Chapter 11: Organisms and Populations

Introduction

Ecology is a subject which studies the interactions among organisms and between the organism and its physical (abiotic) environment. Ecology is basically concerned with four levels of biological organisation: organisms, populations, communities, and biomes.

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 not only survival but also reproduction. Major abiotic factors that dictate the varying conditions of different habitats include:

1. Temperature: The most ecologically relevant environmental factor. Organisms that can tolerate a wide range of temperatures are called eurythermal, while those restricted to a narrow range are stenothermal.
2. Water: Life on earth originated in water and is unsustainable without water. For aquatic organisms, water quality (chemical composition, pH) is important. Tolerance to salinity classifies organisms as euryhaline (wide range) or stenohaline (narrow range).
3. Light: Plants rely on light for photosynthesis. Many animals also use diurnal and seasonal variations in light intensity and photoperiod as cues for timing their foraging, reproductive, and migratory activities.
4. Soil: The nature and properties of soil characterize the vegetation of an area, which in turn dictates the type of animals supported.

Responses to Abiotic Factors

Organisms try to maintain the constancy of their internal environment (a process called homeostasis) despite varying external environmental conditions.

• Regulate: Some organisms (all birds and mammals, very few lower vertebrates and invertebrates) are capable of thermoregulation and osmoregulation.
• Conform: An overwhelming majority (99%) of animals and nearly all plants cannot maintain a constant internal environment. Their body temperature changes with the ambient temperature.
• Migrate: The organism temporarily moves away from the stressful habitat to a more hospitable area and returns when the stressful period is over (e.g., migratory birds to Keoladeo National Park, Bharatpur).
• Suspend: In bacteria, fungi, and lower plants, various kinds of thick-walled spores are formed to survive unfavourable conditions. Animals might undergo hibernation (winter sleep to escape cold, e.g., bears) or aestivation (summer sleep to escape heat and desiccation, e.g., snails, fish). Zooplankton undergo diapause, a stage of suspended development.

Adaptations

Adaptation is any attribute of the organism (morphological, physiological, behavioural) that enables the organism to survive and reproduce in its habitat.

• Kangaroo rat in North American deserts meets water requirements through internal fat oxidation.
• Desert plants have a thick cuticle, sunken stomata, and CAM pathway to minimize water loss (e.g., Opuntia has modified leaves as spines).
• Allen’s Rule: Mammals from colder climates generally have shorter ears and limbs to minimize heat loss.

Populations

A population is a group of individuals belonging to the same species that live in a well-defined geographical area, share or compete for similar resources, and potentially interbreed.

Population Attributes

A population has certain attributes that an individual organism does not:

• Birth rates and Death rates: Refers to per capita births and deaths.
• Sex ratio: The ratio of males to females in a population.
• Age distribution: Portrayed using an age pyramid which helps determine if the population is growing, stable, or declining.

Population Growth

The size of a population (\(N\)) is fundamentally determined by four processes:

1. Natality (B): Number of births.
2. Mortality (D): Number of deaths.
3. Immigration (I): Number of individuals of the same species that have come into the habitat.
4. Emigration (E): Number of individuals who left the habitat. Equation: \(N_{t+1} = N_t + [(B + I) - (D + E)]\)

Growth Models:

1. Exponential Growth: When resources in the habitat are unlimited, each species realizes its full innate potential to grow in number. Equation: \(\frac{dN}{dt} = rN\) (where \(r\) is the intrinsic rate of natural increase). It yields a purely J-shaped curve.
2. Logistic Growth: Resources are limited, leading to competition. A habitat has enough resources to support a maximum possible number, called carrying capacity (\(K\)). Equation: \(\frac{dN}{dt} = rN \left(\frac{K-N}{K}\right)\). It yields a generic Sigmoid (S-shaped) curve.

Population Interactions

In nature, animals, plants, and microbes do not and cannot live in isolation.

• Predation (+/-): Transfer of energy to higher trophic levels; keeps prey population under control. Justifies biological control methods. (e.g., Tiger and Deer).
• Competition (-/-): Fitness of one species is significantly lower in the presence of another species. (e.g., Gause’s Competitive Exclusion Principle states that two closely related species competing for the same resources cannot co-exist indefinitely).
• Parasitism (+/-): One organism benefits at the expense of the other (host). Hosts often evolve mechanisms to reject or resist the parasite. E.g., human liver fluke, ticks on dogs.
• Commensalism (+/0): One species benefits and the other is neither harmed nor benefited. E.g., Orchid growing as an epiphyte on a mango branch.
• Mutualism (+/+): Interacting species derive mutual benefit. E.g., Lichens (fungus and cyanobacteria), Mycorrhizae (fungi and roots of higher plants).

Competency Based Questions

Q1. Analysis of an animal population living in a heavily restricted island ecosystem with a carrying capacity (\(K\)) of 500 individuals currently shows a population size (\(N\)) of exactly 500 individuals. What happens mathematically to the standard logistic growth rate relative to time (\(\frac{dN}{dt}\)) of this stable population?

(A) \(\frac{dN}{dt}\) becomes strictly negative as overpopulation occurs.
(B) \(\frac{dN}{dt}\) is equal to the intrinsic rate of natural increase (\(r\)).
(C) \(\frac{dN}{dt}\) perfectly equals zero.
(D) \(\frac{dN}{dt}\) exponentially approaches infinity.

Q2. During severe summer desiccation, a local freshwater lake completely dries out. Many microscopic zooplankton species within this specific habitat do not die or migrate, but rather spontaneously enter an extreme state of suspended metabolic development to survive the intense heat. Identify this specific biological adaptation term.

Q3. If a rapidly expanding bacterial population grows solely according to the exponential equation \(N_t = N_0 e^{rt}\), and its intrinsic rate of natural increase (\(r\)) is calculated to be \(0.05\) per minute, roughly calculate the time fundamentally required for the initial starting population to completely double mathematically. (Assume \(\ln 2 \approx 0.693\))

Q4. Contrast mathematically the fundamental differences between an expanding population structured by a regular triangular, broad-based age pyramid versus a sharply contracting population exhibiting an urn-shaped, narrow-based age pyramid.

Q5. When five highly competitive species of insectivorous warblers deliberately chose to permanently inhabit the exact same large spruce tree without aggressive competitive exclusion occurring, what specific ingenious behavioral mechanism prominently allowed their seemingly impossible, sustained ecological coexistence?