Organism And Its Environment

Ecology is the scientific study of the interactions among organisms and between organisms and their physical (abiotic) environment. It examines biological organisation at different levels: organisms, populations, communities, and biomes.

At the organismic level, ecology is essentially physiological ecology. This area focuses on how individual organisms adapt to their environments, enabling not only survival but also successful reproduction.

Environmental variations are largely driven by the Earth's rotation around the Sun and the tilt of its axis, causing seasonal changes in temperature, light intensity, and precipitation. These variations lead to the formation of major biomes (large-scale ecosystems characterized by specific climate and vegetation types), such as deserts, rainforests, and tundras.

Within each biome, regional and local differences create a wide variety of distinct habitats. Life exists in diverse habitats, from extreme conditions like hot deserts, deep ocean trenches, polar regions, and thermal springs, to more favourable ones. Even the human intestine serves as a habitat for various microbes.

The key elements contributing to habitat variation are the major abiotic factors: temperature, water, light, and soil. However, an organism's habitat is also defined by biotic components – the other living organisms it interacts with, including pathogens, parasites, predators, and competitors.

Through natural selection over evolutionary time, organisms have developed adaptations to optimise their survival and reproduction in their specific habitats.

Every organism occupies a unique niche within its ecological system. A niche encompasses the range of environmental conditions an organism can tolerate, the resources it utilises, and its specific functional role in the ecosystem.

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Major Abiotic Factors

The most important physical factors in the environment affecting life:

• Temperature: A crucial ecological factor. Temperature varies seasonally, with latitude (decreasing from equator to poles), and with altitude (decreasing from plains to mountains). Extreme temperatures exist in diverse habitats (e.g., subzero in polar regions to over $100^\circ\textsf{C}$ in thermal springs). Temperature impacts enzyme kinetics, metabolism, and other physiological functions, significantly influencing the geographical distribution of species.
  • Organisms tolerating a wide range of temperatures are eurythermal.
  • Organisms restricted to a narrow temperature range are stenothermal.
  Global warming is predicted to alter the distribution range of many species.
• Water: Essential for all life; life originated in water. Its availability is a limiting factor in many habitats, especially deserts. Water quality (chemical composition, pH, salinity) is crucial for aquatic organisms. Salinity levels vary greatly (low in inland waters, 30-35 ppt in sea, >100 ppt in hypersaline lagoons).
  • Organisms tolerating a wide range of salinities are euryhaline.
  • Organisms restricted to a narrow range of salinities are stenohaline.
  Osmotic problems prevent many freshwater organisms from surviving in saltwater and vice versa.
• Light: Vital for autotrophs (plants) for photosynthesis. Light intensity and duration (photoperiod) influence plant distribution and reproductive cycles (flowering). Animals use light/photoperiod variations as cues for foraging, reproduction, and migration. Light quality (spectral composition) also matters; UV radiation is harmful, and different wavelengths of visible light penetrate to different depths in aquatic environments, affecting the distribution of aquatic plants like algae (e.g., red algae are found deepest as they can utilise the green light that penetrates furthest).
• Soil: The characteristics of soil (composition, grain size, water holding capacity, pH, mineral content, topography) determine the type of vegetation that can grow in an area. This, in turn, influences the animal life supported by that vegetation. In aquatic environments, sediment characteristics determine the types of bottom-dwelling (benthic) organisms.

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Responses To Abiotic Factors

Organisms develop various strategies to cope with variable or stressful abiotic conditions in their habitats. Ideally, organisms maintain a constant internal environment (homeostasis) for optimal physiological functioning, but achieving this in a changing external environment can be challenging or costly.

Organisms respond to environmental stress through different mechanisms:

• Regulate: Some organisms maintain a constant internal environment (homeostasis) through physiological or behavioural means, despite external variations. This includes thermoregulation (maintaining constant body temperature) and osmoregulation (maintaining constant osmotic concentration).
  • Birds and mammals are excellent regulators. Humans maintain a body temperature of $37^\circ\textsf{C}$ by sweating in summer (evaporative cooling) and shivering in winter (generating heat).
  • Regulation is energetically expensive, especially for small animals with a high surface area-to-volume ratio, leading to faster heat loss. This is why very small animals are rarely found in cold regions.
  • Some species can regulate only within a limited range of conditions and conform outside that range.
• Conform: The majority of animals (99%) and almost all plants cannot maintain a constant internal environment. Their body temperature changes with ambient temperature (poikilotherms or ectotherms), and the osmotic concentration of body fluids changes with the environment. They simply 'conform' to the external conditions.
• Migrate: The organism moves away temporarily from a stressful habitat to a more favourable area and returns when conditions improve. This is a behavioural response, like birds migrating in winter to warmer regions (e.g., Siberian cranes visiting Keolado National Park).
• Suspend: Organisms in stressful conditions may enter a state of suspended metabolic activity or dormancy.
  • In bacteria, fungi, and lower plants, thick-walled spores are formed to survive unfavorable conditions.
  • In higher plants, seeds and vegetative propagules serve to survive stress and disperse, germinating when conditions are favourable.
  • In animals, if migration is not possible, they may escape in time: Hibernation (winter sleep, e.g., bears) to avoid cold stress; Aestivation (summer sleep, e.g., some snails and fish) to avoid heat and desiccation; Diapause (a stage of suspended development) in many zooplankton in lakes/ponds under unfavorable conditions.

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Adaptations

Adaptation is any characteristic (morphological, physiological, or behavioural) of an organism that enables it to survive and reproduce effectively in its specific habitat. Adaptations are products of evolution and are genetically fixed.

Examples of adaptations:

• Desert animals: Kangaroo rat in North America meets water needs through internal fat oxidation and has highly concentrated urine to minimise water loss.
• Desert plants: Thick cuticle on leaf surface, sunken stomata in pits to reduce transpiration (e.g., Opuntia). Some (like Opuntia) lack leaves (reduced to spines) and perform photosynthesis via flattened stems (phylloclades). Many use the CAM photosynthetic pathway, keeping stomata closed during the day.
• Animals in cold climates: Mammals have shorter ears and limbs to reduce heat loss (Allen's Rule). Aquatic mammals in polar seas (seals) have a thick layer of fat (blubber) under the skin for insulation.
• Physiological adaptations to high altitude: Experiencing altitude sickness (nausea, fatigue, heart palpitations) at high altitudes due to low oxygen pressure. Acclimatisation involves physiological adjustments: increased red blood cell production, decreased hemoglobin binding affinity for oxygen, increased breathing rate. People living at high altitudes often have higher RBC counts/hemoglobin.
• Physiological adaptations to extreme temperatures: Archaebacteria thriving in hot springs ($>100^\circ\textsf{C}$) have enzymes adapted to high temperatures. Fish in Antarctic waters (below $0^\circ\textsf{C}$) have antifreeze proteins in body fluids to prevent freezing.
• Physiological adaptations to high pressure: Marine invertebrates and fish in deep oceans ($>100$ times atmospheric pressure) have biochemical adaptations (e.g., special enzymes) to function under high pressure.
• Behavioural adaptations: Desert lizards regulate body temperature behaviourally: bask in sun to gain heat when cold, move to shade to avoid overheating when hot. Some burrow into the soil to escape heat.

Populations

In nature, organisms of the same species typically live in groups within a defined geographical area, interacting (sharing/competing for resources, interbreeding), forming a population. While individuals cope with the environment, natural selection acts on populations, leading to evolutionary changes.

Population ecology links ecology to population genetics and evolution, studying the dynamics and structure of populations.

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Population Attributes

Populations possess characteristics that individual organisms do not:

• Birth rates (Natality): Per capita number of births over a given period.
• Death rates (Mortality): Per capita number of deaths over a given period.
• Sex ratio: Proportion of males and females in the population.
• Age distribution: Proportion of individuals in different age groups (pre-reproductive, reproductive, post-reproductive) at a given time, often represented graphically as an age pyramid. The shape of the age pyramid indicates if the population is growing, stable, or declining.
• Population size (Population density, N): The number of individuals per unit area or volume. Population density reflects the status of the population in its habitat.

Population density measurement: Total number is a common measure, but per cent cover or biomass might be more appropriate for certain species (e.g., large tree vs. many small plants, dense bacterial culture). Relative density (e.g., fish caught per trap) or indirect estimation (e.g., tiger census based on pug marks) can also be used.

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Population Growth

Population size is not static but changes over time influenced by factors like food availability, predation, and weather. Changes in density provide insights into population dynamics.

Population density fluctuates due to four basic processes:

• Increase: Natality (births), Immigration (individuals entering).
• Decrease: Mortality (deaths), Emigration (individuals leaving).

Change in population density over time ($N_{t+1}$) is given by the formula: $N_{t+1} = N_t + [(B + I) - (D + E)]$, where $N_t$ is density at time t, B is births, I is immigration, D is deaths, E is emigration.

Births and deaths are usually the most significant factors. Immigration and emigration are important in special cases, like colonization of a new habitat.

Population growth patterns can sometimes follow predictable models:

1. Exponential growth (Geometric growth): Occurs when resources (food, space) are unlimited, allowing a species to reproduce at its maximum potential.
   • Rate of change in population size ($dN/dt$) is proportional to the current population size (N) and the intrinsic rate of natural increase (r): $\frac{dN}{dt} = rN$.
   • 'r' is a measure of the inherent potential for population growth. Higher 'r' means faster growth.
   • When N is plotted against time, exponential growth yields a J-shaped curve.
   • The integral form of the equation is $N_t = N_0e^{rt}$, where $N_t$ is density at time t, $N_0$ is density at time zero, r is intrinsic rate of natural increase, and e is the base of natural logarithms.
   Exponential growth can lead to enormous population sizes quickly if resources remain unlimited, as demonstrated by the story of wheat grains on a chessboard or rapid microbial growth.
2. Logistic growth (Verhulst-Pearl Logistic Growth): Occurs when resources are limited, which is the case for most populations in nature. Limited resources lead to competition.
   • In a habitat with limited resources, there is a maximum population size the environment can support sustainably. This limit is called the carrying capacity (K) for that species in that habitat.
   • Population growth in a limited resource environment typically shows a lag phase (slow initial growth), followed by acceleration, deceleration, and finally reaches an asymptote (stable phase) when the population density approaches K.
   • A plot of N versus time results in a sigmoid (S-shaped) curve.
   • The equation for logistic growth is $\frac{dN}{dt} = rN (\frac{K - N}{K})$. Here, the growth rate slows down as N approaches K.
   The logistic growth model is considered more realistic for most natural populations because resources are rarely unlimited indefinitely.

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Life History Variation

Populations evolve strategies to maximise their reproductive success (Darwinian fitness, high 'r') in their specific habitats. Organisms adopt different life history traits based on the environmental constraints (abiotic and biotic factors).

Examples of contrasting reproductive strategies:

• Some organisms breed only once in their lifetime (e.g., Pacific salmon fish, bamboo).
• Others breed many times (e.g., most birds and mammals).
• Some produce a large number of small offspring (e.g., Oysters, pelagic fish).
• Others produce a small number of large offspring (e.g., birds, mammals).

The evolution of these diverse life history patterns is an active area of ecological research, investigating how trade-offs (e.g., between number and size of offspring, or between current reproduction and future survival) are shaped by natural selection in different environments.

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Population Interactions

No species lives in isolation in nature. Organisms interact with other species in their habitat, forming biological communities. Even minimal communities require interactions (e.g., a plant needs microbes for nutrient cycling and often animals for pollination).

Interspecific interactions occur between populations of different species. These interactions can have different outcomes for each species involved – beneficial (+), detrimental (-), or neutral (0).

Types of interspecific interactions:

Species A	Species B	Name of Interaction
+ (benefits)	+ (benefits)	Mutualism
– (harmed)	– (harmed)	Competition
+ (benefits)	– (harmed)	Predation
+ (benefits)	– (harmed)	Parasitism
+ (benefits)	0 (unaffected)	Commensalism
– (harmed)	0 (unaffected)	Amensalism

• Predation (+/-): One species (predator) benefits by feeding on another species (prey), which is harmed.
  • Predators transfer energy to higher trophic levels.
  • They keep prey populations under control, preventing them from reaching densities that destabilise the ecosystem. Introduction of exotic species often leads to rapid spread due to absence of natural predators (e.g., prickly pear cactus in Australia controlled by a moth predator).
  • Predators can maintain species diversity by reducing competition among prey species (e.g., starfish *Pisaster* in intertidal communities).
  • Predators are often 'prudent' to avoid overexploiting prey, which could lead to their own extinction.
  • Prey evolve defenses: camouflage, being poisonous (Monarch butterfly acquires toxin by feeding on weed), warning coloration.
  • Herbivores feeding on plants are ecologically considered predators.
  • Plants evolve defenses against herbivores: morphological (thorns like in Acacia, Cactus) and chemical (producing toxins like cardiac glycosides in Calotropis, or commercially valuable substances like nicotine, caffeine, quinine, strychnine, opium which are defenses).
• Competition (-/-): Both interacting species are harmed (experience reduced fitness). Occurs when species compete for shared, limiting resources.
  • Competition can occur between closely related or even unrelated species for the same resource (e.g., flamingoes and fish competing for zooplankton).
  • Resources don't necessarily have to be limiting; interference competition occurs when one species' efficiency is reduced by the mere presence/activity of another.
  • Gause's Competitive Exclusion Principle states that two closely related species competing for the same limiting resources cannot coexist indefinitely; the competitively inferior one will be eliminated. Evidence exists (e.g., Abingdon tortoise extinction after goat introduction, competitive release of barnacle *Chathamalus* by *Balanus*).
  • However, species can evolve mechanisms to promote coexistence, such as resource partitioning (dividing the resource or using it at different times/ways), as seen in MacArthur's warblers.
• Parasitism (+/-): One species (parasite) benefits by living on or inside the host, deriving nourishment, while harming the host.
  • Parasites often evolve to be host-specific and may co-evolve with their hosts.
  • Adaptations: loss of sense organs, presence of adhesive organs/suckers, loss of digestive system, high reproductive capacity.
  • Complex life cycles often involve intermediate hosts or vectors (e.g., human liver fluke uses snail and fish; malarial parasite uses mosquito vector).
  • Parasites harm hosts by reducing survival, growth, reproduction, population density, and making them vulnerable to predators.
  • Ectoparasites: Live on the external surface (lice on humans, ticks on dogs, copepods on fish). Cuscuta (dodder) is a parasitic plant living on others. (Female mosquito seeking blood is not considered a parasite, but a predator/vector, as it doesn't live *on* the host permanently).
  • Endoparasites: Live inside the host body (liver flukes, malaria parasite, tapeworms). Have simplified morphology but high reproductive potential.
  • Brood parasitism: Parasitic birds (e.g., cuckoo) lay eggs in the host bird's nest, relying on the host to incubate and raise their young. Parasitic eggs often mimic host eggs.
• Commensalism (+/0): One species benefits, and the other is neither harmed nor benefited.
  • Examples: Orchid growing as epiphyte on mango branch, barnacles on whale's back, cattle egret foraging near grazing cattle (cattle stir up insects that egrets eat), clown fish living among stinging tentacles of sea anemone (fish gets protection, anemone is unaffected).
• Mutualism (+/+): Both interacting species benefit. Interactions are often obligate and evolve together (co-evolution).
  • Examples: Lichens (fungus and algae/cyanobacteria). Mycorrhizae (fungi and plant roots - fungus helps in nutrient absorption, plant provides carbohydrates).
  • Plant-animal mutualisms for pollination and seed dispersal: Plants provide rewards (pollen, nectar, fruits), animals provide services. These interactions are often highly specific and involve co-evolution (e.g., fig trees and their specific pollinator wasps, orchids and their pollinators).
  • In the fig-wasp mutualism, the wasp pollinates the fig flowers and lays eggs in the fruit; the wasp larvae develop by feeding on some of the developing seeds provided by the fig.
  • Orchids like Ophrys use sexual deceit: the flower petal mimics a female bee, attracting male bees for pseudocopulation, leading to pollination. Co-evolution is crucial here, as changes in the female bee's appearance must be matched by changes in the orchid flower for successful pollination.

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