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Chapter 4.1 Population Ecology Population Characteristics • Population Density: The number of living entities per unit of area Population Characteristics • Spatial Dispersion: the pattern of spacing of a population within an area – Uniform – Clumped (groups or herds) – Random • Dispersion patterns depend on the availability of resources Limiting Factors • Limiting Factors: any abiotic or biotic factor that restricts the number, reproduction, or distribution of organisms Limiting Factors • Two types of limiting factors: – Density independent – Density dependent Chapter 4 Population Ecology 4.1 Population Dynamics Density-independent factor: Any factor (that limits population size) in an environment that does not depend on the number of members in a population per unit area. • Examples? Density-Independent Factors Weather events Fire Human alterations of the landscape Air, land, and water pollution • Can wipe out populations of species regardless if there are a few or a lot in a given area Chapter 4 Population Ecology 4.1 Population Dynamics Density-dependent factor: Any factor, that limits population size, in an environment that depends on the number of members in a population per unit area The greater the number of species, the more that are wiped out • Examples? Density-Dependent Factors Usually biotic factors and may include: Disease Parasites Competition for resources Predation Density-Dependent Factors • Example: • If one deer in a heavily populated community contracts tuberculosis, a communicable disease, what will happen to the deer population? • Parasite? Examples of Density-Dependent Factors: • Competition: for food, space, mates • Causes a lower birth rate (natality), increases death rate (mortality), or both Density-Dependent Factors • Predation: interaction between species on different trophic levels • Almost every species is food for another species • Predators (hunters) and prey (the hunted) have, in most cases, existed together for years Predator-Prey Graph • What do you notice about the populations? Chapter 4 Population Ecology 4.1 Population Dynamics Population Growth Rate Population growth rate (PGR) explains how fast a given population grows. What are some factors that would affect how quickly a populations grow? Population Growth Rate • Depends on several factors: • Natality: birthrate (during a given time) • Mortality: death rate (during the same given time) • These two factors affect PGR the most • Emigration: the number of individuals moving away from a population • Immigration: the number of individuals moving into a population • These two factors are usually about the same • Two mathematical models that represent population growth rate: • 1. Exponential Growth Model • 2. Logistic Growth Model Chapter 4 Population Ecology 4.1 Population Dynamics Exponential Growth Model Exponential growth : the growth rate is proportional to the size of the population. All populations grow exponentially until some limiting factor slows the population’s growth. • If all offspring survive and reproduce, the population grows slowly at first • Lag phase: slow growth period • The rate of population growth begins to increase rapidly • Exponential growth phase: rapid population growth due to total number of organisms without any limiting factors • Exponential growth creates a J-shaped curve • Second Model: Chapter 4 Population Ecology 4.1 Population Dynamics Logistic Growth Model The population’s growth slows or stops following exponential growth, at the population’s carrying capacity. • Carrying capacity: the maximum number of organisms in a population that an environment can support for the long term Carrying capacity is limited by the energy, water, oxygen, and nutrients available (resources). Logistic Growth Model • Similar to the exponential growth model, but with an S-shaped curve • Occurs when the population’s growth slows or stops after exponential growth at the population’s carrying capacity Chapter 4 Population Ecology 4.1 Population Dynamics Exceeding the carrying capacity of an area results in death to the organisms living there Boom and Bust Graph • A = “old” carrying capacity • B = exponential growth • C = massive death • D = “new” carrying capacity Chapter 4 Population Ecology Chapter 4 Population Ecology 4.1 Population Dynamics Reproductive Patterns Species vary in the number of births per reproduction cycle, in the age that reproduction begins, and in the life span of the organism. Two main patterns: R-strategy K-strategy Chapter 4 Population Ecology 4.1 Population Dynamics R-strategy: (Rate Strategy) is a population pattern seen where fluctuations in biotic or abiotic factors occur in the environment. An r-strategist is generally a small organism. Short life span Produces many offspring • Examples of r-strategists: • Fruit fly • Mice • Locusts Chapter 4 Population Ecology 4.1 Population Dynamics k-strategy : (carrying-capacity strategy) a population reproduction pattern that is adapted for living in stable environments. A k-strategist is generally a larger organism. Long life span Produces few offspring • Examples: • Bears • Elephants Factors that Contribute to Biotic Potential High (lots of babies) r-strategists Maternal age Low (few babies) k-strategists older Number of offspring 1-2 (low) 1,000s (high) Maternal care months/years short/none Gestation period long short Organism’s size large small younger Human Population 4.2 Chapter 4 Population Ecology 4.2 Human Population Human Population Growth Demography: The study of human population size, density, distribution, movement, and birth and death rates. Chapter 4 Population Ecology 4.2 Human Population Technological Advances For thousands of years, the size of the human population remained relatively constant and below the environment’s carrying capacity. Why is it increasing in population now? Populations are Increasing Because: • Advances in technology: • Farming and domestication of animals • Medicine—antibiotics and vaccines • Better shelter Chapter 4 Population Ecology 4.2 Human Population Human Population Growth Rate Although the human population is still growing, the rate of its growth has slowed. • What might have happened in the 1960s? • What might be responsible for the decline in percent increase in human population (world wide)? Chapter 4 Population Ecology 4.2 Human Population 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. Why not? Demographic Transition • Demographic Transition: a change in growth rate resulting from high birth and death rates to low birth and death rates • Consists of 3 stages • Most countries have not yet undergone a complete demographic transition – 80% of the world’s population lives in these areas • Stage 1: high birth rate and a high death rate – Families have lots of children • Stage 2: Improvements in living conditions – Increased food production – Medical advances – Improved sanitation • Stage 3: The birth rate decreases – Families have fewer children – Population growth slows and may stabilize – Undergone an industrial revolution • Future population growth depends somewhat on how many people of different ages are living in that country today. Chapter 4 Population Ecology 4.2 Human Population Zero Population Growth Zero population growth (ZPG) occurs when the birthrate and immigration rate equal the deathrate and emigration rate. Chapter 4 Population Ecology 4.2 Human Population Age Structure Age structure: the number of males and females in each of three age groups: pre-reproductive stage, reproductive stage, and postreproductive stage. • At zero population growth, an age structure graph should appear more balanced with numbers at pre-reproductive, reproductive, and post-reproductive ages being approximately equal. Chapter 4 Population Ecology 4.2 Human Population 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 by each person.