Organisms and Populations | CBSE Class 12 Biology Notes

This note covers organisms and populations: what ecology studies, the definition of a population, population attributes such as birth rate, death rate, sex ratio and age pyramids, ways of measuring population density, the four processes that change density, exponential and logistic growth, carrying capacity, life history variation, and the population interactions of predation, competition, parasitism, commensalism, amensalism and mutualism.

What does ecology study, and what is a population?

Ecology is the study of the interactions among organisms and between an organism and its physical environment. It deals with the relationships of living organisms with the abiotic components (physico-chemical factors) and the biotic components (other species) of their environment.

Life can be studied at many levels of biological organisation: macromolecules, cells, tissues, organs, individual organisms, populations, communities, ecosystems and biomes. Ecology is basically concerned with four of these levels: organisms, populations, communities and biomes.

How-type and why-type questions

At any level we can ask two types of question. When a bulbul sings early in the morning, the how-type question "How does the bird sing?" seeks the mechanism; the answer lies in the voice box and the vibrating bone in the bird.

The why-type question "Why does the bird sing?" seeks the significance of the process; the answer may lie in the bird's need to communicate with its mate during the breeding season.

Populations and communities

Definition: A population is a group of individuals of a given species that live in a well-defined geographical area, share or compete for similar resources and potentially interbreed.

In nature we rarely find isolated, single individuals of any species. Interbreeding implies sexual reproduction, but a group of individuals resulting from asexual reproduction is also generally considered a population for ecological studies.

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

An individual has to cope with a changed environment, but it is at the population level that natural selection operates to evolve the desired traits. Population ecology is therefore important, because it links ecology to population genetics and evolution.

Animals, plants and microbes do not and cannot live in isolation. Populations of different species interact in various ways to form a biological community.

Note: Ramdeo Misra (1908 to 1998) is revered as the Father of Ecology in India. He obtained a Ph.D in Ecology (1937) under Prof. W. H. Pearsall at Leeds University, and established ecology teaching and research at Banaras Hindu University, Varanasi.

Due to his efforts, the Government of India established the National Committee for Environmental Planning and Coordination (1972), which later paved the way for the Ministry of Environment and Forests (1984).

What attributes does a population have that an individual does not?

An individual may have births and deaths, but a population has birth rates and death rates. An individual is either male or female, but a population has a sex ratio. A population also has an age distribution.

Birth rate and death rate

In a population, birth and death rates refer to per capita births and deaths. They are expressed as the change in numbers (increase or decrease) with respect to the members of the population.

Worked example 1. A pond had 20 lotus plants last year. Through reproduction 8 new plants were added, taking the current population to 28. What is the birth rate?

Answer: birth rate = 8/20 = 0.4 offspring per lotus per year.

Worked example 2. In a laboratory population of 40 fruitflies, 4 individuals died during one week. What is the death rate?

Answer: death rate = 4/40 = 0.1 individuals per fruitfly per week.

Sex ratio

The sex ratio describes the proportion of females and males in a population, for example 60 per cent females and 40 per cent males.

What is an age pyramid, and what does its shape show?

A population at any given time is made up of individuals of different ages. If the age distribution (the per cent of individuals of a given age or age group) is plotted, the resulting structure is called an age pyramid.

For human populations, age pyramids generally show the age distribution of males and females in one diagram. The age groups are pre-reproductive, reproductive and post-reproductive. The shape of the pyramid reflects whether the population is growing, stable or declining.

Three pyramids, labelled Expanding, Stable and Declining, stand side by side. Each is a stack of horizontal bars split in half by a central vertical line: one yellow bar at the bottom (Pre-reproductive), two blue bars above it (Reproductive) and three pink bars at the top (Post-reproductive).

In the expanding pyramid the yellow bar is the widest and the bars narrow steadily upwards. In the stable pyramid the yellow bar and the lowest blue bar are about equally wide. In the declining pyramid the yellow base is narrower than the blue bars, and the widest bar lies in the reproductive group.

See Fig. 11.1 in your NCERT textbook

The logic is simple: today's pre-reproductive individuals are the next reproductive group. A broad base means more future parents, so the population grows; a narrow base means fewer, so it declines.

How is population size, or population density, measured?

The size of a population tells us a lot about its status in the habitat. The outcome of competition, the impact of a predator or the effect of a pesticide is evaluated in terms of a change in population size.

Population size could be as low as fewer than 10 (Siberian cranes at the Bharatpur wetlands in any year) or go into millions (Chlamydomonas in a pond). It is technically called population density, designated as N, and need not be measured in numbers only.

When numbers are not the best measure

Total number is generally the most appropriate measure, but sometimes it is meaningless or difficult to determine. Suppose an area has 200 carrot grass (Parthenium hysterophorus) plants but only a single huge banyan tree with a large canopy.

Calling the banyan's density low would underestimate its enormous role in that community. Here per cent cover or biomass is a more meaningful measure. Counting is also impractical when a population is huge, such as a dense bacterial culture in a petri dish, where biomass or per cent cover is more practical.

Relative and indirect estimates

Sometimes absolute densities are not needed, and relative densities serve equally well: the number of fish caught per trap is a good enough measure of their total density in a lake. Often sizes are estimated indirectly; the tiger census in our national parks and tiger reserves is often based on pug marks and fecal pellets.

Which four processes change population density?

Population size is not static. It keeps changing with time, depending on factors such as food availability, predation pressure and adverse weather. Whatever the ultimate reasons, density in a habitat fluctuates because of four basic processes:

1. Natality (B) is the number of births during a given period that are added to the initial density. It increases density.
2. Mortality (D) is the number of deaths in the population during a given period. It decreases density.
3. Immigration (I) is the number of individuals of the same species that have come into the habitat from elsewhere during the period. It increases density.
4. Emigration (E) is the number of individuals of the population who left the habitat and went elsewhere during the period. It decreases density.

A central box reads Population Density (N). An oval labelled Natality (B) on the left and a box labelled Immigration (I) above each send an arrow marked + into it. Arrows marked − lead out to an oval labelled Mortality (D) on the right and a box labelled Emigration (E) below.

See Fig. 11.2 in your NCERT textbook

The population density equation

If N is the population density at time t, then its density at time t + 1 is:

Nₜ₊₁ = Nₜ + [(B + I) − (D + E)]

Density increases if births plus immigrants (B + I) exceed deaths plus emigrants (D + E), and decreases if (D + E) is larger.

Worked example 3. Apply the equation to the lotus pond, where Nₜ = 20, B = 8 and there were no deaths, immigrants or emigrants.

Answer: Nₜ₊₁ = 20 + [(8 + 0) − (0 + 0)] = 28 lotus plants.

Under normal conditions, births and deaths are the most important factors; immigration and emigration matter only under special conditions. For instance, if a new habitat is just being colonised, immigration may contribute more to population growth than birth rates.

How does a population grow when resources are unlimited?

Resource (food and space) availability is essential for the unimpeded growth of a population. When resources are unlimited, each species can realise fully its innate potential to grow in number, as Darwin observed while developing his theory of natural selection. The population then grows in an exponential or geometric fashion.

The exponential growth equation

In a population of size N, let b be the per capita birth rate and d the per capita death rate. The change in N during a unit time period t is:

dN/dt = (b − d) × N. Letting (b − d) = r gives dN/dt = rN.

Definition: The intrinsic rate of natural increase (r) is the per capita birth rate minus the per capita death rate. It measures the inherent potential of a population to grow and is used to assess the impact of any biotic or abiotic factor on population growth.

Plotting N against time for exponential growth gives a J-shaped curve. The integral form of the equation is Nₜ = N₀eʳᵗ, where Nₜ is the density after time t, N₀ the density at time zero, r the intrinsic rate of natural increase and e the base of natural logarithms (2.71828).

How fast exponential growth builds up

Under unlimited resources, any species growing exponentially can reach enormous densities in a short time. Darwin showed how even a slow-growing animal like the elephant could reach enormous numbers in the absence of checks. A popular anecdote makes the point:

1. A king, confident of winning a game of chess, agreed to any bet his minister proposed.
2. The minister asked for wheat: one grain on square 1, two on square 2, four on square 3, eight on square 4, doubling on each square until all 64 were filled.
3. The minister won, and the king began placing the grains, thinking the bet easy.
4. By half the board, the king realised that all the wheat in his kingdom would be inadequate to cover all 64 squares.

Similarly, a single Paramecium doubling by binary fission every day would reach a mind-boggling number in 64 days, provided food and space remained unlimited.

What is logistic growth, and what is carrying capacity?

No population in nature has unlimited resources to permit exponential growth. Limited resources lead to competition between individuals, and eventually the "fittest" individual survives and reproduces.

Definition: Carrying capacity (K) is the maximum possible number of individuals of a species that the resources of a given habitat can support, beyond which no further growth is possible.

A population growing in a habitat with limited resources passes through these phases:

1. A lag phase, when numbers rise slowly.
2. A phase of acceleration, when growth speeds up.
3. A phase of deceleration, when growth slows down.
4. An asymptote, when the population density reaches the carrying capacity.

A plot of N against time gives a sigmoid curve. This is Verhulst-Pearl Logistic Growth, described by dN/dt = rN[(K − N)/K], where N is the population density at time t, r the intrinsic rate of natural increase and K the carrying capacity.

The y-axis is Population density (N) and the x-axis is Time (t); a horizontal dashed line marks K. Curve a (red), labelled dN/dt = rN, rises ever more steeply and passes above the K line: the exponential plot, when responses are not limiting growth.

Curve b (blue), labelled dN/dt = rN[(K − N)/K], rises slowly, then steeply, then levels off along the K line: the logistic plot, when responses are limiting growth.

See Fig. 11.3 in your NCERT textbook

The term (K − N)/K acts as a brake. When N is small compared with K it is close to 1, so growth is nearly exponential; as N approaches K it approaches zero, and growth stops at K. Since resources for most animal populations are finite and become limiting sooner or later, the logistic model is considered more realistic.

What is life history variation?

Populations evolve to maximise their reproductive fitness, also called Darwinian fitness (a 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.

Which strategy best maximises fitness? Ecologists suggest that life history traits have evolved in relation to the constraints imposed by the abiotic and biotic components of the habitat in which the organisms live. The evolution of life history traits is currently an important area of ecological research.

How are interactions between populations classified?

No natural habitat on earth is inhabited by a single species; for any species the minimal requirement is one more species on which it can feed. Even a plant needs soil microbes to break down organic matter and return inorganic nutrients, and an animal agent for pollination.

Interspecific interactions arise between populations of two different species. They can be beneficial (+), detrimental (−) or neutral (0) to one or both species.

Both species benefit in mutualism and both lose in competition. In parasitism and predation only one species benefits (the parasite and the predator), and the interaction is detrimental to the other (the host and the prey).

In commensalism one species benefits and the other is neither benefited nor harmed. In amensalism one species is harmed while the other is unaffected. Predation, parasitism and commensalism share one feature: the interacting species live closely together.

What roles does predation play, and how do prey defend themselves?

Predation is nature's way of transferring the energy fixed by plants to higher trophic levels. A tiger eating a deer is predation, but a sparrow eating any seed is no less a predator. Animals that eat plants are called herbivores, but in a broad ecological context they are not very different from predators.

The roles of predators

1. They act as "conduits" for energy transfer across trophic levels.
2. They keep prey populations under control; without them prey could reach very high densities and cause ecosystem instability.
3. They are the basis of biological control of agricultural pests, which relies on the predator's ability to regulate its prey.
4. They maintain species diversity by reducing the intensity of competition among competing prey species.

Exotic species introduced into a new area can become invasive because the land lacks their natural predators. The prickly pear cactus introduced into Australia in the early 1920s spread into millions of hectares of rangeland. It was brought under control only after a cactus-feeding predator (a moth) from its natural habitat was introduced.

The starfish Pisaster is an important predator in the rocky intertidal communities of the American Pacific Coast. When all the starfish were removed from an enclosed intertidal area, more than 10 species of invertebrates became extinct within a year because of interspecific competition.

If a predator overexploits its prey, the prey might become extinct, and the predator would follow for lack of food. This is why predators in nature are "prudent".

Defences of prey animals

Some insects and frogs are cryptically coloured (camouflaged) to avoid detection by predators. Others are poisonous and therefore avoided. The Monarch butterfly is highly distasteful to birds because of a special chemical in its body, which it acquires as a caterpillar by feeding on a poisonous weed.

Defences of plants

For plants, herbivores are the predators. Nearly 25 per cent of all insects are phytophagous (feeding on plant sap and other parts of plants). Plants cannot run away, so they have evolved an astonishing variety of morphological and chemical defences.

What is competition, and does it always lead to exclusion?

Darwin was convinced that interspecific competition is a potent force in organic evolution. It is generally believed that competition occurs when closely related species compete for the same limiting resources, but this is not entirely true.

Firstly, totally unrelated species can compete: in some shallow South American lakes, visiting flamingoes and resident fishes compete for their common food, the zooplankton. Secondly, resources need not be limiting. In interference competition, one species feeds less efficiently because of the inhibitory presence of another, even when food and space are abundant.

Definition: Competition is a process in which the fitness of one species (measured as its r, the intrinsic rate of increase) is significantly lower in the presence of another species.

Evidence for competition

Gause and other experimental ecologists showed in the laboratory that, when resources are limited, the competitively superior species eventually eliminates the other. Evidence for such competitive exclusion in nature is not always conclusive, but strong circumstantial evidence exists in some cases.

The Abingdon tortoise in the Galapagos Islands became extinct within a decade after goats were introduced, apparently because of the goats' greater browsing efficiency.

In competitive release, a species restricted to a small area by a competitively superior species expands its range dramatically when the competitor is experimentally removed. Connell's field experiments on the rocky coasts of Scotland showed that the larger, superior barnacle Balanus dominates the intertidal area and excludes the smaller barnacle Chathamalus from that zone.

In general, herbivores and plants appear to be more adversely affected by competition than carnivores.

Competitive exclusion and resource partitioning

Gause's Competitive Exclusion Principle states that two closely related species competing for the same resources cannot co-exist indefinitely, and the competitively inferior one will be eliminated eventually. This may be true if resources are limiting, but not otherwise.

More recent studies point out that competing species might evolve mechanisms that promote co-existence. One is resource partitioning: choosing, for instance, different feeding times or foraging patterns. MacArthur showed that five closely related species of warblers on the same tree co-existed through behavioural differences in their foraging.

How do parasites live on and in their hosts?

Parasitism ensures free lodging and meals, so it has evolved in many taxonomic groups, from plants to higher vertebrates. Many parasites are host-specific, parasitising only a single host species, and host and parasite tend to co-evolve: if the host evolves ways to reject or resist the parasite, the parasite must evolve ways to neutralise them.

In accordance with their life styles, parasites have evolved these special adaptations:

1. Loss of unnecessary sense organs.
2. Adhesive organs or suckers to cling on to the host.
3. Loss of the digestive system.
4. High reproductive capacity.

Parasite life cycles are often complex. The human liver fluke (a trematode) depends on two intermediate hosts, a snail and a fish, and the malarial parasite needs a vector (the mosquito) to spread to other hosts.

Most parasites harm the host. They may reduce its survival, growth and reproduction, reduce its population density, and make it physically weak and so more vulnerable to predation.

Ectoparasites and endoparasites

Cuscuta has lost its chlorophyll and leaves in the course of evolution and derives its nutrition from its host plant. The female mosquito is not considered a parasite, although it needs our blood for reproduction: it visits a host only briefly for a blood meal and does not live on or in it.

Brood parasitism

In brood parasitism the parasitic bird lays its eggs in the host's nest and lets the host incubate them. Its eggs have evolved to resemble the host's in size and colour, reducing the chance that the host detects and ejects them. The cuckoo (koel) and the crow show this during the breeding season (spring to summer).

What are commensalism and mutualism?

Commensalism

In commensalism one species benefits and the other is neither harmed nor benefited.

• An orchid growing as an epiphyte on a mango branch, and barnacles growing on the back of a whale, benefit; the mango tree and the whale derive no apparent benefit.
• The cattle egret always forages close to grazing cattle, because the moving cattle flush out insects from the vegetation that the egrets might otherwise find hard to catch.
• The clown fish lives among the stinging tentacles of a sea anemone and gets protection from predators; the anemone does not appear to benefit.

Mutualism

Mutualism benefits both species. Lichens are an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria. In mycorrhizae, fungi help the roots of higher plants absorb essential nutrients from the soil, and the plant provides the fungi with energy-yielding carbohydrates.

The most spectacular examples are plant-animal relationships. Plants pay animals "fees" for pollination and seed dispersal: pollen and nectar for pollinators, juicy and nutritious fruits for seed dispersers. The system must be safeguarded against "cheaters" that steal nectar without pollinating, so these interactions often involve co-evolution of flower and pollinator.

The fig and its wasp

Many fig species have a tight one-to-one relationship with a pollinator wasp: a given fig species can be pollinated only by its "partner" wasp species and no other.

1. The female wasp searches the fig for suitable egg-laying sites.
2. While searching, she pollinates the fig inflorescence.
3. She uses the fruit as an oviposition (egg-laying) site.
4. The developing seeds within the fruit nourish her larvae, as the fig's return for pollination.

Two photographs. Part (a) shows a pale, opened fig flower with a tiny wasp on it: the fig flower is pollinated by the wasp. Part (b) shows a fig fruit cut in half, with a red, seed-filled interior around a dark central cavity and several small dark wasps on the cut surface: a wasp laying eggs in a fig fruit.

See Fig. 11.4 in your NCERT textbook

Orchids and sexual deceit

Many orchid floral patterns have evolved to attract the right pollinator insect (bees and bumblebees) and ensure guaranteed pollination. Not all orchids offer rewards. The Mediterranean orchid Ophrys uses sexual deceit to get pollinated by a species of bee:

1. One petal of the flower bears an uncanny resemblance to the female bee in size, colour and markings.
2. The male bee, perceiving a female, "pseudocopulates" with the flower.
3. During this, the bee is dusted with pollen.
4. When it pseudocopulates with another flower, it transfers the pollen and pollinates that flower.

If the female bee's colour patterns change even slightly during evolution, pollination success will fall unless the orchid co-evolves to keep its petal resembling the female bee.

A photograph against a dark background of a bee, with a dark body and spread translucent wings, clinging to a large brown, velvety petal of an orchid flower. Pale greenish-yellow petals spread above it, and a green stem runs down the left side.

See Fig. 11.5 in your NCERT textbook

Glossary

• Ecology — The study of the interactions among organisms and between organisms and their abiotic (physico-chemical) and biotic environment.
• Population — A group of individuals of one species in a well-defined geographical area that share or compete for similar resources and potentially interbreed.
• Community — The populations of different species, such as animals, plants and microbes, that live together and interact in various ways.
• Age pyramid — The structure obtained by plotting the per cent of individuals in each age group; its shape shows whether a population is growing, stable or declining.
• Population density (N) — The technical term for population size, measured as numbers, per cent cover, biomass or relative density depending on the species.
• Intrinsic rate of natural increase (r) — The per capita birth rate minus the per capita death rate (b − d); a measure of a population's inherent potential to grow.
• Carrying capacity (K) — The maximum number of individuals of a species that a habitat's resources can support, beyond which no further growth is possible.
• Interference competition — Competition in which one species' feeding efficiency is reduced by the inhibitory presence of another species, even when food and space are abundant.
• Competitive release — The dramatic expansion of a species' distributional range when a competitively superior species that restricted it is experimentally removed.
• Resource partitioning — Avoidance of competition by species sharing a resource, for example through different feeding times or different foraging patterns.
• Brood parasitism — Parasitism in which a bird lays its eggs in a host's nest and lets the host incubate them, as the cuckoo (koel) does with the crow.
• Co-evolution — Linked evolution of two interacting species, such as a host and its parasite or a flower and its pollinator.

Common errors and misconceptions

• Misconception: Population density always means the number of individuals. Correct: Total number is generally the most appropriate measure, but per cent cover, biomass or relative density (such as fish caught per trap) can be more meaningful.
• Misconception: Only closely related species compete. Correct: Totally unrelated species can compete for the same resource, as flamingoes and resident fishes compete for zooplankton in some shallow South American lakes.
• Misconception: Competition can happen only when resources are limiting. Correct: In interference competition, one species feeds less efficiently because of the presence of another, even when food and space are abundant.
• Misconception: The female mosquito is a parasite because it feeds on human blood. Correct: The female mosquito is not considered a parasite, although it needs our blood for reproduction.
• Misconception: Exponential growth is the more realistic model. Correct: Resources for most animal populations are finite and become limiting sooner or later, so the logistic model is considered more realistic.
• Misconception: An orchid growing on a mango branch is a parasite. Correct: The orchid is an epiphyte; it benefits while the mango tree derives no apparent benefit, so the interaction is commensalism.

Exam-style questions with model answers

Q1. List the attributes that populations possess but individuals do not. [2 marks]

1. Birth rate (per capita births), whereas an individual only has births.
2. Death rate (per capita deaths), whereas an individual only dies.
3. Sex ratio, whereas an individual is either male or female.
4. Age distribution, shown as an age pyramid.

Q2. If a population growing exponentially doubles in size in 3 years, what is its intrinsic rate of increase (r)? [3 marks]

1. Exponential growth follows Nₜ = N₀eʳᵗ.
2. Doubling means Nₜ = 2N₀ when t = 3 years, so 2N₀ = N₀e³ʳ, which gives e³ʳ = 2.
3. Taking natural logarithms: 3r = ln 2 = 0.693.
4. So r = 0.693 ÷ 3 = 0.231 per year (about 0.23 per individual per year).

Q3. What is the ecological principle behind the biological control of pests? Give an example of the principle at work. [3 marks]

1. The principle is predation: a predator regulates the population of its prey, so it can keep a pest population, such as pest insects, under control.
2. Without predators, prey can reach very high densities; exotic species become invasive where their natural predators are absent.
3. Example: the prickly pear cactus, an invasive pest plant introduced into Australia in the early 1920s, spread over millions of hectares of rangeland, and was controlled only after a cactus-feeding moth (a predator) from its natural habitat was introduced.

Q4. Name the important defence mechanisms in plants against herbivory. [3 marks]

1. Morphological defence: thorns, the most common means, as in Acacia and cactus.
2. Chemical defence: stored chemicals that make the herbivore sick, inhibit feeding or digestion, disrupt its reproduction or even kill it.
3. Example: Calotropis produces highly poisonous cardiac glycosides, so cattle and goats are not seen browsing on it.
4. Nicotine, caffeine, quinine, strychnine and opium are produced by plants as defences against grazers and browsers.

Q5. With the help of a suitable diagram, describe the logistic population growth curve. [5 marks]

1. Diagram: plot population density (N) on the y-axis against time (t) on the x-axis. Draw a horizontal dashed line labelled K and an S-shaped curve that rises slowly, then steeply, then levels off at K, labelled dN/dt = rN[(K − N)/K].
2. No population in nature has unlimited resources, so individuals compete. A habitat can support a maximum number, beyond which no further growth is possible: the carrying capacity (K).
3. The population shows a lag phase, then phases of acceleration and deceleration, and finally an asymptote when density reaches K.
4. This sigmoid curve is Verhulst-Pearl Logistic Growth, where N is population density at time t, r is the intrinsic rate of natural increase and K is the carrying capacity.
5. Since resources for most animal populations are finite and become limiting sooner or later, the logistic model is considered more realistic than the J-shaped exponential model.

Q6. State Gause's Competitive Exclusion Principle. Give evidence for competition in nature and explain how competing species may co-exist. [5 marks]

1. Two closely related species competing for the same resources cannot co-exist indefinitely, and the competitively inferior one will be eliminated eventually. This may be true if resources are limiting, but not otherwise.
2. The Abingdon tortoise in the Galapagos Islands became extinct within a decade after goats were introduced, apparently because the goats browsed more efficiently.
3. Competitive release: a species restricted to a small area by a competitively superior species expands its range dramatically when the competitor is experimentally removed. In Connell's experiments on the rocky coasts of Scotland, the larger barnacle Balanus dominates the intertidal area and excludes the smaller Chathamalus from that zone.
4. Co-existence: species may evolve resource partitioning, such as different feeding times or foraging patterns. MacArthur showed that five closely related warbler species on the same tree co-exist through behavioural differences in foraging.

Q7. What is parasitism? Describe the adaptations of parasites and distinguish ectoparasites from endoparasites, with examples. [5 marks]

1. Parasitism is an interaction in which the parasite benefits and the host is harmed. Many parasites are host-specific and co-evolve with their hosts.
2. Adaptations: loss of unnecessary sense organs, adhesive organs or suckers to cling to the host, loss of the digestive system and high reproductive capacity.
3. Ectoparasites feed on the external surface of the host: lice on humans, ticks on dogs, copepods on marine fish, and Cuscuta on hedge plants.
4. Endoparasites live inside the host, in the liver, kidney, lungs or red blood cells, as the human liver fluke and the malarial parasite do. Their life cycles are more complex, and their morphology and anatomy are greatly simplified, with emphasis on reproductive potential.

Key takeaways

• A population has birth rates, death rates, a sex ratio and an age distribution, which an individual organism does not have.
• Population density (N) can be measured as numbers, per cent cover, biomass, relative density or indirect signs such as tiger pug marks and fecal pellets.
• Natality and immigration raise density while mortality and emigration lower it: Nₜ₊₁ = Nₜ + [(B + I) − (D + E)].
• With unlimited resources growth is exponential (dN/dt = rN, a J-shaped curve); with limited resources it is logistic, a sigmoid curve levelling off at K.
• Interactions are mutualism (+ +), competition (− −), predation and parasitism (+ −), commensalism (+ 0) and amensalism (− 0).
• Predators transfer energy across trophic levels, control prey populations and maintain species diversity, while prey animals and plants have evolved defences such as camouflage, poisons, thorns and chemicals.
• Competitive exclusion may occur when resources are limiting, but species can co-exist through resource partitioning, as MacArthur's warblers show.
• Co-evolution links mutualists tightly, as in the fig and its partner wasp and the Ophrys orchid that deceives male bees.

Test yourself

A pond had 20 lotus plants last year, and 8 new plants were added. What is the birth rate?

The birth rate is 8/20 = 0.4 offspring per lotus per year.

Why is per cent cover or biomass sometimes better than numbers for measuring population size?

Numbers mislead when individuals differ greatly in size. Calling one huge banyan tree "low density" beside 200 carrot grass plants underestimates its enormous role in the community.

Give one example each of an organism that breeds only once and one that produces many small offspring.

Pacific salmon fish and bamboo breed only once in their lifetime. Oysters and pelagic fishes produce a large number of small-sized offspring.

What happened when all the starfish Pisaster were removed from an enclosed intertidal area?

More than 10 species of invertebrates became extinct within a year because of interspecific competition.

How does the Monarch butterfly protect itself from birds?

It is highly distasteful to birds because of a special chemical in its body, which it acquires as a caterpillar by feeding on a poisonous weed.

Why is the cattle egret and grazing cattle association called commensalism?

The egret benefits because the moving cattle flush insects out of the vegetation, making them easier to catch, while the cattle are neither harmed nor benefited.