Four closely related plant species, P, Q, R and S, each grew naturally in only one isolated valley.

Over 15 years, some grassland in each valley was destroyed. A new, fast-growing plant also reached two of the valleys. The new plant grew over the native plants and competed with them for space.

The table shows the results of surveys before and after these changes. A final recorded number of 0 was confirmed by repeated surveys.

Before the changes, no plants of species Q had an inherited characteristic that allowed them to survive when crowded by the competitor. In species S, 35% of the plants had this characteristic.

Scientists proposed two explanations for the differences in survival:

1. the amount of grassland remaining determined whether a species survived
2. the arrival of the new competitor determined whether a species survived.