Short Notes: Organisms and Population

Table of Contents
1. Ecology
2. Major Abiotic Factors
3. Responses to Abiotic Factors
4. Adaptations
5. Population Attributes
6. Age Distribution (Population Structure)
7. Population Growth Models
8. Life-history Variation
9. Population Interactions
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Ecology

Ecology is the scientific study of the interactions between organisms and their physical and biological environment. It examines how organisms obtain energy and resources, how they respond and adapt to abiotic factors, and how groups of organisms interact to form higher levels of organisation.

• Levels of ecological organisation: Organism → Population → Community → Ecosystem → Biome → Biosphere

Major Abiotic Factors

Abiotic factors are the non-living physical and chemical components of the environment that influence organisms and ecosystems. Important abiotic factors and their ecological relevance are:

• Temperature: The most important abiotic factor because it affects metabolic rates, enzyme activity and kinetic energy of molecules. Species may be eurythermal (tolerate wide temperature range) or stenothermal (tolerate narrow range). Common adaptations to temperature extremes include hibernation, aestivation and migration.
• Water: Essential for all life processes; availability and salinity determine distribution of organisms. Species may be euryhaline (tolerant of wide salinity changes) or stenohaline (tolerant of only narrow salinity range). Osmoregulation is a key physiological adaptation.
• Light: Drives photosynthesis and influences behaviour and photoperiodic responses (flowering, breeding). Plants adapted to light conditions include heliophytes (sun plants) and sciophytes (shade plants).
• Soil: Properties such as pH, texture, organic matter and mineral composition affect plant growth and the organisms that depend on plants. Topography and drainage are important soil-related factors.
• Other factors: Wind, humidity, salinity, oxygen concentration in aquatic habitats, and chemical factors (pollutants, nutrients) also shape species distributions and ecosystem processes.

Responses to Abiotic Factors

Organisms respond to unfavourable abiotic conditions by either maintaining internal constancy, changing with the environment, moving away, or suspending activity.

• Regulate: Maintain a relatively constant internal environment (homeostasis), e.g., thermoregulation in mammals and birds.
• Conform: Internal conditions change to match external conditions, e.g., many marine invertebrates whose body osmolarity tracks the surrounding water.
• Migrate: Seasonal or life-cycle movements to more favourable environments, e.g., many birds, some fishes.
• Suspend: Enter a dormant state to survive unfavourable periods, e.g., diapause, hibernation (winter dormancy) and aestivation (summer dormancy).

Adaptations

Adaptations are traits that increase an organism's fitness in a particular environment. They may be structural, physiological or behavioural.

• Morphological adaptations: Physical features such as thick fur in polar mammals, streamlined bodies in aquatic animals, or spines in cacti that reduce water loss.
• Physiological adaptations: Internal functional changes such as production of antifreeze proteins in cold-water fishes, CAM or C4 photosynthesis in plants of arid zones, and increased red blood cell production (acclimatisation) at high altitude.
• Behavioural adaptations: Actions by organisms like basking for thermoregulation, burrowing to escape heat, nocturnal activity to avoid daytime heat, and social behaviours such as cooperative hunting.
• Acclimatisation versus adaptation: Acclimatisation is a short-term, reversible physiological change within an organism's lifetime; adaptation refers to heritable traits shaped by natural selection over generations.

Population Attributes

A population is a group of individuals of the same species occupying a particular area and capable of interbreeding. Key attributes used to describe populations are:

• Population density: Number of individuals per unit area or volume (commonly denoted as N per unit area).
• Natality (b): Birth rate of the population, usually expressed per individual per unit time.
• Mortality (d): Death rate of the population, usually expressed per individual per unit time.
• Immigration (i): Arrival of individuals into the population from elsewhere.
• Emigration (e): Departure of individuals from the population to other areas.
• Growth equation: The basic change in population size over time can be written as dN/dt = (b + i) - (d + e), where dN/dt is the rate of change of population size.

Age Distribution (Population Structure)

Age distribution or age structure of a population shows the relative number of individuals in different age classes. It influences future growth and stability of the population.

• Expanding population: Pyramid-shaped age structure with a broad base, characteristic of high birth rates and rapid population growth (many young individuals).
• Stable population: Bell-shaped or uniform age structure where birth and death rates are approximately equal; population size remains more or less constant.
• Declining population: Urn-shaped age structure with fewer young individuals, indicating low birth rates and potential future decline.

Population Growth Models

Two simple models describe how populations grow under different environmental assumptions.

• Exponential growth: Occurs when resources are unlimited. Population increases at a rate proportional to its current size. The differential equation is \( \dfrac{dN}{dt} = rN \), where r is the intrinsic rate of increase. The resulting curve is J-shaped.
• Logistic growth: Accounts for resource limitation and carrying capacity. Population growth slows as size approaches the environment's carrying capacity K. The logistic differential equation is \( \dfrac{dN}{dt} = rN\left(\dfrac{K-N}{K}\right) \). The resulting curve is S-shaped (sigmoidal).

Life-history Variation

Species differ in how they allocate resources to growth, reproduction and survival. Life-history strategies are often described along an r-K continuum.

• r-strategists: Characterised by high reproductive rates, small body size, early maturity and short lifespan. They exploit unpredictable or disturbed environments. Examples include many insects, bacteria and annual plants.
• K-strategists: Characterised by low reproductive rates, larger body size, later maturity and longer lifespan. They are adapted to stable environments near carrying capacity. Examples include elephants, whales and many large mammals.
• Trade-offs: Life-history traits involve trade-offs; for example, producing many small offspring versus fewer large offspring with higher survival probability.

Population Interactions

Interactions among species shape community structure and ecosystem function. The common types of interspecific interactions, their signs (+, -, 0) and examples are:

• Mutualism (+/+): Both species benefit. Examples: lichen (alga + fungus), mycorrhiza (plant roots + fungi), pollination mutualisms between flowering plants and pollinators.
• Competition (-/-): Both species are harmed when they compete for the same limited resource. The competitive exclusion principle (G. F. Gause) states that two species competing for identical resources cannot stably coexist; one will exclude the other or they will partition resources.
• Predation (+/-): Predator benefits, prey harmed. Examples: lion-zebra, spider-insect. Predation affects population dynamics, behaviour and evolutionary adaptations.
• Parasitism (+/-): Parasite benefits, host is harmed. Examples: ectoparasites like lice and ticks; endoparasites like tapeworms. Brood parasitism is a special form, e.g., the cuckoo laying eggs in other birds' nests.
• Commensalism (+/0): One species benefits and the other is neither helped nor harmed. Examples: orchids or epiphytic ferns growing on tree branches; cattle egret feeding near cattle.
• Amensalism (-/0): One species is harmed while the other is unaffected. Example: production of antibiotics by some fungi (e.g., Penicillium) that inhibit bacteria.

Summary: Understanding how abiotic factors, adaptations, population attributes and interspecific interactions operate provides the foundation for studying population dynamics and community organisation. The basic population models (exponential and logistic), age-structure concepts, and types of species interactions are essential concepts to analyse and predict ecological outcomes.