(a) Ecological niche (10 marks) (b) Instinct behaviour in animals (10 marks) (c) Pathogenesis of cholera (10 marks) (d)…

(a) Ecological niche

The ecological niche is the functional role of a species in its community together with the range of conditions and resources it needs. Hutchinson defined it as an n-dimensional hypervolume, each axis being an environmental variable such as temperature, moisture or food size, within which the population can survive and reproduce. Fundamental niche: the full range a species could occupy if competitors and predators were absent. Realised niche: the narrower range it actually occupies because of competition, predation and parasitism. By Gause's competitive exclusion principle, two species cannot share the same niche indefinitely, so they divide resources (resource partitioning). Example: the lion-tailed macaque, Macaca silenus, of the Western Ghats lives chiefly in the canopy of evergreen rainforest and eats fruits, seeds and insects. The habitat is its realised niche, and niche differences let it live beside other arboreal species such as the Nilgiri langur.

(b) Instinct behaviour in animals

Instinct is inborn, stereotyped behaviour that is performed correctly without learning or previous experience, and is characteristic of the species. Fixed action pattern: the unchanging motor sequence that, once started, runs to completion even if the stimulus is removed. Sign stimulus or releaser: the simple cue in the environment that sets it off through an innate releasing mechanism in the nervous system. Example: Tinbergen showed that a male three-spined stickleback, Gasterosteus aculeatus, in breeding season attacks any model with a red underside, even a crude one, but ignores a lifelike model without red. Other examples are the web spinning of spiders, the nest building of weaver birds and the pecking of a gull chick at the red spot on the parent's beak. Value: it ensures survival and reproduction in actions needed immediately, without the cost and risk of learning.

(c) Pathogenesis of cholera

Cholera is caused by Vibrio cholerae (O1 and O139), a curved Gram-negative bacterium taken in with contaminated water or food. Step 1: Bacteria survive stomach acid, reach the small intestine and adhere to the epithelium by toxin-coregulated pili without invading it. Step 2: They secrete cholera toxin, made of one A subunit and five B subunits. Step 3: The B subunits bind GM1 ganglioside on the enterocyte, and the A1 fragment enters the cell. Step 4: A1 ADP-ribosylates the Gsα protein, which locks adenylate cyclase on, so cyclic AMP rises greatly. Step 5: High cAMP opens the CFTR channel, so chloride and bicarbonate flow out and sodium absorption is stopped. Water follows by osmosis. Result: profuse watery "rice-water" stools, dehydration, hypovolaemic shock and acidosis. Treatment is prompt oral or intravenous rehydration.

(d) Amniocentesis

Amniocentesis is a prenatal diagnostic test in which amniotic fluid is withdrawn from the uterus for analysis. Procedure: at 15 to 20 weeks of pregnancy, and under ultrasound guidance, a fine sterile needle is passed through the abdominal and uterine walls into the amniotic sac, and about 15 to 20 mL of fluid is drawn out. Analysis: fluid contains shed fetal cells (amniocytes). They are cultured for karyotyping and microarray analysis to detect chromosomal disorders such as Down, Edwards and Patau syndromes, and for enzyme and DNA tests of inborn errors of metabolism. The fluid is also tested for alpha-fetoprotein, which is raised in neural tube defects. Risks: a small risk of miscarriage, infection and leakage of fluid. Law: the PCPNDT Act, 1994 prohibits using the test to determine the sex of the fetus.

(e) Lotka-Volterra model

The Lotka-Volterra model describes the cyclic interaction of a predator and its prey by two differential equations. Prey: dN/dt = rN − aNP. Predator: dP/dt = baNP − mP. Here N and P are prey and predator densities, r is the prey growth rate, a is the capture rate, b is the efficiency of converting prey eaten into new predators, and m is the death rate of the predator. The isoclines are N = m/(ba) for the predator and P = r/a for the prey. Solutions are closed neutrally stable cycles: prey rise, predators follow after a lag and then reduce prey, and predators fall in turn. Assumptions: prey grow exponentially without predators, there is no carrying capacity, predation is linear in prey density, and the environment is uniform. Limitation: real populations show damped or limit-cycle oscillations, so later models add prey self-limitation and saturating functional responses.