Chapter 5 Section 1: How Populations GrowIn the 1950s, a fish farmer in Florida tossed a few plants called hydrilla into a canal. Hydrilla was imported from Asia for use in home aquariums because it is hardy and adaptable. The few plants he tossed in reproduced quickly and kept on reproducing. Today, their tangled stems snag boats in rivers and overtake habitats; native water plants and animals are disappearing. Why did these plants get so out of control? Is there any way to getrid of them?Meanwhile, people in New England who fish for a living face a different problem. Theircatch has dropped dramatically, despite hard work and new equipment. The cod catch in one recent year was 3,048 metric tons. Back in 1982, it was 57,200 metric tons—almost 19 times higher! Where did all the fish go? Can anything be done to increase their numbers? How do ecologists study populations?Researchers study populations’ geographic range, density and distribution, growth rate, and age structure.The stories of hydrilla and cod both involve dramatic changes in the sizes of populations.A population is a group of organisms of a single species that lives in a given area, such as the hydrilla population represented on this map.The area inhabited by a population is called its geographic range.A population’s range can vary enormously in size, depending on the species. A bacterial population in a rotting pumpkin may have a range smaller than a cubic meter, whereas the population of cod in the western Atlantic covers a range that stretches from Greenland down to North Carolina.Humans have carried hydrilla to so many places that its range now includes every continent except Antarctica, and it is found in many places in the United States.Population density refers to the number of individuals per unit area.Populations of different species often have very different densities, even in the same environment.A population of ducks in a pond may have a low density, while fish and other animals in the same pond community may have higher densities. Distribution refers to how individuals in a population are spaced out across the range of the population—randomly, uniformly, or mostly concentrated in clumps.An example of a population that shows random distribution is the purple lupine. These wild flowers grow randomly in a field among other wildflowers. The dots in the illustration represent individual members of a population with random distribution.An example of a population that shows uniform distribution is the king penguin. The dots in the illustration represent individual members of a population with uniform distribution.An example of a population that shows clumped distribution is the striped catfish. These fish organize into tight groups. The dots in the illustration represent individual members of a population with clumped distribution.A population’s growth rate determines whether the population size increases, decreases, or stays the same.Hydrilla populations in their native habitats tend to stay more or less the same size over time. These populations have a growth rate of around zero; they neither increase nor decrease in size.