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 an individual does not.

• Birth rate (natality) and death rate (mortality) are expressed per capita: births or deaths per individual per unit time. Example: 8 new plants added to a pond with 20 lotus plants in a year gives 8/20 = 0.4 offspring per lotus plant per year; 4 deaths among 40 fruit flies in a week gives 0.1 per fly per week.
• Sex ratio: for example 60 percent females and 40 percent males.
• Age distribution: the percentage of individuals of each age group. Plotted for a population, it gives an age pyramid (for humans, males and females are shown in one combined diagram). Shapes: expanding (triangular, broad base — growing), stable (bell-shaped), declining (urn-shaped, narrow base).
• Population density (N): the population size in a given area. It need not always be a count. With 200 Parthenium plants and a single huge banyan tree in an area, stating that banyan density is low would underestimate its enormous role; percent cover or biomass is more meaningful. Relative densities often serve the purpose — e.g. the number of fish caught per trap. Tiger census in national parks and reserves is often based on pugmarks and fecal pellets. Counting bacteria in a Petri dish is impossible, so indirect methods are used.

Changes in density. Four processes: natality (B) and immigration (I) increase density; mortality (D) and emigration (E) decrease it. N(t+1) = N(t) + [(B + I) − (D + E)]. Under normal conditions births and deaths matter most; immigration matters when a new habitat is colonised.

Exponential growth. With unlimited resources, a population grows exponentially. If b and d are per capita birth and death rates, dN/dt = (b − d) × N; writing r = b − d (the intrinsic rate of natural increase), dN/dt = rN, giving $N_t = N_0 e^{rt}$, a J-shaped curve. Values of r: Norway rat 0.015, flour beetle 0.12, human population of India in 1981 0.0205. Even a slow-growing animal like the elephant could reach enormous numbers without checks (Darwin).

Logistic growth. No population has unlimited resources, so competition arises and the fittest survive. A habitat supports a maximum number — its carrying capacity (K). A population in a habitat with limited resources shows a lag phase, then acceleration, deceleration, and an asymptote at K: a sigmoid (S-shaped) curve. This is Verhulst-Pearl logistic growth: dN/dt = rN[(K − N)/K]. Because resources become limiting sooner or later, the logistic model is more realistic.

Life history variation. Populations evolve to maximise reproductive (Darwinian) fitness (a high r value) in their habitat. Some organisms breed only once in their lifetime (Pacific salmon, bamboo); others breed many times (most birds and mammals). Some produce many small offspring (oysters, pelagic fishes); others a few large offspring (birds, mammals).

Interspecific interactions are beneficial (+), detrimental (−) or neutral (0) to each species: mutualism (+, +); competition (−, −); predation and parasitism (+, −); commensalism (+, 0); amensalism (−, 0).

Predation. Transfers energy fixed by plants to higher trophic levels; keeps prey populations under control (the prickly pear cactus introduced into Australia in the early 1930s was controlled only after a cactus-feeding moth from its natural habitat was introduced — the basis of biological control); maintains species diversity by reducing competition among prey (when the starfish Pisaster was removed from rocky intertidal communities of the American Pacific coast, more than 10 invertebrate species became extinct within a year). Prey defences: camouflage (insects, frogs); distastefulness (monarch butterfly, from a chemical its caterpillar takes from a poisonous weed); thorns (Acacia, cactus); poisons (Calotropis — cardiac glycosides; cattle and goats do not browse it); nicotine, caffeine, quinine, strychnine and opium are plant defences. Nearly 25 percent of insects are phytophagous.

Competition. The fitness (r) of one species is significantly lower in the presence of another. Unrelated species can compete (flamingoes and resident fishes for zooplankton in some shallow South American lakes). Interference competition occurs even when resources are not limiting. The Abingdon tortoise of the Galapagos became extinct within a decade of goats being introduced, owing to the goats’ greater browsing efficiency. Competitive release: a species restricted by a superior competitor expands its range when the competitor is removed. Connell: on Scottish rocky coasts the larger, superior barnacle Balanus excludes the smaller Chthamalus from the intertidal zone. Gause’s competitive exclusion principle: two closely related species competing for the same resources cannot coexist indefinitely; the inferior one is eventually eliminated. Resource partitioning: MacArthur showed that five closely related warbler species on the same tree avoided competition by differences in foraging. Herbivores and plants are more adversely affected by competition than carnivores.

Parasitism. Many parasites are host-specific and co-evolve with their hosts. Adaptations: loss of unnecessary sense organs, adhesive organs or suckers, loss of the digestive system, high reproductive capacity. Life cycles are often complex: the human liver fluke needs a snail and a fish as intermediate hosts; the malarial parasite needs a mosquito vector. Parasites reduce host survival, growth and reproduction and may make hosts vulnerable to predators. Ectoparasites: lice on humans, ticks on dogs, copepods on marine fish, Cuscuta (no chlorophyll) on hedge plants. Endoparasites live inside the host, with simplified bodies and high reproductive potential. Brood parasitism: the cuckoo (koel) lays eggs in a crow’s nest; its eggs resemble the host’s in size and colour.

Commensalism. Orchid growing as an epiphyte on a mango branch; barnacles on a whale’s back; cattle egret foraging near grazing cattle, which flush out insects; clown fish living among the stinging tentacles of a sea anemone, gaining protection.

Mutualism. Lichens (fungus with photosynthesising algae or cyanobacteria); mycorrhizae (fungi help plant roots absorb nutrients; the plant supplies carbohydrates); pollination and seed dispersal. Each fig species is pollinated only by its partner wasp species: the female wasp lays eggs in the fruit, the larvae feed on developing seeds, and the wasp pollinates the fig. The Mediterranean orchid Ophrys uses sexual deceit: one petal resembles a female bee in size, colour and markings; the male bee pseudocopulates with it and carries pollen to the next flower. If the female bee’s colour pattern changes, the orchid must co-evolve to keep its pollination success.