Hawaii's Oahu island is one of the most disturbed ecosystems in the world. Non-native plants that stowed away on ships or were brought to the island by early settlers for lawns and gardens have now nearly wiped out native plants.
"If you go to most places in Hawaii, with the exception of some high elevation areas and Kauaʻi and the Big Island, you're going to stand in this forest and say, 'Look at that, this beautiful tropical forest,'" said Corey Tarwater, a University of Wyoming Assistant Professor in the department of zoology and physiology. "The chances are, there's not one native plant around you. That it's all invasive plants that have been brought over for one reason or the other."
Invasive birds that were brought to the island for their beauty or for sport have had even more success - there are no longer any native frugivorous birds on the island. It's long been known that these invasive species interact, but how and how much was mostly unknown until Tarwater's research with the Hawaiian Vine Project. She's one of the Principal Investigators on the project that looked at how invasive birds influenced the spread of both native and non-native plants on Oahu. Their findings were recently published in the journal Proceedings of the National Academy of Sciences.
What the researchers uncovered was a complex network between non-native species. They expected the birds to eat the more abundant fruits on the landscape, then poop the seeds out, and the plant would become more abundant in a self-perpetuating cycle. But after looking at the seeds collected from fecal samples, they noticed that abundance had very little to do with what was being eaten.
"We didn't expect that we would find that these birds are actually preferring certain types of fruits and that abundance isn't playing as big of a role as we thought it would or that is generally as the theory would suggest," said Tarwater. "We didn't realize that it'd be these niche-based processes."
Invasive species of plants on Oahu are more likely to fruit for a longer period of time and their fruits are more likely to be smaller and high in lipids - all traits that are desirable by birds. So even if they aren't the most abundant plant on the landscape, non-native birds will prefer to eat from the invasive plants. And by eating the fruits of these plants, the birds help spread their seeds, making them more common.
But even more surprising than the invasive species network they discovered, was the fact that these species have started to specialize in a relatively short amount of time.
"In these novel ecosystems, even though all these invasive species are interacting with each other and they haven't been around each other that long - it's all been less than 100 years - even though they haven't co-evolved with each other, these networks, these interactions between the birds and the plants are really similar to native dominated networks, meaning they have the same kind of structure," said Tarwater.
According to Tarwater, the fact that species interact like this shows that invasive species can become deeply embedded into communities very rapidly. Similar to native-dominated communities, their existence, preferences, and actions can cause cascading effects that can change and structure the whole community. These findings run counter to previous expectations, particularly for islands.
Their role in the structure may also mean that invasive species are much harder to remove from an area than previously thought. But if we know what traits are driving interactions, species with those traits can be targeted for management.
"We can say, 'invasive species that have these traits, these are the ones that the birds love. So those are the ones that we suggest targeting, for removal from certain areas.' Just like we can say, 'They will eat these native plants. So these are the ones that we know that can do well. It can become a self-sustaining population,'" said Tarwater. "Or, 'Some of these threatened and endangered species, they're never going to touch - the birds are not going to disperse them. And so if you want them to be a sustaining population, you won't be able to have them be a self-sustaining population. You will always have to take part in their management, but there are other native species that the birds will eat just fine.' And we can give them these ideas about how to manage the lands and hopefully make it a little more successful."
In the next part of the Hawaiian Vine Project, the team intends to completely remove certain species of invasive plants from an area to see if the birds will change which fruits they're eating, and thus, dispersing. According to Tarwater, this could be an effective management strategy for invasive plants.
Have a question about this story? Contact the reporter, Ivy Engel, at email@example.com.