Rapa Nui, or Easter Island, is currently experiencing the most extreme ecological catastrophe of any inhabited island. Human encroachment and habitat destruction, including catastrophic deforestation, are devastating the island’s environment, exacerbated by invasive species. Recent studies point to the decisive role played by the Polynesian rat (Rattus exulans) on the environmental catastrophe. These pesky rodents completely altered the island’s environment.
In the past, Rapa Nui boasted 15 to 19.7 million Rapa Nui palm trees (Paschalococos disperta). By 1600 CE, these grandiose giants had almost all been logged to extinction. When European explorers arrived in 1722 CE, they discovered only a few scattered single palm trees left. With the arrival of humans, and thus the Polynesian rat, the extinction of many palms was fast-approaching.
In a newspaper article, the authors describe how researchers created an ecological model that shows just how quickly Polynesian rats can reproduce. One breeding pair might generate 11.2 million descendants in only 47 years! With the extraordinary capacity for growth, the rat population was able to take over the island completely. Like locusts, they produced awesome devastation through their ravenous diet of Arecaceae seeds. Indeed, they devastated up to 95% of seeds, making it nearly impossible for palm trees to regenerate.
“Critically, palms produce relatively few seeds per tree. When rats consume or cache most of the seeds, regeneration fails,” – Dr. Carl Lipo.
The impressive seeds of Jubaea chilensis, the island’s dominant palm species, are loaded with starch and oil. The protein and other nutritional content is highly attractive to starved rats. Each palm nut is a pretty big energy reward for the rats, motivating them to chew through the hard shells.
As the combined effects of human exploitation and rat predation pushed Rapa Nui past its carrying capacity, ecological collapse was inevitable. As people chopped down forests to plant crops, they outran by driving rats deeper into the hinterlands. These pests had long since put down roots, forming robust populations on the island.
“Palm nuts were highly vulnerable for several reasons. These seeds are rich in oils and carbohydrates. For rats, each palm nut represents a substantial energy reward worth the effort to gnaw through the shell.”
This research published in the Journal of Archaeological Science does a fantastic job of illustrating why that’s a problem. To fully learn from historic ecological catastrophes, we must take a fully nuanced approach.
These findings push back against the prior assumption that solely blamed deforestation on human activity. Rather, they highlight the need to look beyond one cause or solution to the complex factors at play in ecological crashes.
“Rats reached every corner of the island within decades of human arrival, chewing through palm seeds and stopping the next generation of trees before they could even sprout.”
The research published in the Journal of Archaeological Science serves as a critical reminder that understanding past ecological disasters requires a nuanced perspective.
“For island archaeology, this means we must critically evaluate the relative role of invasives plus people together, not people alone,” – Dr. Terry Hunt and Dr. Carl Lipo.
The findings challenge previous assumptions that solely attribute deforestation to human activity. Instead, they underscore the importance of considering all factors involved in ecological collapses.