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

(i) Regulate

(ii) Conform

(iii) Migrate

(iv) Suspend

(b) If 8 individuals in a population of 80 butterflies die in a week, calculate the death rate of population of butterflies during that period.

OR

(a) What is a trophic level in an ecosystem? What is ‘standing crop’ with reference to it?

(b) Explain the role of the ‘first trophic level’ in an ecosystem.

(c) How is the detritus food chain connected with the grazing food chain in a natural ecosystem?

(a) Responses of different animals to various abiotic factors:

•Regulate - Some organisms are able to maintain homeostasis by physiological or behavioural means, which ensures constant body temperature. All birds and mammals are capable of such regulation (thermoregulation and osmoregulation).

•Conform - Most animals and plants cannot maintain a constant internal environment. Their body temperature changes with the surrounding temperature. In aquatic animals, the osmotic concentration of the body fluids changes according to the osmotic concentration of the water body.

•Migrate - Some organisms temporarily move away from the stressful habitat to a more hospitable area and return when stressful period is over. For example, birds, during winter undertake long-distance migrations to more hospitable areas. Every winter, we see thousands of migratory birds coming from Siberia to Keolado National Park in Rajasthan.

•Suspend - In bacteria, fungi and lower plants, thick walled spores are formed which help them to survive unfavourable conditions. In higher plants, seeds and some vegetative reproductive structures serve as means to cope up periods of stress as well as help them in dispersal. They do so by reducing their metabolic activity and going into a state of ‘dormancy’.

(b) Death rate is defined as the ratio of the number of deaths occurring in a population of a particular area during a particular period of time.

If 8 individuals in a population of 80 butterflies die in a week,

Death rate in the population is given by 8/80 = 0.1 individuals per week.

OR

(a) All Organisms occupy a particular place in their natural surroundings or in a community according to their interaction with other organisms. Based on their feeding habits, they occupy a specific place in the food chain that is known as their trophic level.

The standing crop is measured as the mass of living organisms (biomass) or the number of organisms living in a unit area at a particular time. The biomass of a species is expressed in terms of fresh or dry weight.

(b) The most important trophic level within the ecosystem is the first level: primary producers. These organisms live without feeding off another level. They only need sunlight and water to survive. They convert solar energy into food. All other organisms depend on these primary producers to obtain their nutrition.

(c) The detritus food chain (DFC) is made up of decomposers (fungi and bacteria) which are heterotrophic/saprotrophic organisms. They obtain their energy and nutrition by degrading dead organic matter or detritus. They secrete digestive enzymes that breakdown dead organic matter into simple inorganic materials that are easily absorbed by them.

Contrary to a terrestrial ecosystem, where larger fraction of energy flows through the detritus food chain, in an aquatic ecosystem, grazing food chain (GFC) is the major conduit for energy flow. Detritus food chain is connected with the grazing food chain at some levels: some of the organisms of detritus food chain (DFC) are prey to the grazing food chain (GFC) animals. We observe that in a natural ecosystem few animals like cockroaches and crows are omnivores.