- In 1991, a massive eruption at Mount Pinatubo decimated natural old-growth forests, likely resulting in the natural local extinction of several species, a study notes.
- But surveys carried out 20 years after the eruption show that the landscape is regenerating and is dominated by a possibly endemic rodent species, Apomys sacobianus.
- Biologists know Ap. sacobianus from a single specimen collected in 1956; previous studies conducted by the team show it may be specific only to Mount Pinatubo.
- The rodent species is a “disturbance specialist,” meaning that unlike other Apomys species that thrive on mountaintops, Ap. sacobianus has adapted to living in the lowlands due to eruptions in the past.
MANILA — The eruption of Mount Pinatubo in the Philippines in 1991 spewed enough volcanic ash into the atmosphere to cool the world. In the immediate vicinity of the volcano, a slurry of ash, mud and volcanic debris blanketed 18,000 hectares (44,500 acres) of forest land, triggering fears among biologists that it could have doomed species left undiscovered and unstudied by science.
With limited surveys conducted on the active volcano before the eruption, researchers were particularly interested in how it impacted species, particularly Apomys sacobianus, a mouse species first observed by scientists in 1956 but not seen by them since.
When biologists returned to Mount Pinatubo two decades after the eruption, in 2011 and 2012, what they found surprised them: the landscape had begun to regenerate, and the tiny mouse was the dominant rodent species on the slopes.
Eric Rickart, curator of vertebrate zoology at the National History Museum of Utah and lead author of the new paper detailing the findings, tells Mongabay that the rodent’s presence contradicts conventional wisdom about the fragility of endemic species after a cataclysmic event.
“For many years, we’ve been interested in how Philippine mammals respond to habitat disturbance — both natural and that caused by humans,” Rickart says. “Most conservation biologists think that endemic species, especially those found on tropical oceanic islands, are the most highly threatened species and are least tolerant of disturbance and competition from non-native invasive species. Clearly, Apomys sacobianus doesn’t fit this description.”
In the study, published last September in the special biodiversity edition of the Philippine Journal of Science, Rickart and his team detailed the species that survived the infamous volcanic blast: eight bat species, five native rodents and two native large mammals. The study notes the unique case of Ap. sacobianus, an extreme example of a “disturbance specialist” or one that quickly adapts to natural events.
From 2011 to 2012, the group surveyed five localities covered in the Pinatubo eruption to study how species adapted to the eruption. This “resilience” among rodents, researchers say, is “instrumental in regenerating soil and promoting early revegetation.” Similarly, understanding the natural way the environment recovers from a natural disturbance could also help conservation biologists and forest managers identify ways to reclaim deforested areas.
Before apex predators like raptors dominate a jungle, it’s the rodent species that sneak into landscapes struck by severe disturbances, Rickart says, adding that it’s common for exotic or non-native species to colonize the area and become the dominant species there.
Previous studies showed that non-native plants thrived in the Pinatubo area 15 years after the eruption, leading the team to “expect that non-native pest rats would be most abundant, along with a few hardy native species,” Rickart says. “Only native species present are widespread generalists that can tolerate disturbance.”
But in surveying Pinatubo’s weedy landscape, the researchers found that Ap. sacobianus rules the mountain. The sturdy mouse may have pushed back pest rodent species and restricted them to the upland farms of the Indigenous Aetas people, the study says. “Native small mammals are resilient, moving into areas of regenerating habitat and ultimately displacing non-native species as vegetation matures,” it says.
The study builds upon surveys conducted by Filipino biologist Danny Balete, field survey leader for the Field Museum in Chicago, who died in 2017. The survey used standardized techniques developed for Philippine small mammals. The group spent a total of 4,507 nights laying traps garnished with coconut or earthworms as bait for rodents and used mist nets for bats.
The team noted that it’s possible that some species have moved to different portions of the mountain; they used the same techniques to record the presence of two rodent species, Ap. zambalensis and Bullimus luzonicus, in localities where they were not recorded before.
While there are scattered patches of mammalian studies on Pinatubo, the researchers estimate that several species may have been lost due to the eruption. Pinatubo’s upper slopes were covered with old-growth forests and would have hosted at least six to eight species before the eruption blew its top off.
As is the case with other mountains, it’s possible too that the peak of Pinatubo has at least one endemic species — and it may be Ap. sacobianus, researchers speculate, since the genus is known to thrive on mountaintops. But unlike other Apomys species, Ap. sacobianus might have adapted to the lowlands due to Pinatubo’s long history of explosive eruptions.
“The evidence is very clear that Apomys sacobianus is thriving on Pinatubo and we believe it is adapted to living in disturbed environments — a legacy of having evolved in a place that was regularly disturbed by volcanic eruptions, some of which were much more powerful than the 1991 event,” Rickart says.
While there’s no immediate threat to the species, besides maybe a bigger eruption, researchers say it will probably rule the Pinatubo landscape until the forest regenerates and other native species return.
“If Apomys sacobianus is indeed a ‘disturbance specialist’ as we think it is, it may become less common as forest develops and species that have a greater requirement for forested habitat increase,” Rickart says.
But it’s still possible to see the species in areas with low levels of disturbance, researchers say, as the mouse is living proof that while big species dominate, it’s the small ones that endure.
Rickart, E. A., Heaney, L. R., & Balete, D. S. (2020). Mammals of Mt. Pinatubo, Luzon Island, Philippines: Extreme resilience following catastrophic disturbance. Philippine Journal of Science, 150, 121-133.
Banner image of the eruption-proof Apomys sacobianus in Mount Pinatubo, Philippines. Image by Danny Balete/Field Museum of Natural History.
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