Population Biotic Community and Succession


  •  A population consists of individuals of the same species living in a particular area and interacting with one another. 
  • It represents a fundamental unit of ecology, where individuals compete for resources, reproduce, and contribute to the gene pool of the species. 
  •  A biotic community encompasses all the populations of different species coexisting and interacting within a specific habitat or ecosystem. 
  • It reflects the intricate network of relationships between organisms, including competition, predation, and mutualism, that shape community dynamics. 
  • Succession refers to the process of gradual and sequential changes in the composition and structure of communities over time. 
  • It occurs in response to environmental disturbances or through natural ecological processes, leading to the establishment of distinct stages of vegetation and species diversity. 
  • Population dynamics influence the structure and functioning of biotic communities, while succession reflects the long-term patterns of change within communities. 
  • Understanding these concepts provides insights into the dynamics of ecosystems and the mechanisms driving ecological patterns and processes.


Population Attributes

A population comprises individuals of the same species living in a specific area, sharing or competing for resources, and potentially interbreeding. 

 Examples include cormorants in a wetland, rats in an abandoned dwelling, or lotus plants in a pond. 

Population ecology links ecological principles to population genetics and evolution, as it is at the population level that natural selection operates to evolve desired traits.


Attributes of a Population:

  • Birth and Death Rates:

- Unlike individual organisms, populations have birth rates and death rates expressed per capita, indicating changes in population size over time. 

- Example: A pond with 20 lotus plants adds 8 new plants, resulting in a birth rate of 0.4 offspring per lotus per year.


  • Sex Ratio:

- Populations exhibit a sex ratio, indicating the proportion of males and females within the population.


  •  Age Distribution:

- Populations consist of individuals of different ages, reflected in an age pyramid, which indicates the growth status of the population (growing, stable, or declining). 

- Population pyramids, also known as age-sex pyramids, graphically represent the age distribution of individuals within a population. 

- The shape of a population pyramid reflects the age structure and growth status of a population.

- It typically consists of two back-to-back bar graphs, one representing males and the other females. 

-      Growing Population:

A pyramid with a broad base and narrow apex indicates a growing population. 

In such populations, the number of young individuals (pre-reproductive age) is larger than older individuals (post-reproductive age). 

This shape suggests high birth rates, low death rates, and potential for future population growth. 

- Stable Population:

A population pyramid with relatively uniform width across all age groups indicates a stable population. 

The number of individuals in each age group remains relatively constant over time. 

Birth rates are roughly equal to death rates, resulting in little to no population growth. 

- Declining Population:

A pyramid with a narrow base and broad apex suggests a declining population. 

In such populations, there are fewer young individuals compared to older individuals. 

This shape indicates low birth rates, high death rates, and potential population decline in the future.




  • Usage: Population pyramids are valuable tools for demographers, policymakers, and researchers to understand population dynamics, plan for future resource allocation, and develop appropriate policies for healthcare, education, and social services. 
  • Population Density:

- Population density, technically referred to as population density (N), represents the size of a population relative to its habitat. 

- It can be measured in various ways, including total number, percent cover, or biomass. 

- Example: In a dense laboratory culture of bacteria, population density may be reported based on biomass. 

- Relative densities, such as the number of fish caught per trap in a lake, can serve as proxies for total population density in ecological investigations.


  • Estimating Population Size:

- Population sizes are often estimated indirectly, especially for large populations or those difficult to count directly. 

- Techniques include using markers like pug marks and fecal pellets for tiger census in national parks and reserves.


Population Growth

  • Population size for any species is not fixed but constantly changing due to various factors such as food availability, predation pressure, and weather conditions. 
  • Changes in population density provide insights into the population's status—whether it's thriving or declining.


Four Basic Processes:

  • Natality: The number of births during a specific period in the population, contributing to an increase in population density. 
  • Mortality: The number of deaths in the population during a specific period. 
  • Immigration: The arrival of individuals of the same species into the habitat from elsewhere during the considered time period. 
  • Emigration: The departure of individuals of the population from the habitat to elsewhere during the considered time period.





Population Density Equation:

  • The population density at time t + 1 (Nt+1) is calculated using the equation:

Nt+1 = Nt + (B + I) – (D + E) 

Where  B is Natality,

D is Mortality,

E is Emigration, and

I is Immigration.


  • Population density increases if (B + I) exceeds (D + E), otherwise it decreases. 
  • Under normal conditions, births and deaths are the primary factors influencing population density, while immigration and emigration become significant under special conditions. 
  • For instance, during habitat colonization, immigration may contribute more significantly to population growth than birth rates.


Growth Models

  • Population growth exhibits specific and predictable patterns over time. 
  • Understanding these patterns helps in addressing concerns related to uncontrolled population growth, akin to the issues faced with human populations.


Exponential Growth:

  • In an ideal scenario with unlimited resources like food and space, each species can maximize its growth potential. 
  • This leads to exponential or geometric growth, as observed by Darwin while formulating his theory of natural selection.


Equation for Exponential Growth:

  • The increase or decrease in population density (dN/dt) during a unit time period (t) is given by:

dN/dt = (b – d) × N 

Here, (b – d) represents the intrinsic rate of natural increase (r).


Magnitude of Intrinsic Rate (r):

  • The intrinsic rate of natural increase (r) is crucial for assessing the impact of various factors on population growth. 
  • Examples: For Norway rat, r is 0.015; for flour beetle, it's 0.12; in 1981, India's human population had an r value of 0.0205.


Population Growth Curve:

  • The exponential growth pattern results in a J-shaped curve when population density (N) is plotted against time. 
  • The equation for exponential growth in integral form is:

Nt = N0 × ert


Example Anecdote:

  • An anecdote illustrates the dramatic consequences of exponential growth, like the doubling of wheat grains on a chessboard squares or the rapid multiplication of Paramecium through binary fission.


Logistics growth

  • No population in nature has access to unlimited resources, leading to competition among individuals for limited resources. 
  • Eventually, the fittest individuals survive and reproduce, reflecting the principles of natural selection.


Nature's Carrying Capacity (K):

  • Each habitat has a maximum capacity, termed as the carrying capacity (K), which determines the maximum population size sustainable in that habitat. 
  • Governments worldwide recognize this and implement measures to limit human population growth.


Phases of Logistic Growth:

  • A population in a habitat with limited resources initially experiences a lag phase, followed by phases of acceleration and deceleration. 
  • Eventually, the population reaches an asymptote when it stabilizes at the carrying capacity. 
  • The growth pattern is represented by a sigmoid curve when plotting population density (N) against time (t).


Verhulst-Pearl Logistic Growth Equation:

  • The logistic growth model is described by the equation:

dN/dt = (K - N) × rN/K 

  • Here, N represents population density at time t, r is the intrinsic rate of natural increase, and K is the carrying capacity.




  • Since resources are finite and become limiting for most animal populations, the logistic growth model is considered more realistic than exponential growth. 
  •  Government census data for India over the last 100 years can be analyzed to identify the growth pattern evident in the population. 
  • Plotting the population figures against time will help determine whether the growth pattern aligns more with exponential or logistic growth.


Life History Variation:

Maximizing Reproductive Fitness:

  • Populations evolve to maximize their reproductive fitness, known as Darwinian fitness, which is often indicated by a high intrinsic rate of increase (r value). 
  • Organisms adapt their reproductive strategies to efficiently utilize resources and survive in their habitat.


Variation in Reproductive Strategies:

  • Different species exhibit diverse reproductive strategies tailored to their environment. 
  • Some species breed only once in their lifetime, such as Pacific salmon and bamboo, while others breed multiple times, like most birds and mammals. 
  • Variation also exists in offspring size and number, with some species producing a large number of small-sized offspring (e.g., oysters, pelagic fishes) and others producing fewer but larger offspring (e.g., birds, mammals).


Factors Influencing Life History Traits:

  • Ecologists propose that life history traits evolve in response to constraints imposed by both abiotic and biotic factors in the habitat. 
  • These traits are shaped by selective pressures such as resource availability, predation, competition, and environmental stability. 
  • For example, species in environments with unpredictable resources may adopt a strategy of producing many small offspring to increase the chances of survival, while species in stable environments may invest more in producing fewer but better-cared-for offspring. 
  •  Understanding the evolution of life history traits in different species is a crucial area of research in ecology. 
  • Ecologists investigate how environmental factors influence the development of reproductive strategies and how these strategies contribute to the overall fitness and survival of populations.


Population Interactions:

No Single-Species Habitat:

  • In natural habitats, no species exists in isolation; interactions between different species are fundamental to ecosystem dynamics. 
  • Even species capable of producing their own food, like plants, rely on interactions with other organisms for essential functions such as nutrient cycling and pollination.


Formation of Biological Communities:

  • Animals, plants, and microbes interact in various ways to form biological communities, creating complex webs of interactions. 
  • These interactions contribute to the structure and function of ecosystems, influencing population dynamics, resource utilization, and species distribution.


Types of Interspecific Interactions:

  • Interspecific interactions occur between populations of different species and can be categorized based on their effects on each species involved. 
  •  Mutualism (+/+): Both species benefit from the interaction, such as in pollination or symbiotic relationships between organisms. 
  • Competition (-/-): Both species compete for limited resources, which can negatively impact population growth and survival. 
  • Parasitism (+/-): One species benefits at the expense of the other, such as in parasite-host relationships where the parasite benefits while the host is harmed. 
  • Predation (+/-): One species (predator) benefits by consuming another species (prey), which is negatively impacted by the interaction. 
  • Commensalism (+/0): One species benefits while the other is unaffected, as seen in interactions like hitchhiking or organisms utilizing the shelter of another species without causing harm. 
  • Amensalism (-/0): One species is negatively impacted while the other remains unaffected, such as when one species releases chemicals that inhibit the growth of another species without any benefit or harm to itself.





Characteristics of Interactions:

  • Predation, parasitism, and commensalism often involve close physical proximity between interacting species, facilitating direct contact and exchange of resources or energy. 
  • These interactions play crucial roles in shaping population dynamics, community structure, and ecosystem functioning, highlighting the interconnectedness of species within ecosystems.



  • Mutualism is an interspecific interaction where both interacting species benefit from the relationship.



  • Lichens: Represent an intimate mutualistic relationship between a fungus and photosynthesizing algae or cyanobacteria. The fungus provides structure and protection, while the algae/cyanobacteria conduct photosynthesis, providing nutrients. 
  • Mycorrhizae: Associations between fungi and plant roots where fungi aid in nutrient absorption from the soil, and in return, receive energy-yielding carbohydrates from the plant. 
  • Plant-Animal Relationships: Plants rely on animals for pollination and seed dispersal. Animals receive rewards such as pollen, nectar, or nutritious fruits from plants in exchange for their services.



  • Mutualistic interactions often involve co-evolution, where the evolution of one species is tightly linked with that of its mutualistic partner. 
  • Example: Fig trees and pollinator wasps exhibit a one-to-one relationship, where each fig species is pollinated exclusively by its partner wasp species. The wasp pollinates the fig while laying eggs, and in return, receives food for its larvae from the developing seeds.





  • Orchids display diverse floral patterns to attract specific pollinators, such as bees. Some orchids employ "sexual deceit," resembling female bees to attract males for pollination. This highlights how co-evolution operates to maintain successful pollination.





Safeguarding Mutualistic Relationships:

  • Mutualistic systems must guard against "cheaters," organisms that exploit benefits without providing reciprocal advantages. 
  • Plants may evolve mechanisms to ensure pollinators receive rewards, such as offering nectar and pollen only accessible through pollination. 
  • Co-evolutionary adaptations ensure the mutualistic relationship remains beneficial for both partners, even as environmental conditions and species interactions evolve over time.



  • Competition occurs when organisms vie for the same limited resources, leading to reduced fitness of one or both species involved.


Types of Competition:

  • Interference Competition: Occurs when one species hinders the feeding efficiency of another species, even if resources are abundant. 
  • Exploitative Competition: Arises when species compete directly for limited resources like food and space.



  • In some South American lakes, visiting flamingoes and resident fishes compete for zooplankton as food.
  • Competitive exclusion principle: States that closely related species competing for the same resources cannot coexist indefinitely; the competitively inferior species will be eliminated eventually.
  • Evidence from nature:

- Introduction of goats in the Galapagos Islands led to the extinction of Abingdon tortoises due to greater browsing efficiency. 

- Connell’s experiments showed the dominance of larger barnacles over smaller ones in rocky sea coasts. 

- Competitive release: Occurs when a species expands its distributional range after the removal of a competitively superior species. 

Resource Partitioning:

  • Some species evolve mechanisms like resource partitioning to promote co-existence rather than exclusion. 
  • Resource partitioning involves dividing resources such as food and space to reduce competition. 
  • Example: Closely related species of warblers co-exist by foraging at different times or in different patterns to avoid direct competition.



  • Gause’s Competitive Exclusion Principle suggests that competitively inferior species will be eliminated, but recent studies show exceptions. 
  • Species may evolve mechanisms to promote co-existence, such as behavioral differences in foraging activities to reduce competition.
  • Generalizations about competition should consider the complexities of ecological interactions and evolutionary adaptations that promote co-existence.



  • Predation is nature's way of transferring energy from autotrophic organisms (plants) to higher trophic levels through consumption by animals (predators).



  • Predators include not only large carnivores like tigers but also smaller animals such as sparrows that eat seeds. 
  • Herbivores, which consume plants, are considered predators in an ecological context because they consume primary producers.


Roles of Predators:

  • Energy Transfer: Predators act as conduits for energy transfer across trophic levels, transferring energy fixed by plants to higher trophic levels. 
  • Population Control: Predators regulate prey populations, preventing them from reaching unsustainable densities and causing ecosystem instability. 
  • Biological Control: Predators can control invasive species, maintaining ecosystem balance. For example, introducing a cactus-feeding moth helped control the spread of invasive prickly pear cactus in Australia. 
  • Maintaining Species Diversity: Predators reduce competition among prey species, contributing to species diversity in a community. Removal of starfish predators led to the extinction of multiple invertebrate species due to increased interspecific competition. 
  • Predator-Prey Dynamics:

Predators must balance their predation efficiency to prevent overexploitation of prey, which could lead to prey extinction and subsequently, predator extinction.

Prey species have evolved various defenses against predation, including cryptic coloration, toxicity, and chemical defenses.


Plant-Herbivore Interactions:

  • Herbivores serve as predators to plants, posing significant challenges as plants cannot escape from predation. 
  • Plants have evolved morphological defenses like thorns and chemical defenses such as toxins to deter herbivores. 
  •  Many commercially important chemicals extracted from plants, like nicotine and caffeine, are actually produced by plants as defenses against grazers and browsers.



  • Parasitism is a mode of life where one organism (parasite) benefits at the expense of another organism (host) by residing on or inside the host organism and obtaining nutrients or other resources.



  • Host-Specificity: Many parasites are host-specific, co-evolving with their host species. If the host develops defenses, the parasite evolves countermeasures to overcome them. 
  • Adaptations: Parasites often possess adaptations such as adhesive organs, suckers, or the loss of unnecessary sense organs to cling to the host and obtain nutrients. 
  • Complex Life Cycles: Parasites may have complex life cycles involving one or more intermediate hosts or vectors to complete their life cycle. For example, the human liver fluke requires a snail and a fish to complete its life cycle. 
  • Harm to Host: While some parasites may not harm the host, the majority reduce the host's survival, growth, and reproduction. They may also make the host more vulnerable to predation.


Types of Parasites:

  • Ectoparasites: These parasites live on the external surface of the host organism, such as lice on humans or ticks on dogs. 
  • Endoparasites: Endoparasites live inside the host's body, occupying various sites like the liver, kidney, or red blood cells.



  • Brood Parasitism: In birds, brood parasitism occurs when a parasitic bird lays its eggs in the nest of a host bird, which then incubates the eggs. The eggs of the parasitic bird evolve to resemble those of the host to avoid detection and rejection.


Special Cases:

  • Cuscuta: Cuscuta, a parasitic plant, has lost its chlorophyll and leaves, obtaining nutrition from its host plant. 
  • Female Mosquito: The female mosquito, while feeding on blood for reproduction, is not considered a parasite because it does not directly harm the host's survival or reproductive success.


Evolutionary Perspective:

  • Natural selection did not lead to the evolution of completely harmless parasites because they rely on the host for resources. While reducing harm to the host may be advantageous, parasites still need to obtain resources from the host to survive and reproduce.



  • Commensalism is an interaction between two species where one species benefits, and the other is neither harmed nor benefited.



  • One-sided Benefit: In commensalism, one species gains advantages, while the other species remains unaffected. 
  • Examples of Interaction: The interacting species may involve one species using the resources or services provided by the other species without causing harm or benefit.



  • Orchid and Mango Tree: An orchid growing as an epiphyte on a mango branch benefits from the elevated position for better access to sunlight and air, while the mango tree is neither harmed nor benefited. 
  • Cattle Egret and Grazing Cattle: Cattle egrets forage near grazing cattle, benefiting from insects stirred up by the movement of the cattle. The cattle are not affected by the presence of the egrets. 
  • Sea Anemone and Clownfish: Clownfish live among the tentacles of sea anemones, gaining protection from predators due to the anemone's stinging tentacles. The anemone does not appear to benefit directly from hosting the clownfish.


Role in Ecosystems:

  • Commensal interactions contribute to the diversity and stability of ecosystems by providing opportunities for species to occupy different ecological niches without directly competing or harming each other.



  • While commensalism involves one-sided benefits, it does not necessarily imply a lack of interaction between the species involved. Instead, it highlights the complexity of relationships in nature, where organisms may exploit resources or services provided by others without causing harm.