On the Curious Case of Insurance Populations, Introductions, and Immigration
by Lakea Lin
Feature Image: Parrot Paradise by Wu Guangzhong
There is a concept in invasion ecology (yes, that’s a real field of study with some established books on the topic, although you are unlikely to find a degree for it… for now…) known as the 10% rule. Of all the new species that are transported to a new environment, only 10% are expected to survive.
If that number seems low, it is unfortunately in line with many wild-caught species as many captive breeding efforts and exotic animal rescues may sadly attest (I could go on about how this is all the more reason to conserve species in situ before the need for captive breeding while discouraging importation, but that would be a much longer article). Of the survivors, 10% are expected to breed. This includes unsuccessful attempts at breeding, such as the production of sterile hybrids which threaten many native species (such as the Koloa Maoli) while complicating efforts to conserve them.
Of those that breed, 10% are expected to establish themselves in an ecosystem, becoming “self-sufficient” enough to persist (although not necessarily sustainably). Lastly, of the 10% that establish, 10% are expected to become invasive. That is only 0.01% of all introduced species, or one in 10,000. It may not seem like much, unless you consider the number of species that could be introduced together (such as the sheer number of microbes that may cling to the bottom of a shoe) and the growing body of evidence that introduced species help other introduced species to establish themselves (especially if they are ecosystem engineers with the power to alter the environment to suit their needs, like earthworms helping pigs in a phenomena known as “invasional meltdown.”
Of course, this is just a model, and real life is far from being this simple. There are factors such as how well intact biodiversity can resist invasion (nature abhors a vacuum, and creating a vacuum by eliminating a perceived pest may invite something far worse, as China’s Four Pests campaign soon discovered) as well as diseases (such as Lyme disease) and many new papers on how invasions tend to go from areas of high biodiversity to areas of low biodiversity (which Hawaii used to be. Is it hard to believe? There were relatively few species compared to the mainland, but the few that made it to Hawaii were far more unique, something we would call high endemism. Unfortunately, that makes the rate of extinction far higher for a small landmass far from other landmasses when compared to a large landmass near another landmass. This is known as the equilibrium theory of island biogeography.
All that being said, many of us are familiar with the many ways that an invasive species can make the headlines. But what about the other 99.99% of introduced species, 0.1% of which (1 in a thousand) are expected to stay in the environment? There are more and more surprising stories about how introduced species have found a way to “fit in.” Urban parrots seem to be sticking to urban jungles, eating ornamental palms and remaining surprisingly free of the diseases that plague their wild relatives whose populations face many threats.
In addition to many escaped parrots, Hawaii has also become home to many geckos. Of these miniature mo’o, the orange-spotted gecko is also endangered in its native range. This does not rule out the possibility that they may be competing with native species for other resources like nesting space, but it does pose some complicated questions for conservationists. Can these introduced populations be considered “insurance populations” that can be used in a repopulation effort if anything happens to the original wild population? If so, should they be protected? As a child of immigrants, I can’t help but think it fitting that species from a home I will never know may have also found a new home while escaping the same threats we did.
Captive bred falcons kept by zoos and falconers were used in successful breeding and reintroduction efforts to restore the Peregrine falcon after their population was decimated by DDT. But species that were introduced to a new environment, especially through the pet trade, may have been exposed to diseases that their wild counterparts have no immunity to. They may have forgotten some important knowledge that their ancestors had, as the captive-bred ‘alalā seemed to be missing the knowledge of how their ancestors contended with the ‘io (they have since been trained using videos of how their American cousins mobbed birds of prey).
It is also possible that they may have become genetically distinct from their wild relatives, like the notorious marbled crayfish which appears to have developed the ability to clone itself while in captivity. This ability, known as parthenogenesis, is a key to becoming invasive, as there is little to control their spread. Many of Hawaii’s geckos also have this ability. Which leads me to my conclusion:
As with many things in life, moderation is key. The most important thing that sets an invasive species apart from other introduced species is the lack of restriction on their spread and activity. Whether that takes the form of having no natural predators or no limits on how much they can consume and reproduce, or no adequate competition for resources, it is easy to liken an invasive species to a malignant tumor that has metastasized. There is an example close to home of a species that is being pushed to the brink due to development pressures and disease. And yet, the Monterey Pine, named for how picky it seems to be when choosing where to lay down its roots, has become aggressively invasive in many other parts of the world. Its roots can reach a depth of three stories, draining water in already arid landscapes. It is difficult to remove and difficult to categorize. After all, we may be asking the wrong questions when determining whether or not a species should be protected, and under what conditions. Life is not so simple.