In the presence of abundant resources, populations can grow at geometric or exponential rates. Population growth by organisms with pulsed reproduction can be described by the geometric model of population growth. Population growth that occurs as a continuous process, as in human or bacterial populations, can be described by the exponential model of population growth. Examples of exponential growth from natural populations suggest that this type of growth may be very important to populations during establishment in new environments, during recovery from some form of exploitation, or during exploitation of transient, favorable conditions.As resources are depleted, population growth rate slows and eventually stops; this is known as logistic population growth. As population size increases, population growth eventually slows and then ceases, producing a sigmoidal, or S-shaped, population growth curve. Population growth stops when populations reach a maximum size called the carrying capacity, the number of individuals of a particular population that the environment can support. Sigmoidal population growth can be modeled by the logistic growth equation, a modification of the exponential growth equation that includes a term for environmental resistance. In the logistic model, the rate of population growth decreases as population density increases. Research on laboratory populations indicates that zero population growth at carrying capacity may be attained by many combinations of reduced birthrates and increased death rates.The environment limits population growth by changing birth and death rates. The factors affecting population size and growth include biotic factors such as food, disease, and predators and abiotic factors such as rainfall, floods, and temperature. Because the effects of biotic factors, such as disease and predation, are often influenced by population density, biotic factors are often referred to as density-dependent factors. Meanwhile, abiotic factors such as floods and extreme temperature can exert their influences independently of population density and so are often called density-independent factors. As we have already seen, both abiotic and biotic forces have important influences on populations. The significant effects of biotic and abiotic factors on populations have been well-demonstrated by studies of Galÿpagos finches and their major food sources.On average, small organisms have higher rates of per capita increase, rmax, and more variable populations, while large organisms have lower rates of per capita increase and more stable populations. The intrinsic rate of increase, rmax, is the maximum rate of increase for a given species. This rate of increase would occur in the absence of negative environmental influences. The per capita rate of increase, r, which is the realized rate of increase, is generally less than rmax. One of the best predictors of rmax is body size. In general, rmax decreases with increasing body size, from over 100,000 times from viruses to large vertebrate animals. Though the biological implications of this variation in intrinsic rate of increase are difficult to grasp, we can use the tools of population biology to compare the tempo of population growth by small and large organisms.
The present state of the human population can be examined using the conceptual tools of population biology discussed in chapters 9 and 10 and in chapter 11. Though humans live on every continent, their population density differs by several orders of magnitude in different regions. In 2003, 60.6% of the global population, or about 3.8 billion people, were concentrated in Asia. The remainder of the human population was spread across Africa (13.6%), Europe (11.6%), North America (7.9%), South America (5.8%), and Oceania (0.5%). Population densities in different regions vary from less than 1 person per square kilometer to nearly 1,000 persons per square kilometer. While the populations of some countries are stable, and some are declining, the global population is expected to continue growing past the year 2050. One of the greatest environmental challenges of the twenty-first century will be to establish a sustainable global human population.
