Mains Paper II · 4 Environment & Ecology · 4.1 Ecology and ecosystems
4.1.6 Population and community ecology: growth, interactions and keystone species
Populations grow by births and immigration and shrink by deaths and emigration, within limits set by resources, while species in a community interact in ways that can help, harm or leave each other unaffected.
- APPSC Group 2 · Mains Paper II
- Advanced level
- 9 min read
- Free
Core facts
- Population attributes (an individual does not have them): birth rate (natality), death rate (mortality), sex ratio, age distribution.
- Population density is written as N.
- Population change: Nt+1 = Nt + [(B + I) − (D + E)]. Density rises when births plus immigration exceed deaths plus emigration.
- Exponential growth (unlimited resources): dN/dt = rN. It gives a J-shaped curve. r is the intrinsic rate of natural increase.
- Logistic growth (limited resources): dN/dt = rN (K − N)/K. It gives a sigmoid (S-shaped) curve. This is the Verhulst–Pearl model. K is the carrying capacity.
- Interactions between two species:
| Interaction | Species A | Species B |
|---|---|---|
| Mutualism | + | + |
| Competition | − | − |
| Predation | + | − |
| Parasitism | + | − |
| Commensalism | + | 0 |
| Amensalism | − | 0 |
- Keystone species: a species whose effect on the community is far larger than its numbers suggest. The classic case is the starfish Pisaster.
Explained
Measuring population size
- Plain numbers are not always meaningful. If an area has 200 Parthenium plants and one large banyan tree, percent cover or biomass describes the banyan better.
- Relative density is often enough. For example, the number of fish caught per trap is a measure of the fish population.
- Birth rate example (NCERT): 20 lotus plants last year, 8 new ones added: birth rate = 8/20 = 0.4 offspring per lotus per year.
- Age pyramids show the share of pre-reproductive, reproductive and post-reproductive individuals:
- triangular (broad base): expanding
- bell-shaped: stable
- urn-shaped (narrow base): declining
Growth models
- NCERT r values: Norway rat 0.015; flour beetle 0.12; India's human population in 1981 0.0205.
- Exponential: Nt = N0 e^(rt). A population that doubles in 3 years has r = ln 2 / 3 ≈ 0.23 per year.
- Logistic: the curve has a lag phase, then acceleration, then deceleration, and finally an asymptote at K. It is more realistic, because resources are always finite.
- Life-history patterns (NCERT):
- Some breed once in a lifetime: Pacific salmon, bamboo.
- Most birds and mammals breed many times.
- Some produce many small offspring: oysters, pelagic fishes.
- Others produce a few large offspring: birds and mammals.
- Populations evolve to maximise reproductive (Darwinian) fitness, shown by a high r.
Examples of each interaction (NCERT)
- Mutualism:
- lichens (fungus + alga or cyanobacterium)
- mycorrhiza (fungus + roots of higher plants)
- fig and its pollinator wasp: the female wasp lays eggs in the fruit, and the larvae feed on the developing seeds
- the orchid Ophrys, pollinated through sexual deceit of a bee
- Commensalism: orchid growing on a mango branch; barnacles on a whale; cattle egret and grazing cattle; sea anemone and clown fish.
- Amensalism: Penicillium secretes penicillin, which harms bacteria while the fungus is unaffected.
- Parasitism: lice on humans, ticks on dogs, copepods on marine fish (ectoparasites); liver fluke (endoparasite); Cuscuta on hedge plants.
- Brood parasitism: the koel (cuckoo) lays its eggs in a crow's nest.
- Competition: flamingoes and resident fishes compete for zooplankton in South American lakes.
Deeper
Predation
- Predators transfer energy, keep prey numbers in check and help keep species diversity.
- The prickly pear cactus, an introduced plant in Australia, had spread over millions of hectares of rangeland by the 1920s (NCERT dates the problem to the early 1920s). It was controlled only by introducing a cactus-feeding moth from its native area. (Biological control.)
- Prudent predators do not over-exploit prey. If prey went extinct, the predator would starve too.
- Prey defences:
- camouflage
- poison: the Monarch butterfly is distasteful to birds because of a chemical its caterpillar takes in from a poisonous weed
- thorns (Acacia, cactus)
- plant chemicals: Calotropis makes poisonous cardiac glycosides; nicotine, caffeine, quinine, strychnine and opium are plant defences against grazers
- Nearly 25% of all insects are phytophagous (they feed on plant sap and other plant parts).
Competition
- NCERT defines competition as a process in which the fitness of one species (measured by its r) is significantly lower in the presence of another. It occurs between related and unrelated species, and even when resources are not limited.
- Interference competition: the feeding efficiency of one species is reduced by the interfering and inhibitory presence of the other, even when resources are plentiful.
- Abingdon tortoise (Galapagos) died out within a decade of goats arriving, apparently because the goats browsed more efficiently.
- Competitive release: a species whose range is restricted by a superior competitor expands when that competitor is removed. Connell's experiments on the Scottish coast showed that the larger barnacle Balanus excludes the smaller Chthamalus from the intertidal zone.
- Gause's competitive exclusion principle: two closely related species competing for the same resource cannot coexist indefinitely; the weaker is eventually eliminated.
- Resource partitioning: MacArthur showed that five closely related warblers live on the same tree by foraging in different parts of it.
Parasites
- Many are host-specific, so host and parasite co-evolve.
- Adaptations: loss of unnecessary sense organs, adhesive organs or suckers, loss of the digestive system, high reproductive capacity.
- Life cycles of endoparasites are more complex than those of ectoparasites.
- In brood parasitism, the parasite's eggs resemble the host's in size and colour, which reduces the chance that the host detects and ejects them.
- NCERT notes that the female mosquito is not considered a parasite, although it needs our blood to reproduce.
Keystone species
- Robert Paine removed all Pisaster starfish from an enclosed intertidal area on the American Pacific coast. Within a year, more than 10 species of invertebrates became extinct, because of interspecific competition once the predator was gone.
- Other examples: tigers and wolves (top predators), elephants (open up forest), fig trees (fruit through the year).
Common traps
- ✗ "Commensalism benefits both species." ✓ That is mutualism. In commensalism one benefits and the other is unaffected.
- ✗ "One harmed, the other unaffected, is commensalism." ✓ That is amensalism.
- ✗ "Endoparasites have simpler life cycles than ectoparasites." ✓ Endoparasites have more complex life cycles.
- ✗ "The logistic curve is J-shaped." ✓ Logistic is S-shaped (sigmoid); exponential is J-shaped.
- ✗ "Gause showed warbler coexistence." ✓ MacArthur showed warbler resource partitioning; Gause gave competitive exclusion.
- ✗ "Lichen is commensalism." ✓ Lichen is mutualism between a fungus and an alga or cyanobacterium.
Quick revision
- Population attributes: birth rate, death rate, sex ratio, age distribution.
- Nt+1 = Nt + [(B + I) − (D + E)].
- Exponential: dN/dt = rN (J-shaped). Logistic: dN/dt = rN(K − N)/K (S-shaped, Verhulst–Pearl).
- r: Norway rat 0.015; flour beetle 0.12; India 1981 0.0205.
- Breed once: Pacific salmon, bamboo.
- +/+ mutualism; −/− competition; +/− predation, parasitism; +/0 commensalism; −/0 amensalism.
- Koel in crow's nest = brood parasitism.
- Gause: exclusion. MacArthur: warblers. Connell: barnacles.
- Pisaster removed → more than 10 invertebrate species lost: keystone species.
- Prickly pear in Australia controlled by a cactus-feeding moth.
Go deeper: Mains writing (beyond what APPSC Group 2 usually asks)
Mains writing Mains writing
- Frame: carrying capacity and species interactions explain both population crashes and the success of invasive species.
- Analysis points:
- Invasive species often grow exponentially because their natural enemies are absent. Lantana, Prosopis juliflora and water hyacinth are examples in India.
- Biological control uses predation or parasitism (the Australian cactus moth), but a careless introduction can itself become invasive.
- Loss of keystone and top predators causes cascades: prey multiply and over-graze, and diversity falls.
- Human population: the logistic model raises the question of Earth's carrying capacity, which depends on technology and consumption as well as numbers.
- Way forward:
- early detection and control of invasives
- protect apex predators and keystone species
- test biocontrol agents carefully before release
- manage harvests (fish, forest produce) below the maximum sustainable yield
- Example: the spread of Lantana camara in Indian forests has reduced native fodder and altered fire behaviour, showing a successful competitor freed from its natural controls.
Practise this topic. Questions on Ecology & ecosystems, with answers explained.
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