3.6 Summary of Section 3 Many people wish to limit the number of their offspring, and so resort to contraceptive measures. Chemical contraceptives interrupt the production of gametes, or prevent implantation. Mechanical or barrier contraceptives prevent egg and sperm from meeting and, in the case of IUDs, prevent implantation. Surgical methods of contraception involve physical alteration of the reproductive tract so as to prevent e
1.3.5 Natural methods of contraception Many people with particular religious beliefs are fundamentally opposed to the use of artificial methods of contraception. In the developing world, where, as you saw above, the population is frequently increasing at an unsustainable rate, this is a particular problem. For Muslims and Roman Catholics (and others), who may nonetheless wish to limit their families, the preferred option is to use natural family planning methods. The most commonplace method, which involves estimating the ‘fertil
1.3.4 Surgical methods of contraception Surgical methods are by and large the most drastic and irreversible ones, ranging from castration to relatively untraumatic tube-tying. Because of the psychological and physiological side-effects, surgical removal of the testes or ovaries is not generally carried out for contraceptive reasons alone, although these operations may be carried out for other reasons, such as the presence of malignant tumours. Any kind of surgical sterilization can be physiologically traumatic for a woman, as it in
2 How has the human population grown? For most of human history there have been relatively few people in the world. Figure 2 shows that only over the last 50 years have numbers really shot up, and that, at the turn of the century, the population reached over six billion. 1 What is development? In this unit we begin to look at the human being in the context of an individual life cycle, examining some of the processes that contribute to the formation of a new person. This is the first time that many of you will have encountered this level of biological detail; we would ask that you take the time to understand it fully at this stage. We hope to show you that, far from being a dry academic subject, the study of biology allows us to glimpse a dimension of dynamic sophistication and eleg Learning outcomes After completing this unit you should be able to:
Define and use, or recognize definitions and applications of, each of the terms printed in bold in the text.
Explain the scientific basis for the main methods of contraception. (Question 1)
List the factors affecting fertilization. (Questions 2 and 5)
Describe with the help of diagrams the early stages of embryonic development. (Question 3)
References 4 Summary Many rivers are fed by springs, which occur at points where groundwater reaches the surface. Springs can occur in different geological settings, forming valley springs, stratum springs or solution channel springs. The water in a river originates from overland flow, from interflow and from baseflow. Baseflow forms a higher proportion of river water in summer than in winter, and in rivers flowing over good aquifers. River discharg 3.5 Big dams in the future? Throughout the 20th century, reservoir construction to improve water resources was considered a key component of development. It was undertaken universally by industrialized nations, and by the later 20th century, increasingly by developing nations, building bigger and bigger dams. More recently the debate over the environmental impact of big dams and their questionable cost-benefit analysis has been universally recognized. In the 1990s the Narmada scheme caused the World Bank to rethink its 3.4.6 Soil salinization The change from annual flooding by a river to perennial irrigation that can be provided from a reservoir can cause soil salinization, if salts normally present in the river water accumulate in the soil as the water evaporates. These salts were previously washed away by the flooding, but the reduced supply of water by irrigation leaves them in the soil. The water is taken in by plants, or evaporated by the sun, leaving the salts behind. This causes a decline in crop yields until eventually the 3.4.5 Sediment loss to agriculture The trapping of sediment behind dams may also affect agriculture. The Nile Valley, for example, used to flood naturally once a year, and the sediment in the waters was deposited on the land, forming a fertile soil. These floodwaters and the sediment they carry are now trapped behind the Aswan Dam, and artificial fertilizers must be used down river in the valley. Without the yearly supply of sediment in floodwater, the banks of the Nile are eroding downstream of the dam, and the Nile delta is 3.4.2 Ecological changes Creation of a reservoir produces ecological changes not only to the area of the reservoir itself, by destroying the natural vegetation, but also upstream and downstream of the reservoir. The gradient of a river upstream of a reservoir may be reduced, so the water will slow down, changing the character of the river, causing deposition of sediment, and changes to the natural vegetation and animal life. Downstream of the reservoir the discharge will change, as well as the sediment load, also aff 3.4 Environmental effects of reservoir construction Reservoirs may totally alter the water resources of a country. Before the Aswan Dam was completed in Egypt, more than half of the 8 × 1010 m3 of water that flowed down the River Nile through Egypt each year ran into the sea. Most of the water can now be used in Egypt, mainly for irrigation, and instead of a single annual crop grown after seasonal flooding, more than one crop can be grown each year. However, advantages such as these must be considered in conjunction with 2 River flow The total land area drained by a river system, including all its tributaries, is called a river catchment. The water in a river comes not only from direct precipitation, springs and overland flow (i.e. water flowing across the ground surface, excluding that in streams and rivers; this is rare in temperate vegetated areas) but also from the underground flow of water, directly to the river. Part of this underground flow is interflow, that part of infiltration which moves th 1 Springs We have seen that where precipitation reaches the ground, some runs off the surface into streams and rivers and some of it infiltrates, passing through the soil. Water that reaches the water table to become groundwater may eventually re-emerge at the surface as springs where the water table intersects the surface. Almost all streams and rivers have springs or seepages as their ultimate source, or are fed by them at various points along their courses. Artesian springs that are associated Learning outcomes By the end of this unit you should be able to: list the types of springs, and how each type relates to a different geological setting; use hydrographs to distinguish overland flow and interflow from baseflow, and make inferences about the climate of an area; expain how various changes in land use in a river catchment will change the hydrograph of a river; distinguish the different types of reservoir construction, and decide whether a Acknowledgements Grateful acknowledgement is made to the following sources for permission to reproduce material in this unit: The content acknowledged below is Proprietary and used under licence (not subject to Creative Commons licence). See Terms and Conditions. Figure 1 Copyr 8 Summary The rate at which water infiltrates into the ground depends on the permeability of the rocks and the state of the ground surface. Below the ground surface there is an unsaturated zone which has air in the pore spaces, and a saturated zone which has all the pores filled with water. The water table is the boundary between the unsaturated zone and the saturated zone, and is the level at which water stands in wells. Water below the water table is called groundwa 7 Aquifers A layer of rock that is sufficiently porous to store water, and permeable enough to allow water to flow through it, is called an aquifer. Consolidated porous and permeable rocks, for example, sandstone and limestone, can form important and extensive aquifers (e.g. Figure 15). Unconsolidated sands and gravels may also be good 6 Permeability It is important to distinguish clearly between porosity and permeability. Porosity is a measure of how much water can be stored in a rock, whereas permeability is a measure of the properties of a rock which determine how easily water and other fluids can flow through it (see Section 4). Permeability depends on the exte
Figures
