Organisms and Populations | ISC Class 12 Biology Notes

This note covers populations, population density, birth and death rates, sex ratio, age pyramids, changes in population size, exponential and logistic growth, reproductive fitness, and interactions involving competition, predation, parasitism, commensalism, amensalism and mutualism.

What is a population, and which attributes describe it?

Ecology studies interactions among organisms and between organisms and their physical environment. The abiotic environment consists of non-living physical and chemical factors; biotic components are the living organisms with which an organism interacts.

Definition: A population is a group of individuals of a species occupying a defined geographical area and sharing or competing for similar resources. Its members can potentially interbreed.

Interbreeding refers to sexual reproduction, but groups produced by asexual reproduction, reproduction without the fusion of reproductive cells, are also generally treated as populations in ecological studies. Bacteria in a culture plate therefore provide a population example alongside lotus plants in a pond.

How does a population differ from an individual?

An individual can be born or die, whereas a population has a birth rate and a death rate. These rates express births and deaths relative to the number of members of the population over a specified time.

A population also has a sex ratio, the relative representation of males and females, and an age distribution, the proportions belonging to different ages or age groups. A single individual does not possess these group attributes.

For example, a population may contain 60 per cent females and 40 per cent males. This describes its composition, rather than assigning a sex ratio to each individual. Similarly, an age distribution summarises many individuals with different ages.

A biological community comprises interacting populations of different species. Keep this distinct from a population of one species. Population ecology connects ecological conditions with evolution: individuals cope with environmental change, while evolutionary change through natural selection occurs at the population level.

How is population density measured?

Population density expresses population size in a specified area or volume. The symbol N denotes population density or size in the habitat being studied. Changes in this measure help reveal the effects of competition, predators and other environmental influences.

Total number is generally the most appropriate measure, but counting individuals can sometimes be misleading or impractical. The chosen measure should represent the population meaningfully. Biomass means the mass of living material; percentage cover expresses the percentage of an area covered by the organisms.

When are alternatives to counting useful?

Consider an area with 200 carrot grass plants, Parthenium hysterophorus, and one huge banyan tree. A count alone understates the banyan's enormous role in the community. Biomass or percentage cover provides a more meaningful comparison in this situation.

Relative density can serve an investigation when an absolute population count is unnecessary. Fish caught per trap provides an indicator of the fish population in a lake. Pug marks are footprints, while faecal pellets are droppings used as indirect evidence.

Counting is also difficult when a population is extremely large, as in a dense bacterial culture. The important distinction is between population size itself and the practical way used to estimate or express it.

How are natality, mortality and age structure studied?

Natality refers to births added to a population during a given period. Mortality refers to deaths during that period. A per capita rate is expressed per member of the population, so the total number of events must be related to population size.

How do the birth-rate and death-rate calculations work?

1. Identify the initial population and the number of births or deaths during the stated interval.
2. Divide births by the initial population to calculate the per capita birth rate for that interval.
3. Divide deaths by the initial population to calculate the per capita death rate for that interval.
4. State the result with the organism and time interval, rather than reporting an unexplained decimal.

For 20 lotus plants producing eight new plants in a year, the birth rate is 8/20 = 0.4 offspring per lotus per year. The population becomes 28, but 28 is not the denominator used for this calculation.

For four deaths among 40 fruitflies during a week, the death rate is 4/40 = 0.1 individuals per fruitfly per week. Birth rate and death rate cannot be compared meaningfully without attending to their time intervals.

What does an age pyramid show?

An age pyramid represents the proportions of individuals in different age groups. Pre-reproductive individuals have not entered the reproductive stage; reproductive individuals are in it; post-reproductive individuals are beyond it. Human age pyramids generally show the age distributions of males and females.

Three stacked profiles are labelled Expanding, Stable and Declining. The age bands are labelled Pre-reproductive, Reproductive and Post-reproductive. The expanding profile has a broad base, the stable profile has similar lower widths, and the declining profile has a narrower base than the bands just above it.

See Fig. 11.1 in your NCERT textbook

The diagram relates age composition to growth status. A broad younger group characterises the expanding profile; a constricted younger group characterises the declining profile. The shapes summarise population structure without requiring a count of every individual's age in the description.

How do births, deaths and migration change population size?

Population size changes with food availability, predation, the consumption of prey by a predator, and adverse weather. Whatever the underlying environmental cause, additions and losses can be organised into four basic demographic processes, meaning processes that change population numbers.

Alongside natality and mortality, immigration is the arrival of individuals of the same species from elsewhere. Emigration is the departure of individuals to another habitat. Both terms must be understood from the viewpoint of the population being studied.

How is the population balance written?

Let t denote the starting time and t + 1 the end of the chosen interval. Let Nₜ and Nₜ₊₁ be the respective population sizes. During that interval, B denotes births, I immigrants, D deaths and E emigrants.

Nₜ₊₁ = Nₜ + [(B + I) − (D + E)]

1. Start with the population present at the beginning of the specified interval.
2. Add the individuals born and the individuals arriving from elsewhere during that interval.
3. Subtract the individuals dying and the individuals leaving during the same interval.
4. Interpret the balance: the population increases when births plus immigrants exceed deaths plus emigrants.

A central box labelled Population Density (N) receives arrows from Natality (B) on the left and Immigration (I) above, each marked plus. Arrows lead out to Mortality (D) on the right and Emigration (E) below, each marked minus.

See Fig. 11.2 in your NCERT textbook

Under normal conditions, births and deaths are the most important influences on population density. Migration becomes important under special conditions. During the colonisation of a new habitat, immigration may contribute more significantly to population growth than birth rates.

Note: B and D are event totals for the interval. Per capita birth and death rates express events relative to the population. Do not substitute a per capita rate directly for an event total in the population-balance equation.

How does exponential population growth work?

Exponential growth describes growth in which the rate of increase is proportional to the population present. Ideally, unlimited food and space allow a species to realise its innate potential to increase. Under unlimited resources, growth is usually exponential.

Let b be the per capita birth rate and d the per capita death rate, measured over the same time unit. The expression dN/dt means the rate of change of population size with time; its letter d denotes a differential, distinct from the death-rate symbol d.

dN/dt = (b − d) × N = rN, where r = b − d is the intrinsic rate of natural increase. It represents the population's inherent potential to grow and helps assess effects of living and non-living environmental factors.

What does the integrated equation mean?

The integrated form expresses size after an elapsed time: Nₜ = N₀eʳᵗ. Here N₀ is the initial population size at time zero, Nₜ is the size after time t, and e is the base of natural logarithms, given as 2.71828.

The expression eʳᵗ means e raised to the power (r × t). A natural logarithm is a logarithm to base e. The time unit used for t must match that used for the rates.

Plotting population size against time for exponential increase produces a J-shaped curve. As the population enlarges, the absolute increase becomes greater even while the same per capita growth rate is retained.

Even an animal that reproduces slowly, such as an elephant, could reach enormous numbers in the absence of checks. Another illustration is a hypothetical Paramecium population doubling daily through binary fission, division of one individual into two, provided food and space remain unlimited.

The unlimited-resource condition is essential to the illustration. Exponential growth describes reproductive potential under those conditions; it does not establish that a natural population can continue increasing without environmental limits.

How does logistic growth differ from exponential growth?

Logistic growth describes population increase under limited resources. Competition develops as individuals require the same finite supplies. Carrying capacity, represented by K, is the maximum population a particular habitat can support with its available resources.

The Verhulst-Pearl logistic equation is dN/dt = rN[(K − N)/K]. N is population density at time t, r is the intrinsic rate of natural increase, and dN/dt is the rate of population change. The bracketed expression accounts for the resource limit represented by K.

Which phases produce the sigmoid curve?

1. The population initially passes through a lag phase, when its increase is slow.
2. It enters an acceleration phase, during which population increase becomes faster.
3. It passes into deceleration as resource limitation increasingly restrains its growth.
4. It approaches an asymptote at carrying capacity, where the curve levels off.

An asymptote is a limiting line approached by a curve. The complete logistic curve is sigmoid, or S-shaped. Resources for most animal populations are finite and become limiting sooner or later, so logistic growth is considered a more realistic model.

Population density (N) is on the vertical axis and Time (t) on the horizontal axis. Curve a rises steeply in a J shape. Curve b is S-shaped and levels towards a horizontal dotted line labelled K.

See Fig. 11.3 in your NCERT textbook

For a symbolic example, set N equal to K in the logistic equation. The expression K − N becomes zero, so the model gives zero net growth. This is a consequence of the model, rather than a claim that births and deaths have individually stopped.

What do life history variations reveal about reproductive fitness?

Life history traits concern an organism's reproductive pattern, including how often it breeds and the number and size of offspring it produces. Populations evolve towards maximising reproductive fitness, also called Darwinian fitness, expressed here in terms of a high intrinsic rate of natural increase.

The reproductive strategy that evolves depends on the selection pressures, environmental influences affecting survival and reproduction, operating in the habitat. It is therefore important to connect reproductive patterns with environmental conditions rather than assume that one pattern is universally best.

Which reproductive patterns illustrate this variation?

Pacific salmon fish and bamboo breed only once in their lifetime. Most birds and mammals breed many times. This comparison concerns the frequency of reproductive episodes, not simply whether an organism can reproduce.

Oysters and pelagic fishes produce a large number of small-sized offspring. Pelagic fishes are fishes of open water. Birds and mammals provide examples of producing a small number of large-sized offspring. Offspring number and offspring size are therefore separate features of a reproductive strategy.

Ecologists suggest that life history traits have evolved in relation to constraints imposed by the biotic and abiotic components of the habitat.

Reproductive fitness is consequently different from ordinary physical strength. The comparison concerns success in reproduction under the habitat's constraints. A large body or many offspring alone does not provide the complete description of a life history.

How are interactions between populations classified?

Interspecific interactions occur between populations of different species. An interaction can benefit a species, harm it or have a neutral effect. Here + denotes benefit, − denotes harm and 0 denotes neither benefit nor harm.

The two signs must be read together. They refer to the two interacting species, called species A and species B in the table. A benefit to one participant does not, by itself, show whether the other participant benefits, suffers or remains unaffected.

Mutualism benefits both participants, whereas competition adversely affects both. In parasitism, a parasite obtains resources from a host organism. Its sign pattern matches predation, although the biological relationship differs.

Commensalism benefits one species without benefiting or harming the other. Amensalism harms one while the other remains unaffected. The zero sign records neutrality for a participant; it does not mean that the two species lack an interaction.

These classifications describe relationships within communities. Plants, animals and microbes form many interactive links, although all may not be readily apparent. Identifying both participants and the effect on each is the starting point for explaining any example.

How does predation affect communities and prey defences?

A predator obtains food by consuming another organism, its prey. The relationship includes a tiger eating a deer and, in the broad ecological sense, a sparrow eating seeds. Herbivores, animals that feed on plants, are therefore plant predators in this context.

Predation transfers energy fixed by plants to higher trophic levels, the feeding positions in a food chain. It also keeps prey populations under control. Biological control uses such ecological relationships to regulate organisms that cause problems, including agricultural pests.

What examples show the importance of predators?

Prickly pear cactus spread rapidly after its introduction into Australia in the early 1920s. Its spread was controlled after a cactus-feeding moth from its natural habitat was introduced. The example connects the control of an introduced plant with a feeding interaction.

Predators also help maintain species diversity by reducing competition among prey species. When the starfish Pisaster was removed from an enclosed intertidal area, the shore between high and low tide levels, on the American Pacific Coast, more than 10 invertebrate species, animals without backbones, became extinct within a year because of interspecific competition.

If a predator overexploits its prey, the prey might become extinct, followed by the predator for lack of food. Predator success therefore cannot be understood simply as consuming the greatest possible number of prey without regard to the continuing food supply.

Which adaptations help prey avoid predation?

Camouflage is colouring that makes an organism difficult to detect against its surroundings. Some insects and frogs have such colouring. Some prey are poisonous. The Monarch butterfly is highly distasteful to birds because of a chemical acquired while its caterpillar feeds on a poisonous weed.

Plants have structural and chemical defences against herbivory, feeding on plants. Thorns in Acacia and cactus are structural defences. Calotropis produces poisonous cardiac glycosides, toxic defensive chemicals. Nearly 25 per cent of insects are known to be phytophagous, meaning they feed on plant sap or other plant parts.

Many plants produce chemicals that inhibit feeding or digestion, disrupt reproduction, cause sickness or even kill herbivores. Nicotine, caffeine, quinine, strychnine and opium are examples of plant substances associated with defence against grazers and browsers.

When does competition cause exclusion or allow coexistence?

Interspecific competition is a process in which the fitness of one species, measured through its intrinsic rate of increase, is significantly lower in the presence of another species. Competitors need not be closely related: visiting flamingoes and resident fishes compete for zooplankton in some shallow South American lakes.

Zooplankton are small drifting aquatic animals. Competition is also not restricted to a shortage of resources. In interference competition, the interfering or inhibitory presence of one species might reduce another's feeding efficiency even when food and space are abundant.

What are competitive exclusion and competitive release?

Gause's Competitive Exclusion Principle states that two closely related species competing for the same resources cannot coexist indefinitely; the competitively inferior species is eventually eliminated. This may be true if resources are limiting, but not otherwise.

Evidence for competitive exclusion in nature is not always conclusive. The Abingdon tortoise became extinct after goats were introduced into the Galapagos island it occupied, apparently because the goats browsed more efficiently. The word apparently preserves the distinction between circumstantial evidence and a demonstrated cause.

Competitive release occurs when a species expands its distribution after a superior competitor is experimentally removed. Connell's experiments on Scotland's rocky coasts showed that the larger barnacle Balanus excludes the smaller barnacle Chathamalus from the intertidal zone, the shore between high and low tide levels.

How can resource partitioning reduce competition?

Resource partitioning means using a shared resource in different ways, such as feeding at different times or adopting different foraging patterns. Foraging means searching for and obtaining food. Such differences can permit coexistence rather than exclusion.

MacArthur showed that five closely related warbler species living on the same tree avoided competition through behavioural differences in foraging. The example illustrates how species facing competition might evolve mechanisms that promote coexistence.

In general, herbivores and plants appear to be more adversely affected by competition than carnivores, animals that eat other animals. This is a qualified comparison, not a claim that carnivores do not compete. Likewise, competitive exclusion must retain its resource limitation condition.

How do parasites depend on hosts?

A parasite obtains food and a place to live through association with a host, the organism supporting it. Many parasites are host-specific, meaning they parasitise a single host species. Host and parasite tend to co-evolve, undergoing linked evolutionary changes.

If a host evolves resistance, the parasite must counter that resistance to remain successful with the same host species. Majority of parasites harm their hosts; they may reduce host survival, growth or reproduction and thereby reduce population density. They might also weaken hosts and increase vulnerability to predators.

Which adaptations and life cycles support parasitism?

Parasitic adaptations include adhesive organs or suckers for attachment, loss of unnecessary sense organs, loss of the digestive system and high reproductive capacity. These are adaptations found among parasites, not a requirement that every parasite possess every feature.

Life cycles are often complex. An intermediate host supports part of a parasite's development before completion of its life cycle. The human liver fluke depends on a snail and a fish as intermediate hosts. A vector carries a parasite between hosts; a mosquito carries the malarial parasite.

Ectoparasites feed externally; endoparasites live within the host, for example in organs or blood cells. Endoparasites have greatly simplified structural features and emphasised reproductive potential. The parasitic plant Cuscuta lacks chlorophyll, the green photosynthetic pigment, and leaves and derives nutrition from its host plant.

How does brood parasitism work?

In brood parasitism, a bird lays its eggs in a host bird's nest and leaves incubation to the host. Incubation is the maintenance of conditions needed for eggs to develop. Koel and crow illustrate this relationship.

The parasitic bird's eggs have evolved resemblance to host eggs in size and colour. This reduces the chances that the host will detect and eject them. Resemblance reduces rejection risk; it does not justify saying that rejection is impossible.

How do commensalism and mutualism differ?

Commensalism benefits one species while the other remains unaffected; mutualism benefits both. To distinguish them, identify what each participant obtains. Close physical association alone does not establish that both species gain from the relationship.

Which associations illustrate commensalism?

An orchid growing as an epiphyte, a plant growing on another plant for support, benefits on a mango branch. Barnacles benefit by growing on a whale. Neither the mango tree nor the whale derives any apparent benefit from these associations.

Cattle egrets forage near grazing cattle because the cattle disturb insects in vegetation, making them easier to catch. Clown fish gain protection among a sea anemone's stinging tentacles because predators avoid the tentacles. The anemone does not appear to derive any benefit from hosting the fish.

Which associations illustrate mutualism?

Lichens are intimate associations between fungi and photosynthesising algae or cyanobacteria. Photosynthesis uses light energy to make food; cyanobacteria are bacteria capable of this process. Mycorrhizae are associations between fungi and plant roots. The fungus helps absorb essential soil nutrients; the plant supplies energy-yielding carbohydrates.

Plants also reward pollinators with pollen or nectar, and seed dispersers with nutritious fruits. Pollination is transfer of pollen to a flower's receptive female surface; seed dispersal is movement of seeds away from their place of production. Plant-animal relationships often involve co-evolution.

In many fig species, a particular wasp species is the partner pollinator. The female uses the fig as an egg-laying site, and developing seeds nourish its larvae, the immature wasps. While seeking egg-laying sites, it pollinates the fig's inflorescence, a cluster of flowers.

Do all orchid pollinators receive rewards?

Not all orchids offer rewards. The Mediterranean orchid Ophrys uses sexual deceit: a petal resembles a female bee in size, colour and markings. A male attempts mating, called pseudocopulation, and receives pollen that it transfers during a similar encounter with another flower.

If the female bee's colour patterns change even slightly during evolution, pollination success will be reduced unless the orchid co-evolves to preserve the resemblance. This example illustrates specialised pollination and linked evolution; it should not be presented as proof that every pollinator obtains a food reward.

Glossary

• Population — Individuals of one species sharing or competing for similar resources in a defined geographical area.
• Population density — Population size in a specified habitat, expressed through numbers or another suitable measure of abundance.
• Natality — Births added to the initial population during a specified period of time.
• Mortality — Deaths occurring among members of a population during a specified period.
• Immigration — Arrival of individuals of the same species from elsewhere into the population's habitat.
• Emigration — Departure of individuals from the population's habitat to another place during a specified period.
• Age pyramid — Graphical representation of the proportions of individuals in different age groups of a population.
• Carrying capacity — Maximum population that a particular habitat can support with its available resources.
• Intrinsic rate of natural increase — Difference between per capita birth and death rates, representing the inherent potential for population growth.
• Reproductive fitness — Reproductive success under habitat conditions, expressed here through a high intrinsic rate of natural increase.
• Competitive release — Expansion of a species' distribution following experimental removal of a competitively superior species.
• Resource partitioning — Use of shared resources through different times or patterns of feeding, allowing competitors to coexist.
• Brood parasitism — A bird's use of another bird's nest and incubation for its own eggs.
• Commensalism — Interaction in which one species benefits while the other is neither benefited nor harmed.
• Amensalism — Interaction in which one species is harmed while the other species remains unaffected.

Common errors and misconceptions

• Misconception: Every population must reproduce sexually. Correct: Groups produced through asexual reproduction are also generally considered populations for ecological study.
• Misconception: Population density must be a count. Correct: Biomass, percentage cover, relative density and indirect estimates can be more meaningful or practical.
• Misconception: Eight new plants among 20 original plants give a birth rate of 8/28. Correct: The calculation uses the initial population: 8/20 = 0.4 offspring per plant over the stated interval.
• Misconception: Exponential growth continues indefinitely in nature. Correct: Unlimited resources are an ideal condition; natural resources impose limits on population growth.
• Misconception: Competition requires related species and scarce food. Correct: Unrelated species can compete, and interference competition may occur even when resources are abundant.
• Misconception: Gause's principle means competing species must always exclude one another. Correct: The resource limitation condition matters, and resource partitioning can promote coexistence.
• Misconception: All close associations are mutualistic. Correct: Both species must benefit; commensalism benefits one, whereas the other is unaffected.
• Misconception: Brood parasitism means feeding on the host's blood. Correct: It involves laying eggs in a host bird's nest and leaving incubation to that host.

Exam-style questions with model answers

Q1. A pond initially has 20 lotus plants. Reproduction adds eight plants during one year. Calculate the per capita birth rate, stating the calculation and its meaning. [2 marks]

1. The per capita birth rate is births divided by the initial population: 8/20 = 0.4.
2. This means 0.4 offspring per lotus per year; the denominator is the initial 20 plants.

Q2. Four individuals in an initial laboratory population of 40 fruitflies die during one week. Calculate the per capita death rate and state its meaning. [2 marks]

1. The death rate is deaths divided by initial population size: 4/40 = 0.1.
2. This represents 0.1 individuals dying per fruitfly per week, rather than a total of 0.1 deaths in the culture.

Q3. Using Nₜ for initial population, Nₜ₊₁ for population after one interval, B for births, I for immigrants, D for deaths and E for emigrants during that interval, write the population-balance equation, identify additions and losses, and state the condition for increase. [4 marks]

1. The population-balance equation is Nₜ₊₁ = Nₜ + [(B + I) − (D + E)], using event totals for the same interval.
2. Births and immigration add individuals, so B + I is the total addition to the original population.
3. Deaths and emigration remove individuals, so D + E is the total loss from the population.
4. Population size increases when B + I exceeds D + E, because additions are greater than losses during that interval.

Q4. Compare exponential and logistic growth in five points: resource conditions, equations, curve shapes, later growth and relevance to natural populations. Use N for population density, t for time, r for intrinsic rate of natural increase, K for carrying capacity and dN/dt for rate of population change. [5 marks]

1. Exponential growth assumes ideal unlimited resources; logistic growth describes increase when resources become limiting and competition develops among individuals.
2. The exponential equation is dN/dt = rN. The logistic equation is dN/dt = rN[(K − N)/K], which includes carrying capacity.
3. Exponential increase produces a J-shaped curve of population density against time. Logistic growth produces a sigmoid, or S-shaped, curve.
4. Under the ideal conditions, exponential increase continues rapidly. Logistic growth passes through lag, acceleration and deceleration before levelling towards carrying capacity.
5. Exponential growth demonstrates the potential for unimpeded increase. Logistic growth is considered more realistic because resources for most animal populations are finite and become limiting.

Q5. State Gause's Competitive Exclusion Principle, explain its resource condition, and explain how resource partitioning can allow coexistence. [3 marks]

1. Gause's principle states that two closely related species competing for the same resources cannot coexist indefinitely; the competitively inferior one is eventually eliminated.
2. This may be true if resources are limiting, but not otherwise. Evidence for such exclusion in nature is not always conclusive.
3. Resource partitioning can reduce competition through different feeding times or foraging patterns, allowing species using a shared resource to coexist.

Q6. Classify each described association and explain its outcome: an orchid benefits by growing on a mango branch without affecting the tree; a fungus supplies soil nutrients to plant roots and receives carbohydrates; a bird lays eggs in another bird's nest and leaves incubation to that host. [3 marks]

1. The orchid association is commensalism: the orchid benefits from growing on the branch while the mango tree remains unaffected by the association.
2. The root-fungus association is mutualism, specifically mycorrhiza: the plant gains help with nutrient absorption and the fungus receives energy-yielding carbohydrates.
3. The nesting association is brood parasitism: the parasitic bird benefits by leaving incubation of its eggs to the host bird.

Q7. Explain five defences against predation, using each supplied example: camouflaged insects or frogs; poisonous prey; Monarch butterflies containing a distasteful chemical acquired from a poisonous weed during the caterpillar stage; Acacia or cactus thorns; and plant chemicals that inhibit feeding or digestion. [5 marks]

1. Camouflaged insects or frogs are difficult for predators to detect because their colouring conceals them against their surroundings, reducing the chance of discovery.
2. Poisonous prey are avoided by predators. Their harmful properties therefore provide a defence distinct from merely remaining difficult to see.
3. The Monarch butterfly's chemical makes it highly distasteful to its bird predator. The butterfly acquires the chemical while feeding as a caterpillar on a poisonous weed.
4. Thorns in Acacia and cactus provide a structural defence against herbivores. These are physical features of the plant rather than defensive chemicals.
5. Plant chemicals can inhibit feeding or digestion in herbivores. This interferes with the herbivore's use of plant food and provides a chemical defence.

Q8. Explain the four life history patterns represented by these groups: Pacific salmon and bamboo; most birds and mammals; oysters and pelagic fishes; birds and mammals in relation to offspring size. [4 marks]

1. Pacific salmon and bamboo illustrate organisms that breed only once during their lifetime, rather than repeatedly over several reproductive episodes.
2. Most birds and mammals illustrate organisms that breed many times during their lifetime; this comparison concerns the frequency of reproduction.
3. Oysters and pelagic fishes produce large numbers of small-sized offspring, illustrating one combination of offspring number and size.
4. Birds and mammals illustrate production of a small number of large-sized offspring, contrasting with the many small offspring of the preceding group.

Key takeaways

• Populations possess birth rates, death rates, sex ratios and age distributions that describe groups rather than individual organisms.
• Population density can be expressed through counts, biomass, percentage cover or suitable indicators, depending on the ecological investigation.
• Births and immigration increase population size, whereas deaths and emigration decrease it during the chosen interval.
• Exponential growth represents ideal unlimited resources; logistic growth includes carrying capacity and produces a sigmoid curve.
• Reproductive strategies vary in breeding frequency, offspring number and offspring size in relation to habitat constraints.
• Predation transfers energy, regulates prey populations and can help maintain species diversity by reducing competition among prey.
• Competition need not cause exclusion: resource partitioning can allow species using a shared resource to coexist.
• Classify interactions by their effects on both participants, preserving distinctions between benefit, harm and neutrality.

Test yourself

Why can biomass be more useful than a count for comparing a banyan with carrot grass?

A single large banyan can play an enormous role that a count understates. Biomass represents living mass rather than merely the number of individuals.

Which two processes add individuals to a population?

Natality adds individuals through births, while immigration adds individuals of the same species arriving from elsewhere.

What is carrying capacity?

It is the maximum population that a particular habitat can support with its available resources.

What does a declining age pyramid look like?

Its youngest age bands form a narrower base than the bands immediately above them.

Can unrelated species compete for the same food?

Yes. Visiting flamingoes and resident fishes compete for zooplankton in some shallow South American lakes.

What distinguishes ectoparasites from endoparasites?

Ectoparasites feed on the host's external surface, whereas endoparasites live inside the host's body.

Why do parasitic birds' eggs resemble host eggs?

Resemblance in size and colour reduces the chances of detection and ejection by the host bird.

How does amensalism differ from commensalism?

Amensalism harms one species while leaving the other unaffected; commensalism benefits one while leaving the other unaffected.