Organisms and Populations Class 12 Biology Chapter 11 Notes

Organisms and Populations Class 12 Biology Chapter 11 Notes

Population Attributes:

• In ecology, populations are groups of individuals of the same species that share a defined geographical area, compete for resources, and may interbreed. This concept includes groups formed through both sexual and asexual reproduction. Examples include cormorants in a wetland, rats in an abandoned dwelling, and lotus plants in a pond.
• Population ecology is crucial because it connects ecological studies to population genetics and evolution. Natural selection primarily acts at the population level to shape desirable traits.
• Populations have distinct attributes that individual organisms lack. Instead of individual births and deaths, populations have birth rates and death rates, calculated per capita. For example, if a pond had 20 lotus plants last year and added 8 new plants this year, the birth rate would be 0.4 offspring per lotus per year.
• Sex ratio is another population attribute. While individuals are either male or female, populations have a distribution of these sexes, such as 60% females and 40% males.
• Populations are composed of individuals of different ages, which can be depicted in an age pyramid. The shape of the pyramid reflects the population’s growth status: growing, stable, or declining.
• Population size (or density) is a vital parameter. Ecological processes, such as competition, predation, or pesticide effects, are evaluated based on changes in population size. Population size can vary greatly, from under 10 individuals to millions.
• Measuring population size can be challenging. While total numbers are often appropriate, in some cases, other metrics like per cent cover or biomass are more meaningful. Sometimes, relative densities are sufficient for ecological investigations, especially when absolute counts are impractical.
• Estimating population sizes is often done indirectly, using methods like tracking pug marks and fecal pellets for tiger census in national parks and reserves.

Population Growth:

The size of a population is dynamic and changes over time due to various factors such as food availability, predation, and weather conditions. These changes in population density provide insights into the population’s well-being. Population density fluctuates because of four basic processes:

1. Natality: This refers to the number of births within the population during a specific period, contributing to an increase in population density.
2. Mortality: Mortality represents the number of deaths within the population during a given period.
3. Immigration: Immigration involves the arrival of individuals of the same species from other areas into the habitat during the observed time frame, leading to an increase in population density.
4. Emigration: Emigration occurs when individuals from the population leave the habitat and move elsewhere during the observed period, resulting in a decrease in population density.

The change in population density from time t (Nt) to time t + 1 (Nt+1) is calculated using the formula:

Nt+1 = Nt + [(B + I) – (D + E)]

In this equation, population density increases if the sum of births and immigration (B + I) exceeds the sum of deaths and emigration (D + E). Typically, births and deaths are the primary factors influencing population density, while immigration and emigration become significant under special circumstances. For instance, when a new habitat is being colonized, immigration may play a more critical role in population growth than birth rates.

Growth Models:

The growth of a population over time follows specific and predictable patterns, which can be broadly categorized into two main models: exponential growth and logistic growth.

1. Exponential Growth:
   • Exponential growth occurs when a population has unlimited access to resources such as food and space.
   • Under these ideal conditions, the population realizes its full potential for growth.
   • The population grows at a constant rate, resulting in a J-shaped curve when plotted over time.
   • The formula for exponential growth is: dN/dt = rN, where r is the intrinsic rate of natural increase.
   • Exponential growth can lead to rapid increases in population size. For instance, even a slow-growing species can reach enormous numbers quickly if resources are unlimited.
2. Logistic Growth:
   • In nature, no population has unlimited resources, leading to competition for limited resources among individuals.
   • Eventually, the fittest individuals survive and reproduce, but the population’s growth is constrained by available resources.
   • Each habitat has a maximum carrying capacity (K), which represents the maximum population size that can be supported by available resources.
   • Logistic growth involves a lag phase, followed by phases of acceleration and deceleration, and eventually, the population density stabilizes at the carrying capacity (K).
   • When plotted over time, logistic growth forms a sigmoid or S-shaped curve.
   • The formula for logistic growth is: dN/dt = rN (K – N) / K, where r is the intrinsic rate of natural increase, N is the population density at time t, and K is the carrying capacity.
   • Logistic growth is considered a more realistic model because it accounts for finite and limiting resources.

In the context of human population growth, government census data for India over the last 100 years can be analyzed to determine which growth pattern is evident. This analysis helps in understanding whether India’s population growth has followed an exponential or logistic pattern, considering the availability of resources and constraints on growth.

Life History Variation:

Populations evolve to maximize their reproductive fitness, which is also known as Darwinian fitness, often measured by a high r-value, in their specific habitats. Organisms develop distinct reproductive strategies influenced by selection pressures. These strategies can include:

1. Reproductive Frequency: Some organisms, like Pacific salmon and bamboo, breed only once in their lifetime, while others, such as most birds and mammals, breed multiple times during their lifetime.
2. Offspring Quantity and Size: Species also vary in the number and size of offspring they produce. Some species produce a large number of small-sized offspring, as seen in oysters and pelagic fishes, while others produce a small number of large-sized offspring, such as birds and mammals.

The choice of reproductive strategy is influenced by the abiotic (non-living) and biotic (living) factors in their habitat. Organisms adapt their life history traits to optimize their chances of survival and reproduction in their specific environment. This adaptation to different life history traits in various species is a significant area of research for ecologists, helping to uncover the complex interplay between biology and the environment.

Population Interactions:

Natural habitats on Earth are not inhabited by single species; instead, various species interact with one another to form biological communities. Even plants, which produce their own food, rely on soil microbes for nutrient cycling and animal agents for pollination. These interactions, known as interspecific interactions, can be beneficial, detrimental, or neutral to the involved species. Here are the possible outcomes of interspecific interactions:

1. Predation:

Predation is a fundamental ecological interaction where one organism (the predator) feeds on another organism (the prey). It plays a crucial role in shaping ecosystems and has several ecological implications:

1. Energy Transfer: Predation serves as nature’s way of transferring energy fixed by autotrophic organisms (plants) to higher trophic levels. Herbivores, which consume plants, are also considered predators in an ecological context.
2. Population Control: Predators help control prey populations. Without predators, prey species can experience explosive population growth, leading to ecosystem instability and imbalances.
3. Biodiversity Maintenance: Predators contribute to maintaining species diversity in a community. By reducing the population of competing prey species, predators prevent a single species from dominating an ecosystem.
4. Biological Control: In certain situations, predators can be used as a biological control method to manage pest populations. For example, introducing a predator from the pest’s natural habitat can help control invasive species.
5. Coexistence Strategies: Prey species have evolved various strategies to reduce the impact of predation. These strategies include cryptic coloration (camouflage), poison production, and the development of unpalatable or toxic defenses.
6. Plant-Herbivore Interactions: For plants, herbivores act as predators. Many plants have evolved morphological and chemical defenses against herbivores, such as thorns, toxic chemicals, and deterrent compounds. These defenses help protect plants from being consumed.

Overall, predation is a critical ecological process that influences the dynamics of populations and communities and contributes to the balance and diversity of ecosystems.

2. Competition:

Competition is a critical ecological interaction where organisms compete for limited resources, and it plays a significant role in shaping ecosystems and influencing evolutionary processes. Here are key points about competition:

1. Nature of Competition: Competition occurs when organisms, whether closely related or unrelated, vie for the same resources, including food, space, and other essential factors. It’s not solely limited to closely related species.
2. Resource Limitation: While competition often arises when resources are limited, it can also occur in scenarios where resources are abundant. In interference competition, one species can hinder the feeding efficiency of another species, even if resources are not scarce.
3. Definition of Competition: Competition is defined as a process where the fitness of one species, measured by its intrinsic rate of increase (r), is significantly reduced in the presence of another species. This means that the presence of a competitor negatively affects the reproductive success and survival of one or both species involved.
4. Laboratory vs. Natural Evidence: Laboratory experiments have demonstrated that when resources are limited, the competitively superior species may eventually exclude the other. However, evidence for such competitive exclusion in nature is not always conclusive. Some examples, like the extinction of the Abingdon tortoise in the Galapagos Islands after the introduction of goats, provide persuasive evidence of competition.
5. Competitive Release: Competitive release occurs when a species with a restricted distribution expands its range dramatically when a competitively superior species is removed. This phenomenon supports the occurrence of competition in nature.
6. Resource Partitioning: Some species facing competition may evolve mechanisms to promote co-existence rather than exclusion. Resource partitioning is one such mechanism where species choose different strategies, such as feeding at different times or in different patterns, to avoid direct competition. Behavioral differences can facilitate the co-existence of closely related species, as observed in warblers foraging in the same tree.
7. Competitive Exclusion Principle: Gause’s Competitive Exclusion Principle states that two closely related species competing for the same resources cannot co-exist indefinitely, and the competitively inferior one will eventually be eliminated. However, more recent studies suggest that competition in nature can be complex, and co-existence mechanisms may evolve to prevent complete exclusion.

In summary, competition is a multifaceted ecological process that can influence the distribution, abundance, and behavior of species in ecosystems, with various factors and mechanisms at play in different situations.

3. Parasitism:

Parasitism is a common ecological interaction in which one organism (the parasite) benefits at the expense of another organism (the host). Parasitism has evolved across various taxonomic groups, ranging from plants to higher vertebrates. Here are some key points about parasitism:

1. Host-Specific Parasites: Many parasites have evolved to be host-specific, often co-evolving with their hosts. If hosts develop mechanisms to resist the parasite, the parasite may evolve countermeasures to overcome host defenses. This co-evolution can lead to complex interactions.
2. Parasitic Adaptations: Parasites have evolved specific adaptations for their parasitic lifestyle, such as the loss of unnecessary sense organs, adhesive organs or suckers for attachment to hosts, and, in some cases, the loss of a digestive system. Parasites often have high reproductive capacity.
3. Complex Life Cycles: Many parasites have complex life cycles involving one or more intermediate hosts or vectors to facilitate parasitization of their primary host. For example, the human liver fluke requires two intermediate hosts to complete its life cycle.
4. Harm to Hosts: Most parasites harm their hosts. They can reduce the host’s survival, growth, and reproduction, as well as increase vulnerability to predation by weakening the host. Parasitism generally has negative effects on host populations.
5. Ectoparasites: Ectoparasites live on the external surface of the host and include organisms like lice, ticks, and some parasitic plants like Cuscuta. These parasites derive their nutrients from the host’s external tissues.
6. Endoparasites: Endoparasites live inside the host’s body at various sites, such as the liver, kidney, or red blood cells. They often have simplified anatomical features, emphasizing their reproductive potential.
7. Brood Parasitism: Brood parasitism is observed in birds where a parasitic bird lays its eggs in the nest of another host bird, which then incubates the foreign eggs. The parasitic bird’s eggs often resemble those of the host to avoid detection and ejection.
8. Mosquitoes and Blood Feeding: The female mosquito, despite needing blood for reproduction, is not considered a parasite. This is because mosquitoes do not permanently reside within their hosts but only feed on them temporarily for a blood meal.

While the idea of an ideal parasite that thrives within a host without harming it may seem appealing, natural selection has favored parasites that extract resources from their hosts, often causing harm. This harm can be a result of competition for resources or the physiological and immunological effects of parasitism. Parasitism is a complex ecological phenomenon that has shaped the co-evolution of many species in ecosystems.

4. Commensalism:

Commensalism is an ecological interaction in which one species benefits, while the other species neither benefits nor is harmed. This interaction results in an asymmetrical relationship where one organism gains an advantage without any apparent impact on the other. Here are some key points about commensalism:

1. Benefit to One, No Effect on the Other: In commensalism, one organism benefits from the relationship, typically by gaining resources or support, while the other organism is unaffected, neither harmed nor benefited.
2. Examples of Commensalism: Examples of commensalism include:
   • Epiphytic Orchids: Orchids growing as epiphytes on tree branches benefit by obtaining support and access to light, while the host tree is generally not affected.
   • Barnacles on Whales: Barnacles attaching to the skin of whales benefit by being transported to different feeding areas, while the whale does not seem to benefit or suffer from their presence.
   • Cattle Egret and Grazing Cattle: Cattle egrets foraging near grazing cattle benefit by catching insects stirred up by the cattle’s movement. The cattle are not influenced by the presence of the egrets.
   • Sea Anemone and Clownfish: Clownfish find protection from predators by living among the stinging tentacles of sea anemones. The anemone does not appear to gain any apparent benefit from hosting the clownfish.
3. Asymmetrical Relationship: Commensalism results in an unequal relationship where one organism gains an advantage, often related to access to resources or protection, while the other organism remains unaffected.

Commensalism is one of several types of ecological interactions that shape the dynamics of ecosystems, and it demonstrates the variety of ways species can interact with one another in nature.

5. Mutualism:

Mutualism is an ecological interaction in which both interacting species benefit from the relationship. It is characterized by cooperation and mutual assistance between organisms of different species. Here are some key points about mutualism:

1. Benefits to Both: In mutualistic relationships, both species involved receive advantages or rewards that enhance their fitness or survival.
2. Examples of Mutualism:
   • Lichens: Lichens are a classic example of mutualism, consisting of a partnership between a fungus and photosynthetic algae or cyanobacteria. The fungus provides protection and a suitable environment, while the photosynthetic partner produces food through photosynthesis.
   • Mycorrhizae: Mycorrhizal associations involve fungi and plant roots. The fungi assist plants in nutrient absorption from the soil, while plants supply the fungi with energy-rich carbohydrates.
   • Plant-Animal Mutualism: Many plants rely on animals for pollination and seed dispersal. Plants offer rewards such as nectar, pollen, or nutritious fruits to attract pollinators and seed dispersers. In return, these animals aid in the reproduction and dispersal of plant offspring. Co-evolution often occurs in these relationships, where both the plant and animal evolve traits that enhance their mutual benefits.
     • Example: Fig trees have a one-to-one relationship with specific wasp species for pollination. The wasp uses the fig as a place to lay its eggs and pollinates the fig flowers in the process. In return, the fig offers some of its seeds as food for the developing wasp larvae.
     • Orchids exhibit diverse floral patterns that have evolved to attract specific pollinator insects, ensuring effective pollination. Some orchids use “sexual deceit” to trick male bees into pseudo-copulation, transferring pollen in the process.
3. Co-Evolution: Mutualistic interactions often lead to co-evolution, where the evolutionary paths of both species become intertwined. This co-evolution ensures that the mutualistic relationship remains effective and stable.
4. Maintenance of Mutualism: To prevent “cheating” by one partner and maintain the mutualistic relationship, mechanisms may evolve to ensure that both partners continue to provide benefits.

Mutualism plays a vital role in ecosystems by facilitating interactions that benefit multiple species and contribute to ecological stability and diversity.