- Population growth is determined by birth, death, immigration, and emigration rates and can take the form of exponential or logistic growth. - Exponential growth occurs when resources are unlimited, while logistic growth occurs when resources become limited, causing the population to level off at the carrying capacity of the environment. - Population regulation occurs through density-dependent factors like competition and disease, which increase mortality at high population densities, and density-independent factors like weather. - R-strategist species are adapted to unstable environments through high reproduction rates, small body size, and short lifespans. K-strategist species are adapted to stable environments through slower growth, larger body size, and investment in fewer offspring.

1. Population ecology
Topic- Population Dynamics/ Population growth
The term population growth refers to the change in the number of individuals in a
population with time.
Change in population is determined by four factors:
• Birth
• Death
• Immigration
• Emigration
In a close to population the change in population is only related to birth and death
of individuals where as in case of open population birth death immigration and
emigration results in the change of population.
Population growth can be exponential growth or logistic growth.
Exponential growth
Exponential growth is defined as growth with no limitation.
All members have abundance resource availability and population to place with
intrinsic rate of increase.

2. Exponential growth in the number increase in geometric progression 20
, 21
, 22
,
23
… it is constant fraction.
By contrast, a pattern of growth that increases at constant amount per unit of time
called arithmetic growth.
Exponential growth can be expressed by the given equation:
Where, n= Population size
r= intrinsic rate of natural increase
In a closed population, r is defined as the instantaneous per capita birth rate (b)
minus the instantaneous per capita death rate (d).
Similarly, in an open population r is equal to (Birth + Immigration) - (Death +
Emigration).
When resources such as food and space in a habitat are unlimited, all members of a
species have the ability to grow exponentially. The population size that increases
exponentially at a constant rate, results in a J- shaped growth curve when population size
(N) is plotted over time (t).

3. Logistic growth
As population density increases, each individual has access to fewer resources. It
means a particular environment can only support a maximum population size.
The number of individuals of a particular species that a particular environment can
support indefinitely is defined as carrying capacity (represented by the letter K).
Carrying capacity is not fixed, but varies over space and time with the abundance
of limiting sources resources. It is determined by various factors including
predation, competition and climatic conditions.
(a) Change in population size (N) through time as predicted by the logistic model of population
growth. Initially (low values of N), the population grows exponentially as N increases the rate of
population growth decreases, eventually reaching zero as the population size approaches the
carrying capacity (K).
(b) The relationship between the rate of population growth, dN/dt and population size N, takes
the form of a parabola, reaching a maximum value at a population size of N = K/2. The rate of
population growth (dN/dt) is at its highest when N=K/2 (called the inflection point) and then
decreases as it approaches the carrying capacity (K).

4. Population regulation
Population ecologists have identified a number of mechanisms by which
populations could be regulated.
Broadly speaking, factors regulating population growth are either density-
dependant or density-dependant.
Density-dependent factors are those whose effects on the birth rate or death rate
change as a function of the population density. These factors include disease,
competition for space, water and nutritions and predation. For example, a
population of rabbits may increase exponentially until competitive intraspecific
interactions cause either the birth rate to decrease or the death rate to increase,
leading to a net decline in reproductive rate and subsequent decrease in population
density.
Density-dependent factors can have either a positive or a negative correlation to
population size. As population size increases, either birth rate decline or mortality
rate increases or both. It is a negative feedback.
However, not always density dependent factors are negatively related to
population size. In some cases, growth rate increases with population size. This
phenomenon is referred to as the Allee effect (after W. Allee, who first described
it) and is an example of positive feedback. Positive relationships between fitness
and population size can be caused by a variety of mechanisms that affect
reproductive and survival.
Density-independent factors affect population growth, irrespective of the density
of the population. These factors are usually associated with abiotic events-
changes in the physical environment that either promotes or repress population
growth, but their effects are independent of population density it may include
natural catastrophes such as hurricanes, floods and seasonal variation in weather
pattern.

5. r-strategists and k-strategists
The concept of r-strategists and k-strategists links population dynamics to life
history.
A strongly variable and unpredictable environments lead to a highly fluctuating
population density.
These conditions select for a high intrinsic growth rate r which is achieved by a
distinctive life -history strategy consisting of rapid development, a small body size,
early reproduction, semelparity and a short life span (usually with a life span of
less than a year).
Species with this reproductive pattern overcome the massive loss of their offspring
by producing so many unprotected young that a few will survive to reproduce
many offspring to begin the cycle again. Species with these characteristics are
called r-strategists. Algae, bacteria, rodents, annual plants (such as dandelions) and

6. most insects are r-strategists. Such species tend to be opportunities. They
reproduce and disperse rapidly when conditions are favorable.
On the other hand, relatively constant or predictable environmental conditions
allow a population to reach its carrying capacity K and thus a high average
population density.
The concept says that these conditions select for a high K, which is achieved by
slow development (associated with great competition ability), a large body size,
delayed reproduction, iteroparity and a long life span. Species with these
characteristics are called K-strategists. These species spent fairly little energy into
reproduction and put most of their energy into nurturing and protecting their
young until they reach reproductive age.