Organisms and Populations — Notes

A population is a group of individuals of a species living in a well-defined geographical area, sharing or competing for similar resources and potentially interbreeding. A population has attributes that individual organisms do not.

Population density (N) — the number of individuals per unit area or volume. It may be measured by: a total count (large, visible animals); quadrats (plants; count in sample squares); percentage cover or biomass when a simple count is misleading (a single huge banyan tree versus 200 carrot-grass plants); relative density (fish caught per trap); indirect signs such as pug marks and faecal pellets (tiger census); and mark–recapture: N = (number marked × number in second catch) ÷ number recaptured marked (for example, 60 marked, 50 caught, 12 recaptured → about 250).

Natality is the number of births during a given period that are added to the initial density; the birth rate is expressed per individual per unit time (for example 0.3 per plant per year). Mortality is the number of deaths; the death rate is per individual per unit time (for example 0.075). Immigration is the number of individuals of the same species that come into the habitat from elsewhere; emigration the number that leave. Hence N(t+1) = N(t) + [(B + I) − (D + E)].

Sex ratio is the proportion of females to males (for example 60 percent female, 40 percent male).

Age structure: plotted as an age pyramid of pre-reproductive, reproductive and post-reproductive individuals: expanding (triangular, broad base — growing), stable (bell-shaped) and declining (urn-shaped).

Carrying capacity (K) — the maximum number of individuals of a species that a habitat can support, set by its limited resources.

Exponential growth: when resources are unlimited, each species can realise its full innate potential to grow. If the per-capita birth rate is b and death rate d, the intrinsic rate of natural increase is r = b − d, and dN/dt = rN, integrating to N_t = N₀e^{rt}. The graph of N against t is J-shaped. Values of r: Norway rat 0.015, flour beetle 0.12, human population of India in 1981 0.0205. Doubling time = ln 2 ÷ r.

Logistic growth: no population has unlimited resources; competition for food and space leads to survival of the fittest. A population growing in a habitat with limited resources shows a lag phase, then acceleration, then deceleration, and finally an asymptote when density reaches the carrying capacity K. This sigmoid (S-shaped) curve is the Verhulst–Pearl logistic growth model: dN/dt = rN (K − N)/K. As N approaches K, (K − N)/K approaches zero, so growth slows and stops. Since resources for most animal populations are finite, the logistic model is considered more realistic.

Populations evolve to maximise their reproductive fitness, also called Darwinian fitness (high r value), in the habitat in which they live. Under a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy.

• Some organisms breed only once in their lifetime (Pacific salmon fish, bamboo); others breed many times (most birds and mammals).
• Some produce a large number of small-sized offspring (oysters, pelagic fishes); others produce a small number of large-sized offspring (birds, mammals). Ecologists suggest that life-history traits of organisms have evolved in relation to the constraints imposed by the abiotic and biotic components of the habitat.

Population interactions (+ beneficial, − detrimental, 0 neutral): mutualism (+, +), competition (−, −), predation (+, −), parasitism (+, −), commensalism (+, 0), amensalism (−, 0).

Mutualism: both species benefit. Lichens — a fungus and a photosynthesising alga or cyanobacterium. Mycorrhizae — fungi and roots of higher plants: the fungus helps absorb nutrients; the plant provides carbohydrates. Plant–animal mutualism: plants reward pollinators and seed dispersers with nectar, pollen or fruit. Every fig species is pollinated only by its partner wasp species; the female wasp uses the fruit as an egg-laying site and pollinates the fig inflorescence, and the developing seeds feed its larvae. The Mediterranean orchid Ophrys uses sexual deceit: one petal resembles the female of a bee species, and the male “pseudocopulates” with it and transfers pollen.

Competition: both species are harmed. It is commonly between closely related species competing for the same limited resources, but this is not always so: flamingoes and resident fishes in some shallow South American lakes compete for zooplankton. Resources need not be limiting: in interference competition, the feeding efficiency of one species may be reduced by the interfering and inhibitory presence of the other. The Abingdon tortoise of the Galapagos became extinct within a decade after goats were introduced, owing to the goats’ greater browsing efficiency.

• Competitive release: a species whose distribution is restricted by a superior competitor expands dramatically when the competitor is removed. Connell showed that on the rocky sea coasts of Scotland, the larger and competitively superior barnacle Balanus dominates the intertidal area and excludes the smaller Chthamalus.
• Gause’s competitive exclusion principle: two closely related species competing for the same resources cannot coexist indefinitely; the competitively inferior one will eventually be eliminated (shown with two species of Paramecium).
• Resource partitioning: MacArthur showed that five closely related species of warblers on the same tree avoided competition and coexisted because of differences in their foraging activities.

Predation: nature’s way of transferring energy fixed by plants to higher trophic levels. Predators keep prey populations under control — the prickly pear cactus introduced into Australia spread over millions of hectares until a cactus-feeding moth was introduced. Predators help maintain species diversity by reducing competition among prey: when the starfish Pisaster was removed from an American intertidal community, more than 10 species of invertebrates became extinct within a year owing to interspecific competition. Prudent predators avoid over-exploiting their prey. Anti-predator defences: camouflage (cryptic colouration) in some insects and frogs; the Monarch butterfly is distasteful because of a chemical it acquires by feeding on a poisonous weed as a caterpillar; thorns (Acacia, cactus); chemicals — Calotropis produces highly poisonous cardiac glycosides; nicotine, caffeine, quinine, strychnine and opium are produced as defences against grazers and browsers.

Parasitism: the parasite gains food and shelter at the host’s expense. Many parasites are host-specific and have evolved adaptations: loss of unnecessary sense organs, adhesive organs or suckers, loss of the digestive system, and high reproductive capacity. Life cycles are often complex, with intermediate hosts — the human liver fluke depends on a snail and a fish; the malaria parasite needs a mosquito. Parasites may reduce host survival, growth and reproduction and make the host more vulnerable to predators.

• Ectoparasites feed on the external surface: lice on humans, ticks on dogs, copepods on marine fish; Cuscuta on hedge plants (it has lost its chlorophyll and leaves).
• Endoparasites live inside the host: tapeworm, liver fluke, Plasmodium; they have simplified morphology and high reproductive potential.
• Brood parasitism: the cuckoo (koel) lays eggs in the nest of its host, the crow, which incubates them; the parasitic eggs resemble the host’s eggs in size and colour, reducing the chance that the host detects and ejects them.

Commensalism: one species benefits; the other is neither harmed nor benefited — an orchid growing as an epiphyte on a mango branch; barnacles on the back of a whale; cattle egrets foraging near grazing cattle, which stir up insects; sea anemone and clown fish (the fish gains protection from predators among the stinging tentacles).

Amensalism: one species is harmed and the other is unaffected — for example Penicillium secretes penicillin, which inhibits bacteria.