NEET Biology Class 12 Organisms And Populations Notes

Organisms And Populations

Ecology is the study of interactions among organisms and between the organism and its physical (abiotic) environment. Ecology is concerned with 4 levels of biological organization: Organisms, Populations, Communities, and biomes.

Organism And Its Environment

Physiological ecology:

(Ecology at the organismic level) is the study of the adaptation of an organism to environments in terms of survival and reproduction.

• The rotation of the earth and the tilt of its axis cause annual variations in temperature and seasons. Major biomes (desert, rainforest, tundra, etc.) are formed due to these variations and precipitation (rain and snow).

Biology Class 12 Notes For Neet

Read And Learn More: NEET Biology Class 12 Notes

• Regional and local variations within a biome lead to the formation of different habitats.
• Life exists even in extreme and harsh habitats.
• For example, the Rajasthan desert, rain-soaked Meghalaya forests, deep ocean trenches, torrential streams, Polar Regions, high mountain tops, thermal springs, and compost pits.
• Our intestine is a habitat for many microbes.
• The physicochemical (abiotic) components (water, light, temperature, soil, etc.) and biotic components (pathogens, parasites, predators, competitors, etc.) lead to variation in different habitats.

“class 12 organisms and population notes “

Abiotic Factors

Biology Class 12 Notes For Neet

1. Temperature:

• The most ecologically relevant environmental factor.
• The temperature on land varies seasonally. It gradually decreases from the equator towards the poles and from plains to mountain tops. It ranges from subzero levels (in polar areas and high altitudes) to >500C (in tropical deserts).
• The average temperature in thermal springs and deep-sea hydrothermal vents is above 1000 C.
• Mango trees cannot grow in temperate countries (Canada, Germany, etc.). There is no Snow leopard in Kerala forests.
• Tuna fishes are rare beyond tropical latitudes in the ocean.
• Temperature affects the kinetics of enzymes, basal metabolism, and other physiological functions of the organism.

Based on a range of thermal tolerance, organisms are of 2 types:

1. Eurythermal: They can tolerate a wide range of temperatures.
2. Stenothermal: They can tolerate only a narrow range of temperatures

2. Water:

• It is the second most important factor.
• Desert organisms have special adaptations to limited water.
• The productivity & distribution of plants is dependent on water.
• For aquatic organisms, water quality (pH, chemical composition) is important. The salt concentration (salinity in parts per thousand) is less than 5 in inland waters, 30-35 in the sea, and > 100 in some hypersaline lagoons.

“organism and population neet pyq “

Based on the tolerance to salinity, organisms are 2 types:

1. Euryhaline: Tolerates a wide range of salinities.
2. Stenohaline: Tolerates only a narrow range of salinity. Many freshwater animals cannot live for long in seawater and vice versa because of osmotic problems.

Biology Class 12 Notes For Neet

3. Light:

• Plants need sunlight for photosynthesis.
• Small forest plants (herbs and shrubs) are adapted to photosynthesize optimally under very low light because they are overshadowed by tall, canopied trees.
• Many plants depend on sunlight for photoperiodism (For example, flowering). Many animals use the diurnal and seasonal variations in light intensity and photoperiod for timing their foraging, reproductive, and migratory activities. The sun is the ultimate source of light and temperature on land.
• Deep (>500m) in the oceans, the environment is dark and there is no energy available from the Sun. The spectral quality of solar radiation is also important for life. The UV spectrum is harmful to many organisms.
• Not all the color components of the visible spectrum are available for marine plants.

4. Soil:

• Nature & properties of soil differ due to climate, weathering process, sedimentation, method of soil development, etc.
• Soil composition, grain size, and aggregation determine the percolation and water-holding capacity of the soils. These characteristics and parameters like pH, mineral composition, and topography determine the vegetation and animals in an area. In the aquatic environment, the sediment s determine the type of benthic animals.

Responses to Abiotic Factors

Organisms maintain a constant internal environment (homeostasis) despite varying external environmental conditions. This is possible by following processes.

1. Regulate:

It is the maintenance of homeostasis by physiological and behavioral means. It ensures constant body temperature (thermoregulation), constant osmotic concentration (osmoregulation), etc. For example, All birds and mammals, have very few lower vertebrates and invertebrates.

Thermoregulation in mammals:

The success of mammals is mainly due to their ability to maintain a constant body temperature. In summer, when the outside temperature is higher than the body temperature (370C), sweating occurs.

This results in evaporative cooling and brings down body temperature. In winter, when the temperature is below 370C, shivering occurs. It produces heat and raises the body temperature.

2. Conform:

• 99% of animals and nearly all plants cannot maintain a constant internal environment. Their body temperature or osmotic concentration changes with the surrounding conditions. They are called conformers.
• In aquatic animals, the osmotic concentration of body fluids changes with that of the ambient osmotic concentration.
• Thermoregulation is energetically expensive, especially for small animals (shrews, hummingbirds, etc.). They have a larger surface area relative to their volume. So they lose body heat very fast when it is cold outside.
• Then they have to expend much energy to generate body heat. Therefore very small animals are rare in polar regions.

“organisms and population class 12 ppt “

3. Migrate:

• Many animals like birds move away temporarily from stressful habitats to a more hospitable area and return when the stressful period is over.
• For example, During winter, Keolado National Park (Bharatpur,
• Rajasthan) hosts migratory birds coming from Siberia and other extremely cold northern regions.

4. Suspend:

• In bacteria, fungi, and lower plants, thick-walled spores help to survive unfavorable conditions. Under suitable conditions, they germinate.
• In higher plants, seeds and some vegetative reproductive structures serve to tide over periods of stress by reducing their metabolic activity. They germinate under favorable moisture and temperature.

In animals: Examples are

• Hibernation of bears during winter.
• Aestivation of some snails and fishes during summer.
• Diapause (a stage of suspended development) of many zooplankton in lakes & ponds.

Adaptations

Biology Class 12 Notes For Neet

• Adaptation is the morphological, physiological & behavioral attribute that enables an organism to survive and reproduce in its habitat.
• Many adaptations have evolved over a long evolutionary time and are genetically fixed.

Adaptations of kangaroo rat in North American deserts:

• Internal fat oxidation gives water as a byproduct if there is no external source of water.
• Ability to concentrate urine so that a minimal volume of water is used to remove excretory products.

Adaptations of desert plants:

• Presence of thick cuticles on leaf surfaces. Their stomata are arranged in deep pits to minimize water loss through transpiration.
• A special photosynthetic pathway (CAM) that enables their stomata to remain closed during day time.
• Desert plants like Opuntia have no leaves (they are reduced to spines). Photosynthesis is done by stems.

“organisms and population class 12 one shot “

Adaptations of mammals:

• Mammals from colder climates have shorter ears and limbs to reduce heat loss. (This is called Allen’s Rule).
• Aquatic mammals like seals have a thick layer of fat (blubber) below their skin that acts as an insulator and reduces loss of body heat.

Physiological and biochemical adaptations:

• Archaebacteria are found in hot springs and deep-sea hydrothermal vents where the temperature is >1000C. Many fish thrive in Antarctic waters (temperature is below 00C).
• Many marine invertebrates and fishes live at great depths in the ocean where the pressure is >100 times the normal atmospheric pressure.
• At a high-altitude place (>3,500 m) we feel altitude sickness. Its symptoms are nausea, heart palpitations, and fatigue. This is due to low atmospheric pressure. So the body does not get enough O₂.
• Gradually, we acclimatize to the situation and the body compensates for low O₂ availability by increasing RBC and breathing rate and decreasing the binding capacity of hemoglobin.

Behavioural adaptations:

• Desert lizards bask in the sun and absorb heat when their body temperature is low, but move into shade when the ambient temperature starts increasing. Some species burrow into the soil to hide and escape from the above-ground heat

Populations

• A population is a group of individuals of the same species that live in a given geographical area, share or compete for similar resources, and potentially reproduce.
• For example, All the cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate lotus plants in a pond, etc.
• Population ecology is an important area of ecology as it links ecology to population genetics & evolution.

Population Attributes

1. Birth rates: Refer to per capita births. For example, In a pond, there were 20 lotus plants last year, and through reproduction 8 new plants were added.

• Hence, the current population = 28
• The birth rate = 8/20 = 0.4 offspring per lotus per year.

2. Death rates: Refer to per capita deaths.

For example,4 individuals in a laboratory population of 40 fruit flies died during a week. Hence, the death rate = 4/40 = 0.1 individuals per fruit fly per week.

3. Sex ratio: A population has a sex ratio. For example,60% of the population is females and 40% males.

Class 12 Biology Notes For Neet

4. Age pyramid:

• It is the structure obtained when the age distribution (% of individuals of a given age or age group) is plotted for the population.
• For the human population, age pyramids generally show the age distribution of males and females in a combined diagram.

“organism and population class 12 ppt “

Population size or population density (N):

• It is the number of individuals of a species per unit area or volume. For example, the population density of Siberian cranes at Bharatpur wetlands in any year is <10. It is millions for Chlamydomonas in a pond.
• Population size is also measured in % cover or biomass. For example, In an area, 200 Parthenium plants and a single huge banyan tree are seen. In such cases, measuring % cover or biomass is meaningful to show the importance of the banyan tree.
• The total number is a difficult measure for a huge population.
• In such cases, relative population density (without knowing absolute population density) is used. For example, the number of fish caught per trap indicates the total population density in the lake.
• In some cases, indirect estimation of population sizes is performed. For example, Tiger census in national parks & tiger reserves based on pug marks & fecal pellets.

Class 12 Biology Notes For Neet

Population Growth

• The population size changes depending on factors like food availability, predation pressure & weather.
• Changes in population density give some idea about the population – whether it is flourishing or declining.

4 basic processes that fluctuate the population density:

1. Natality (B): It is the number of births in a population during a given period.
2. Mortality (D): It is the number of deaths in a population during a given period.
3. Immigration (I): It is the number of individuals of the same species that have come into the habitat from elsewhere during a given period.
4. Emigration (E): It is the number of individuals in the population who left the habitat and went elsewhere during a given period.

Natality and immigration increase the population density and mortality and emigration decreases the population density

If N is the population density at time t, then its density at time t +1 is

N_t+1 = N_t+ [(B + I) – (D + E)]

• Population density increases if B+I is more than D+E.
• Otherwise, it will decrease.

Under normal conditions, births & deaths are important factors influencing population density. The other 2 factors have importance only under special conditions. For example, for a new colonizing habitat, immigration may be more significant to population growth than birth rates.

Growth Models

Class 12 Biology Notes For Neet

1. Exponential growth:

• Resources (food and space) are essential for unimpeded population growth.
• If resources are unlimited, each species shows its full innate potential to grow in number. Then the population grows exponentially or geometrically.
• If population size = N, birth rates (per capita births) = b, and death rates (per capita deaths) = d, then the increase or decrease in N during a unit period t (dN/dt) will be

dN/dt = (b – d) × N

Let (b–d) = r, then

dN/dt = rN

The r (‘intrinsic rate of natural increase) is an important parameter for assessing the impacts of any biotic or abiotic factor on population growth.

r value for the Norway rat = 0.015

r value for the flour beetle = 0.12

r value for human population in India (1981) = 0.0205

The integral form of the exponential growth equation is

N = N₀e_rt

“organism and population /class 12 ppt “

Where,

N_t = Population density after time t

N₀ = Population density at time zero

r = Intrinsic rate of natural increase

e = The base of natural logarithms (2.71828)

a = Exponential growth (J-shaped curve)

b = Logistic growth (Sigmoid curve)

Class 12 Biology Notes For Neet

2. Logistic growth:

• No population in nature has unlimited resources for exponential growth. This leads to competition among individuals for limited resources.
• Eventually, the ‘fittest’ individuals survive and reproduce.
• In nature, a given habitat has enough resources to support the maximum possible number, beyond which no further growth is possible. It is called carrying capacity (K).
• A population with limited resources shows initially a lag phase, phases of acceleration & deceleration, and finally an asymptote. This type of population growth is called Verhulst-Pearl Logistic Growth.

It is described by the following equation:

⇒ \(\mathrm{dN} / \mathrm{dt}=\mathrm{rN}\left(\frac{\mathrm{K}-\mathrm{N}}{\mathrm{K}}\right)\)

Where,

N = Population density at time t

r = Intrinsic rate of natural increase

K = Carrying capacity

Since resources for growth for most animal populations are finite the logistic growth model is a more realistic one.

Life History Variation

• Populations evolve to maximize their reproductive fitness or Darwinian fitness (high r-value). Under a particular set of selection pressures, organisms evolve toward the most efficient reproductive strategy.
• Some organisms breed only once in their lifetime (Pacific salmon fish, bamboo) while others breed many times (most birds and mammals).
• Some produce a large number of small-sized offspring (Oysters, pelagic fishes) while others produce a small number of large-sized offspring (birds, mammals).
• These facts indicate that life history traits of organisms have evolved due to limited abiotic and biotic components of the habitat.

Population Interactions

Class 12 Biology Notes For Neet

• Organisms interact in various ways to form a biological community.
• Interaction between two species is called Interspecific interactions. They include

Name of the Interaction And Species:

• In predation, parasitism, and commensalisms, the interacting species live closely together.
• In a broad ecological context, all carnivores, herbivores, etc. are predators. About 25 % of insects are phytophagous.
• If a predator overexploits its prey, then the prey might become extinct. It results in the extinction of predators.
• Therefore, predators in nature are ‘prudent’.

Importance of predators:

Predators control prey populations:

• When certain exotic species are introduced into a geographical area, they spread fast due to the absence its natural predators in the invaded land.
• For example, the prickly pear cactus introduced into Australia in the early 1920s caused havoc by spreading.
• Finally, the invasive cactus was brought under control only after a cactus-feeding predator (a moth) was introduced into the country.

Biological control:

• Biological control methods are based on the ability of the predator to regulate the prey population.
• Predators maintain species diversity in a community, by reducing the intensity of competition among competing prey species.
• For example, the starfish Pisaster is a predator in the rocky intertidal communities of the American Pacific
• Coast. In an experiment, when all the starfishes were removed from an enclosed intertidal area, more than 10 species of invertebrates became extinct within a year, due to interspecific competition.

Defenses of prey species to lessen the impact of predation:

• Camouflage (cryptic coloration) of some insects and frogs.
• Some are poisonous and so avoided by predators.
• The monarch butterfly is highly distasteful to its predator bird. It is due to a special chemical in its body. It is acquired during its caterpillar stage by feeding on a poisonous weed.
• Thorns (Acacia, Cactus, etc.) are the most common morphological means of defense of plants. Many plants produce chemicals that make the herbivore sick, inhibit feeding or digestion, disrupt its reproduction, or kill it.
• For example, Calotropis produces highly poisonous cardiac glycosides. Therefore cattle or goats do not eat it.
• Nicotine, caffeine, quinine, strychnine, opium, etc. are defenses against grazers and browsers.

2. Competition:

• It is a process in which the fitness of one species (‘r’ value) is significantly lower in the presence of another species. Interspecific competition is a potent force in organic evolution.
• Competition occurs when closely related species compete for the same limited resources.
• Unrelated species can also compete for resources.
•  For example, Flamingoes and fishes in some shallow South American lakes compete for zooplankton.
• Competition occurs in abundant resources also. For example, In interference competition, the feeding efficiency of one species is reduced due to the interfering and inhibitory presence of other species, even if resources are abundant

Class 12 Biology Notes For Neet

Evidence for competition:

• The Abingdon tortoise in the Galapagos Islands became extinct within a decade after goats were introduced on the island, due to the greater browsing efficiency of the goats.
• ‘Competitive release’: A species, restricted to a small geographical area (due to the presence of competitively superior species), expands its distributional range when the competing species is experimentally removed.
• Connell’s field experiments: Connell’s field showed that on the rocky sea coasts of Scotland, the larger & competitively superior barnacle Balanus dominates the intertidal area, and excludes the smaller barnacle Chathamalus from that zone.

Gause’s ‘Competitive Exclusion Principle:

• It states that two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will be eliminated eventually.
• This may be true in limited resources, but not otherwise.
• Species facing competition may evolve mechanisms that promote co-existence rather than exclusion. Example, ‘resource partitioning’.

Resource partitioning:

• If two species compete for the same resource, they could avoid competition by choosing different times for feeding or different foraging patterns.
• For example, MacArthur showed that five closely related species of warblers living on the same tree were able to avoid competition and co-exist due to behavioral differences in their foraging activities.

3. Parasitism:

Many parasites are host-specific (they can parasitize only a single host species).  They tend to co-evolve. i.e., if the host evolves special mechanisms against the parasite, the parasite has to evolve mechanisms to counteract them, to be successful with the same host species.

Adaptations of parasites: Loss of sense organs, presence of adhesive organs or suckers to cling on to the host, loss of the digestive system, high reproductive capacity, etc.

• The life cycles of parasites are often complex. Example,
• Human liver fluke depends on 2 intermediate hosts (a snail and a fish) to complete its life cycle.
• The malarial parasite needs mosquitoes to spread to other hosts.
• Parasites harm the host. They may reduce the survival, population density, growth, and reproduction of the host.
• They may make the host physically weak and more vulnerable to predation.

Types of parasites:

1. Ectoparasites:

• Parasites that feed on the external surface of the host. Example,
  • Lice on humans.
  • Ticks on dogs.
  • Ectoparasitic Copepods on many marine fishes.
  • Cuscuta plant on hedge plants.
• Cuscuta has no chlorophyll and leaves. It derives its nutrition from the host plant.
• Female mosquito is not considered a parasite, because it needs our blood only for reproduction, not as food.

2. Endoparasites:

• Parasites that live inside the host body at different sites (liver, kidney, lungs, RBC, etc).
• The life cycles of endoparasites are more complex.
• They have simple morphological and anatomical features and high reproductive potential.

Brood parasitism in birds:

• Here, the parasitic birds lay eggs in the nest of their host and let the host incubate them.
• During evolution, the eggs of the parasitic bird have evolved to resemble the host’s egg in size and color to reduce the chances of the host bird detecting the foreign eggs and ejecting them from the nest.
• For example, Brood parasitism between cuckoo and crow.

4. Commensalism:

Examples:

• Orchid (+) growing as an epiphyte on a mango branch (0).
• Barnacles (+) growing on the back of a whale (0).
• Cattle egret (+) and grazing cattle (0). The egrets forage close to where the cattle are grazing. As the cattle move, the vegetation insects come out. Otherwise, it is difficult for the egrets to find and catch the insects.
• Sea anemone (0) and clown fish (+). Stinging tentacles of sea anemones give protection to fish from predators.

5. Mutualism:

Examples:

• Lichen: It is a mutualistic relationship between a fungus and photosynthesizing algae or cyanobacteria.
• Mycorrhizae: Associations between fungi and the root of higher plants. The fungi help the plant in the absorption of essential nutrients from the soil while the plant provides the fungi with carbohydrates.
• Mutualism between plants and animals through pollination and seed dispersion: Examples:
  • Fig trees and wasps: The fig species is pollinated only by its ‘partner’ wasp species and no other species. The female wasp pollinates the fig inflorescence while searching for suitable egg-laying sites in fruits. The fig offers the wasp some developing seeds, as food for the wasp larvae.
  • Orchids show a diversity of floral patterns: They can attract the right pollinator insects (bees and bumblebees) to ensure pollination. Not all orchids offer rewards.
  • ‘Sexual deceit’ of Ophrys (Mediterranean orchid): One petal of its flower resembles a female bee in size, color and markings. So male bees ‘pseudocopulates’ with the flower and is dusted with pollen. When this bee ‘pseudocopulation’ with another flower, it transfers pollen to it.

If the female bee’s color patterns change slightly during evolution, pollination success will be reduced unless the orchid flower co-evolves to maintain the resemblance of its petal to the female bee