It’s not only giant alien snakes or feral cats that can threaten entire ecosystems.
Now scientists have confirmed that exotic coconut palm trees can also devastate native birds and other species. The large-scale introduction of the Cocos nucifera palm has been a slow-occurring invasion that has disrupted nutrient cycles and caused a cascading effect on birds, other trees, and plant-eaters throughout the Pacific.
In a nutshell, the coconut palms that are the postcard-picture of tropical tranquility are not the kind of trees that seabirds like to nest in. No nests means no constant shower of poop, less nutrients for the soil under the trees, less fertile ground for plants, less nourishing fodder for herbivores.
Photographs such as this one made in the 1940s on the Gilbert Islands helped make coconut palms the icon of tropical paradise. But research has confirmed that the trees may be deleterious to native plants and animals.
NGS stock photo by W. Robert Moore
“Seabirds are shunning the palms as nesting sites, favoring other tree species instead, sending a ripple through island ecosystems,” Stanford University said in a recent news release about a study of coconut palms on Palmyra, an atoll in the South Pacific between Hawaii and Tahiti.
The research was funded in part by the National Geographic Society’s Committee for Research and Exploration.
“With the birds has gone the rich cargo of guano that they normally dispense so freely to the earth under their abodes. The absence of that precious input has caused the soil around the palms to become nutritionally deficient,” Stanford added.
“That, in turn, is lowering the nutritional content of plant species growing around the palms and is causing the creatures that feed on those plants, such as crabs and grasshoppers, to forage elsewhere.”
NGS stock photo of red-footed boobies by James L. Stanfield
“We found that you can get a five- to twelvefold decline in important soil nutrients such as nitrate and phosphate when coconut palms are present, mainly because the birds aren’t there depositing nutrients to that system,” said Hillary Young, a stanford doctoral candidate in biology and member of the research team that conducted a study on Palmyra.
Young is lead author of a paper describing the study, published online this week in the Proceedings of the National Academy of Sciences.
“This is an unusual example of an introduced or spreading plant that causes wide nutrient declines in ecosystems,” Young said. Typically, introduced plants enrich the nutrient content of the ecosystems, Young said.
NGS stock photo of coconut palm trees by W. Robert Moore
How long the palms have been growing on Palmyra, or how they arrived, isn’t clear, according to Stanford’s statement. “Most researchers agree that coconut palms originated in Asia. Coconuts can travel long distances by floating on the ocean currents, but the palm was probably introduced in much of its current range, including areas like Hawaii and the Americas, by early human travelers a few thousand years ago.”
Whether coconuts were brought to Palmyra by oceans or by people, they have clearly proliferated in modern times, and now seem to be causing widespread changes to surrounding plant and animal communities, and the ecosystem as a whole, the university explained.
NGS stock photo of black noddy terns by Bates Littlehales
Nesting seabirds are most likely bypassing the palms on architectural grounds, Young said. “The palms have relatively small canopies with spiky, sharp leaves, so I don’t think they make particularly good nesting habitat for these birds,” she said.
Red-footed boobies form the largest contingent of forest-dwelling seabirds on Palmyra, but black and brown noddies, terns and frigatebirds also nest in the atoll’s forests, Young added.
“Most of these birds are also colonial species, so they like to nest in large groups,” she said. “If you think about it, the coconut palm only has space for maybe one or two nests.”
NGS stock photo of frigatebirds by Bianca Lavies
Young and her colleagues, including senior author Rodolfo Dirzo, professor of biology and the National Geographic grantee, compared the nutrient content of several species of native trees favored by the seabirds, as well as the coconut palms, on some of the different islets that make up the atoll. The islets are typically dominated either by the palms or by native trees, with relatively few mixed species forests, Stanford said.
“Being able to conduct our studies on multiple islets sharing the same general ecological conditions, but with different forest types, was an ideal experimental setup to investigate the cascading ecological consequences of invasive or spreading plants–currently a serious global environmental problem,” Dirzo said.
“All of the tree species we analyzed showed these nutrient changes,” Young said. “Whenever coconuts are present, the nutrient levels decline in the leaves of each species.” Even the palms themselves had lower nutrient levels when growing in a palm forest than when they grew in mixed or native-dominated forests. “Coconut palms don’t increase nutrient levels of anything,” she said.
The researchers tested leaves with different nutrient levels on strawberry hermit crabs and longhorned grasshoppers, species that are widely distributed across Palmyra.
“We would put a crab in a bucket and offer it two different leaves,” Young said. In each test, the researchers used two leaves from the same plant species, but one leaf was from a specimen growing in a coconut palm forest, the other from a tree in a forest dominated by native trees favored by the seabirds. They used the same approach with the grasshopper tasters, but placed them in yogurt containers rather than buckets, Stanford explained.
NGS stock photo of frigatebird by Bianca Lavies
“You couldn’t tell anything was different between these two leaves; we had to mark them as to what type of forest they came from,” Young said. After leaving the taster alone with both leaves for 24 hours, the researchers would measure the change in the size of each leaf.
“I was shocked at the results,” Young said. “There was dramatically higher consumption of leaves that came from plants in native forests, even though they were the same species from the same atoll.”
That was true even for leaves where the difference in nutrient levels only differed by 10 percent, she said. “Nutrient levels are so important to these herbivores that they can detect that and select for only those leaves that have high nutrient levels. It was really impressive.”
The researchers also assessed differences in leaf consumption in the field, using two different methods, each of which also indicated herbivores on the atoll had a profound preference for leaves of trees growing in forests dominated by the native trees.
“The field findings were consistent with the results of the taste tests, all of which showed that levels of leaf consumption by herbivores is reduced in coconut-dominated forests and that nutrient depletion driven by the spread of coconut palms ripples through the ecosystem’s food chain,” Stanford said.
“Seabirds can move a large amount of nutrients to land ecosystems and those movements, if you disrupt them, can have a lot of impacts on the ecosystem where the seabirds live,” Young said.
“The coconut palm is this iconic tree that is everywhere in the tropical world and we all love it, but this study suggests they are actually having deleterious effects on ecosystems where they become dominant.”
NGS stock photo by Luis Marden
Young said the broader implication from the study is that changes in plant communities change connections among ecosystems. “Since humans are changing plant communities the world over in a myriad of ways–invasive species, land use change, resource extractions–it is critical to realize that changes to these plant communities are not isolated,” she said. “An apparently innocuous change to these plant communities can disrupt invisible connections among ecosystems and potentially trigger a cascade of change that can fundamentally alter those ecosystems.”
Once these changes have happened, it can be difficult or even impossible to repair the damage, Young added. “The emphasis needs to be on protecting native plant communities and preventing damaging disruptions from happening in the first place.”
Rob Dunbar, the W. M. Keck Professor in Environmental Earth System Science and senior fellow at the Woods Institute for the Environment, and Douglas McCauley, doctoral candidate at Stanford’s Hopkins Marine Station, are also authors of the PNAS paper.
The research was also funded by the National Science Foundation.