Lesson 13 of 13 · 13 min

Chapter review

Watch a class

The whole chapter on YouTube

Whole chapter one-shot with past questions
NCERT Wallah · Hinglish · Whole chapter · Open on YouTube

Whole chapter read line by line
Competition Wallah · Hinglish · Lecture · Open on YouTube

Must-know facts

20 facts

1Ecology works at four levels: organisms, populations, communities, biomes.
2Natural selection acts at the population level, which links population ecology to genetics and evolution.
3Population attributes: birth rate, death rate, sex ratio, age distribution; rates are per capita.
4Lotus: 8 new on 20, birth rate 0.4 per lotus per year. Fruit flies: 4 of 40 die, death rate 0.1 per fly per week.
5Age pyramid shapes show growing, stable or declining populations.
6Population density N can be a number, per cent cover or biomass; relative density (fish per trap) often suffices.
7Tiger census uses pug marks and fecal pellets.
8Nt+1 = Nt + [(B + I) − (D + E)]; natality and immigration add, mortality and emigration subtract.
9Exponential: dN/dt = rN, r = b − d; Nt = N0 e^(rt); J-shaped curve.
10r values: Norway rat 0.015, flour beetle 0.12, India (1981) 0.0205.
11Logistic: dN/dt = rN (K − N)/K; sigmoid curve with lag, acceleration, deceleration, asymptote at K.
12Logistic growth (Verhulst-Pearl) is the more realistic model.
13Breed once: Pacific salmon, bamboo. Many small offspring: oysters, pelagic fishes.
14Signs: mutualism +/+, competition −/−, predation and parasitism +/−, commensalism +/0, amensalism −/0.
15Prickly pear in Australia controlled by a cactus-feeding moth; Pisaster removal led to over 10 invertebrate species going extinct.
16Nearly 25 per cent of insects are phytophagous; Calotropis makes cardiac glycosides.
17Gause: Competitive Exclusion Principle; MacArthur: five warbler species co-exist by resource partitioning.
18Connell: Balanus excludes Chthamalus on the Scottish coast (competitive release when removed).
19Human liver fluke needs a snail and a fish; the koel lays eggs in the crow's nest (brood parasitism).
20Commensalism: orchid on mango, barnacles on whale, cattle egret, clown fish; mutualism: lichen, mycorrhiza, fig-wasp, Ophrys.

Common traps

Where marks are lost

Calling the J-shaped curve logistic growth.

The J-shaped curve is exponential growth (unlimited resources); the logistic curve is sigmoid (S-shaped) and levels off at K.

Using total births instead of per capita rate.

Divide the births or deaths by the number of members: 8 births among 20 lotus plants is 0.4 per lotus per year.

Mixing up commensalism (+, 0) and amensalism (−, 0).

In commensalism one species gains; in amensalism one species is harmed. The other species is unaffected in both.

Saying competition happens only between closely related species.

Unrelated species compete too, as flamingoes and fishes do for zooplankton in South American lakes.

Saying competition needs limiting resources.

In interference competition one species lowers another's feeding efficiency even when food and space are abundant.

Calling the female mosquito a parasite.

NCERT does not treat the female mosquito as a parasite, even though she needs blood to reproduce.

Calling the orchid on a mango tree a parasite.

The orchid is an epiphyte using the branch for support: commensalism, not parasitism.

Writing Nt+1 = Nt + (B + E) − (D + I).

Immigration adds and emigration subtracts: Nt+1 = Nt + [(B + I) − (D + E)].

Thinking predators are always harmful to prey communities.

Predators keep prey in check and maintain diversity; removing Pisaster caused more than 10 invertebrate species to go extinct.

Key terms

18 terms

Population: Individuals of one species in a defined area that share or compete for resources and can interbreed.
Birth rate: Births per member of the population in a given period.
Sex ratio: The proportion of females and males in a population.
Age pyramid: A plot of the per cent of individuals in each age group, showing growth status.
Population density (N): Population size, measured as numbers, per cent cover or biomass.
Natality: Number of births added to a population in a given period.
Emigration: Individuals leaving the habitat for elsewhere in a given period.
Intrinsic rate of natural increase (r): r = b − d, per capita birth rate minus per capita death rate.
Carrying capacity (K): The maximum population a habitat can support for a species.
Logistic growth: Growth that slows as N approaches K, giving a sigmoid curve.
Darwinian fitness: Reproductive fitness, a high r value.
Mutualism: An interaction in which both species benefit.
Commensalism: One species benefits, the other is unaffected.
Amensalism: One species is harmed, the other is unaffected.
Competitive release: A species expands its range when a superior competitor is removed.
Resource partitioning: Competing species co-exist by using a resource at different times or in different ways.
Brood parasitism: A bird laying eggs in another species' nest for it to incubate, like the koel in a crow's nest.
Co-evolution: Linked evolution of two interacting species, such as a flower and its pollinator.

Test yourself: 10 questionsExam-style questions on Organisms and Populations, with full solutions.Start