Exercises
List the attributes that populations possess but not individuals.
Answer
(i) Birth rate (natality) – per capita births; an individual is only born once. (ii) Death rate (mortality) – per capita deaths. (iii) Sex ratio – e.g. 60% females and 40% males; an individual is either male or female. (iv) Age distribution / age pyramid – showing whether the population is growing, stable or declining. (v) Population density (size, N). (vi) Population growth and growth rate (intrinsic rate of natural increase, r), along with immigration and emigration.
If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population?
Answer
Given: Nt = 2N₀, t = 3 years. Exponential growth: Nt = N₀ e^(rt).
2N₀ = N₀ e^(3r)
⇒ e^(3r) = 2
⇒ 3r = ln 2 = 0.693
⇒ r = 0.693/3 ≈ 0.231 per year.
The intrinsic rate of increase is about 0.23 per year (23% per year).
Name important defence mechanisms in plants against herbivory.
Answer
Morphological defences: thorns and spines (e.g. Acacia, cactus), as well as hairs, tough or waxy leaves. Chemical defences: plants produce and store chemicals that make the herbivore sick, inhibit feeding or digestion, disrupt reproduction or kill it – e.g. Calotropis produces highly poisonous cardiac glycosides, so cattle and goats avoid it; nicotine, caffeine, quinine, strychnine and opium are also produced as defences against grazers and browsers.
An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?
Answer
It is commensalism. The orchid grows as an epiphyte on the mango branch and benefits by getting support and better access to sunlight, while the mango tree is neither harmed nor benefited (+, 0 interaction). The orchid makes its own food and does not draw nutrients from the mango tree.
What is the ecological principle behind the biological control method of managing with pest insects?
Answer
It is based on predation (and parasitism): predators and parasites naturally regulate the population of their prey/host. Introducing or encouraging natural enemies of a pest keeps the pest population under control without chemicals. Example: the prickly pear cactus that spread over millions of hectares in Australia was controlled only after a cactus-feeding moth from its native habitat was introduced.
Define population and community.
Answer
Population: a group of individuals of the same species living in a well-defined geographical area at a given time, sharing or competing for similar resources and potentially interbreeding (e.g. lotus plants in a pond).
Community: an assemblage of populations of different species (plants, animals and microbes) living together and interacting with each other in a particular area.
Q7. Define the following terms and give one example for each
(a) Commensalism
(b) Parasitism
(c) Camouflage
(d) Mutualism
(e) Interspecific competition
Ans. (a) Commensalism: interaction in which one species benefits and the other is neither harmed nor benefited. Example: cattle egret and grazing cattle; clown fish living among the tentacles of a sea anemone.
(b) Parasitism: interaction in which one species (parasite) benefits by getting food and shelter from the other (host), which is harmed. Example: Cuscuta on hedge plants; lice on humans; brood parasitism by the cuckoo (koel) in the crow's nest.
(c) Camouflage: cryptic colouration by which an organism blends with its surroundings to avoid detection by predators. Example: some insects and frogs.
(d) Mutualism: interaction in which both species benefit. Example: lichens (fungus + alga/cyanobacterium); mycorrhiza; fig tree and its pollinator wasp.
(e) Interspecific competition: interaction between two different species in which the fitness (r) of one is significantly lowered in the presence of the other, both being affected. Example: flamingoes and resident fishes competing for zooplankton in South American lakes; goats and Abingdon tortoise on Galapagos.
With the help of suitable diagram describe the logistic population growth curve.
Answer
Diagram (in words): plot population density N on the y-axis against time t on the x-axis. The curve is S-shaped (sigmoid) and levels off at a horizontal dashed line marked K (carrying capacity). (For comparison, the exponential curve is J-shaped and keeps rising.)
In nature, resources are limited, so no population grows exponentially indefinitely. A population growing with limited resources shows: (i) a lag phase – slow initial growth; (ii) an acceleration (exponential) phase – rapid growth; (iii) a deceleration phase – growth slows as competition for resources rises; (iv) an asymptote – population density reaches the carrying capacity K, the maximum number the habitat can support.
This is the Verhulst-Pearl logistic growth, described by: dN/dt = rN [(K − N)/K], where N = population density at time t, r = intrinsic rate of natural increase and K = carrying capacity. It is considered more realistic than the exponential model.
Select the statement which explains best parasitism.
(a) One organism is benefited.
(b) Both the organisms are benefited.
(c) One organism is benefited, other is not affected.
(d) One organism is benefited, other is affected.
Answer
(d) One organism is benefited, other is affected. The parasite benefits while the host is harmed.
List any three important characteristics of a population and explain.
Answer
(i) Birth rate (natality): the number of births per individual of the population in a given period. Example: 8 new lotus plants added to 20 in a year
⇒ birth rate = 8/20 = 0.4 offspring per lotus per year.
(ii) Death rate (mortality): the number of deaths per individual in a given period. Example: 4 of 40 fruit flies die in a week
⇒ death rate = 4/40 = 0.1 per fruit fly per week.
(iii) Age distribution: the percentage of individuals of different ages; plotted as an age pyramid, its shape shows whether the population is growing (broad base), stable (bell-shaped) or declining (urn-shaped, narrow base).
Other characteristics include sex ratio and population density (measured as numbers, per cent cover or biomass).
Important Extra Questions
Write the equation for change in population density.
Answer
Nt+1 = Nt + [(B + I) − (D + E)], where B = births (natality), I = immigration, D = deaths (mortality), E = emigration.
State Gause's Competitive Exclusion Principle.
Answer
Two closely related species competing for the same limiting resources cannot co-exist indefinitely; the competitively inferior one will eventually be eliminated.
What is resource partitioning? Give an example.
Answer
A mechanism by which competing species avoid competition by using resources differently, e.g. different feeding times or foraging patterns. MacArthur's five warbler species on the same tree co-existed due to different foraging behaviour.
Why are predators described as "prudent"?
Answer
If a predator overexploits its prey, the prey may become extinct and the predator will then die out for lack of food; so predators in nature do not overexploit their prey.
What is brood parasitism?
Answer
A parasitic bird (cuckoo/koel) lays its eggs in the nest of a host bird (crow), which incubates them; the parasite's eggs have evolved to resemble the host's eggs in size and colour.