Organisms and Populations: common doubts, answered
The questions students ask most often about Organisms and Populations, each with a short answer. For the full chapter, read the Organisms and Populations notes.
About the chapter
Why are population ecology and natural selection connected?
Natural selection acts on traits that affect survival and reproduction, and its effect shows up as a change in the population, as individuals with favourable traits leave more offspring. Population ecology studies the factors, such as birth and death rates and resources, that decide how many leave offspring. Hence the field links ecology with genetics and evolution.
Ecology and populations
Read this section in the notes →What is a population and how is it different from a community?
A population is a group of individuals of the same species living in a given area at a given time. A community is all the populations of different species living together in that area. So a population has a single species, while a community has many species interacting with one another.
Population attributes
Read this section in the notes →What are the main attributes of a population?
A population has attributes that an individual does not have: birth rate, death rate, sex ratio and age distribution. Birth and death rates are expressed as per capita, that is, per individual per unit time. These attributes define the population and also decide whether it will grow, stay stable or decline.
What do the three shapes of age pyramids indicate?
A pyramid with a wide base indicates a growing population, with many young individuals. A bell shape indicates a stable population, with similar numbers of young and middle-aged individuals. An urn shape, narrow at the base, indicates a declining population. The pyramid compares three age groups: pre-reproductive, reproductive and post-reproductive.
Population density
Read this section in the notes →How is population density measured when counting is not possible?
Density is not always the number of individuals. It can be expressed as per cent cover or biomass, as for a huge banyan tree or algae in a pond. Relative density, such as the number of fish caught per trap, is often enough. Tigers are censused through indirect signs such as pug marks and faecal pellets.
Four processes of population growth
Read this section in the notes →What are the four factors that change population size?
Natality (births) and immigration increase the population, while mortality (deaths) and emigration decrease it. The expression is Nt+1 = Nt + [(B + I) − (D + E)]. Immigration adds and emigration subtracts, so a common mistake is to switch their signs.
Exponential growth
Read this section in the notes →What is exponential growth and what does its curve look like?
Exponential growth occurs when resources are unlimited and the population grows in proportion to its size. It is expressed as dN/dt = rN and gives a J-shaped curve. The growth keeps accelerating because each new member also reproduces. The r value is the intrinsic rate of natural increase, equal to birth rate minus death rate.
What is the meaning of r in population growth?
r is the intrinsic rate of natural increase, equal to birth rate (b) minus death rate (d) per individual. A positive r means the population is growing and a negative r means it is shrinking. Examples given are 0.015 for the Norway rat and 0.12 for the flour beetle, so the beetle population grows much faster.
Logistic growth and life history
Read this section in the notes →What is the difference between exponential and logistic growth?
Exponential growth assumes unlimited resources and gives a J-shaped curve, whereas logistic growth assumes limited resources and gives an S-shaped (sigmoid) curve. In logistic growth, the population passes through a lag phase, acceleration, deceleration and finally reaches an asymptote at the carrying capacity K. It is the more realistic of the two.
What is carrying capacity?
Carrying capacity (K) is the maximum population size that the environment can sustain, because food and space are limited. In the logistic curve the population levels off near K, and the equation is dN/dt = rN (K − N)/K. When N approaches K, growth slows to zero.
What are life history variations, with examples?
Different organisms have different reproductive strategies adapted to their habitat. Some, such as Pacific salmon and bamboo, breed only once in a lifetime, while others, such as birds and mammals, breed many times. Some, such as oysters and pelagic fishes, produce a large number of small offspring, whereas others produce a few large ones.
Population interactions
Read this section in the notes →How do you remember the +, − and 0 signs of interspecific interactions?
Mutualism is +/+, competition is −/−, predation and parasitism are +/− (the predator or parasite gains and the prey or host loses), commensalism is +/0 and amensalism is −/0. In the last two the second species is unaffected. So 0 means no effect on that species.
Predation
Read this section in the notes →How do predators help maintain a community?
Predators keep prey populations under control and prevent a single prey species from dominating, so they maintain species diversity. In an experiment, when the starfish Pisaster was removed from a rocky coast, more than 10 species of invertebrates went extinct within a year because of interspecific competition. In Australia a cactus-feeding moth controlled prickly pear.
How do plants defend themselves from herbivores?
Plants have evolved morphological defences such as thorns (acacia, Cactus) and chemical ones. Many produce toxic substances: Calotropis produces cardiac glycosides that make it poisonous, and others make nicotine, caffeine, quinine and strychnine. These chemicals make the plant unpalatable or harmful to herbivores.
Competition
Read this section in the notes →What is the Competitive Exclusion Principle?
Gause's principle says that two closely related species competing for the same limited resource cannot coexist indefinitely, and the competitively inferior one is eliminated. Species can avoid it by resource partitioning, as MacArthur showed with five warbler species feeding on different parts of the same tree.
What did Connell's experiment on barnacles show?
On the Scottish coast the larger barnacle Balanus dominates the lower zone and excludes the smaller Chthamalus, which is confined to the upper zone. When Balanus was removed, Chthamalus spread to the lower zone too. This showed competition and the effect of competitive release.
Parasitism
Read this section in the notes →What are the adaptations of parasites?
Parasites show loss of unnecessary sense organs, presence of adhesive organs or suckers to cling to the host, loss of digestive system, and high reproductive capacity. Their life cycles are often complex and include intermediate hosts, like the human liver fluke that needs a snail and a fish to complete its life cycle.
What is brood parasitism, with an example?
In brood parasitism a bird lays eggs in the nest of another species and the host incubates them. The koel lays its eggs in the nest of the crow. The parasitic eggs resemble the host's eggs in colour and size, so the host does not detect and eject them.
Commensalism
Read this section in the notes →What is the difference between commensalism and parasitism?
In commensalism one species benefits and the other is neither harmed nor benefited, whereas in parasitism one benefits at the cost of the other. An orchid growing on a mango branch, barnacles on a whale, and the cattle egret near grazing cattle are commensal. Clown fish living among sea anemones is another example.
Mutualism
Read this section in the notes →What are some examples of mutualism?
Lichens, a fungus living with an alga or cyanobacterium, and mycorrhizae, a fungus with plant roots, are standard examples. The fig tree and its wasp pollinator depend on each other: the wasp pollinates the fig flowers while laying eggs in the fruit, and its larvae feed on some of the developing seeds. Many flowering plants and the animals that pollinate them or disperse their seeds are mutualists too. Both partners benefit in each case.
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