As we saw in Chapter 54, the size of Earth's human population is a factor in many environmental problems, and it is increasing rapidly (see Figure 54.20). Anyone 40 years old or
58.15 Earth from Space The view from space reminds us that Earth is a small planet. The future of humanity, as well as that of other species, will depend on how we function as stewards of Earth and its resources.
older has lived through a doubling of the human population. But around 1965, the population growth rate peaked and began to decline. The most recent decline before that time, in the fourteenth century, was caused by increased death rates from plagues, wars, and famines. In contrast, the current decline is being caused by voluntary reductions in fertility. Fertility rates have been declining in rich countries for more than a century, but they began to fall in poor countries as well in 1965. Nobody had predicted this change, but the fact that it was voluntary gives us hope that humanity may be able, at least in part, to choose the kind of world in which its children and grandchildren will live.
If humans are to live on Earth in a sustainable manner that provides opportunities for the development of human creativity and rewarding lifestyles, major changes will need to be made in how we use environmental resources. Because we have limited abilities to regenerate and restore degraded natural ecosystems, we will need to develop an increased appreciation of the roles of living organisms in Earth's system. We must not load our ecosystems so heavily that their Plim-soll lines sink below the water line.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.