat can support, the surplus must either die or leave the area.
Because the number of survivors is more important than the number of dying
individuals, mortality is better expressed as survival or as life expectancy. The
life expectancy refers to the average number of years the members of a
population have left to live.
Plant natality
Plants’ natality is more difficult to determine than animals. The factors that
make it difficult to measure are:
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Seed production of individual plants varying year to year.
Seed production will also vary age class to age class.
The seeds will become dormant for long periods of time before germinating
Plant natality is an uncertain factor to measure. The time it takes for a plant
to germinate its seeds may be extended over too long of a time period for
accurate measurement.
Immigration
It is the movement of new ones into a population leading to increase in
population size.
Emmigration
Emigration is the permanent movement or departure of individuals of
same species out of the location due to lack of food, scarcity of space
(overcrowding) etc. Due to emigration, species spread into new areas which
results the decreasing of local population.
If the number of individuals added is greater than lost, the population will
show positive growth. The population show negative growth when the added
individual is lesser than lost. If the added and lost individuals are equal, the
population will become stationary and is called zero growth.
These processes are very dominant and can be easily calculated for animal
populations while in case of plant populations these processes are not distinctly
visible. In Plant Population seeds or propagule dispersal play an important role
in these processes especially in emmigration.
The Influence can be expressed as:
Nt = N0 + B– D + I – E
Where N0 = the population size at some arbitrary starting time (time zero)
Nt = the population size at some future time t
B = Birth rate / Natality
D = Death rate/ Mortality
I = Immigration
E = Emigration
Density
Population density is defined as the numbers of individuals per unit area
or per unit volume of environment. Larger organisms as trees may be expressed
as 100 trees per hectare, whereas smaller ones like phytoplanktons (as algae) as
1 million cells per cubic metre of water.
In terms of weight it may be 50 kilograms of fish per hectare of water
surface. Density may be numerical density (number of individuals per unit area
or volume) when the size of individuals in the population is relatively uniform,
as mammals, birds or insects or biomass density (biomass per unit area or
volume) when the size of individuals is variable such as trees.