Chapter 5.1 How Populations Grow

Characteristics of populations Three important characteristics of populations are: geographic distribution, density, and growth rate. Geographic distribution describes the area inhabited by a population. The range can be very different based on the animals we are looking at. Bacteria may have a very small geographic distribution but migrating whales have a very large distribution. Characteristics of populations

Population density This is the number of individuals per unit area. This number varies on the species and the ecosystem. A saguaro cactus has a low density (meaning there are a few cacti in an area) vs. other plants in that community. Population density

There are 3 things that affect population growth – the number of births, the number of deaths, and the number of individuals to enter or leave the population. A population usually grows if it’s birthrate is higher than it’s death rate. It will decrease if the death rate is higher than the birthrate. Immigration, movement of individual into a population, will cause the population to grow. Emigration – movement of individuals out of a population, will cause the population to shrink. Population growth

If a population has more than enough space and food, is protected from predators and disease, that population will increase in size. A J-shaped curve on a graph of number of individual and time shows exponential growth. This occurs when individuals in a population reproduce at a constant rate. Under ideal conditions, which unlimited resources, a population will grow exponentially. Exponential growth

Population growth slows or stops because resources become less available. This produces an S-shaped curve called logistic growth. Logistic growth occurs when a population’s growth slows or stops following a period of exponential growth. This can happen as a result of birthrate decreases, death rate increases, or both events occurring at the same time. When birthrate and death rate are the same population growth stops. Logistic growth

Carrying capacity is the largest number of individuals a certain environment can hold.