How Populations Grow

How Populations Grow Three important characteristics of a population are: 1) Population density – number of individuals per unit area 150 bullfrogs = 50 bullfrogs per square kilometer 3 sq. kilometers – 2) Geographic distribution – range – area inhabited by population (where do you find the population on Earth?) – 3) Growth rate – net gain or loss to number of individuals in the population Birth rate > death rate; population increases (↑) Birth rate = death rate; population equalizes (=) Birth rate < death rate; population decreases (↓) – 4) Other factors affecting population growth – migration/movement of organisms Immigration vs. Emigration (movement into a population) (movement out of the population) think in think exit

Exponential Growth – aka “J-curve” - Occurs when the individuals in a population reproduce at a constant rate. * Under ideal conditions, with unlimited resources (no limiting factors), a population will grow exponentially. = infinite population size – BUT in reality, this doesn’t continue in nature…

Logistic Growth – aka “S-curve” Occurs when a population’s growth slows or stops following a period of exponential growth * Exponential growth until a population is limited by a factor and the carrying capacity is reached for that habitat Carrying capacity (aka “k”) – largest number of individuals that a given environment can support. Resources become less available – slow or limit population growth

Logistic Growth of Yeast Population Carrying capacity Number of Yeast Cells Time (hours)

Limits to Growth Limiting Factors – factor that causes population growth to decrease Competition, predation, parasitism and disease, drought and other climate extremes, human disturbances

Density-Dependent Factors A limiting factor that depends on population size Affect large populations the most Competition – food, water, space, mates (birds for nesting sites) interspecies – between different species intraspecies – within the same species Predation (moose and wolves) Parasitism and disease (wasp cocoons and larval sphinx moth)

Density-Dependent - Predation 60 2400 50 2000 40 1600 30 1200 20 800 10 400 1955 1960 1965 1970 1975 1980 1985 1990 1995 Moose Wolves

Density-Independent Factors Affect all populations in similar ways, regardless of the population size - unusual weather, natural disasters, seasonal cycles, human activities (damming rivers, clear-cutting forests, etc.)

Human Population Growth Human population increasing over time 3 people per second, 6 billion now, 9 billion in your lifetime Exponential growth following improvements in: medicine, sanitation, agriculture, energy, technology Human population cannot continue to grow exponentially – at some point, it will slow – but when? Around 1800s – English economist Thomas Malthus predicted exponential growth could not occur forever – war, famine, and disease would control human populations * Birth rates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly

Human Population Growth Industrial Revolution begins Agriculture begins Bubonic plague Plowing and irrigation

Demographic Transition Demography – study of human populations Demographic transition – a change in birth and death rates Throughout history, birth rates = death rates; populations were stable Modern countries’ advances in nutrition, sanitation, and medicine cause exponential growth; birth rate exceeds death rates; populations grow As countries modernize, birth rates fall, population growth slows Transition completes when birth rate falls to meet death rate and population growth stops

Age Structure These graphs show the number of people in different age groups

Age Structure Diagrams U.S. Population Rwandan Population Males Females Males Females U.S. age structure predicts slow, but steady growth rate for future = Developed country Rwanda age structure predicts a population to double in 30 years due to many young people = Underdeveloped country