The chapter Organisms and Populations discusses key ecological concepts like how organisms interact with their environment and how populations grow and change. It explains the role of abiotic factors such as temperature and water in survival, and that most organisms thrive between 0°C and 40°C.
It also introduces population dynamics, covering features like population size, density, birth rate, and death rate. Models like logistic growth (dN/dt = rN[(K–N)/K]) help explain how populations stabilise over time. Real-life examples, such as Gause's competition experiments and mutualism between clownfish and sea anemones, help illustrate ecological relationships clearly. 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
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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:
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 response to abiotic factors:
Regulate their internal conditions only to some limited extent but adapt when environmental conditions exceed their regulatory ability.
Spores are formed by certain microorganisms, like bacteria and fungi to survive hostile environmental conditions. Some animals go into dormancy during winter or aestivate when it gets very hot and dry.
Adaptations are characteristics that enhance the survival chances or ability of an organism to reproduce within its habitat. Some of the most significant are:
Opuntia and other desert plants have thick stems, spiny leaves, and sunken stomata to minimise water loss. These plants adopt CAM photosynthesis at arid temperatures to ensure survival.
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.
Some animals have developed certain habits based on particular environmental factors, such as migrating or hibernating.
The population has its specific features that make it different from the individual organism or the whole species. Among the Organisms and Population Attributes, the following can be described:
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.
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.
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.
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.
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 number of species that died over a period of time.
(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 their 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.
Population 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. It 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 organisms in which both organisms compete for a variety of resources including food, water, habitat, etc. 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.
Population regulation prevents size and growth in the process of keeping the ecosystem stable.
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.
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 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.
Loss of habitats, hunting, global warming, and pollution are the reasons which drive species to extinction. Conservation measures involve the creation of reserves, anti-poaching legislation, captive breeding, and habitat restoration.
Ecology | Parasitic Symbiosis |
Adaptations and Habitats | Difference between Mutualism and Commensalism |
Population Interaction | Symbiosis |
Any quality of an organism that allows it to live and reproduce in its own place is called adaptation. It is a fixed attribute and living things tend to adapt to their environment. Desert plants with a thick cuticle or submerged stomata to reduce water loss by changing the air are an example of adaptability. Organisms also show physical adaptations such as people exhibiting high altitude illnesses such as nausea, vomiting, heart palpitations, etc. and they become accustomed to the environment after some time.
Different types of biological responses include:
Population density changes mainly because of the following factors:
One of the great abiotic features is Temperature.
Temperature is one of the strongest forces affecting living and nonliving things on Earth. Next, water is also a necessary abiotic factor in living organisms and the environment. Many organisms are altered due to the availability of less or more water. Finally, light. Light is the most important abiotic factor in plant survival and the last aspect of abiotic soils that greatly affects biodiversity and plant growth.
The Morphological Defense Mechanism is one of the most important protective mechanisms in plants such as the cactus whose leaves have been transformed into sharp thorns to prevent weeds from eating them. Some plants, such as using a morphological defense mechanism, have the edges of their spiny leaves or sharp edges that prevent insects from eating them.
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