Organisms and Population

Organisms and Populations focus on the interactions of organisms with their surroundings and the way populations grow, adapt, and regulate themselves within ecosystems. It includes important concepts like population characteristics, ecological interactions such as mutualism, predation, and competition; determinants of population dynamics. This chapter forms a base for advanced studies in Biology like ecology, conservation biology, and environmental management, making it important for Class 12 to NEET level.

Important Topics of Chapter Organisms and Population

• Population interactions
• Population attributes
• Ecology
• Biotic and Abiotic Factors

Overview of the Chapter

The branch of science that studies the relationship between living things and the environment is called ecology. The various levels of organization are:
(i) Organism: each animal/bird who is part of a species.
(ii) Population: A group of species that can reproduce.
(iii) Communities: A diverse group of people come together to form communities.
(iv) Biomes: A large number of plants and animals found in the climate zone.

Understanding a single organism and its population attributes becomes very important in the learning of biology and ecology. It involves studies on single organisms, their features, interactions among them, and finally how they go on to form populations with unique features.

An understanding of the population attributes would be useful in learning about ecosystem dynamics, projecting the changes that would happen in population sizes, and hence putting in place conservation strategies.

Also Read:

• MCQ Practice on Organisms and Populations
• Abiotic Factors
• Biotic Factors
• Ecosystem

Major Abiotic Factors Affecting Organisms and Populations

Abiotic factors are key factors that form the environment and influence the survival, distribution, and adaptation of organisms. Major abiotic factors include temperature, water, light, and soil, each having unique impacts on living organisms:

Temperature

• Usually closely associated with metabolic and physiological processes in organisms, Enzyme activity changes with temperature.
• Temperature varies widely around the globe: at times less than 0°C in the polar regions, or more than 50°C in deserts.
• Eurythermal organisms (dogs, cats, red algae) can tolerate wide ranges of temperatures, whereas stenothermal organisms (anaconda, crocodile, penguin) exist within very narrow temperature ranges.

Water

• Required by all forms of life; fewer species exist in drier areas with low levels of water, like deserts.
• The survival characteristics of an organism are affected by the characteristics of the water (pH and salt levels).
• Euryhaline species (such as crabs, and mollyfish) tolerate varying salt concentrations and stenohaline species (like haddock, and goldfish) exist only in a small range of salt concentrations.

Light

• Essential for the photosynthesis of autotrophs; the sun is the primary source of light.
• Variation in the level of requirement among the organisms leads to changes in the growth patterns of plants and migratory behaviours in animals to reproduce.
• Levels of solar irradiance, especially ultraviolet radiation, impact the health and condition of organisms primarily depending on the depth of the ocean.

Soil

• Hence, soil type influences biodiversity but is rather determined by climatic factors, topography, and sedimentation processes.
• Features such as pH, mineral and texture determine the species of plants and animals which thrive in a given locality
• Collectively, these abiotic factors form an ecosystem, determining what's there, how abundant it is and in what adaptations the organisms exist.

Responses to Abiotic Factors

All organisms have different adjustments to withstand various abiotic factors within the environment. The reactions help them survive and live under changing conditions. There are two major categories of reactions to abiotic factors:

Regulators

• In this process, a stable internal environment is maintained irrespective of what happens outside.
• Universally present among birds and mammals and some vertebrates and invertebrates.
• For example, human beings maintain a constant body temperature at 37°C by sweating during hot weather and shivering in cold weather.

Conformers

• Regulate their internal body temperature according to external temperatures, and often lack complex thermoregulation.
• Small animals, because of their high surface-to-volume ratio, lose heat rapidly and consequently are not usually found in polar regions.

Partial Regulators

Regulate their internal conditions only to some limited extent but adapt when environmental conditions exceed their regulatory ability.

Migration

• Seasonal movement of animals to avoid tough environmental conditions.
• For instance, many birds migrate from Siberia to Keoladeo National Park in Rajasthan during winter.

Dormancy and Spore Formation

Spores are formed by certain microorganisms like bacteria and fungi to survive hostile environmental conditions. Some animals go into dormancy during winter or aestivating when it gets very hot and dry.

NCERT Books Link:

• NCERT book for class 11 Biology
• NCERT book for class 12 Biology

Adaptations

Adaptations are characteristics that enhance the survival chances or ability of an organism to reproduce within its habitat. Some of the most significant are:

Desert Plant Adaptations

Opuntia and other desert plants have thick stems, spiny leaves, and sunken stomata to minimize water loss. These plants adopt CAM photosynthesis at arid temperatures to ensure survival.

Human High-Altitude Adaptations

High-altitude people often experience symptoms of altitude sickness such as nausea and fatigue.

To cope, the body raises its count of red blood cells, thus increasing oxygen binding and breathing rates.

Behavioural Adjustments

Some animals have developed certain habits based on particular environmental factors, such as migrating or hibernation.

Population Attributes

The population has its specific features that let it be different from the individual organism or the whole species. Among such features, one can mention the following:

Birth And Death Rates

Birth Rate

• The number of live births per 1,000 individuals in a population per year.

• It shows the reproductive capacity and growth potential of the population.

Death Rate

• The number of deaths per 1,000 individuals in a population per year.

• It reflects the mortality rate and the general health condition of the population.

Sex Ratio

• The ratio of males to females in a population.

• Affects mating patterns, reproductive rates and social structure.

• This may affect the population growth and dynamics.

Age Distribution

• Proportion of individuals in different age groups.

• Usually divided into pre-reproductive, reproductive and post-reproductive.

• Affects the growth rate of the population and future reproduction potential.

• Indicates dependency ratio with implications for economic and social support systems.

Population Density

• Number of individuals per unit area/volume.

• High densities can lead to increased competition for resources, elevated disease transmission, and social interaction, while low densities may mean difficulties finding mates and increased vulnerability to environmental changes.

NCERT Notes Subject Wise Link:

• NCERT notes Class 12 Maths
• NCERT notes Class 12 Physics
• NCERT notes Class 12 Chemistry
• NCERT notes Class 12 Biology

Characteristics of Population

Population is the total number of individuals in a given area. This is affected by the birth/death rates, immigration, and emigration. Population density says how individuals are spread within a habitat, telling about either resource availability or competition. There's high density indicating abundant resources, but there is always competition. Perhaps low density might indicate reduced resource availability.

Distribution describes the spacing of individuals and can be:

• Clumped: Distributed in patches as a result of uneven distribution of resources (e.g. elephants).
• Uniform: Spaced evenly apart due to reasons like territorial habits or competition for resources (e.g. plants).
• Random: Spaced individually apart as the resource is evenly distributed. This phenomenon is very rarely encountered in nature.

Examples:

• Human Population: Depends upon birth rates as well as death rates and immigration and emigration. It may exhibit clumped, uniform or random dispersion.
• Animal Populations: Deer are clumped near water, but their fish counterparts distribute evenly because of territorial needs. These distributions are very important for conservation and management.

Population Growth

The size of the population of any species may never stop, depending on the availability of food sources, weather conditions, and predators. There are four main reasons for the change in population density:

(i)Natality: Described as the number of children born over a period of time.

(ii) Death: Described as the death toll in a given period.

(iii) Immigration: It is defined as the number of people who come to a habitat from different places for a period of time.

(iv)Emigration: It is defined as the number of types of populations that exit the habitat for other places.

If the Population density is N at time t, then its density at time t+1 is represented as

Nt + 1 = Nt + [(B + I) - (D + E)]

When overcrowding is represented by N during t, the birth rate is represented by B + I while the mortality rate is represented by D + E.

NCERT Solutions Subject-wise link:

• NCERT solutions for class 12 Physics
• NCERT solutions for class 12 Chemistry
• NCERT solutions for class 12 Mathematics
• NCERT solutions for class 12 Biology

Population Interactions

Interactions between different organisms can be divided into two categories, namely, interspecific interactions and intraspecific interactions. When an interaction occurs between similar species then it is called an Intraspecific interaction while an interaction between different organisms then it is called an interspecific interaction.

(i) Predation: Cooperation when a deer is killed by an invader. This is important in keeping predators afloat. In plants, herbivores act as predators. In some plants, a variety of habits are found that help them to protect themselves from predators. Familiarity may include thorns such as preservatives or protective substances such as caffeine, nicotine, quinine, etc., used for commercial purposes.

(ii) Competition: An interaction between an organism in which both organisms compete for a variety of resources including food, water, habitat, etc. Meat may be of the same type or different types. This interaction occurs when resources are limited. According to the Gause Policy on Competitive Exclusion. The two adjacent models will compete for the same services and are not competitive so the lower one will be released during the competition.

(iii) Parasitism: An interaction where one type is dependent on another type which leads to the benefit of one type and harms another. Those parasites feed on the outer surface of living organisms called ectoparasites. For example, marine fish include ectoparasitic copepods. In the case of birds, there is Brood parasitism where birds lay their eggs in the bird's nest while the host incubates them.

(iv) Commensalism: An interdependence of species where it is recognized that one species benefits and the other is neither benefited nor harmed. For example, an orchid grows like an epiphyte on a mango branch.

(v) Mutualism: An interaction where both species will benefit from each other. Examples are lichens (the relationship between algae and fungi), and mycorrhiza (the symbiotic relationship between fungi and high plant roots).

(vi) Amensalism: An interaction between an organism in which one species is harmed while another species is harmless or harmless.

NCERT Exemplar Solutions Subject-wise link:

• NCERT exemplar solutions for class 12 Physics
• NCERT exemplar solutions for class 12 Chemistry
• NCERT exemplar solutions for class 12 Mathematics
• NCERT exemplar solutions for class 12 Biology

Population Regulation

Population regulation prevents size and growth in the process of keeping the ecosystem stable.

Density Dependent Factors

These factors, for example, diseases, predation, and competition increase as the population density increases. For instance, diseases spread fast in dense populations, and competition may limit growth and reproduction.

Density Independent Factors

These factors, such as natural disasters and climate change, occur regardless of the population's density. Hurricanes or shifting of climate affected polar bears resulting in a sudden decline in their population.

Human Impact on Populations

Human factors affect populations through habitat destruction due to paving and pollution, leading to a loss of biodiversity. Significant conservation measures include protected areas, the use of renewable resources, and wildlife legislation.

Endangered Species

Loss of habitats, hunting, global warming, and pollution are the reasons which drive species to extinction. Conservation measures involve the creation of reserves, antic-poaching legislation, captive breeding, and habitat restoration.

Human Population Growth

Exponential growth in the human population, a result of the advance of industry and medicine, strains resources and infrastructure. Answers to this question include family planning measures, education on sustainability measures, and policies geared toward renewable energy to be implemented in the pursuit of a sustainable future.

Tips, Tricks, and Strategies for Preparing for Organisms and Populations

Here are some tips and tricks to prepare for the organisms and population chapter:

Mnemonics

Use "CUP" to recall types of population distributions: C for Clumped, U for Uniform, and P for Random.

Study Aids

Charts and Tables: Make tables to help you summarize types of population interactions and their effects on species.

Diagrams: Make diagrams of population growth curves and survivorship curves to aid in memory recall.

Multimedia

View videos or animations about ecological interactions and adaptations to grasp the ideas visually.

Flashcards

Use flashcards to memorize terms such as carrying capacity, biotic potential, and population dispersion.

Types of Questions and Weightage of Organisms and Populations in Exams

The table below indicates the weightage and types of questions asked on Organisms and Populations in exams:

Also Read:

Prescribed Books for Organisms and Population

The main and most recommended learning resource for organisms and Populations is the NCERT Biology class 12 textbook covering all concepts as per the CBSE syllabus. For further understanding, "Trueman's Elementary Biology, Volume 1" can be referred to. The other useful book is "Pradeep's Biology" which is also known for its detailed explanation and plenty of solved examples. You can access a variety of videos, articles and mock tests on this chapter from the careers360 platform. These books along with proper practice will make a great package for board exams as well as competitive exams like NEET.

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