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Ecology: Populations Characteristics of Populations 1. 2. 3. 4. Geographic distribution Density Growth Rate Age Structure Geographic Distribution • Also called Range • Describes the area inhabited by a population Population Density • The number of individuals per unit area; can vary tremendously depending on the species & its ecosystem How many Cacti are there per square kilometer in this desert? Population Growth Three factors that effect population growth 1. Number of births 2. Number of deaths 3. Number of individuals that enter or leave the population Examples • What happens if the number of births are greater than deaths? • What happens if the number of deaths are greater than the number of births? • Which situation will allow for population growth? Movement within populations • Immigration - The movement of individuals INTO an area • Emigration - The movement of individuals OUT OF an area Which of these will allow a population to grow? Exponential Growth • Occurs when the individuals in a population reproduce at a constant rate • Under ideal conditions with unlimited resources, a population will grow exponentially J-shaped curve Logistic Growth • Occurs when a population’s growth slows or stops following a period of exponential growth • This happens as resources become less available causing the growth of a population to slow or stop. S-shaped curve Carrying Capacity - The largest number of individuals that a given environment can support K X – number of individuals t – time K – carrying capacity Limiting Factors • A factor that causes population growth to decrease such as predation, competition, climate extremes or human disturbances Two Types of Limiting Factors 1. Density-dependent 2. Density-independent Density-dependent • Limiting only when the population density reaches a certain level • Biotic factors include competition, predation, parasitism, disease • Operates most strongly when a population becomes very large and dense Competion • When organisms are crowded this causes more competition • Examples: space, food, water, sunlight • This competition can be between different species which may lead to evolution to occupy different niches Predation • Predator-prey relationships are one of the best mechanisms of population control Parasitism and disease • Parasites take nourishment at the expense of the host • Usually weakens but can cause death • Similar to predation Density-Independent • Affect all populations in similar ways regardless of the population size • Abiotic factors include natural disasters, seasonal cycles, unusual weather • Can lead to a characteristic crash • Most populations can adapt to some change • Some changes can effect entire populations with major upsets leading to long-term declines Human Population Growth • Size of human population tends to increase with time • Human population growth was very slow due to harsh conditions and limiting factors • High death rates for years Why did population growth increase? • Life got easier with improvements in industry and agriculture • Safer food supply • Improved sanitation • Improved health care • Decrease in death rate while birth rate had no change • Exponential Growth Demography • Scientific study of human populations • Birthrates, death rates, and the age structure help predict countries growth rates Demographic Transition • The hypothesis as to why population growth has slowed dramatically in countries such as US, Japan, and Europe • A dramatic change in birth and death rates • Historically human societies have had both high birth and death rates Age Structure Diagrams • Help predict future population growth • Bar graph of the number of people in each age group in the population • United States has a slow but steady growth rate • What is the shape of Rwanda’s age structure diagram? • Indicates that there more young children than teenagers and more teenagers than adults. • Predicts a population that will double in approx. 30 years Future Population Growth • Estimated human population by 2050 will be more than 9 billion. • Human population is still growing exponentially. • Will it level out to logistic growth? • How might that happen?