Chapter 11: Organisms and Populations | Summary

Population density changes over time due to four processes: natality (births) and immigration increase population density, while mortality (deaths) and emigration decrease it. The equation Nt+1 = Nt + [(B + I) – (D + E)] describes how population density changes over a unit time period. Under normal conditions, births and deaths are the most influential factors, while immigration and emigration assume importance only under special circumstances such as colonisation of a new habitat.

Two primary growth models describe population growth. Exponential (J-shaped) growth occurs when resources are unlimited, described by dN/dt = rN, where r is the intrinsic rate of natural increase. Logistic (S-shaped or sigmoid) growth, described by the Verhulst-Pearl equation dN/dt = rN[(K–N)/K], occurs when resources become limiting. The population eventually reaches the carrying capacity (K) — the maximum number the habitat can sustainably support.

Populations evolve to maximise their reproductive fitness (Darwinian fitness, measured by a high r value) in the habitats they occupy. Life history traits — such as the number of times an organism breeds in its lifetime and the size and number of offspring — have evolved in response to the abiotic and biotic constraints of the habitat.

Some organisms, like Pacific salmon and bamboo, breed only once in their lifetime, while others like most birds and mammals breed multiple times. Organisms such as oysters and pelagic fishes produce large numbers of small-sized offspring, whereas birds and mammals produce fewer but larger offspring. These differences reflect distinct evolutionary strategies for maximising survival and reproductive success.

In nature, no species lives in complete isolation; they interact with other species in various ways. Interspecific interactions — interactions between populations of two different species — are classified based on their effect on each species using '+' (beneficial), '–' (detrimental), or '0' (neutral) signs. The major types are: mutualism (+/+), competition (–/–), predation (+/–), parasitism (+/–), commensalism (+/0), and amensalism (–/0).

Predation facilitates energy transfer across trophic levels and regulates prey populations, preventing ecosystem instability. Competition, especially among closely related species sharing the same resources, can lead to competitive exclusion of the inferior species (Gause's Competitive Exclusion Principle). However, species may also evolve mechanisms like resource partitioning to coexist. Parasitism and commensalism involve close physical association between species, with one gaining benefits at the host's expense or with the host being unaffected, respectively.

Predators play crucial roles beyond energy transfer: they maintain prey population sizes, support species diversity in communities, and prevent dominance by any single prey species. The removal of the starfish Pisaster from rocky intertidal communities of the American Pacific Coast led to the extinction of over 10 species of invertebrates within a year due to unchecked interspecific competition, illustrating the keystone role of certain predators.

Prey species have evolved diverse defences against predation. Animals may use camouflage (cryptic colouration), chemical toxins (Monarch butterfly acquiring toxins from poisonous weeds), or other avoidance strategies. Plants, unable to escape herbivores, have evolved morphological defences such as thorns (Acacia, Cactus) and chemical defences including toxic glycosides (Calotropis), alkaloids (nicotine, caffeine, quinine, strychnine, opium), and other substances that deter, harm, or disrupt the reproduction of herbivores.

Competition occurs when the fitness of one species is significantly reduced in the presence of another, especially when resources are limiting. Gause's Competitive Exclusion Principle states that two closely related species competing for the same resources cannot coexist indefinitely — the competitively inferior species will eventually be eliminated. Classic examples include the extinction of the Abingdon tortoise after goat introduction in Galapagos, and the exclusion of the barnacle Chathamalus by the larger Balanus on Scottish rocky coasts.

However, species facing competition may evolve mechanisms that promote coexistence rather than exclusion. Resource partitioning is one such mechanism — species competing for the same resource avoid direct competition by choosing different times to feed or different foraging strategies. MacArthur's study of five closely related warbler species living on the same tree showed that behavioural differences in foraging activities allowed these species to coexist without competitive exclusion.

Parasitism is an interaction where the parasite benefits at the expense of the host. Parasites have evolved complex adaptations including loss of sense organs, presence of adhesive organs or suckers, reduced or absent digestive systems, and high reproductive capacity. Many have complex life cycles involving intermediate hosts or vectors. The human liver fluke requires two intermediate hosts (a snail and a fish), while the malarial parasite requires a mosquito vector.

Ectoparasites live on the external surface of the host (e.g., lice on humans, ticks on dogs, Cuscuta on hedge plants), while endoparasites live inside the host body. Brood parasitism, as seen in the cuckoo (koel) laying eggs in a crow's nest, is a fascinating behavioural form of parasitism where the parasite's eggs have evolved to resemble the host's eggs in size and colour to avoid detection and ejection.

Commensalism benefits one species while the other remains unaffected. Examples include orchids growing as epiphytes on mango trees, barnacles on whale backs, cattle egrets foraging near grazing cattle (benefiting from insects flushed out by cattle movement), and clown fish living among the stinging tentacles of sea anemones for protection.

Mutualism benefits both interacting species. Classic examples include lichens (fungus + algae/cyanobacteria), mycorrhizae (fungi + plant roots), and plant-pollinator relationships. The fig-wasp mutualism is especially tight — each fig species is pollinated exclusively by a specific wasp species that also uses the fig fruit for oviposition. Orchids show diverse floral adaptations to attract pollinators, with the Mediterranean orchid Ophrys employing 'sexual deceit', having a petal that mimics a female bee to attract male bees for pseudocopulation, ensuring pollination.

Chapter 11 provides a comprehensive understanding of ecology at the population and community levels. It establishes that populations are dynamic entities characterised by birth rates, death rates, sex ratios, and age distributions, and that their growth is governed by exponential or logistic models depending on resource availability. Interspecific interactions — predation, competition, parasitism, commensalism, amensalism, and mutualism — shape the structure and dynamics of biological communities, driving evolution and coexistence strategies.

For CBSE Board exams, this chapter is of high importance. Students must be thorough with the mathematical aspects of population growth models (exponential and logistic equations, the concept of r and K), the graphical representation of growth curves, Gause's Competitive Exclusion Principle, resource partitioning, the fig-wasp mutualism, types of parasites, and examples of all six interspecific interactions. These topics consistently appear across 2-mark, 3-mark, and 5-mark questions.