Q.(a) Following are the responses of different animals to various abiotic factors. Describe each one with the help of an example.

(a) Regulate = keep internal environment constant (birds/mammals); Conform = internal state varies with environment (most aquatic animals); Migrate = move temporarily to a favourable place (migratory birds); Suspend = enter dormancy/reduced metabolism (bear hibernation, seeds). (b) Death rate $= 8/80 = 0.1$ (10%). OR: trophic level = feeding step; standing crop = biomass present; first trophic level = producers; DFC and GFC are linked by exchange of dead matter and organisms.

Concept (main). No organism can maintain a completely stable internal state against a fluctuating external environment forever; there are four strategies organisms use to cope with abiotic stress.

(a) Responses to abiotic factors (with examples).

1. Regulate: The organism maintains homeostasis — a constant internal environment (body temperature, osmotic concentration) despite changes in the external environment, by physiological (or behavioural) means. Example: Birds and mammals (including humans) thermoregulate to keep body temperature constant (~37°C in humans).
2. Conform: The organism cannot regulate; its internal (body) condition changes with the external environment. Example: The majority of animals and nearly all plants — e.g. aquatic invertebrates and most fishes — are conformers whose body temperature/osmotic concentration varies with the surroundings.
3. Migrate: The organism temporarily moves away from the stressful habitat to a more hospitable area and returns when conditions improve. Example: Migratory birds that come to Keoladeo National Park (Bharatpur) in winter from colder regions like Siberia.
4. Suspend: The organism reduces its metabolic activity and passes the unfavourable period in a dormant/resting state. Examples: Bears hibernate in winter (some snails/fish aestivate in summer); thick-walled spores in bacteria/fungi and seeds/dormant stages in many organisms suspend development until conditions become favourable.

(b) Calculation of death rate. Death rate = number of deaths per individual of the population per unit time.

Deaths = 8; initial population = 80; time = 1 week.

$\text{Death rate} = \frac{8}{80} = 0.1 \text{ individual per butterfly per week} \;(= 10\%)$

OR — Ecosystem: trophic levels, standing crop and food chains.

1. A trophic level is a specific step or level in a food chain, occupied by organisms that obtain their food from the same source (i.e. that are the same number of steps from the producers) — e.g. producers form the first trophic level, herbivores the second, and so on. The standing crop is the amount of living material (biomass) or the number of organisms present in a trophic level at any given time, measured as biomass (fresh or dry weight) per unit area or number per unit area.
2. Role of the first trophic level. The first trophic level is that of the producers — green plants (and autotrophs). They capture solar energy and fix it into chemical energy (food) by photosynthesis, forming the base of every food chain. All other (consumer) trophic levels ultimately depend on the producers for their energy and food; hence the producers determine the total energy available in the ecosystem. …