Revision: Ecology and Environment >> Organisms and Populations Biology (Theory) ISC (Science) ISC Class 12 CISCE

A complex of several plant and animal communities in a region, shaped by and adapted to similar climatic conditions (Clements and Shelford, 1939).

Ecology is the branch of biology that studies the interactions among organisms and between organisms and their environment.

Organisms and their environment are always interdependent and mutually reactive.

The study of the relationship of a single species with its environment is called autecology.

The study of the relationship of a group of different species (a community) with their environment is called synecology.

Camouflage, also called cryptic colouration, is a defense mechanism or tactic that organisms use to disguise their appearance, usually to blend in with their surroundings. Organisms use camouflage to mask their location, identity, and movement.

Species that tolerate narrow range of salinity are called stenohaline species.

A population is a group of individuals of the same species that are capable of interbreeding and are found in a geographical area. E.g., all the frogs identified as Rana tigrina living in a given pond constitute a population. Similarly, all water hyacinth plants (Eichhornia) growing in that pond form another population. Organisms of the same kind may form several populations inhabiting different geographical areas.

The population is defined as a group of individuals of a species occupying a definite geographic area at a given time.

An interaction in which one species benefits while the other is neither benefited nor harmed is called commensalism.

Commensalism is the interaction between two living individuals of different species in which one is benefited while the other is neither harmed nor benefited except to a negligible extent.

Parasitism is generally defined as a relationship between the two living species in which one organism is benefitted at the expense of the other. The organism that is benefitted is called the parasite, while the one that is harmed is called the host.

Interspecific competition occurs when closely related species compete for limited resources such as food and habitat.

A community is an assemblage of populations of different species living in the same area and interacting with one another.

A community is an association of a number of different interrelated populations belonging to different species in a common environment. A community is characterised by species diversity, coexistence and interdependence.

Niche describes the position of a species in an environment and also the functional role played by an organism.

The type of interaction in which both species are benefitted is called mutualism.

Mutualism refers to an interaction between two different species in which both benefit.

Interactions occurring among individuals of the same species within a community.

Interaction between populations of two different species living in the same habitat.

An interspecific relationship in which one organism benefits, while the other is neither benefited nor harmed.

It is a non-obligatory interaction between two different species in which both are benefited, but neither depends completely on the other for survival.

Mutualism is an interaction between two species in which both benefit.

Parasitism is an interaction between two species in which one (known as parasite) obtains its food in ready‑made form from the other living organism (said to be host).

The rivalry between two or more organisms for the same limited resource, where the use of the resource by one reduces its availability to the other, is called competition.

Competition that occurs between individuals of different species for the same limited resources is called interspecific competition.

Competition that occurs between individuals of the same species for limited resources is called intraspecific competition.

A type of competition in which one organism directly prevents another from accessing a resource is called interference competition.

A type of competition in which organisms share a resource, but one uses it more efficiently than the other, is called exploitative competition.

A group of organisms of the same species occupying a particular area at a particular time is called a population.

The number of individuals of a species present per unit area (or volume) at a given time is called population density.

The inherent capacity of a population to increase in number under ideal environmental conditions is called biotic potential.

The scientific study of the size, structure and growth of a population, generally human population, is called demography.

The increase in the number of individuals of a species in a given area over a period of time is called population growth.

The rate of change in population size per individual per unit time is called the population growth rate.

The reproductive capacity of a population, measured as the number of births during a given period, is called fecundity.

The movement of individuals out of a population to other areas is called emigration.

The rate at which new individuals are added to a population through reproduction is called natality.

The maximum growth rate of a population under ideal conditions with maximum reproduction and minimum mortality is called the intrinsic rate of growth.

The rate at which individuals die in a population over a given period is called mortality.

The addition of individuals to a population from neighbouring populations is called immigration.

The maximum number of individuals that an environment can support or sustain with available resources is called carrying capacity.

When the number of individuals in a population is plotted against time, it yields a specific mathematical pattern called a population growth curve. The shape depends entirely on the presence and intensity of environmental resistance.

The proportion of individuals belonging to different age groups in a population is called age distribution.

The arrangement or composition of a population based on different age groups is called age structure.

The graphical representation showing the proportion of individuals in different age groups of a population is called an age pyramid.

The proportion or ratio of males to females in a population is called sex ratio.

The flow and transformation of energy within a population is called bioenergetics

\[\frac{\mathrm{dN}}{\mathrm{dt}}=\mathrm{rN}\]

Integral form: \[\mathbf{N_{t}}=\mathbf{N_{0}}\mathbf{e^{rt}}\]

\[\mathbf{N_{t+1}}=\mathbf{N_{t}}+[(\mathbf{B+I})-(\mathbf{D+E})]\]

Verhulst-Pearl Logistic Growth:

\[\frac{\mathrm{d~N}}{\mathrm{d~t}}=\mathbf{rN}\left[\frac{\mathrm{K}-\mathrm{N}}{\mathrm{K}}\right]\]

• Ecology, derived from Greek words meaning 'the study of the house', is a branch of biology focused on the interactions among organisms and their environment.
• While Henry David Thoreau likely used the term first in 1858, German biologist Ernst Haeckel formally defined its scientific meaning in 1886.
• The subject is primarily concerned with five levels of biological organisation: organisms, populations, communities, ecosystems, and the biosphere.
• Each organisational level builds in complexity, progressing from individual life forms up to the biosphere, which encompasses all ecosystems on Earth.
• Clements and Shelford introduced the concept of a "biome" in 1939, defining it as a large complex of various plant and animal communities sharing a common climate.

1. Organism: An individual living entity that interacts with its surroundings.
2. Population: A group of individuals of the same species living in a defined area.
3. Community: Different populations living and interacting together in an area.
4. Ecosystem: A functional unit where organisms interact with the physical environment.
5. Landscape: A land area containing a group of different ecosystems.
6. Biome: A large ecological region defined by climate and dominant vegetation.
7. Biosphere: The entire zone of life on Earth where all ecosystems exist.

Population interactions refer to the ways organisms influence one another within a shared ecosystem, categorised by positive (+), negative (-), or neutral (0) effects.

1. Mutualism (+/+): Both species benefit. (Example: Lichen – algae and fungi helping each other)
2. Competition (-/-): Both species are harmed when fighting for limited resources. (Example: Two plant species competing for sunlight)
3. Predation (+/-): Predator kills and eats the prey. (Example: Tiger hunting a deer)
4. Parasitism (+/-): Parasite slowly harms a living host for nutrients. (Example: Tapeworm inside human intestines)
5. Commensalism (+/0): One benefits; the other is entirely unaffected. (Example: Cattle egrets eating insects stirred up by grazing cattle)
6. Amensalism (-/0): One is harmed; the other is entirely unaffected. (Example: Penicillium fungus killing nearby bacteria)
7. Neutralism (0/0): Neither species affects the other. (Example: Rabbits and deer sharing the same forest)

• The ecological hierarchy scales from the individual organism, which is the fundamental unit of study, up to the biosphere, which encompasses the entire global ecosystem.
• Autecology focuses on the relationship between a single species and its environment, whereas synecology examines complex interactions among a group of different species.
• Ecological relationships are categorised as either intraspecific (occurring between members of the same species) or interspecific (occurring between two or more different species).
• Primary interactions include competition for limiting resources, predation or cannibalism for consumption, mutualism for shared benefit, and parasitism at the expense of a host.
• Populations can be strictly localised (demes), dispersed but interconnected (metapopulations), or entirely exclusive to one specific geographic region (endemic species).
• Keystone species serve as the critical foundation of a biological community; their removal fundamentally disrupts and alters the entire ecological structure.

• Intraspecific relationships encompass all interactions among individuals of the same species, driven by factors such as reproduction rates, resource competition, and mutual assistance.
• The complexity of these interactions spans a broad spectrum, ranging from solitary existences with minimal contact to highly organised societies featuring a distinct division of labour.
• Organisms maintain group cohesion, express dominance, and coordinate collective behaviours through diverse communication methods, including physical posturing, calls, and facial expressions.
• Cohesive interactions function to draw individuals together, providing collective advantages such as shared warmth, coordinated movement, and enhanced safety.
• Dispersive interactions act to separate individuals across an environment, primarily serving to defend established territories and secure exclusive control over limited resources.

Sign Key: (+) = Benefited, (-) = Harmed, (0) = Unaffected

• Commensalism (+/0) is an interaction where one species (the commensal) gains a benefit – such as food, shelter, or transport – while the other (the host) remains completely unaffected.
• It occurs in various forms, such as phoresy for transport (e.g., barnacles on a whale) or inquilinism for shelter (e.g., clownfish living safely inside a stinging sea anemone).
• Epiphytic plants, like orchids, grow on host plants (such as a mango tree) strictly for physical support and better sunlight, without absorbing any of the host's nutrients.
• Cattle egrets exhibit this relationship by closely following grazing cattle to easily catch insects that are flushed out of the grass by the cattle's movement.
• Remora fish attach themselves to the underside of sharks to gain free oceanic transport and scavenge leftover food scraps, causing absolutely no impact on the shark.

• Proto-cooperation is a mutually beneficial but non-obligatory ecological interaction between two distinct species.
• The relationship is not essential for survival, meaning both participating organisms can live independently without the other.
• It differs fundamentally from mutualism, which is an obligatory association, and commensalism, where only one species derives a benefit.
• A prominent example of this interaction is the temporary association between the hermit crab (Eupagurus prideauxi) and the sea anemone (Adamsia pallida).
• In this dynamic, the sea anemone gains mobility to access new feeding grounds, while it provides the hermit crab with defense against predators via its stinging cells.

• Mutualism is an interaction where both species benefit from each other, represented as (+, +).
• Lichens are a mutualistic association between fungi and algae/cyanobacteria, where fungi provide shelter and algae provide food through photosynthesis.
• Mycorrhizae are associations between fungi and plant roots, where fungi help the plant absorb nutrients from the soil, and the plant provides carbohydrates to the fungi.
• Fig trees and wasps share a one-to-one relationship where the wasp pollinates the fig, and the fig provides developing seeds as food for the wasp larvae.
• The orchid Ophrys uses "sexual deceit" by resembling a female bee, causing male bees to pseudocopulate with it and transfer pollen from flower to flower.

• Predation (+/-) involves a predator killing and eating its prey, which acts as nature's main channel for transferring energy up the food chain.
• Predators naturally regulate prey numbers, preventing overpopulation – a principle used effectively in biological pest control.
• By consuming dominant prey species, predators prevent resource monopolisation and maintain overall species diversity.
• Predator and prey populations cycle together naturally; predators are "prudent" and do not hunt their food source to extinction.
• To survive, prey species continuously evolve physical defences (like camouflage or thorns) and chemical defences (like toxins).

• Parasitism is an interaction where one organism (the parasite) derives ready-made nourishment directly from another living organism (the host).
• Ectoparasites (like ticks and lice) live on the host's outer surface and retain functional organs, whereas endoparasites (like tapeworms and Plasmodium) live inside the host with reduced sensory and digestive systems.
• In brood parasitism, a bird lays its eggs in the nest of a host species, often using egg mimicry and earlier hatching times to ensure the host raises its young (e.g., the Koel bird and the crow).
• Plant parasites use specialised structures to extract nutrients; they can be total parasites lacking chlorophyll entirely (like the leafless Cuscuta) or partial parasites that still photosynthesise (like Mistletoe).
• Parasites possess unique survival adaptations, including specialised attachment organs (hooks and suckers), the ability to absorb food directly through their skin, anaerobic respiration, and massive reproductive capacities involving intermediate hosts.

• Competition is an interaction (-/-) where organisms struggle for the same limited resources, resulting in a negative impact on the biological fitness of both participants.
• It is generally more intense when it occurs between members of the same species (intraspecific) rather than between members of different species (interspecific) because their resource requirements are identical.
• The struggle can occur directly, in which one organism actively prevents another from accessing a resource (interference competition), or indirectly, in which one organism rapidly consumes a shared resource (exploitative competition).
• Gause's Competitive Exclusion Principle states that two closely related species competing for the exact same resource cannot coexist indefinitely; the competitively weaker species will eventually be eliminated.
• To avoid elimination, competing species often adapt to coexist through resource partitioning (using different parts of a resource or hunting at different times) or character displacement (evolving distinct physical traits to reduce overlap).
• Competition usually restricts a species to a smaller realised niche rather than its full-potential fundamental niche; however, if the dominant competitor is removed, the restricted species will rapidly expand its range in a process called competitive release.

• Amensalism is an interspecific ecological interaction characterized by the notation (-, 0), where one species is harmed while the other remains entirely unaffected.
• The interaction typically operates through antibiosis in microbes or allelopathy in higher plants by releasing chemical inhibitors into the environment.
• A classic microbial example involves the fungus Penicillium secreting penicillin, which actively suppresses the growth of Staphylococcus bacteria.
• In terrestrial plants, the Black Walnut tree (Juglans nigra) releases the toxic chemical juglone through its roots, causing lethal wilting in nearby apple and tomato plants.
• Certain crop plants, including barley, sorghum, and sunflower, naturally exude soil chemicals that prevent competitive weeds from germinating nearby.

• A population comprises individuals of the same species occupying a designated geographical area at a specific time, sharing a common gene pool and exhibiting collective demographic properties.
• Demography is the scientific study of population size, structure, and growth dynamics, primarily applied to human populations.
• Population density (D) measures the number of individuals (N) per unit area or volume (A), calculated using the formula D = `"N"/"A"` to reflect a truer ecological impact than raw headcounts.
• Density measurement methods vary by species: direct counts for small/visible populations, percent cover or biomass for plants, and indirect signs (like pugmarks or fecal pellets) for elusive wildlife.
• Biotic potential represents the theoretical maximum inherent capacity of a population to increase numerically under ideal, unlimited environmental conditions.
• Real-world population growth follows two primary mathematical curves: a J-shaped exponential growth curve under unlimited resources, or an S-shaped (sigmoid) logistic growth curve when limited by carrying capacity (K).

• Population growth is studied mainly by two models: exponential growth and logistic growth.
• Population density (N) changes due to four factors: natality (B), mortality (D), immigration (I), and emigration (E).
• Exponential growth → Occurs when resources are unlimited; population increases rapidly and forms a J-shaped curve.
• Logistic growth → Occurs when resources are limited; growth slows down and forms an S-shaped (sigmoid) curve.
• Phases of logistic growth: lag phase → log (exponential) phase → diminishing growth phase → stationary phase.
• Carrying capacity (K) → Maximum population size that the environment can support; population stabilises at this level.

• Carrying capacity (K) represents the maximum population size an environment can sustainably support based on its most critical limiting resource.
• At the carrying capacity threshold, population growth ceases entirely, mathematically represented as \[\frac{\Delta N}{\Delta t}=0\].
• The concept is dynamic rather than static, fluctuating over time due to environmental shifts, climate changes, and natural evolutionary forces.
• It is determined by supportive capacity, which dictates the safe withdrawal limits of raw resources without causing permanent ecological damage.
• It is equally determined by assimilative capacity, which measures the environment's ability to absorb and neutralise waste products without ill effects.
• Industrial and developmental activities strain this equilibrium by simultaneously over-extracting resources and overloading the environment's waste-absorption limits.

• The carrying capacity of an environment constantly fluctuates in response to unpredictable external conditions, such as shifting weather patterns.
• Real populations undergo erratic numerical shifts driven by alternating favourable and unfavourable years, a phenomenon defined as environmental stochasticity.
• These fluctuations are regulated by random environmental variables rather than predictable, deterministic density-dependent processes.
• Small, short-lived organisms are highly vulnerable to these environmental changes, leading to dramatic and rapid population spikes or crashes.
• Large, long-lived organisms possess higher physiological tolerance and resilience, making their population numbers significantly less susceptible to extreme environmental variations.

• Sex ratio is a defining population attribute that represents the proportion of males to females, whereas an individual organism possesses a specific biological sex.
• In various species, the sex ratio is dynamic and shifts automatically in response to changes in population density to optimize survival.
• Environmental variables can override or dictate genetic sex determination, with temperature regulating the incubation outcome and offspring sex in reptiles like turtles and crocodiles.
• Photoperiod, mineral availability, and specific plant growth hormones serve as critical external regulators controlling sex expression in certain organismal populations.

• Bioenergetics is the study of energy flow and transformations within a population over time.
• Tracking energy flow provides a highly reliable baseline for evaluating fluctuations in population density and establishes a population's specific trophic role within its community.
• Population dispersal is the movement of individuals or their propagules in and out of a habitat, regulating geographic distribution and actively preventing overcrowding.
• Emigration (permanent outward movement) decreases local population density, whereas immigration (permanent inward movement) increases it.
• Migration is a temporary, two-way seasonal movement – typically triggered by harsh environmental conditions – that causes periodic fluctuations in population size.

• Organisms evolve life history strategies to maximise reproductive (Darwinian) fitness in their environment.
• Different species adopt different strategies based on selection pressures.
• Some organisms reproduce once in a lifetime (e.g., salmon, bamboo), while others reproduce multiple times (e.g., birds, mammals).
• Species may produce many small offspring (e.g., oysters) or few large offspring (e.g., mammals), depending on survival needs.
• These variations are shaped by biotic and abiotic factors, and studying them is an important area in ecology.