Organisms and Populations

Populations possess the following attributes that individual organisms do not:

- Birth rate and death rate (as per capita rates)
- Sex ratio
- Age distribution
- Population density / size

An individual may have births and deaths, but only a population has these measurable collective properties.

If a population doubles in 3 years, then
$$N_t = 2N_0$$
and from exponential growth,
$$N_t = N_0 e^{rt}$$
So,
$$2 = e^{3r}$$
Taking natural log on both sides,
$$\ln 2 = 3r$$
$$r = \frac{\ln 2}{3} = \frac{0.693}{3} \approx 0.231$$
So the intrinsic rate of increase is about 0.231 per year.

Important defence mechanisms in plants against herbivory include:

- Thorns and spines, e.g. Acacia and Cactus
- Chemical defences such as poisonous or bad-tasting substances that make the herbivore sick, inhibit feeding or digestion, or kill it
- Examples of such chemicals include cardiac glycosides in Calotropis

So, plants protect themselves by morphological as well as chemical defences.

An orchid growing on a mango branch is an example of commensalism because the orchid benefits by using the mango tree as support, while the mango tree is neither harmed nor benefited.

The ecological principle behind biological control of pest insects is predation. A predator helps by regulating the prey population, which is why predators are used in biological pest control.