In simple ecological models, the relationship between a predator and its prey is straightforward, Malthusian. In the classic model, there are rabbits and there are lynxes eating rabbits: the lynxes thrive, reproduce, spawn more hungry lynxes—so many that soon there are fewer and fewer rabbits, fewer lynx meals, eventually fewer lynxes. The lynx population declines, the rabbits slowly recover their numbers, and the cycle of eating and eaten, supply and starvation, begins again. The situation on Guam is altogether different. Even after the forests were emptied of birds, the snake continued to thrive. Its numbers are down: approximately twenty-four snakes per hectare, from a hundred per hectare in the late 1980s. That is a major drop, yet twenty-four snakes per hectare is still four times more dense than even the most snake-infested plot of Amazon jungle. Campbell said, "That's like having a gob versus a big gob." Now the snakes subsist on skinks and geckos. And the skinks and geckos are not disappearing. In fact, because they are no longer preyed upon by birds, they are more abundant than ever. The snakes have found a renewable resource, the gustatory equivalent of solar energy. And there is evidence to suggest that the snakes are reproducing faster too, giving birth at a younger age. Few biological invaders, once they have gained such a solid foothold in their new habitat, subsequently disappear from it entirely. Any hope that the snake would eat itself out of existence appears equally groundless.
In On the Origin of Species, Darwin meditated on the impact that house cats might be having on the surrounding countryside. If there were more cats, there would be fewer mice. With fewer mice burrowing into the hives of their favorite snack, the bumblebee, there would perforce be more bees buzzing about, gathering nectar, pollinating the local blossoms—most notably the blossoms of Trifolium pratense, the common red clover, which depends exclusively on the bumblebee to complete its reproductive cycle. In short, more cats would mean more red clover. "Plants and animals, most remote in the scale of nature, are bound together by a web of complex relations," Darwin wrote.
11 June 2010
Guam: New Predator, New Prey
From Out of Eden: An Odyssey of Ecological Invasion, by Alan Burdick (Farrar, Straus and Giroux, 2005), p. 31: