53.1 Organisms must cope with a varied environment.
The Environmental Challenge
• Many plants and animals actively employ physiological, morphological, or behavioral mechanisms to maintain homeostasis. Such mechanisms are the result of natural selection. (pp. 1138-1139)
53.2 Populations are groups of individuals of the same species that live in the space.
Population Ranges
• Boundary edges between populations can range from sharp and impermeable to indistinct and easily permeable. (p. 1140)
• Three aspects of populations are especially important: the range in which a population occurs, the pattern of spacing of individuals throughout their range, and the size a population eventually attains. (p. 1140)
• Population ranges are not static and can change through time, usually as a consequence of environmental change or physiological adaptation. (p. 1141)
Pattern of Spacing of Individuals in a Population
• Individuals are randomly spaced when interactions between individuals are weak or when microenvironmental aspects of the habitat are nonuniform. (p. 1142)
• Uniform spacing often results from resource competition. (p. 1142)
• Clumping or clustering usually occurs as a response to uneven distribution of resources in the immediate environment. (p. 1143)
• Natural dispersal can occur through many avenues, including individuals or their gametes traveling via wind, water, or other organisms. (p. 1143)
Metapopulations
• Metapopulations are a network of distinct populations interacting and exchanging gametes and/or individuals. Such networks usually occur in habitat patches separated by inhospitable habitat. (p. 1144)
• At any given time, not all suitable habitat patches are inhabited, and periodically an inhabited patch will go extinct, only to be colonized later by individuals from a nearby patch. (p. 1144)
• Source-sink metapopulations occur when only some habitats are suitable for long-term population maintenance. In these cases, more permanent populations (sources) continually send individuals to less permanent, poorer habitat patches (sinks), which are periodically purged, often by environmental conditions, and then recolonized from the source population. (p. 1144)
53.3 Population dynamics depend critically upon age distribution.
Demography
• Demography is the statistical study of populations and their changes through time Populations can be studied as a whole or broken down into constituent parts. (p. 1145)
• Many factors affect population growth, including sex ratio, generation time, and age structure of the populations. (pp. 1145-1146)
• Life tables are used to assess how populations change through time, and are often constructed by following a cohort of individuals from birth until death. (p. 1146)
• Survivorship curves can be constructed by graphing the percentage of an original population, or cohort, that survives to a given age. These curves can be divided into three basic categories based on the period of life in which individuals are most likely to die. (p. 1147)
53.4 Life histories often reflect trade-offs between reproduction and survival.
The Cost of Reproduction
• The complete life cycle of an organism constitutes its life history, and all life histories involve significant trade-offs. (p. 1148)
• Natural selection will favor life histories that maximize lifetime reproductive success. (p. 1148)
• A key reproductive trade-off concerns how many resources to invest in producing any single offspring; thus, a trade-off must exist between the number of offspring produced and the size of each offspring. (p. 1149)
• The trade-off between age and fecundity also plays a key role in many life histories. (p. 1149)
• Organisms that invest all their energy into a single large event and die afterwards are termed semelparous, while iteroparous organisms produce offspring several times over many seasons. (p. 1149)
53.5 Population growth is limited by the environment.
Population Growth
• The actual rate of population increase is defined as the difference between the birthrate and the death rate corrected for movement of animals into and out of a population. (p. 1150)
• The innate capacity of growth for any population is exponential. (p. 1150)
• In logistic growth, as a population approaches carrying capacity, its rate of growth slows as fewer resources remain available for use, resulting in a sigmoidal growth curve. (pp. 1150-1151)
Factors That Regulate Populations
• Density-dependent effects are factors that exert increasing pressure on a population as the population increases in size. (p. 1152)
• Density-independent effects are factors that affect the rate of population growth independent of the size of the population. (p. 1153)
53.6 The human population has grown explosively in the last three centuries.
The Advent of Exponential Growth
• Starting in the early 1700s, changes in technology have given humans more control over their food supply and death rate and made them less vulnerable to climatic uncertainties.The human population has thus been able to escape the confines of logistic growth and has grown explosively over the past 300 years. (p. 1155)
• Population pyramids graphically depict the number of people in each age category. (p. 1156)
• Population growth has been more rapid in developing countries, and the age structures of developing countries indicate that this trend will increase in the near future. (p. 1157)
• Per capita resource consumption is a significant factor determining resource consumption, which is highest in the industrialized countries. (p. 1158)