Unit 11: Organisms and Populations

1. Organism and its Environment

Ecology at the organismic level is essentially physiological ecology, which tries to understand how different organisms are adapted to their environments in terms of survival and reproduction. The rotating earth and its tilted axis cause annual variations in the intensity and duration of temperature, resulting in distinct seasons. These variations, together with annual precipitation (mean annual rainfall and temperature), combine to form major biomes such as desert, rain forest, and tundra.

Key Environmental Factors (Abiotic Factors)

The physico-chemical (abiotic) components of the environment do not remain constant. The major abiotic factors include:

• Temperature: The most ecologically relevant environmental factor. Average temperature varies seasonally, decreasing progressively from the equator towards the poles and from plains to mountain tops. Some organisms can tolerate a wide range of temperatures (eurythermal), while others are restricted to a narrow range (stenothermal).
• Water: Life on earth originated in water and is unsustainable without it. For aquatic organisms, the quality (chemical composition, pH) of water is important. Productivity and distribution of plants are heavily dependent on water. Organisms living in water may be euryhaline (tolerant of a wide range of salinities) or stenohaline (restricted to a narrow range of salinities).
• Light: Plants need light for photosynthesis. Sunlight is also the source for photoperiodic cues required for timing foraging, reproductive, and migratory activities. Many animals use the diurnal and seasonal variations in light intensity and duration as cues. The availability of light on land is closely linked with temperature since the sun is the source for both.
• Soil: The nature and properties of soil in different places depend on the climate, the weathering process, whether soil is transported or sedimentary, and how soil development occurred. Various characteristics such as soil composition, grain size, and aggregation determine the percolation and water holding capacity of the soils along with pH, mineral composition, and topography.

Responses to Abiotic Factors

During the course of evolution, many species have evolved a relatively constant internal (physiological) environment that permits all biochemical reactions and physiological functions to work with maximum efficiency. Organisms maintain internal homeostasis through various means:

• Regulate: Some organisms are able to maintain homeostasis by physiological (and sometimes behavioural) means which ensures constant body temperature, constant osmotic concentration, etc. (e.g., Birds and mammals).
• Conform: A vast majority of animals and nearly all plants cannot maintain a constant internal environment. Their body temperature or osmotic concentration changes with the ambient temperature or concentration in the external environment. These are conformers.
• Migrate: The organism can move away temporarily from the stressful habitat to a more hospitable area and return when stressful period is over (e.g., Birds undertaking long-distance migrations during winter).
• Suspend: In bacteria, fungi, and lower plants, various kinds of thick-walled spores are formed which help them to survive unfavourable conditions. In higher plants, seeds and some other vegetative reproductive structures serve as means to tide over periods of stress. In animals, hibernation (e.g., bears), aestivation (e.g., some snails and fish), and diapause (e.g., many zooplankton species) are examples of suspension.

Adaptations

Adaptation is any attribute of the organism (morphological, physiological, or behavioural) that enables the organism to survive and reproduce in its habitat. Many adaptations have evolved over a long evolutionary time and are genetically fixed. Examples include:

• Kangaroo rats in North American deserts meet their water requirement through internal oxidation of fat, have the ability to concentrate their urine, and excrete solid dry feces to minimize water loss.
• Mammals from colder climates generally have shorter ears and limbs to minimize heat loss (known as Bergmann's Rule).
• In the polar regions, aquatic mammals like seals have a thick layer of fat (blubber) below their skin that acts as an insulator against heat loss.
• Altitude sickness at high altitudes is caused by low atmospheric pressure. The body compensates by increasing red blood cell production, decreasing binding capacity of hemoglobin, and increasing breathing rate.

2. Populations

A population is a group of individuals of the same species living in a well-defined geographical area, sharing or competing for similar resources, and potentially interbreeding. Although an individual organism is the one that has to cope with a changing environment, it is at the population level that natural selection operates to evolve the desired traits.

2.1 Population Attributes

Unlike an individual organism, a population has certain attributes that an individual does not possess:

• Birth rates (Natality): Refers to per capita births. It is the rate of production of new individuals in a population per unit time.
• Death rates (Mortality): Refers to per capita deaths. It is the rate of loss of individuals due to death in a population per unit time.
• Sex Ratio: An individual is either a male or a female, but a population has a sex ratio (e.g., the percentage of males or females in a population).
• Age Distribution: A population at any given time is composed of individuals of different ages. If the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called an age pyramid. The shape of the pyramids reflects the growth status of the population: growing, stable, or declining.
• Population Density: The size of a population in relation to some unit of space is termed population density (designated as N). It is not necessarily measured by numbers alone; biomass or percentage cover is also used depending on the organism.

2.2 Population Growth

The size of a population for any species is not a static parameter. It keeps changing in time, depending on various factors including food availability, pressure from predation, and weather. The density of a population in a given habitat during a given period fluctuates due to changes in four basic processes:

• Natality (B): Number of births during a given period in the population that are added to the initial density.
• Mortality (D): Number of deaths in the population during a given period.
• Immigration (I): Number of individuals of the same species that have come into the habitat from elsewhere during a given time period.
• Emigration (E): Number of individuals of the population who left the habitat elsewhere during a given time period.

Formula for Population Density (N):

N(t+1) = Nt + [(B + I) - (D + E)]

Where N(t) is population density at time t, B is Natality, I is Immigration, D is Mortality, and E is Emigration.

Growth Models

• Exponential Growth: When resources in the habitat are unlimited, each species has the ability to realize fully its innate potential to grow in number. The population grows in an exponential or geometric fashion.

  Equation: dN/dt = rN

  Integral form: Nt = N0 * e^(rt)

  Where N = Population density at time t, r = intrinsic rate of natural increase, e = the base of natural logarithms. The 'r' value is a crucial parameter for assessing impacts of population growth.
• Logistic Growth: Resources for growth for most animal populations are finite and become limiting sooner or later. Therefore, unlimited growth leads eventually to resource competition and a slower growth rate. This gives rise to a sigmoid growth curve.

  Equation: dN/dt = rN * [(K - N) / K]

  Where N = Population density at time t, r = intrinsic rate of natural increase, K = carrying capacity. Since resources are finite, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. This limit is the carrying capacity (K).

2.3 Population Interactions

In nature, animals, plants, and microbes do not live in isolation but interact in various ways to form a biological community. Interspecific interactions arise from the interaction of populations of two different species. They could be beneficial, detrimental, or neutral to one of the species or both.

Note: (+) denotes beneficial interaction, (-) denotes detrimental interaction, and (0) denotes neutral interaction.

Detailed Review of Interactions

• Predation: While predators are thought of as nuisance species, they are nature's way of transferring energy to higher trophic levels, keeping prey populations under control, and helping in maintaining species diversity in a community by reducing the intensity of competition among competing prey species. If a predator is too efficient and overexploits its prey, the prey might become extinct and following it, the predator will also become extinct. Prey species have evolved various defenses (e.g., camouflage, toxic chemicals like monarch butterfly).
• Competition: A process in which the fitness of one species is significantly lower in the presence of another species. It can occur among individuals of the same species (intraspecific) or different species (interspecific). According to Gause's Competitive Exclusion Principle, two closely related species competing for the same resources cannot co-exist indefinitely and the competitively inferior one will eventually be eliminated. Species facing competition may evolve mechanisms that promote co-existence rather than exclusion, such as resource partitioning.
• Parasitism: In this interaction, one organism (parasite) is dependent on the other (host) for food and shelter, and is thus harmed. Many parasites have evolved specialized adaptations such as the loss of unnecessary sense organs, presence of adhesive organs or suckers, loss of digestive system, and high reproductive capacity. Parasites can be ectoparasites (living on the surface of the host) or endoparasites (living inside the host body). Brood parasitism in birds (e.g., cuckoo laying eggs in the crow's nest) is a fascinating example of parasitism.
• Commensalism: An interaction in which one species benefits and the other is neither harmed nor benefited. Examples include an orchid growing as an epiphyte on a mango branch, and barnacles growing on the back of whales.
• Mutualism: This interaction confers benefits on both interacting species. Examples include lichens (a fungus and photosynthetic algae or cyanobacteria), mycorrhizae (fungi and roots of higher plants), and plant-animal interactions for pollination and seed dispersal (e.g., fig tree and its pollinator wasp).

Exam-Oriented Important Notes and Observations

• Always remember the signs for interactions (+, -, 0) as direct MCQs frequently evaluate these combinations.
• Distinguish clearly between exponential growth (J-shaped curve, unlimited resources) and logistic growth (sigmoid curve, carrying capacity K, limited resources).
• Understand that adaptations can be physiological (e.g., high altitude response), morphological (e.g., opuntia leaves modified to spines), or behavioural (e.g., desert lizards basking in the sun).
• Recognize that population density is the fundamental metric used in demographic studies and resource management.