LOGAN — Puerto Ricans love it. Hawaiians hate it. Scholars from Utah State University learn from it.
It’s the coqui, a small frog native to Puerto Rico, where it is controlled by predators, and an accidentally introduced nonnative to Hawaii, where its lack of predators has allowed it to become invasive.
“In Puerto Rico, it is well loved by the people, like a national mascot,” said Karen Beard, associate professor in USU’s Ecology Center and Department of Wildland Resources.
“The coqui has an extremely loud mating call, which everyone in Puerto Rico grew up with. It was introduced to Hawaii in the late 1980s, and they don’t like it as much. It’s not native. It’s nocturnal. It’s loud.”
In some areas of Hawaii, coqui populations have reached about 7.5 frogs per square yard, Beard said.
“That’s higher than anything I’ve ever seen,” she said.
The unchecked coquis are a serious threat to the native insect population, including pollinators of specific Hawaiian plants. Coquis also compete for food with Hawaii’s native bird population.
Beard was a graduate student about the time the coquis were becoming a problem in Hawaii, and she has studied the frogs to learn about evolutionary biology and has guided her graduate students in doing the same.
Beard, former student Eric O’Neill and former Utah State faculty member Mike Pfrender, now at the University of Notre Dame, published a paper about differences between the Puerto Rico and Hawaii populations of coquis in the June 20 online edition of Biology Letters.
Their study will not help control the frog population, nor will it end the males’ lovesick croaking. What it does show is what factors contribute to the genetic makeup of each population.
In the Hawaiian populations of the Caribbean frogs, coquis are born with one of five patterns on their backs, which vary from plain brown to striped, thin stripes to thick and different stripe directions.
“The paper talks about how, in Puerto Rico, the different patterns seem to be maintained by selection pressure, or predation pressure,” Beard said.
“The different patterns allow the frogs to hide successfully from predators in very different terrains, such as tall grass or forest leaf litter. If you hide out in your habitat, you are less likely to get eaten and (more likely) to pass on your genes. So you would expect, and you would find, more coquis with the pattern like leaf litter to be living in the forest.”
Authorities believe the first coquis came to Hawaii hidden in a load of greenhouse plants. Genetic testing from another study has determined that, in all, fewer than 20 coqui individuals are responsible for spawning Hawaii’s current, vast population, and the individuals originated from the forest area near San Juan, Puerto Rico.
Hawaii’s coquis represent only two of the marking patterns found in Puerto Rico, Beard said.
O’Neill’s findings are that, in Puerto Rico, the coqui population’s genetic makeup is determined by natural selection in response to predators, such as birds and snakes. Those individuals with the markings that allow them to hide effectively in their environment survive to reproduce.
In Hawaii, the population is determined by genetic drift, a more neutral, passive process. Without natural predators, any frog that can find a mate contributes to the gene pool, and any uncommon genes of the frogs that don’t mate are lost, making the population even less genetically diverse.
“The take away is that the mechanisms that underlie evolution can really change when the environment is changed,” said O’Neill, now pursuing his doctorate at the University of Kentucky.
Genetic drift is often illustrated with an example using bottles and marbles. If the first bottle contains 20 red and 20 blue marbles, red and blue traits start out equal. If someone moves 10 random marbles (representing offspring) to the next bottle, it could result in more of one color than the other.
Repeating the process with three more bottles, either the red or blue marbles could disappear.
Applying that model to coquis, if selection were random (genetic drift), rather than based on successful traits (natural selection), many diverse genetic traits of the species could be wiped out within a few generations.
Hear the mating call of the male coqui.
Hear the mating call of the male coqui.