Ecology
Ecology studies the interactions of organisms with each other and with their physical (abiotic) environment. It works at four levels: organisms, populations, communities and biomes.
Major abiotic factors
Factor | Why it matters |
|---|---|
Temperature | affects enzyme activity and metabolism. Organisms tolerating a wide range are eurythermal; most are stenothermal (narrow range) |
Water | life is impossible without it. Organisms tolerating a wide range of salinity are euryhaline; others are stenohaline (many freshwater animals can't live in the sea) |
Light | needed for photosynthesis; many plants and animals use day length (photoperiod) to time flowering and migration |
Soil | its composition, grain size and water-holding capacity decide which plants grow |
Responses to abiotic factors
How do organisms cope with stressful conditions?
Regulate: maintain a constant internal environment (homeostasis). Birds and mammals keep a constant body temperature (thermoregulation) and osmotic balance. About 99% of animals and nearly all plants cannot do this.
Conform: let their body temperature or osmotic concentration change with the surroundings. Most animals are conformers.
Migrate: move temporarily to a better area. The Keoladeo National Park (Bharatpur) in Rajasthan receives migratory birds from Siberia and other cold regions every winter.
Suspend: pause activity. Bacteria, fungi and lower plants form thick-walled spores; seeds go dormant; bears hibernate in winter; snails and fish aestivate in summer; many zooplankton enter diapause.
Small animals such as shrews and hummingbirds are rare in polar regions: with a large surface area relative to volume, they lose body heat very fast.
Adaptations
An adaptation is any attribute of an organism that enables it to survive and reproduce in its habitat.
Desert plants: thick cuticle and sunken stomata to reduce water loss; CAM photosynthesis, with stomata open at night; leaves reduced to spines, and photosynthesis in flattened stems (Opuntia).
Kangaroo rat (North American desert): meets its water needs through internal oxidation of fat and produces very concentrated urine.
Allen's rule: mammals from colder climates have shorter ears and limbs, reducing heat loss.
Seals in polar seas have a thick layer of fat (blubber) under the skin for insulation.
High altitude: a person moving to a place like Rohtang Pass gets altitude sickness (nausea, fatigue, heart palpitations) because of low oxygen. The body gradually compensates by making more red blood cells, lowering the binding affinity of haemoglobin, and breathing faster.
Archaebacteria living in hot springs and deep-sea vents above 100 °C show biochemical adaptations.
Population attributes
A population is a group of individuals of the same species living in a defined area at the same time, sharing resources and able to interbreed.
Attribute | Meaning |
|---|---|
Birth rate (natality) | number of births per member per unit time |
Death rate (mortality) | number of deaths per member per unit time |
Sex ratio | proportion of females and males |
Age distribution | proportion of pre-reproductive, reproductive and post-reproductive individuals, shown as age pyramids: expanding (triangular, wide base), stable (bell-shaped) and declining (urn-shaped) |
Population density (N) | number of individuals per unit area. Sometimes measured by percent cover or biomass, or estimated indirectly (pugmarks for tigers, fecal pellets) |
Example. A pond had 25 water lilies last year, and reproduction added 10 new ones. The birth rate is 10/25 = 0.4 offspring per plant per year.
Population growth
Population density changes through four processes:
Natality (B) and immigration (I) increase it;
Mortality (D) and emigration (E) decrease it.
N(t+1) = N(t) + [(B + I) − (D + E)]
Exponential growth. When resources are unlimited, a population grows faster and faster:
dN/dt = rN, so N(t) = N₀ e^(rt)
where r is the intrinsic rate of natural increase. Plotting N against time gives a J-shaped curve. For the human population of India in 1981, r was about 0.0205.
Logistic growth. Resources are always limited. Every habitat can support only a maximum number of individuals, its carrying capacity (K). Growth slows as N approaches K. This is described by the Verhulst-Pearl logistic equation:
dN/dt = rN (K − N)/K
Plotting N against time gives an S-shaped (sigmoid) curve: a slow lag phase, rapid growth, then a levelling off at K. This model is more realistic than the exponential one.
Life history variation
Populations evolve to maximise their reproductive fitness. Some breed only once in their lifetime (Pacific salmon, bamboo); others breed many times (most birds and mammals). Some produce many small offspring (oysters, pelagic fishes); others produce a few large offspring (birds, mammals).
Population interactions
Interaction | Species A | Species B | Example |
|---|---|---|---|
Mutualism | + | + | lichens (fungus and alga); mycorrhizae (fungi and plant roots); fig trees and fig wasps; orchids and the bees that pollinate them |
Competition | − | − | species competing for the same limited resource |
Predation | + | − | tiger eating a deer; also herbivores eating plants |
Parasitism | + | − | lice on humans; Cuscuta on hedge plants |
Commensalism | + | 0 | an orchid growing on a mango branch; barnacles on a whale; cattle egrets feeding near grazing cattle |
Amensalism | − | 0 | one species is harmed, the other unaffected (e.g. Penicillium inhibiting bacteria) |
(+ means benefit, − harm, 0 no effect.)
Predation transfers energy up food chains, keeps prey populations under control, and helps maintain species diversity. When the prickly pear cactus was introduced to Australia in the 1920s, it spread over millions of hectares because it had no natural predators there; it was controlled only by introducing a cactus-feeding moth from its native habitat. Prey defend themselves through camouflage, being poisonous (the monarch butterfly is distasteful to predators because of chemicals from the weed it eats as a caterpillar), and, in plants, thorns and chemicals.
Competition. Gause's competitive exclusion principle states that two closely related species competing for the same resources cannot coexist indefinitely; the competitively inferior one is eliminated. Species can coexist through resource partitioning: MacArthur showed that five closely related warbler species lived on the same tree by feeding in different parts of it.
Parasitism. Parasites are often host-specific and adapted to their hosts: loss of unnecessary sense organs, hooks and suckers to cling, loss of the digestive system, and high reproductive capacity. Ectoparasites live on the host's surface (lice, ticks); endoparasites live inside (tapeworm, liver fluke). In brood parasitism, the cuckoo lays its eggs in the nest of a crow, which raises its chicks; the cuckoo's eggs resemble the host's in size and colour.
Summary
Organisms cope with abiotic stress by regulating, conforming, migrating or suspending activity.
Populations have birth rate, death rate, sex ratio, age distribution and density.
Exponential growth (dN/dt = rN) gives a J-curve; logistic growth (dN/dt = rN(K − N)/K) gives an S-curve levelling at carrying capacity K.
Interactions: mutualism (+/+), competition (−/−), predation and parasitism (+/−), commensalism (+/0), amensalism (−/0).