How Much Food? Resources and Population Size

Limits on Growth

1This summer, students in the Canyon Ridge Middle School science program visited a university lab that studies how populations grow. They worked with yeast, a single-celled fungus that reproduces by budding, as long as it has enough food. Yeast can use sugar as its food. Because yeast cells are tiny and reproduce quickly, scientists often use them as a model for how populations of larger organisms change when resources run short.

2The students set up three flasks. Each flask held 100 mL of water and the same small amount of yeast, and all three were kept at 30 °C. The only difference was the sugar: 1 g, 2 g or 4 g. Every day for five days, the students placed a drop from each flask under a microscope, counted the cells and calculated the number of yeast cells in each milliliter. Their results are in Table 1.

3Later in the summer, the group visited the Desert Hollow Nature Preserve. There, biologists study kangaroo rats, small desert rodents that eat mostly seeds and store extra seeds in their burrows. Many desert plants produce large seed crops only after a rainy winter. Each year, the biologists measure the winter rain, collect and weigh the seeds on sample plots in late spring, and count kangaroo rats in live traps in the fall before releasing them. A hectare is an area of 10,000 square meters, roughly the size of two football fields. Their results are in Table 2 and the graph in Section 2.

4The biologists also keep notes on the animals they trap. In the dry years, many of the kangaroo rats they caught weighed less than usual, and fewer young kangaroo rats were trapped across the preserve. In the wet years, most trapped animals were heavier, and young ones were common. The biologists did not notice any other major change at the preserve, such as new predators or a disease, during these five years.