Organism (one tiger) → population (all tigers in a forest) → community (all species in the forest) → biome (a large region, e.g. tropical forest).
Organism and its environment
Major abiotic (non-living) factors
Factor
Key points
Temperature
Eurythermal organisms tolerate a wide range; stenothermal ones tolerate only a narrow range
Water
Euryhaline organisms tolerate a wide range of salt concentration; stenohaline ones a narrow range
Light
Needed for photosynthesis. Controls flowering (photoperiodism). Red algae live deepest in the sea
Soil
Its type, grain size and drainage decide which plants grow
How organisms respond to stress
Regulate: keep the body constant (homeostasis). Birds and mammals (thermoregulation).
Conform: the body changes with the surroundings. 99% of animals and almost all plants.
Migrate: move away for a while. Siberian birds visit Keoladeo National Park, Bharatpur.
Suspend: slow down and wait. Hibernation (winter), aestivation (summer), diapause (zooplankton), spores and seed dormancy.
Adaptations: cool survival tricks
Organism
Adaptation
Kangaroo rat (desert)
Never drinks water! It gets water from breaking down fat in its body (internal fat oxidation), and makes very concentrated urine
Desert plants
Thick cuticle, sunken stomata, CAM photosynthesis (stomata open at night), leaves reduced to spines; Opuntia photosynthesises with its flat stem
Mammals of cold places
Shorter ears and limbs to lose less heat (Allen's Rule)
Seals
Thick layer of fat called blubber under the skin
Humans at high altitude
Altitude sickness (nausea, tiredness) at places like Rohtang Pass. The body adjusts by making more RBCs, lowering haemoglobin's binding affinity and breathing faster
Archaebacteria
Live in hot springs and deep-sea vents, where the temperature is above 100 °C
Populations
A population is a group of individuals of the same species living in an area.
Population attributes
Birth rate (natality) and death rate (mortality): per capita births and deaths.
Sex ratio: e.g. 60% females, 40% males.
Population density (N): how many live in an area. It can be measured by counting, per cent cover, biomass, or indirectly (e.g. tiger pug marks and faecal pellets).
Age pyramids
Shape
Meaning
Triangle (wide base)
Expanding: many young, growing fast
Bell
Stable: not changing much
Urn (narrow base)
Declining: few young, shrinking
What changes population size?
Increase the population: natality (births) and immigration (coming in).
Decrease the population: mortality (deaths) and emigration (going out).
N(t+1) = Nt + [(B + I) − (D + E)]
Population growth models
Exponential growth
Logistic growth
Resources
Unlimited
Limited
Curve shape
J-shaped
S-shaped (sigmoid)
Equation
dN/dt = rN; Nt = N₀eʳᵗ
dN/dt = rN (K − N)/K
Realistic?
Rare in nature
More realistic
Proposed by
—
Verhulst and Pearl
r = intrinsic rate of natural increase. Examples: Norway rat 0.015, flour beetle 0.12; for India's human population in 1981, r = 0.0205.
K = carrying capacity: the maximum population the environment can support.
Life history variation
Some species breed only once in their life (Pacific salmon, bamboo). Most breed many times (birds, mammals).
Some make many small offspring (oysters, pelagic fishes). Others make few large offspring (birds, mammals).
Population interactions
Interaction
Species A
Species B
Examples
Mutualism
+
+
Lichens (fungus + alga), mycorrhiza (fungus + plant roots), fig and wasp, orchid Ophrys and bee
Competition
−
−
Goats vs Abingdon tortoise (Galápagos), flamingos vs fish for zooplankton
Predation
+
−
Tiger and deer, sparrow eating seeds
Parasitism
+
−
Liver fluke, Cuscuta on hedge plants, cuckoo (brood parasite)
Commensalism
+
0
Orchid on a mango branch, barnacles on a whale, cattle egret with cattle, clownfish and sea anemone
Amensalism
−
0
Penicillium stopping bacterial growth
Predation
Predators control prey populations and keep species diversity high.
Prickly pear cactus spread wildly in Australia until a cactus-eating moth was brought in to control it.
When the starfish Pisaster was removed from a coast in America, more than 10 species of invertebrates died out within a year.
How prey defend themselves:
Camouflage: insects and frogs blend in.
Poison: the monarch butterfly is distasteful to birds because of chemicals it gets by eating a poisonous weed as a caterpillar.
Thorns: Acacia and cactus.
Toxic chemicals: Calotropis makes cardiac glycosides, so cattle don't eat it.
Competition
Gause's Competitive Exclusion Principle: two closely related species competing for the same resource can't live together forever. The weaker one is eliminated.
Resource partitioning: species avoid competition by sharing differently. MacArthur showed that 5 species of warblers live on the same tree by feeding at different heights and times.
Parasitism: tricky parasites
Brood parasitism: the cuckoo (koel) lays its eggs in a crow's nest. Its eggs look like the crow's eggs, so the crow raises the cuckoo chicks!
Ectoparasites live outside the host (lice on humans, ticks on dogs, Cuscuta on plants). Endoparasites live inside (liver fluke, tapeworm).
The human liver fluke needs two intermediate hosts (a snail and a fish) to complete its life cycle.
Mutualism: best friends
Fig and wasp: the female wasp pollinates the fig flower while laying eggs in it. The fig feeds the wasp's larvae.
Ophrys orchid (sexual deceit): one petal looks like a female bee. Male bees try to mate with it and carry pollen to the next flower.