Population Ecology Chapter 4

Population Ecology Population Dynamics Population Density ▪ The number of organisms per unit area Spatial Distribution ▪ Dispersion is the pattern of spacing of a population. Population Ranges ▪ A species might not be able to expand its population range because it cannot survive the abiotic conditions found in the expanded region. Population-Limiting Factors ▪ There are two categories of limiting factors—density-independent factors and density-dependent factors. Density-Independent Factors ▪ Any factor in the environment that does not depend on the number of members in a population per unit area is a density-independent factor. o Weather events o Fire o Human alterations of the landscape o Air, land, and water pollution Density-Dependent Factors ▪ Any factor in the environment that depends on the number of members in a population per unit area is a density-dependent factor. o Biotic factors o Disease o Competition o Parasites Population Growth Rate ▪ The population growth rate (PGR) explains how fast a given population grows. ▪ The natality of a population is the birthrate in a given time period. Exponential Growth Model ▪ Exponential growth occurs when the growth rate is proportional to the size of the population. ▪ All populations grow exponentially until some limiting factor slows the population’s growth. Logistic Growth Model ▪ The population’s growth slows or stops following exponential growth, at the population’s carrying capacity. ▪ A population stops increasing when the number of births is less than the number of deaths or when emigration exceeds immigration. Carrying Capacity ▪ The maximum number of individuals in a species that an environment can support for the long term is the carrying capacity. ▪ Carrying capacity is limited by the energy, water, oxygen, and nutrients available. Reproductive Patterns Species of organisms vary in the number of births per reproduction cycle, in the age that reproduction begins, and in the life span of the organism. ▪ An r-strategist is generally a small organism. ▪ Short life span ▪ Produces many offspring The carrying-capacity strategy, or k-strategy, is an adaptation for living in stable environments. ▪ A k-strategist is generally a larger organism. ▪ Long life span ▪ Produces few offspring Human Population Growth The study of human population size, density, distribution, movement, and birth and death rates is demography Technological Advances For thousands of years, environmental conditions kept the size of the human population at a relatively constant number below the environment’s carrying capacity. Humans have learned to alter the environment in ways that appear to have changed its carrying capacity. Human Population Growth Rate Although the human population is still growing, the rate of its growth has slowed. Trends in Human Population Growth ▪ Population trends can be altered by events such as disease and war. ▪ Human population growth is not the same in all countries. Zero Population Growth ▪ Zero population growth (ZPG) occurs when the birthrate equals the death rate. ▪ The age structure eventually should be more balanced with numbers at pre-reproductive, reproductive, and post-reproductive ages being approximately equal. Age Structure ▪ A population’s age structure is the number of males and females in each of three age groups: pre-reproductive stage, reproductive stage, and postreproductive stage Human Carrying Capacity ▪ Scientists are concerned about the human population reaching or exceeding the carrying capacity. ▪ An important factor is the amount of resources from the biosphere that are used each person.