(a) The prickly pear cactus was íntroduced into Australia in the early 1920's and caused havoc by spreading rapidly into millions of hectares of rangeland due to lack of any of its natural predators in the new land.
b) The invasive prickly pear cactus was brought under control by introducing a cactus feeding predator (a moth) from its natural habitat.

Orchid is an epiphytic plant that grows on a mango tree branch and they together show commensalism. In this interaction, orchid is benefitted and the mango tree is neither harmed nor benefitted at all.

The ecological principle behind the biological control of pest insects is based on the ability of predator to reduce prey population in that habitat

Population. A population is a unit of biotic community made up of near permanent group of interbreeding individuals of a species found in a geographical area or space at a particular time.
Community. It is a grouping of different but independent and interacting populations of different species which live in a specific locality, e.g., pond community.

Defence mechanism of plants against herbivorous animals
(i) Formation of thick cuticle on their leaf surface.
(ii) Formation of leaf spines e.g. leaf spines in Opuntia
(iii) Modification of leaves into thorns. e.g, Bougainvillea and Duranta
(iv) Development of spiny margins on leaves.
(v) Development of sharp silicated edges in leaves.
(vi) Many plants produce and store toxic chemicals which cause discomforts to herbivores e.g. cardiac glucoside by Calotropis, Nicotine by tobacco.

Population attributes :
(i) Birth rate and death rate.
(ii) Age-Sex ratio.

(a) Commensalism:
It is the interspecific interaction, where one species is benefitted by the association while the other is neither benefitted nor harmed, e.g. Sucker fish attached to a shark.
(b) Parasitism:
It is the interspecific interaction, where one species (called parasite) takes food and shelter from another living species (host) and damages the host in due course of time; the parasite is benefitted and the host is harmed,e.g. Tapeworm in the intestine of man.
(c) Camouflage:
It is an adaptation, where the animal (organism blends with the surroundings to escape detection by the predator, e.g. Frogs have olive green skin with patches of chromatophores to camouflage with the grasses.
(d) Mutualism:
It is the interspecific interaction, where both the interacting species are mutually benefitted, e.g. An alga and a fungus in a lichen. Rhizobium and legume plant.
(e) Interspecific competition: It is an interspecific interaction, where organisms of two different species compete with each other for certain resources, e.g. Flamingoes and fishes competing for their common food, i.e. zooplanktons. 

The population can grow exponentially for a certain period of time and then assumes a steady state as the resource availability becomes limiting at some point of time.  

Every environment has resources to support a particular maximum number of individuals, called its carrying capacity; beyond that there is no increase in the size/density of a population.
A population showing logistic growth shows a sigmoid curve, when the number of individuals is plotted as a function of time.
It is described by the equation,
dN/dt = \(rN\left( \frac { K-N }{ K } \right) \), 
Where, N = Population density at time t.
r = Intrinsic rate of natural increase.
K = Carrying capacity.
This growth model is more realistic in nature because no population can sustain the exponential growth indefinitely.

Three important characteristics of a population are:
(i) Population size and population density
(ii) Birth or natality rate
(iii) Death or mortality rate.
(i) Population size.  The size of population keeps changing with time depending on the factors like:
(a) Food availability
(b) Weather
(c) Predation pressure and
(d) Competition.
The population density in a given habitat during a given period, changes due to four basic processes namely
(a) Natality,
(b) Mortality,
(c) Immigration and
(d) Emigration.
While natality and immigration contribute to an increase in the size and density of a population, mortality and emigration contribute to a decrease in them.
So the equation for population growth is:
\(N_{ t+1 }=N_{ t }+[(B+I)-(D+E)],\)
where, N = population density at time t.
B = Birth rate.
I = Immigration.
D = Death rate.
E  = Emigration.
If B + I is more than D + E, the population density increases.
If B + I is less than D + E, the population density decreases.
(ii) Birth or Natality rate. It is generally expressed as number or births per 1,000 individuals of a population per year. It increases the population size (total number of individuals of a population) and population density.
Natal or Birth Rate                  
If B + I is more than D + E, the population density increases.
\(=\frac { Total\quad Number\quad of\quad Live\quad Births\quad in\quad a\quad year }{ Total\quad Population\quad in\quad the\quad Middle\quad of\quad the\quad year } X1000\)
(iii) Death or Mortality rate. It is opposite of Mortality (Death) Rate natality rate. It is commonly expressed as the number of deaths per 1,000 individuals of a population per year.
\(=\frac { No.of\quad Deaths\quad in\quad a\quad year\qquad }{ Total\quad Population\quad in\quad the\quad Middle\quad of\quad the\quad year } X1000\\ \)

(i) Feigning death or conscious mimicry
(ii) Concealing type of aggressive mimicry​
(iii)​​​​​​ Batesian mimicry