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Today, we’re taking a look at the successes and failures of biological control, the arguments for and against, and what we have learned that can help us do a better job of it. Biological control covers a broad category of techniques, but it usually refers to introducing new predators to an ecosystem to control a particular pest species, usually an invasive one.
Humans have had this habit of introducing new pests to different areas, on purpose or accidentally, and in the places they originally evolved, rats might not have been a big problem. But in a new place, like the islands of Hawai’i, those naturally rat-eating predators aren’t present, and without the pressure those native predators put on the rats’ numbers, their population can explode and start really messing things up.
We can try to control rat numbers with traps, or hunting, or even poison, but these can be difficult to use. So one idea to control rat numbers is to bring in some of those rat-eating predators instead, since not having them around kind of led to the problem in the first place.
To be fair, it doesn’t always work out that way. One study, looking at using insects to control other insects, found that only 11% of attempts achieved complete control. But when it does work, biological control can be very effective and, better yet, it can be self-sustaining, meaning you only have to do it once. If you were using rat poison, you might have to hire a bunch of people every year to go out and spread bait. However, predators can just kind of refresh the program themselves every time they have babies, with their numbers expanding and contracting as prey populations change. Now, that might all sound really nice; you’re restoring balance by having both prey and predator, right? But as elegant as this may sound, if you ask Hawai’ians about it, they may have some other ideas.
In fact, let’s talk about Hawai’i in depth real quick. Because Hawai’i has some of the biggest history with this whole scheme, for both good and bad. Now, Hawai’i, of course, is an island chain very far out in the ocean away from everything else, and it consequently has a lot of really unique species there: endemic species, or the kind of things that aren’t found anywhere else. You’ve got birds like the i’iwi and the pueo, mammals like the Hawai’ian monk seal, and a ton of different plants like the koa tree or the hāpuʻu tree fern. These islands may in fact be home to more than 10,000 unique forms of life.
But, notably, none of those species are mongooses, which are originally found in Africa, Southern Europe, and the southerly parts of Asia. They are never found in the middle of the Pacific Ocean. The story goes that, back in the 1880’s, there were a bunch of plantation owners on the Big Island who were all in an association together. They were upset about rats in their sugar cane fields and got the idea from basically some dude in the Caribbean to introduce mongooses to their fields. So they did. And the idea spread to other Hawai’ian islands. But then the whole idea backfired. Mongooses don’t just eat rats; they eat a lot of things, including some of those really cool native species. And once the mongoose was out of the bag, so to speak, there wasn’t any putting them back. They spread and went off into the wild and had little mongoose babies and they are now just kind of there, even today. And at the time it was totally legal to do this! No regulations existed until 1890, when King Kalākaua of Hawai’i passed some laws, but by then it was too late.
And this wasn’t the only example. One person, William Hillebrand, privately introduced the mynah bird to Hawai’i in 1865 to control army worms. Today, they compete with native birds for food. And of course, Hawai’i isn’t the only place this has played out. Cane toads in Australia may be the most famous example of biological control gone wrong. Introduced from South America to control beetle infestations, they instead killed a bunch of native creatures. They didn’t even eat the beetles. In fact, they may have made it worse by eating the few species that were eating the beetles already.
So yes, there’s a lot of examples of biological control going bad, often due to the hasty introduction of generalists, species who are able to eat or affect multiple prey species. Cane toads are often said to eat anything they can fit their mouth around. And today, we know in hindsight that even without direct predation for a control agent to mess up an ecosystem. For example, the control agents themselves might end up as unexpected food sources for nearby predators. There was a case where scientists introduced a type of fly to an area, only for the nearby deer mice to end up developing a taste for them. The influx of new food made mouse numbers more than double, much to the dismay of everything else that the mice liked to eat.
It’s also possible that, even though a certain agent might be fine here and now, that things will change in the future. Critters can spread beyond the original release area, or climate change may force them to move. Today, some of the worst invasive species anywhere were introduced as biological control agents. So it makes sense that people would be hesitant.
But in kind of a mid-episode twist, it has worked sometimes. But the fact remains that it’s a gamble.
Even in Hawai’i, an invasive species, the prickly pear cactus, has been introduced. Native to the American tropics, it is believed to have been brought to the islands around 1809, and spread quickly to the drier parts of the islands. In the 1940s and 50s, scientists tested a few species to try and control the cactus, including the cactus moth Cactoblastis. This time, they tested a number of candidates before selecting one to release, as some of them had a taste for pineapple, which is not native to Hawai’i. The cactus moths worked and have been keeping the prickly pear in check ever since. The same insect has been used in other places, like India, to control an invasive plant, strawberry guava. In this case, researchers spent 15 years researching before releasing the insect. Ladybugs and a type of fly have also been credited with saving the California citrus industry from cottony cushion scale, and a biological control agent was used to save cassava, a staple food for 200 million people in sub-Saharan Africa from the cassava mealybug.
These examples of biological controls appear to have worked well, without major negative consequences to the environment. But what made these biological controls work where the others didn’t? For one thing, unlike the early releases in the 1800s, biological control is now usually subject to regulation and enforcement. In the United States, laws such as the National Environmental Policy Act were passed in the 1970s. Many countries now require risk assessments before approving any real-life experiments. Scientists have also become more particular about what makes a good biological control agent. For example, they must test it with relevant native species to make sure it won’t affect them, and they must also look at how the critters behave and the chemicals they give off to understand potential interactions with the environment.
Despite these precautions, mistakes can still be made, as seen in the 1960s when a type of weevil was introduced to Canada to control an invasive thistle, but ended up attacking native thistles as well. This leads to the question of whether biological control is a good idea. Ideally, we would stop introducing invasive species, but some are already here and cannot be undone. Proponents of biological control say it is better than letting invasive species wreak havoc, and it appears to have saved Hawai’i from a few species. However, it is still a gamble. We have examples that we can refer to which show that things have worked out well in the past when it comes to biological control. Modern examples have a much better track record than in the past when people would just do whatever came to mind. However, this requires a lot of study to get it right, and if you don’t, it is usually a mistake that cannot be undone, as the mongooses in Hawai’i know all too well.
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