Ecology is the study of the interactions among organisms and between the organism and its physical (abiotic) environment. The basic levels of ecological hierarchy are: Organisms → Populations →  Communities →  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. The rotation of the Earth around the Sun and the tilt of its axis cause annual variations in temperature and duration of day and night, resulting in the formation of major Biomes (e.g., deserts, forests, grasslands).

The most ecologically relevant environmental factors are:

1. Temperature: The most important environmental factor. Organisms that can tolerate a wide range of temperatures are eurythermal (e.g., humans, cats, dogs), and those restricted to a narrow range are stenothermal (e.g., polar bears, most reptiles).

2. Water: Life originated in water. Organisms that can tolerate a wide range of salinity are euryhaline (e.g., salmon, eels), and those restricted to a narrow range are stenohaline (e.g., freshwater fishes, most marine invertebrates).

3. Light: Source of energy for autotrophs. Many plants are dependent on sunlight to meet their photoperiodic requirement for flowering.

4. Soil: Affected by climate, weathering process, and development of the topsoil. Soil characteristics (like pH, mineral composition, and topography) determine the vegetation in an area.

Responses to Abiotic Factors:

• Regulate: Organisms that maintain a constant internal body temperature and osmotic concentration (Homeostasis) despite changes in the external environment. All birds and mammals, and a very few lower vertebrate and invertebrate species, are regulators. The success of mammals is largely due to their ability to maintain a constant body temperature.

• Conform: Organisms whose body temperature or osmotic concentration changes with the ambient temperature or concentration. Most animals and nearly all plants are conformers.

• Partial Regulators: Can regulate up to a limited external range, beyond which they conform.

• Suspend: Organisms can cope with stressful conditions by temporarily suspending metabolic activities.

  • Migration: Temporary movement from the stressful area to a more hospitable area (e.g., Keoladeo National Park, Bharatpur, hosts Siberian Cranes).

  • Hibernation: Winter sleep to escape cold (e.g., Bears).

  • Aestivation: Summer sleep to escape heat and desiccation (e.g., Snails and fish).

  • Diapause: A stage of suspended development in zooplanktons in lakes and ponds under unfavourable conditions.

Adaptations:

• Kangaroo rat in North American deserts can meet all its water requirements through internal fat oxidation (a metabolic process) without drinking water.

• Desert plants (e.g., Opuntia) have thick cuticles, sunken stomata, and their leaves are reduced to spines (to minimize transpiration). They use the CAM pathway to keep stomata closed during the day.

• Allen’s Rule: Mammals from colder climates generally have shorter ears and limbs to minimise heat loss.

• Polar seals have a thick layer of fat (blubber) below their skin to act as an insulator and reduce heat loss.

• Altitude Sickness (in humans at high altitudes >3,500m) is caused by the body not getting enough oxygen due to low atmospheric pressure. The body compensates by increasing red blood cell production, decreasing binding affinity of hemoglobin, and increasing breathing rate.

Population Ecology

Population Attributes:

• • Birth Rates (Natality) and Death Rates (Mortality): Expressed as change in number (increase or decrease) with respect to members of the population.

  • Sex Ratio: Expressed as the ratio of male to female individuals (e.g., 60% of the population are females).

  • Age Pyramids: Plotting the percentage of individuals of a given age group (pre-reproductive, reproductive, post-reproductive) against age. The shape of the pyramid indicates the growth status of the population (Expanding, Stable, Declining).

Population Growth: The size of a population changes depending on four processes: Natality, Mortality, Immigration, and Emigration.

• If $N_t$ is population size at time $t$, then $N_{t+1} = N_t + [(B+I) – (D+E)]$.

• Growth Models:

  • Exponential Growth: When resources are unlimited, the growth is exponential. Represented by the equation: $\frac{dN}{dt} = rN$. Where $r$ is the intrinsic rate of natural increase. The resulting curve is J-shaped.

  • Logistic Growth: When resources are limited (realistic). The population exhibits the Verhulst-Pearl Logistic Growth model. $\frac{dN}{dt} = rN \left(\frac{K – N}{K}\right)$. Where $K$ is the carrying capacity (maximum population size the environment can sustain). The resulting curve is S-shaped. The initial phase is lag phase, followed by acceleration and deceleration, and finally the asymptote (where $N=K$).

Species Interactions: Organisms interact with each other in various ways:

Interaction Type :  Species A →  Species B 

Mutualism (+ , + )               →   Lichens (algae + fungi), Mycorrhizae (fungi + plant roots) 

Commensalism  (+ , 0 )     →   Orchid on a mango branch, Cattle egret and grazing cattle 

Predation  (+ , – )                 → Tiger and deer, Carnivorous insects and prey 

Parasitism  (+ , -)                → Human liver fluke (endoparasite), Lice on human hair (ectoparasite) 

Competition ( – , – )              →  Finches competing for seeds 

Amensalism ( – , 0 )            → Penicillium secretes antibiotic that kills bacteria 

• Predation: Predators play three important roles: transfer energy, keep prey populations under control (biological control), and maintain species diversity by reducing the population of a single superior competitor (e.g., Starfish Pisaster removal caused extinction of 10 invertebrate species in an area).

• Competition: Gause’s Competitive Exclusion Principle states that two closely related species competing for the same limited resources cannot coexist indefinitely; the competitively inferior one will be eliminated. Resource partitioning (choosing different times for foraging) allows co-existence (e.g., MacArthur’s Warblers).

• Parasitism: Ectoparasites (e.g., lice, Cuscuta plant on hedge plants) and Endoparasites (e.g., Plasmodium, liver fluke). Parasites have evolved specialized adaptations like loss of sense organs, presence of adhesive organs, and loss of digestive system. Brood parasitism is seen in cuckoo (koel) laying eggs in crow’s nest.

• Mutualism: Orchids have evolved to have specific pollination mechanisms with insects. Mediterranean orchid Ophrys employs sexual deceit to get pollinated by a species of bee (one petal mimics the female bee).