UGC NTA NET/JRF Exam, Environmental Sciences, September-2022

Secondary succession, type of ecological succession (the evolution of a biological community's ecological structure) in which plants and animals recolonize a habitat after a major disturbance-such as a devastating flood, wildfire, landslide, lava flow, or human activity (e.g., farming or road or building construction)-significantly alters an area but has not rendered it completely lifeless.

Secondary succession is distinguished from primary succession, in which a biological community develops where no life had existed previously. Some of the phases of secondary succession are Nudation, Initiation, Continuation, Aggregation, Termination. Secondary succession takes place where a disturbance did not eliminate all life and nutrients from the environment.

The concept of r- and K-selection was originally given by MacArthur and Wilson (1967) and developed by Pianka (1970) to focus on life-history evolution due to selection pressures on species colonizing an island.

In uncrowded, resource-rich environ ments, natural selection would favour organisms which are able to achieve rapid population growth. On the other hand, in crowded and not so resource-rich conditions natural selection would favour organisms which are competitive and maintain population growth near the carrying capacity.

The former are termed r-strategists and the latter K-strategists. In the r-strategists, the population remains on the ascending part of the logistic growth curve (i.e., dN/dt = rN), while in the K-strategists, population is maintained near the upper asymptote of the logistic growth curve (i.e., when N = K or close to it).

The tolerance curve is a measure of organism's fitness and survival. A bell shaped curve results when measures of survival or fitness are plotted against the environmental gradient (Fig. 1). Those environmental conditions under which an organism grows and reproduces maximally comprise the optimum range.

Fig.1 : The bell shaped curve shows the response of an organism to a range of single environmental variable (Temperature, sunlight or nutrient concentration)

The prefix "steno" (narrow) and "eury" (wide) have often been used for comparing the relative degree of tolerance of an organism to a given environmental factor (Fig. 2).

For example, a stenothermal organism will mean that the organism can tolerate only a narrow range of temperature. With narrow range of tolerance, organisms are concentrated at either end of the curve; having the optimum, minimum and maximum in a very narrow range.

The organisms which are confined towards low temperature are called oligo thermic and those towards higher temperature are polythermal. Even a small change in temperature can have marked effect on the stenothermic organisms. Euthermal organisms have a wide range of tolerance for temperature.

Similarly, stenohydric and euhydric refer to tolerance range of an organism to water. Euryhaline would describe a species with a wide range of tolerance for salinity and stenohaline a narrow salinity tolerance.
Fig. 2: Comparison of the relative limits of tolerance of stenothermal and eurythermal organisms

The Hadley cell, named after George Hadley, is a global-scale tropical atmospheric circulation that features air rising near the equator, flowing poleward at a height of 10 to 15 kilometers above the earth's surface, descending in the subtropics, and then returning equatorward near the surface.

This circulation creates the trade winds, tropical rain-belts and hurricanes, subtropical deserts and the jet streams. A Hadley cell's relatively low-altitude overturning circulation has air sinking at roughly 30 degrees latitude after traveling from near the equator.

The Coriolis effect influences the paths of surface-ocean currents just as it does the winds. Earth's rotation from west to east causes surface-ocean currents to swerve to the right in the Northern Hemisphere, helping establish the circular, clockwise pattern of water currents. In the Southern Hemisphere, ocean currents swerve to the left, thereby moving in a circular, counterclockwise pattern.

The Lower Cape Cod land mass is small, thereby limiting freshwater recharge, aquifer area, and aquifer depth. Consequently, freshwater in the Lower Cape Cod aquifer system does not extend to the underlying granitic bedrock, but instead is primarily bounded below by saltwater.

Depths to the boundary, or interface, between fresh and saltwater within the aquifer system differ among the flow lenses, and are directly proportional to the altitude of the overlying water table.

Because of the density difference between fresh and saltwater, the depth to the freshwater/saltwater interface below sea level is about 40 times greater than the altitude of the water table above sea level.

Therefore, if the water table is 5 ft above sea level, the depth to the freshwater/saltwater interface would be about 200 ft below sea level.

The solar radiation that reaches the Earth's surface without being diffused is called direct beam solar radiation. The sum of the diffuse and direct solar radiation is called global solar radiation. Atmospheric conditions can reduce direct beam radiation by 10% on clear, dry days and by 100% during thick, cloudy days.

Silicon is, by far, the most common semiconductor material used in solar cells. representing approximately 95% of the modules sold today. It is also the second most abundant material on Earth (after oxygen) and the most common semiconductor used in computer chips.

Crystalline silicon cells are made of silicon atoms connected to one another to form a crystal lattice. This lattice provides an organized structure that makes conversion of light into electricity more efficient.

Destructive distillation can be defined as the chemical process through which unprocessed materials are subjected to decomposition by heating them and thereby producing 'cracks' in the relatively large molecules.

Generally, the term 'destructive distillation' is used to refer to the processing of certain organic materials along with limited amounts of oxygen and certain reagents, solvents, or catalysts such as phenols and steam.

The process of destructive distillation is known to create 'cracks' in relatively large molecules. It is not uncommon for these large molecules to break apart completely when subjected to destructive distillation.

For example, when coal is subjected to destructive distillation, a wide range of commercially important products are formed (such as coke, coal oil, ammonium hydroxide, coal tar, gas carbon, and coal gas).

Transesterification gained much acceptance in recent years for the conversion of vegetable oils into products with technically more compatible fuel properties. Transesterification is an imperative process for biodiesel production, as it can reduce the viscosity of the feedstock/vegetable oils to a level closer to the conventional fossil-based diesel oil.