10.5 The pace of evolution is generally slow but is responsive to selective pressures.

In addition to genetic diversity, the size of the population also makes a difference in how quickly natural selection can produce a change in a population: beneficial traits can spread more quickly in smaller populations simply because it is more likely that the individuals with the trait will find each other and mate. Reproductive rate and generation time also influence how quickly a population can adapt to changes. Many problem species, like insect pests, are r species and have fast generation times, which means they can often stay one step ahead of our efforts to control them. Many endangered species, on the other hand, are K species, with longer generation times; therefore they take longer to recover if population numbers fall (for more on r and K species, see Chapter 7).

The strength of the selective pressure also affects how quickly natural selection might produce a change in a population. One of the reasons the demise of birds in Guam was so stupefying was that it happened so quickly—particularly for the small birds, which were easiest for the snakes to eat. Larger birds disappeared later, when the snakes started eating their nestlings and eggs. While speciation can take thousands or millions of years, extinction can occur much more quickly if the rate of change exceeds the ability of the population to adapt.

To strengthen the case that the brown tree snake was indeed at the root of the bird extinction in Guam, Savidge had to perform a few more experiments. She needed to be sure, of course, that brown tree snakes actually liked eating birds. To do so, she set bird-baited traps all around the islands for the snakes and waited. “I put them in half a dozen locations throughout the island,” she recalls. What she found shocked her: “In one area where the birds were extinct, 75% of my traps got hit within 4 nights,” she recalls. On the other hand, in the baited traps she had set on Cocos Island, an island off the coast of Guam that is not populated by the snakes, all the birds survived.

Savidge knew, however, that birds weren’t brown tree snakes’ only prey. The reptiles also ate small mammals, so she checked to see whether these animals were also being adversely affected by the snakes’ presence. In the 1960s, before the tree snakes had arrived, scientists on Guam had done a survey of small mammal density and had found, on average, 40 small mammals per hectare of land on the island. When Savidge did the same thing in the mid-1980s, she found only 2.8 animals per hectare. “My prediction was that I would see a decline in these rats and mice and shrews, and indeed, it was like a 94% decline,” she explains. The findings suggested that brown tree snakes were devastating small mammal populations in addition to the birds. All in all, Savidge’s three pieces of evidence—the fact that the geographic location of the snakes correlated strongly with the birds’ disappearance, that brown tree snakes liked to eat birds, and that other small mammals also went missing after the snakes’ arrival—convinced Savidge that she had finally solved the mystery of Guam’s disappearing birds. Brown tree snakes, she concluded, were definitely the culprit.