Solutions-Class-12-Biology-Chapter-13-Organisms and Population-Maharashtra Board

(a) Biotic factor

(a) Biotic community

(b) Commensalism

(d) One species is benefited, other is harmed.

(c) Verhulst-Pearl logistic growth

Commensalism refers to the interaction between two species where one species benefits while the other is neither harmed nor benefited.

The interaction between two species in which one parasitic species derives benefit from the other host species by harming it, is called parasitism.

Camouflage is the disguising colouration or behaviour to merge with the surrounding so that prey or predator can remain hidden.

Lichen, a mutualistic association between alga (cyanobacteria) and fungus, cannot survive independently. Alga synthesizes food through photosynthesis, while fungus absorbs moisture.

Leopard and lion competing for a same prey. Sheep and cow competing for grazing in the same land.

Commensalism

Brood parasiytism

Mutualism

• Pest insects act as prey to predator birds or frogs.
• The biological control method consists of releasing the predators in the farms so that they can control the pest population in the natural way.
• This method releases predators to naturally control pest populations, thereby saving agricultural produce.
• Frogs, being natural predators of locusts, also help control the pest population, thereby reducing the need for chemical pesticides.

Dormancy and Hibernation in Seeds :

• Seeds in dormancy lack metabolic activity and can only reappear with suitable moisture and sunlight.
• Hibernation is a suspended state with reduced metabolic reactions.
• Animal arousal after winter sleep is spontaneous and depends on ambient temperature.
• Dormant seed doesn't show change unless planted or placed in moist conditions.

• Marine fish have unique osmoregulation compared to freshwater fish.
• Marine water has higher salinity than ion concentration, unlike freshwater.
• Marine fish have unique mechanisms to cope with high saline environments.
• Therefore, marine fish cannot survive in freshwater due to their unique osmoregulation mechanisms.

Herbivores feed on plants and thus they are predators for the vegetations.

Plants therefore have evolved variety of morphological and chemical defences against herbivores, which are as follows :

• Cactus and acacia horns serve as morphological defenses against herbivores.
• Many plants produce toxic and unpalatable chemicals that can affect the feeding herbivore.
• Consumption of these chemicals can inhibit predator feeding, digestion, reproduction, or even kill it.
• Calotropis plants produce toxic cardiac glycosides that can harm grazers.
  Secondary metabolites like nicotine, caffeine, quinine, strychnine, and opium act as defenses against grazers and browsers.

• Orchid is an epiphyte. It gets the support from the mango tree. But it does not cause any harm to the mango tree.
• Mango tree does not derive any benefit from this association. Therefore, this interaction is of type of commensalism.

• Desert animals are xerocoles, adapted for water conservation or heat tolerance.
• They have low basal metabolic rates and rely on moisture from plants, rarely drinking water.
• Desert animals like camel produce concentrated urine and dry dung.
• Many hot desert animals are nocturnal, seeking shade or dwelling underground.
• Smaller animals emerge from burrows at night.
• In cold deserts, animals have warmer fur and insulating layers of fat.
• Few desert reptiles can cool themselves by sweating, so they shelter during the day.
• Some animals have bodies designed to save water, such as scorpions and wolf spiders with thick outer coverings.
• Desert animals' kidneys concentrate urine, reducing water excretion.

• Desert plants have thick cuticles on their leaf surfaces and sunken stomata to minimize water loss through transpiration.
• They have a special photosynthetic pathway called CAM-Crassulacean acid metabolism, allowing stomata to remain closed during the day.
• Some plants, like Opuntia, have reduced leaves or spines to prevent transpiration.
• The photosynthetic function is taken over by flattened stems called phylloclade.

• Desert lizards maintain body temperature through basking in the sun and moving into shade when temperature drops.
• Snakes also bask during winter months due to their ectothermic nature.
• Smaller animals use burrowing to adapt to temperature extremes.
• Some species burrow into sand to hide from heat.
• Birds and mammals migrate to cope with severe winter temperatures.

• Population : A population is a group of organisms from the same species that can interbreed and live together in a specific geographical area, sharing or competing for resources.
• Community : Several populations of different species in a particular area makes a community.

• Populations of any species have limited resources, leading to competition for resources. The most fit organisms succeed through survival and reproduction.
• A habitat has enough resources to support a maximum number, but beyond a certain limit, the species' carrying capacity (K) is reached.
• Population growth in limited resources shows phases: lag phase, acceleration phase, deceleration phase, and an asymptote when the density reaches the carrying capacity.
• Verhulst-Pearl Logistic Growth, a plot of N relative to time (t), is considered a more realistic model due to finite resources for animal populations.
• Logistic growth always shows a sigmoid curve.

Important characteristics of a population are as follows :

Natality, Mortality, Density, Sex ratio, Age distribution and age Pyramids depicting the same and Growth.

Natality :

• Natality refers to the birth rate of a population, which can increase population density.
• It can be crude birth rate, calculated by the number of births per 1000 population/year.
• Specific birth rate is relative to a specific criterion like age.
• Absolute natality is the number of births under ideal conditions with abundant resources.
• Realized natality is the number of births under different environmental pressures, always greater than absolute natality.

Mortality :

• Mortality is the death rate of a population, measured in deaths per 1,000 individuals per year.
• A mortality rate of 9.5 in a population of 1,000 means 9.5 deaths per year or 0.95% of the total.
• Absolute Mortality: The number of deaths under ideal conditions with abundant resources.
• Realized Mortality: The number of deaths under environmental pressures.
• Absolute mortality always outweighs realized mortality.

Density :

The density of a population in a given habitat during a given period fluctuates due to changes in four basic processes, viz.

• Natality i.e. Birth rate: Adds to initial population density.
• Mortality: Decreases population density.
• Immigration: Number of same species entering the habitat.
• Emigration: Number of individuals leaving the habitat.

Natality and immigration increase population density, mortality and emigration decrease it.

Sex ratio : The sex ratio is the ratio of male to female individuals in a population, typically 1:1, and is considered the evolutionary stable strategy of ESS for each population, ensuring a balanced distribution.

Age distribution and age pyramid :

• The age distribution and age pyramid are crucial for understanding human populations.
• Each population consists of individuals of different ages, and the age distribution is plotted to create an age pyramid.
• The pyramid divides the population into three age groups: Pre-reproductive (ages 0-14), reproductive (15-44), and post-reproductive (ages 45-85+).

Growth:

Population Growth and Density Analysis :

• Population growth leads to increased density.
• Size and density are dynamic parameters.
• Factors include food, predation pressure, and weather.
• Density indicates population flourishing or decline.