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In this De-Natured segment of Nature League, Brit breaks down a recent scientific journal article about the impact of biodiversity on extinction vulnerability in terms of trophic redundancy.

Article citation:

Dirk Sanders, Elisa Thébault, Rachel Kehoe and F. J. Frank van Veen
Trophic redundancy reduces vulnerability to extinction cascades.
Proceedings of the National Academy of Sciences, 2018

Article link:
http://www.pnas.org/content/pnas/early/2018/02/20/1716825115.full.pdf

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On Nature League, we spend the third week of each month exploring a current trending article from the peer-reviewed literature. Scientific information isn’t /just/ for scientists- it’s for everyone!

It just requires a bit of a break down. [CHEERY MUSIC THEME]. For this month’s De-Natured segment, we’re going to look at an article released in February 2018 in the Proceedings of the National Academy of Sciences. In this month’s Lesson Plan, we talked about the different uses and values of biodiversity.

I mentioned at the end that scientists aren’t sure if biodiversity actually helps stabilize ecosystems, because this is really hard to measure. Well, this article investigated exactly that. The title of the paper is “Trophic redundancy reduces vulnerability to extinction cascades”.

I’ve definitely seen worse when it comes to titles using dense language, but there’s still some vocab we should go through to get off to a strong start. “Trophic redundancy” is the concept of having multiple species in each trophic, or food, level. Basically, a situation where you have multiple producers and multiple consumers at each level of the food chain, or in each available niche, or job. What about “extinction cascades”?

This phrase refers to losing species in a sort of chain reaction process- imagine dominos knocking each other over, but replace those dominos with living species. Directly losing one species is called a “primary extinction”, and if that extinction causes another, we call it a “secondary extinction”, and so on. To understand the connection between redundancy and cascades, let’s consider a grocery store.

There are all kinds of workers in a grocery store- stockers, managers, cashiers, baggers, etc. Trophic redundancy in this situation would be having several stockers, maybe 2 managers, 10 cashiers, something like that. Basically, more than one worker in each role, but each worker only knows how to do /their/ job.

So what happens if you only have a single cashier, and that person decides to quit? There’s no one available to take their place, so that function of the store is gone. Does that then affect other workers?

And this is the question that the authors of the study are asking. If there are multiple species occupying a trophic level, or niche, does that decrease how vulnerable other species will be to extinction? I mean...it’s seems like a simple yes, right?

But that’s the fascinating thing. Even though it makes sense intuitively, it turns out that understanding the relationship between food web complexity and stability is really complicated, and no study has even been able to test any of these predictions with experimental data. Here’s how the authors of this study went about investigating it.

They set up a series of outdoor controlled environments called mesocosms. In these, they put together communities of four trophic, or food web, levels. Here are what these webs looked like:.

There was one plant at the bottom level…that’s the dark green dot. Three species of aphids, which are insects that eat the plant…those are the three light green dots. Three parasitoid wasps that each eat one of the aphids…those are the blue dots, and each one only eats the aphid shown by the line.

And hyperparasitoid wasps that eat all of the parasitoid wasps- that’s the black dot up top. Let’s make our lives a bit easier and shorten some names. For the sake of this segment, we’ll call the parasitoid wasps /wasps/, and call the hyperparasitoid wasps the /top predator/.

So now, we have a plant, some aphids, some wasps, and a top predator. Since there’s only one species of wasp that eats each species of aphic, there is no trophic redundancy in this setup- each species only does one job. In the grocery store version of this, there aren’t any substitutes if the cashier dips out.

The researchers also set up a mesocosm /with/ trophic redundancy. This higher complexity food web looked like the simpler version except they added another plant species, another aphid species, and an additional wasp that ate more than one type of aphid. The communities in all of the replicates were left to grow for about three generations.

At that point, the researchers removed one species of wasps in half of the setups- this represents a primary extinction, and on the picture it’s the wasp that’s crossed out. Then, they watched the other wasps on that trophic level to see if any /secondary/ extinctions occurred. So, what did they find?

Were the redundant, more biodiverse communities more resilient to secondary extinctions? The team found that secondary extinctions were significantly more likely in the simple communities than in the complex ones. This is what they predicted, which is pretty cool; /but/ to explain it, they needed to figure out /how/ it happened?

In all of the experiments, removing the species of wasp made the population of aphids that they eat get denser. Makes perfect sense...it’s way easier to reach a higher population density if you aren’t being, well, eaten. This means the other two aphid species were struggling to compete with these aphids for resources.

Aphid species 2 and 3 were sitting there like, “man, that aphid species is soooo lucky it’s predator was removed, must be nice...” while aphid species 1 just kept on eating. But how does this affect the other wasps? Well, the two aphids getting out-eaten started having smaller populations.

This means the two wasps that only feed on those two aphids have less to eat, and might run into trouble. However, in the complex community with redundancy, that successful aphid was occasionally eaten by the extra wasp the researchers added in. So, the competition for resources wasn’t /as/ bad for the other two aphids, and the wasps that eat them did okay.

Overall, removing one wasp indirectly caused declines in the other wasps at the same trophic level. However, the effect was significantly bigger in the simpler, less biodiverse communities. So, in this study, it turned out that losing biodiversity made the communities more vulnerable to secondary extinctions.

And, since secondary extinctions can make communities simpler, this could cause even more extinctions down the line. So why did this particular article make it into this prestigious journal? Well, here’s what I think.

First off, journals /love/ to publish studies that are novel. The editors like being able to say, “check it out, in our journal there’s this awesome study and it’s /never/ been done before!” That’s certainly part of what’s happening here. Ecologists have long considered the effects of having more species on reducing the risk of extinctions, but the authors state: “Previously, all knowledge on the relationship between food web structure and extinction cascades was based on theory alone.

Our experimental evidence demonstrates that these effects are real.” So, yeah, that’s a big deal. Not only did the team move a concept from theory into real evidence, but they were the first to do it. Another reason this paper made the cut is because it addresses something that is very relevant.

Extinction rates are the highest they’ve been in recent history, and understanding the domino effect of extinctions is absolutely critical moving forward. For example, if we’re going to choose where to put our conservation efforts, we need to know if ecosystems with less redundancy are at more risk. You’ve probably heard that the Devil’s in the details, but in scientific articles he’s definitely in the methods section.

Whenever I read papers in high profile journals, I like to look out for potential pitfalls in the methodology and conclusions. One potential issue with the study is that there are other things that could have caused the secondary extinctions. For example, it might have been something like the sheer number of species present, and nothing to do with the food web redundancy.

Well, the authors had this criticism in mind. To account for confounding variables, they ran simulations in addition to their mesocosm experiments. And sure enough, the simulations also provided clear evidence that redundancy played a significant role, while species richness had no effect.

A separate issue I have with this study is that small and controlled situations set up by researchers are exactly that: small and controlled. I feel like it’s impossible to ever truly capture, much less predict, the unbelievably complex interactions between species in the wild. I mean, it’s possible these results only apply to aphids and wasps, and not to biodiversity overall.

Therefore, I remain open minded and skeptical about the role biodiversity plays in preventing extinction cascades. Thanks for watching this episode of De-Natured! Here on Nature League, we believe that breaking scientific news and articles aren’t /just/ for scientists- they’re for everyone!

And we’ll be here to serve as the translator. We’ll see you next week on “From A to B”, where my friend Adrian and I will continue this month’s discussion of biodiversity.