A second generality that has been described for assemblages of communities of island fauna derives from competition between species there. Based on the fact that “complete competitors cannot coexist“, a species that tries to colonize an island must be, or do, something different than the ones already there.
Take doves, for example:
Doves are strong fliers, able to cross vast spaces of open water or land in search of food and water. They snack almost exclusively on seeds and fruit (and maybe a few insects in the litter), but usually aren’t too fussy about the types of those food items they exploit. On almost every Caribbean island, there is at least one species of dove present, and often more. But there are rules determining which dove species will colonize islands, because, being generalist seed consumers, there is potential competition for the limited food resources of an island.
White-winged Doves are common in the southwestern deserts of the U.S. where they raise their young, even under the most extreme arid conditions. They time their nesting with the fruiting of the saguaro cacti, whose fruits (and seeds) they devour. However, they migrate through Caribbean islands in the non-breeding season. They could stay there and breed, but for the competition already present.
Island rule #2 suggests that on small islands of limited geographic diversity, species occupying similar ecological niches will differ from each other in body weight by approximately a factor of 2. Some argue that the size differences translate to more efficient use of the resources with less inter-species competition; others argue that size differences prevent potential species hybridization; yet another suggestion is that size variation within the “guild” of seed-eaters prevents further colonization by other, potential competitors. Whatever the reason, the size ratio rule seems to work well for some groups of species on some islands (see Case et al. 1982 for more details)
So, let’s test the validity of this rule with the dove species I saw on Culebra.
Sure enough, there is almost an exact doubling of body weight between Common Ground Dove and Zenaida Dove, and also between Zenaida Dove and Scaly-naped Pigeon.
White-winged Doves are also found on Culebra, and on Puerto Rico as well as many other Caribbean islands, but they don’t breed there. One reason might be their fondness for the seeds of Saguaro cactus found only in the Sonoran desert of southwestern U.S.and Mexico, but another might be the fact that they are roughly the same size as the Zenaida Dove, which has already established its presence during the nesting season on Culebra and Puerto Rico.
“Hey! Get outta’ here, WWD. This place is already taken.”
Traveling Two 