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Over the centuries, to fight invasive species, some people have considered using... more invasive species. It's called biological control, and even though many early attempts were disastrous, it can actually work to protect agriculture and native plants and animals.

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Velivelli et al 2014, "Biological control agents: from field to market, problems, and challenges"
https://Hawai'ibirdingtrails.Hawai' (Myna birds)'i%2C%20biocontrol%20has%20been,Klamath%20weed%2C%20and%20ivy%20gourd.

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As a SciShow viewer, you can keep building   your STEM skills with a 30 day free trial  and 20% off an annual premium subscription   at [♪ INTRO]  Look at this cute, adorable, fuzzy little  mongoose. It is a horrendous pest and a   nuisance and anyone who lives in Hawai’i will tell  you: Do not be fooled by those beady little eyes.   Mongooses are not indigenous to the islands,  but they live there now thanks to a failed   attempt at pest control.

And yet, today, despite  many catastrophic and well-publicized failures,   some people say that using invasive animals as  pest control is cost-effective, self-sustaining,   and preferable to pesticides. Even in Hawai’i.  To call this debate controversial would be an   understatement. But at the end of this video, you  should understand why anybody would advocate for   doing this.

So let’s take a careful look at  the successes and failures of this strategy,   called biological control, the arguments for and  against, and what we have learned that can help us   do a better job of it. The term biological control  covers a broad category of techniques. It can   refer to things like conserving native predators  to keep pests down.

But we’re going to focus on   what people generally mean when they use the term:  an attempt to control a particular pest species,   usually an invasive one, by introducing new  predators to the ecosystem. You see, humans   have had this habit of introducing new pests to  different areas, on purpose or accidentally. Rats,   for instance.

We’ve brought rats to many islands  via ships, including Hawai’i. Now in the places   they originally evolved, rats might not have  been a big problem. “Pests” is, after all,   a word with a lot of human judgment attached. In  those places, rats were part of a whole ecosystem,   complete with predators to keep them in check,  like owls or foxes or cats.

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. Humans might  dislike rats because they eat the food we’re   growing, but rats also are a danger to many of  Hawai'i’s native birds.

They’ve caused plants to   disappear by eating all their seeds. They even eat  sea turtle eggs. And rats are just one example.   There are plenty of invasive species out there.  Now, we can try to control rat numbers with traps,   or hunting, or even poison, but these can be  difficult to use.

Rats are often pretty good   at avoiding traps, for instance, and putting rat  poison all over the forest would be difficult,   expensive, and, you know, there are other animals  out there who we definitely don’t want to poison.   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. This is the basic   idea behind biological control, as we have defined  it. The predator, or sometimes a disease-causing   pathogen, puts pressure back on the introduced  species.

That’s a key point, since predator-prey   interactions in nature basically never wipe out  the prey species. Think of it more like a thumb on   the scale, where the predator can manage the prey  population enough that it’s no longer a problem.   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. Most of the time,   the control agent couldn’t establish itself well  enough in the ecosystem to work and just kind   of died off.

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.

But 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, well,  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. Even in Hawai'i. For instance,   one of the invasive species that’s come to Hawai'i  is the prickly pear cactus.

Prickly pears are   native to the American tropics. They were likely  introduced to Hawai'i sometime around 1809,   and quickly took over large areas in some  of the drier parts of the islands. In the   1940’s and 50’s, scientists looked at a couple of  different species to try to control the cactus,   including the cactus moth Cactoblastis.

This time  they actually tested a bunch of candidate species   before picking one to release, which is nice since  it turned out some of the candidates also liked   eating pineapple. Which aren’t native to Hawai’i  either, but we like them because it’s pineapple,   but I digress. The cactus moths worked and they’ve  been keeping prickly pear in check ever since.   They’ve even been used in other places too, like  India.

Another invasive plant, strawberry guava,   was controlled after a 2005 release of a scale  insect. In that case, researchers took a whole 15   years to investigate it before daring to release  the insect. Elsewhere, ladybugs and a type of   fly have been credited with saving the California  citrus industry from a pest called cottony cushion   scale.

Then there’s cassava, a staple food that  feeds 200 million people in sub-Saharan Africa.   50% of it was wiped out by the cassava mealybug,  until the bug was brought under control by a   biological control agent. So there are examples of  biological controls seeming to work fairly well,   and without major negative consequences to the  rest of the environment. So the question then   becomes, what made these biological controls  work where the others didn’t?

For one thing,   unlike those early releases in the 1800’s by just,  like, anybody who felt like doing it, biological   control is now usually subject to regulation  and enforcement. In the United States, there   were major laws passed in the 1970’s, such as the  National Environmental Policy Act. Many countries   require risk assessments now before signing  off on any real-life experiments.

We’ve gotten   much pickier about what makes a good biological  control agent as well. Do not use generalists,   for one thing. Today, scientists need to do some  sort of test with relevant native species to see   if the agent will affect them.

If you’re looking  for something to control a plant, for instance,   you’d need to do tests where you give candidates  the option to feed, lay eggs on, or grow on the   target plant. You’d also give them the option to  choose other plants that are present in the area,   and hope they don’t. This may mean looking at  several dozen candidates.

Some scientists have   also suggested expanding this process to include  examining things like how the critters behave,   or the chemicals they might give off, to  understand as many potential interactions   with the environment as possible. Scientists  may also use models to predict what populations   may look like years down the road and follow up  with post-release monitoring to confirm. That is   not to say the process is now foolproof.

In  the 1960’s, a type of weevil was introduced   to Canada to try to control an invasive thistle.  Scientists thought these weevils were specialists,   but it turned out they’re not and it began to  attack native thistles as well. The scientists   had tested the weevil before, but blind spots  in how they set up the tests meant they didn’t   realize there was a problem until it was too late.  So mistakes can still be made. Systems must still   be refined.

And that leaves us with a question:  is biological control a good idea? Well, I mean,   ideally, we’d just stop introducing invasive  species, period. But some are already here,   and we cannot go back in time.

Proponents of  biological control say it’s better than letting   invasive species wreck things. And they do appear  to have helped save Hawai'i from a few invasive   species. We have examples that they can point to  where things seem to have worked out.

What’s more,   modern examples of biological control have a  much better track record than the days of doing   whatever popped into your head. But it requires a  lot of study to get this right. And if you don’t,   it is usually a mistake that you are never able  to take back, as those mongooses know very well.  This SciShow video is supported by Brilliant.  Brilliant is an interactive online learning   platform with thousands of lessons in  science, computer science, and math,   starting with their course on Mathematical  Fundamentals and going literally to Infinity,   like a course about infinity.

And those  mathematical fundamentals can help you calculate   and wrap your mind around how many animals Hawai’i  could end up with when an invasive population goes   unchecked. This course uses puzzles and treasure  hunts to guide you through variables, rates,   and sequences. So you can learn middle school  math concepts in a way that a middle schooler,   or anyone else for that matter, would actually  enjoy.

Brilliant is giving these learning tools to   you for free for 30 days! After that, you can take  20% off an annual premium Brilliant subscription   by clicking the link in the description down  below or going to Thanks   for watching! [♪ OUTRO]