The Invader That Grows in Hawaii Is Erasing a Marker of Natural History
What's crowding the space for macroevolution? Plants from Brazil.
The next time you’re on vacation in Hawaii, look for the strawberry guava.
If you see a lot of them, it’s a decent bet you’re amid the greenery on one of the tropical archipelago’s younger, less diverse, less macroevolution-friendly islands.
Native to Brazil and brought to Hawaii in 1825, partly for its “ornamental attributes,” according the US Forest Service, the cherry-like plant is the most common invasive species in Hawaii. It may one day take over half the island chain’s landmass because it has zero natural enemies. Strawberry Guava is easier to find on younger islands like Hawaii (700,000 years old), as opposed to older islands like Kauai (5.1 million years old).
But as the strawberry guava plant spreads, it becomes more difficult to look at an island’s biodiversity to determine its age. In Hawaii, strawberry guava acts as nature’s eraser.
In a study published this week in the journal Proceedings of the National Academy of Sciences, researchers share the results of their examination of 517 plots of forest on the Hawaiian islands. What they found may change the way you think about biodiversity in the natural world.
“The effects of island age on diversity patterns of native species percolate across spatial scales via the distribution of rare species,” write the researchers in their study. “However, we found that biological invasions are eroding the signal of island age on biodiversity.”
Lead researcher Dylan Craven, Ph.D., a plant ecologist from the University of Göttingen, tells Inverse that alien species like the strawberry guava plant lower the biodiversity in forests. The chart below shows how non-native species have overtaken the native ones on younger islands.
Essentially, older forests claim a richer diversity of trees, shrubs, and vines, creating a more robust and densely populated natural environment. Older islands have more rare species, which are more evenly clumped across the island, giving the island a greater range biodiversity, from shore to shore.
“These forests should have some large trees — probably O’hia, the most common native species in Hawaii — but also a wide variety of small trees, tree ferns, and shrubs,” Craven says.
Increased biodiversity encourages the evolution of new species and, later, macroevolution — that’s evolution that happens above the species level — over a span of many geological ages. Macroevolution is the sort of thing that occurs in forests that are allowed to grow to be millions of years old — without competing with alien species.
Craven and his team observed that the invasive strawberry guava plants diluted the signal of “macroevolutionary processes” on islands where they were more prevalent.
“The trouble with invasive species is that because they grow so quickly, and spread so fast, that they ‘crowd out’ the native species, whose populations are also under pressure from land-use and climate change,” Craven tells Inverse. “These multiple pressures make it difficult for native species to maintain viable populations in Hawaiian forests.”
Can Non-Native Influences Ever Be Helpful?
There are many examples of natural influences that can shape a landscape for the better. Craven, who grew up near Mount St. Helens in Washington state, says that after a volcano erupts, what grows in the aftermath of the lava and mud are new grasslands and forests, replacing what was there before.
Craven also brings up the example of certain species of pine trees that grow in old mining areas, and improve conditions so much that native species eventually return.
Beyond the introduction of alien species like the strawberry guava plant, older forests are subject to other threats, like deforestation. Tree replanting efforts, while admirable, aren’t going to bring things back to normal, even after a thousand years.
"Land-use change, which is one of the major drivers of deforestation globally, is erasing millions of years of evolution in less than a day’s work
“Land-use change, which is one of the major drivers of deforestation globally, is erasing millions of years of evolution in less than a day’s work,” Craven says. “While reforesting can have many positive effects when done correctly, planting trees is not sufficient to replace extinct species or to recreate what forests looked like 100 or 200 years ago.”
Hawaii has seen drastic deforestation in the last 100 years, as is brutally spelled out in this compelling article, “Last Stand: The Vanishing Hawaiian Forest.” Craven’s study of biodiversity in older forests puts into stark relief how important it is to preserve them and not introduce non-native species in vulnerable ecosystems.
In an old forest, the further you walk, the more new species you will find, Craven says, because more species are clustered atop each other.
But in younger forests, there are fewer species and less diversity. If you’re hiking on the unique natural laboratory of an island and spot the same species of plant often, you could infer the island is younger and has more real estate for non-native invasive species to flourish. Enter the strawberry guava plant:
If you’ve not got the most trained eye when it comes to identifying plants, apps like Seek use image recognition technology to put a name to the flower or leaf.
“Citizen scientists — even vacationers on Hawaii — can help improve what we know about biodiversity by helping document where species occur,” Craven says.
Biodiversity patterns emerge as a consequence of evolutionary and ecological processes. Their relative importance is frequently tested on model ecosystems such as oceanic islands that vary in both. However, the coarse-scale data typically used in biogeographic studies have limited inferential power to separate the effects of historical biogeographic factors (e.g., island age) from the effects of ecological ones (e.g., island area and habitat heterogeneity). Here, we describe local-scale biodiversity patterns of woody plants using a database of more than 500 forest plots from across the Hawaiian archipelago, where these volcanic islands differ in age by several million years. We show that, after controlling for factors such as island area and heterogeneity, the oldest islands (Kaua’i and O’ahu) have greater native species diversity per unit area than younger islands (Maui and Hawai’i), indicating an important role for macroevolutionary processes in driving not just whole-island differences in species diversity, but also local community assembly. Further, we find that older islands have a greater number of rare species that are more spatially clumped (i.e., higher within-island β-diversity) than younger islands. When we included alien species in our analyses, we found that the signal of macroevolutionary processes via island age was diluted. Our approach allows a more explicit test of the question of how macroevolutionary factors shape not just regional-scale biodiversity, but also local-scale community assembly patterns and processes in a model archipelago ecosystem, and it can be applied to disentangle biodiversity drivers in other systems.