Chapter 11: Organisms and Populations (Ecology)

• Population density (number per unit area)
• Birth rate (natality) and death rate (mortality)
• Sex ratio (ratio of females to males)
• Age distribution (shown by age pyramids)
• Population growth (rate of change in size), immigration and emigration

Density, birth rate, death rate, sex ratio, age distribution and growth rate.

For exponential growth, $N_t=N_0{e}^{rt}$. Doubling in 3 years: $2N_0=N_0{e}^{3r}$, so

$$e^{3r}=2\Rightarrow r=\frac{\ln 2}{3}=\frac{0.693}{3}\approx 0.231\text{per year}$$

r ≈ 0.231 per year (about 23% per year).

• Morphological: thorns (Acacia, cactus), spines and prickles; tough or hairy leaves.
• Chemical: poisonous or bad-tasting chemicals that sicken or kill herbivores or put them off eating, e.g. cardiac glycosides in Calotropis; nicotine, caffeine, quinine, strychnine, opium; also alkaloids that reduce digestion.

Thorns and spines, and chemicals such as cardiac glycosides (Calotropis), nicotine, caffeine, quinine and strychnine.

This is commensalism. The orchid (an epiphyte) benefits by getting support and better access to sunlight, while the mango tree is neither harmed nor benefited (the orchid takes no food from it).

Commensalism: the orchid benefits; the mango tree is unaffected.

It is based on predation (and parasitism): natural enemies (predators, parasites, pathogens) of a pest keep its population in check. In biological control, such enemies are introduced or encouraged, e.g. ladybirds to eat aphids, Gambusia fish to eat mosquito larvae, Bacillus thuringiensis against caterpillars. It reduces pests without harmful chemicals.

Predator–prey (and parasite–host) relationships: natural enemies keep pest populations under control.

• Population: a group of individuals of the same species living in a particular area at a given time, sharing resources and able to interbreed.
• Community: all the populations of different species living together and interacting in a particular area.

Population: same species in an area. Community: populations of different species interacting in an area.

• (a) Commensalism: one species benefits, the other is neither harmed nor benefited. Example: an orchid growing on a mango branch; barnacles on a whale; cattle egrets and grazing cattle.
• (b) Parasitism: one species (parasite) benefits at the cost of the other (host). Example: Cuscuta on hedge plants; lice on humans; tapeworm in the intestine.
• (c) Camouflage: an organism blends with its surroundings to avoid being detected by predators (or prey). Example: some insects and frogs are coloured like their background; the stick insect.
• (d) Mutualism: both species benefit. Example: lichens (fungus + alga); mycorrhiza (fungus + plant roots); fig and fig wasp.
• (e) Interspecific competition: different species compete for the same limited resource, harming both. Example: flamingos and fish in South American lakes competing for zooplankton; the Abingdon tortoise and goats on the Galapagos.

As defined, with the examples above.

In nature, resources are limited, so a population cannot grow exponentially for ever. Logistic (Verhulst–Pearl) growth has these phases:

• Lag phase: slow growth at first.
• Acceleration (exponential) phase: rapid growth.
• Deceleration phase: growth slows as resources become limiting.
• Asymptote: the population levels off at the carrying capacity (K), the maximum the environment can support.

The curve is S-shaped (sigmoid) when population size N is plotted against time t. It is described by

$$\frac{dN}{dt}=rN\left(\frac{K-N}{K}\right)$$

Diagram: draw N (y-axis) against time (x-axis): a J-shaped curve for exponential growth and an S-shaped curve for logistic growth, with a horizontal dashed line at K that the S-curve approaches.

An S-shaped curve: lag, rapid growth, deceleration, and levelling off at the carrying capacity K; $dN/dt=rN(K-N)/K$.

(d) One organism (the parasite) is benefited and the other (the host) is harmed.

(d)

• Population density: the number of individuals per unit area or volume. It tells us how crowded the population is; it changes with natality, mortality, immigration and emigration.
• Birth rate (natality): the number of births per 1000 individuals per year; it adds to the population.
• Death rate (mortality): the number of deaths per 1000 individuals per year; it reduces the population.

(Others: sex ratio, and age distribution shown by age pyramids that are expanding, stable or declining.)

For example, density, birth rate and death rate (also sex ratio and age distribution), as explained above.