51.1 Distribution and Abundance

• Most species identification guides show the range, or geographic distribution, of different species.

  • Two types of factors determine range:

    • Abiotic factors, such as temperature, rainfall, the presence of geographical structures like mountains and oceans, and large-scale ongoing and historical processes such as continental drift.

    • Biotic factors, such as past and current interactions with other species provide habitat, food, or competition.

• The population density-the number of individuals per unit area of Z. vivipαrαvaries throughout its range.

• If individuals from a species occupy many small patches of habitat so that they form many independent populations, they are said to represent a metapopulation (“beyond population”)-a population of populations connected by dispersal.

51.2 Demography and Life History

• Populations grow as a result of births, here meaning any form of reproduction, and through immigration, which occurs when individuals enter a population by moving from another population.

• Populations decline due to deaths and emigration, which occurs when individuals leave a population to join another population.

• Analyzing birth rates, death rates, immigration rates, and emigration rates are fundamental to demography: the study of factors that determine the size and structure of populations through time.

• To predict the future of a population, biologists have to know something about its makeup.

  • They need to know the population’s age structure, and how many individuals of each age are alive.

  • They also need to know how likely individuals of different ages are to survive to the following year, how many offspring are produced by females of different ages, and how many individuals of different ages immigrate and emigrate during each generation time-the average time between a mother’s first offspring and her daughter’s first offspring.

• An age class is a group of individuals of a specific age-for example, all female lizards between 0 (birth) and 1 year old.

• These individuals represent a cohort-a group of the same age that can be followed through time.

• Survivorship-a key component of a life table-is defined as the proportion of offspring produced that survive, on average, to a particular age (the opposite of mortality).

• Fitness trade-offs are the inescapable compromise between two traits that cannot be optimized simultaneously.

• An organism’s life history describes how an individual allocates resources to growth, reproduction, and activities or structures that are related to survival.

51.3 Population Growth

• The difference between the birth rate and death rate per individual is called the per capita rate of increase and is symbolized r.

• This equation is a model of exponential population growth, which occurs when r does not change over time but the instantaneous growth dN/dt increases as population size N increases due to the product of r times N.

• Carrying capacity, K, is defined as the maximum number of individuals in a population that can be supported in a particular habitat over α sustained period of time.

51.5 Case Study: Human Population Growth

• The UN projections are based on three scenarios, which hinge on different values for fertility rates－the average number of surviving children that each woman has during her lifetime.

• The replacement rate is the average fertility required for each woman to produce exactly enough offspring to replace herself and her offspring’s father.