Biology_Today_-_October_2016

s Population density in a given habitat changes due to four basic processes:

Immigration (I)

Emmigration (E)

Population

Density (N)

Natality (B) Mortality (D)

Number of deaths

in the population

during a given

time period.

Number of births

during a given

period in the

population.

Number of individuals of the

same species that have come

into the habitat from elsewhere

during the time period.

Number of individuals of

the population who left the

habitat and gone elsewhere

during the time period.

+

+–

s If N is the population density of time t, then its density at time t + 1 is:

Nt + 1 = Nt + [(B + I) – (D + E)]

Exponential Growth

s When resources like food and space are unlimited,

population grows in an exponential or geometric ratio,

resulting in J-shape curve.

= rN

dN

dt

s (r is intrinsic rate of natural increase)

Logistic Growth

s Population shows initially a lag phase, followed by phases of

increase and decrease and finally the population density reaches

the carrying capacity (k), beyond which no further growth is

possible, resulting in a sigmoid curve (S - Shape curve).

s Verhulst-Pearl logistic growth equation is- dN

dt

=rNK-N

K

⎛

⎝⎜

⎞

⎠⎟

Time (t)

Population density (N)

Senescent

Phase

K K

Exponential

Lag phase phase

dN= rN

dt

dN= rN

dt

K–N

K

Time (t)

Population density (N)

POPULATION INTERACTIONS

1. Predation

s It is an interaction between 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 known as preys.

s E.g., carnivorous animals, carnivorous plants like Utricularia, Drosera, etc.

Importance of Predation

Prey population

s The predator keeps the population of the prey under check so that an equilibrium

is maintained.

s The prey also maintains its optimum activity and efficiency since mostly weak

and sick individuals are removed by the predators.

Biological control of weeds

s Red locust menace was controlled in Mauritius by the introduction of Mynah

from India.

s Aphids and other pests are kept under check by beetles (e.g., Coccinella or Lady Bird

Beetle).

s Gambusia is introduced in ponds to check growth of mosquito larvae. Cactus (Opuntia) introduced

into Australia in the early 1920’s was brought under control by a cactus-feeding predator (a moth).

Species diversity

s Predators also help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species.

Vegetation

s Predation helps in growth of vegetation all over the globe by restricting the population of herbivores.