Populations have attributes that individual organisms do not.

(i) Birth rate. An individual may have births, but only a population has a birth rate, expressed per capita. If a pond had 20 lotus plants and 8 new plants were added, the birth rate is 8/20 = 0.4 offspring per lotus per year.

(ii) Death rate. Similarly per capita: if 4 individuals in a laboratory population of 40 fruitflies died in a week, the death rate is 4/40 = 0.1 individuals per fruitfly per week.

(iii) Sex ratio. An individual is either a male or a female, but a population has a sex ratio — for example 60 per cent females and 40 per cent males.

(iv) Age distribution. A population at any time is composed of individuals of different ages, and the proportion of different age groups of males and females is presented graphically as an age pyramid, whose shape indicates whether the population is stationary, growing or declining. An individual has only one age.

(v) Population density (N). The size of the population in an area, which may be expressed as numbers, biomass or per cent cover depending on the species. A single organism has no density.

Why these are population-level properties. Each is a statistical property that emerges only from many individuals considered together. This is also why natural selection, although it acts on individuals, is studied at the population level.

Use the integral form of the exponential growth equation.

\( N_t = N_0 e^{rt} \)

Substitute. The population doubles, so N_t = 2N₀, and t = 3 years.

\( 2N_0 = N_0 e^{3r} \quad\Rightarrow\quad 2 = e^{3r} \)

Take natural logarithms.

\( \ln 2 = 3r \quad\Rightarrow\quad r = \dfrac{0.693}{3} = 0.231 \)

Answer: r = 0.231 per year.

Two remarks worth making. First, the doubling time depends only on r and not at all on the starting population size, since t_double = 0.693/r. Second, compare 0.231 with the chapter's examples: 0.015 for the Norway rat, 0.12 for the flour beetle, and 0.0205 for the human population of India in 1981. A value of 0.231 is a very rapid rate of increase — more than ten times the human rate.

Plants have evolved an astonishing variety of morphological and chemical defences against herbivores, because unlike animals they cannot run away from their predators. The problem is severe: nearly 25 per cent of all insects are known to be phytophagous, feeding on plant sap and other parts of plants.

Morphological defences. Thorns are the most common morphological means of defence, as in Acacia and cactus. Related structures include spines, prickles, hairs and a tough or waxy cuticle, all of which deter the herbivore before it can feed.

Chemical defences. Many plants produce and store chemicals that:

• make the herbivore sick when they are eaten;
• inhibit feeding or digestion;
• disrupt its reproduction;
• or even kill it.

The standard example. The weed Calotropis, seen growing in abandoned fields, produces highly poisonous cardiac glycosides, and that is why cattle and goats are never seen browsing on this plant.

A striking consequence. A wide variety of chemical substances that we extract from plants on a commercial scale — nicotine, caffeine, quinine, strychnine and opium among them — are produced by plants actually as defences against grazers and browsers.

This is commensalism — the interaction in which one species benefits and the other is neither harmed nor benefited (+ / 0).

The orchid benefits. It is growing as an epiphyte on the mango branch, which gives it support and a position high enough to reach the light, without having to build a trunk of its own.

The mango tree derives no apparent benefit — and no apparent harm either. This is the key distinction from parasitism: the orchid is an epiphyte, not a parasite. It retains its own chlorophyll and makes its own food, and it does not tap the mango's vascular tissue for nutrition. Contrast Cuscuta, a parasitic plant on hedge plants, which has lost its chlorophyll and leaves and derives its nutrition from the host it parasitises.

Other examples of the same interaction. Barnacles growing on the back of a whale; the cattle egret foraging beside grazing cattle, which stir up and flush out insects as they move; and the clown fish sheltering among the stinging tentacles of a sea anemone, which does not appear to derive any benefit from hosting it.

The principle. Biological control methods adopted in agricultural pest control are based on the ability of the predator to regulate the prey population.

Why predators can do this. Predators keep prey populations under control; but for predators, prey species could achieve very high population densities and cause ecosystem instability. Predation is in this sense a regulating force, not merely a destructive one.

Why pest outbreaks happen in the first place. When certain exotic species are introduced into a geographical area, they become invasive and start spreading fast, because the invaded land does not have their natural predators. The pest is not inherently stronger in the new land — it is simply unchecked, so its growth is exponential where at home it would have been logistic.

The classic case. The prickly pear cactus introduced into Australia in the early 1920s caused havoc by spreading rapidly into millions of hectares of rangeland. The invasive cactus was finally brought under control only after a cactus-feeding predator — a moth — from its natural habitat was introduced into the country. The remedy was to restore the missing half of a natural interaction.

Why the source of the control agent matters. The moth was taken from the cactus's own natural habitat, and so was already specialised on it. A control agent that is not specific to the pest becomes the next invasive species — which is why biological control must be done with great care.

Population. A population is a group of individuals of a given species living in a well defined geographical area, sharing or competing for similar resources, and potentially interbreeding. Although the term interbreeding implies sexual reproduction, a group of individuals resulting from asexual reproduction is also generally considered a population for the purpose of ecological studies.

Examples: all the cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate, lotus plants in a pond.

Community. A community is the assemblage of populations of all the different species that live together and interact in a given area. Animals, plants and microbes do not and cannot live in isolation but interact in various ways to form a biological community, and even in minimal communities many interactive linkages exist, although all may not be readily apparent.

The relationship between the two. A population is one species; a community is many. There is no habitat on earth inhabited by just a single species, and such a situation is even inconceivable — for any species the minimal requirement is one more species on which it can feed. Even a plant, which makes its own food, needs soil microbes to return inorganic nutrients and an animal agent for pollination. Community and population are two of the four levels of organisation ecology deals with, the others being organisms and biomes.

One point about (e) worth adding. Competition need not involve closely related species, and resources need not even be limiting — in interference competition the feeding efficiency of one species is reduced by the inhibitory presence of the other even when food and space are abundant.

The curve. A population growing in a habitat with limited resources shows initially a lag phase, followed by phases of acceleration and deceleration, and finally an asymptote, when the population density reaches the carrying capacity. A plot of N in relation to time results in a sigmoid curve. This type of population growth is called Verhulst-Pearl Logistic Growth, and is described by:

\( \dfrac{dN}{dt} = rN\left(\dfrac{K - N}{K}\right) \)

where N is the population density at time t, r the intrinsic rate of natural increase, and K the carrying capacity.

Why each phase occurs. In the lag phase N is very small, so even a high per capita rate produces little absolute increase. In the acceleration phase N has grown but is still far below K, so (K−N)/K is close to 1 and growth is nearly exponential. In the deceleration phase N is a large fraction of K, so (K−N)/K becomes small and drags dN/dt down as competition for limited resources intensifies. At the asymptote N = K, so (K−N)/K = 0 and growth ceases.

Why this model. No population in nature has unlimited resources at its disposal, and since resources for growth for most animal populations are finite and become limiting sooner or later, the logistic growth model is considered the more realistic one.

The correct answer is (d) — one organism is benefited, the other is affected.

Why. In parasitism only one species benefits, the parasite, and the interaction is detrimental to the other species, the host. On the sign convention it is (+ / −). The majority of parasites harm the host: they may reduce its survival, growth and reproduction, reduce its population density, and render it more vulnerable to predation by making it physically weak.

Why the others are wrong.

(a) One organism is benefited is incomplete rather than false. It is true of parasitism, but it is equally true of predation and of commensalism, so it does not identify parasitism. A definition must exclude the alternatives.

(b) Both organisms are benefited is mutualism (+ / +) — as in lichens, mycorrhizae, or the fig and its wasp.

(c) One organism is benefited, the other is not affected is commensalism (+ / 0) — as in an orchid growing as an epiphyte on a mango branch, or barnacles on the back of a whale.

A caution about (d). Predation is also (+ / −). What distinguishes parasitism from predation is that the parasite lives on or in the host, usually over a long period and usually without killing it outright, whereas a predator consumes its prey.

(i) Population density (N). The size of the population in a given area. The size of the population tells us a lot about its status in the habitat, and whatever ecological process we wish to investigate — competition with another species, the impact of a predator, the effect of a pesticide — we always evaluate it in terms of a change in population size. Density need not be measured in numbers alone: where one huge banyan stands among 200 carrot grass plants, per cent cover or biomass is a more meaningful measure, and where counting is impractical, an indirect estimate is used, as in the tiger census based on pug marks and fecal pellets.

(ii) Birth rate and death rate. An individual may have births and deaths, but a population has birth rates and death rates, and in a population these refer to per capita births and deaths for a stated period. If a pond had 20 lotus plants and 8 were added, the birth rate is 0.4 offspring per lotus per year; if 4 of 40 fruitflies died in a week, the death rate is 0.1 individuals per fruitfly per week. Because they are per capita, populations of very different sizes can be compared directly, and the difference between the two rates gives r, the intrinsic rate of natural increase.

(iii) Age distribution, shown as the age pyramid. A population at any given time is composed of individuals of different ages. Plotting the per cent of individuals of each age group — for humans, males and females together — gives an age pyramid whose shape reflects the growth status of the population: broad-based if growing, straight-sided if stable, narrow-based if declining. It is therefore the one attribute that forecasts the population's future rather than describing only its present.

A fourth, if wanted: sex ratio. An individual is either male or female, but a population has a sex ratio — for instance 60 per cent females and 40 per cent males — and this sets a limit on how fast the population can grow, since in most species it is the number of females that determines the number of offspring.