Too dry for cacti: extreme droughts could push iconic species to extinction

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1:55Welcome back to The Nature Podcast. This week, how cactuses are feeling the heat, and how moving beyond BMI could help in diagnosing obesity. I'm Benjamin Thompson.

2:17First up on the show this week, we've got a climate story that centres on an iconic group of plants, cacti. And I spoke to someone who is a big fan of these often spiky succulents. In terms of how many plants I have, at this point, several tens of thousands. So it's gotten a little bit out of hand. This is Mikhail Pillit from the University of South Florida in the US. So I go back and forth a lot between different parts of the country. Some of my plants are in a hotter area and actually have to come inside during summer because they can't take the heat in Arizona. In Florida, they can. So it's such a tremendously diverse family, and they all have their adaptations just like any other organism.

3:01Cacti are found all over the Americas, and the stereotype of hardy heat lovers that can survive in the driest of conditions isn't always true. In fact, just like any other organism, they're also at risk from climate change, and many cacti have been struggling as a result of intense drought conditions. And situations like these are forecast to become more frequent and more ferocious over the next decades. But extreme climate events are complicated and nuanced, making them hard to model. A lot of work on climate change focuses on long-term averages of things like temperature or rainfall, and the impact of extreme events on biodiversity is understudied.

3:46But this week, Mikhail and his colleagues present a climate modelling study on extreme droughts and their potential impacts on almost a thousand species of cacti. I gave Mikhail a call and he told me why cacti aren't as drought resistant and hardy as people might assume. Even though we think as cacti of these very drought adapted organisms, they are really not that different in a way. And this hurts me to say that they're really not that different from any other plant or even animal in the sense that they're adapted to the conditions they evolved under. And so when you change those conditions rapidly, as under climate change, they might not be able to handle those novel conditions.

4:27On top of that, cacti are also one of the most threatened plant families due to habitat destruction and illegal trade. And so there's a real conservation need to look at them and their responses to drop. You were looking at the effects of extreme droughts. What effects in particular on these cacti were you looking to study? So the models we use, they look at the statistical connection between species and their environment. If I were to ask you why there's no cacti, let's say, in Greenland, you'd probably answer because it's too cold over there. And so instead of you answering those questions, we just have a computer do that and attach quantitative criteria to where these species are expected to occur based on observations of where they are in the wild right now and what the environment, so climate is like over there at the moment.

5:19And so based on that model, we're able to project under future climate change how they might shift. So what you're looking at then is the effect that extreme droughts might have on where cacti could live. That's correct. For each individual species, we have separate models that describe where they could live, what climate is suitable, and how that climate shifts geographically. When we talk about drought, about extreme drought, what are we actually talking about? Because I'm sure it's a technical term that has a technical meaning. It's important to keep in mind that what a drought means differs regionally.

5:52What a severe drought is in the UK might be different from what a severe drought is in the Amazon or the Arizona desert. And so we define what a severe drought is based on the baseline for a specific area in terms of what their typical dry conditions are like. And so you and your colleagues have pulled together essentially two data sets, right? We have the climate data set, but you also have the cacti data set with information on a lot of plants. That's thanks to so many researchers, as well as the general public, because we use the community science platform iNaturalist. So we put together this enormous database of occurrences of where different cactus species occur in the wild.

6:35The size of our data set here was definitely facilitated by the fact that cacti are so iconic. So people love to observe cacti in the wild and share those observations with other people. But so, yes, we put together a database and then cleaned it all up and ended up with nearly 650 ,000 records across the cactus family that we were able to use in our models. So you put these two data sets together then to model what might happen if severe droughts added to climate models more generally. So you have these original models that look at temperature averages, and then these other models that include severe droughts of different severities in different locations.

7:13Yes. What did you see then? So when I presented early results to a subgroup of collaborators, they thought I'd done something wrong with these models because of comparison between models without drought and withdraw. It was such a world of difference that we had to go back and verify everything. Turned out that we were correct, however. And what we found is that when we look at the differences between our models with and without severe drought, in both cases, the majority of species decline under climate change. When you consider drought, the number of species does go up a bit. But where the difference between these models really lies, what's so surprising is how intensely species decline.

8:01When you look specifically at species losing over four-fifths of their area with these models, then that goes up from a few percentage points to almost a quarter of all species losing that much of their area. And so species losing 80 % of their suitable habitat is a tremendous loss. And was this seen equally across the Americas? Cacti in the Americas are not just uniformly distributed. There are hotspots. Our projections have a tremendous impact on ecoregions of cacti in areas such as northeastern Brazil and Chile along the Argentinian border as well. in some regions, exceeding an expected 25 % loss of species, and in a couple of cases, exceeding almost 50 % loss.

8:56And you've said there that they're in trouble, many of these plants, like what sort of things at a species level could be impacted? This gets back at the surprising results when we first looked at our models, when we actually went to talk to some of the co-authors who are very heavily involved in field work to do with cacti. What several of them noted is, you know, we're not too surprised by these results. Yes, they seem very intense, but we are in many cases already seeing the impacts of extended severe draw in the field. They're seeing things like reduced flowering. They're seeing reduced growth or plant sunburning in the field.

9:35And in some cases, they're seeing massive die-offs. And so that's definitely been in the media quite a bit in recent years as well, especially in the US Southwest, where people have noted mass mortality of the saguaro cactus, which are the traditional cliche, quote unquote, cactus from the movies, right? And so those are some of the impacts that we are expecting directly. For folk picking up your paper then, and there will be a lot of climate scientists and botanists who will be interested in this, I'm sure. What questions do you think they would have for you? What do you think this work doesn't necessarily do?

10:11So I think that there's still many other different types of extreme events to be included in this. Draught is not the only kind of extreme event. There's also questions about adaptation. However, there tends to be a bit of a mismatch in terms of how fast species can adapt from an evolutionary perspective and the rate of current climate change. Another big one that we're missing is species interactions. For example, pollination, herbivory, and so on and so on, are almost impossible to model for so many species simultaneously. And so I see this study as, one, emphasizing the importance of severe drought, two, showing that we're likely underestimating the risk of extinction, and three, as a triage tool for directing or limited scientific resources in the right direction.

11:01And you've laid out why you've chosen cacti. Obviously, you have a deep personal respect, insight into these plants, but also you use them as a bellwether. One can only assume that this could happen, and obviously this is a potential, this could happen to other plants too. So even though we didn't look at other plants, and we certainly didn't look at animals, we believe that these general insights are applicable to other groups as well, not only in terms of the direct impact of drought, but also like in terms of the interconnections between different species in nature. And finally, then, what would you like done with this information?

11:36One of the main tools that conservationists have to know where to spend resources is the IUCN red list, which is actually where some of these terms that we use like critically endangered and endangered come from. They actually have technical meanings. And so the red list, thanks to the efforts of several of our co-authors, have an almost complete listing of the conservation status of cacti. But it's been over 10 years since they were assessed. And so we'd like to use some of the insights from this paper to reassess all the species of cacti, considering climate change at the same time. That will be really valuable information.

12:14And we've already seen for a subgroup of cacti that the situation in less than 10 years has gotten much, much worse. Mikhail Pillett from the University of South Florida there. To read his paper, look out for a link in the show notes. Coming up, results of a study that's been evaluating the effectiveness of a change in the way that obesity is diagnosed. Right now, though, it's time for the Research Highlights with Dan Fox.

12:43Be it closing a wound or repairing a submarine, glue that can work in wet conditions is pretty handy. But traditional adhesives often perform poorly underwater. Now researchers have taken inspiration from clusters of cholera-causing bacteria to create a new, sticky peptide. The researchers synthesized a peptide called BAP1, which normally forms part of a protein that helps anchor clusters of the bacterium Vibrio cholerae to a range of surfaces. They observed that the peptide bound strongly to a variety of organic and inorganic materials. To test the peptide's strength, the authors used it to fasten two glass slides together and found that even in salty water, it took roughly as much force per square centimetre to pull the plates apart as it would to separate two layers of heavy-duty Velcro.

13:37The authors say that as the glue can also make nanoparticles clump together, it could be used to help remove contaminants from water. Get stuck into that research. You can read it in Science Advances. The unusual remains of a planetary system have given astronomers a glimpse into a possible future for our solar system. When sun-sized stars die, they swell into giant balls of gas and then shed most of their mass, often breaking apart any closely orbiting planets in the process. Sometimes, broken planetary material forms a disk of gas and dust that stays gravitationally tethered to the dead star.

14:23Researchers examining data from the European Southern Observatory's Very Large Telescope in Chile noticed that the spectrum of light emitted by a faraway dead star had an unusual pattern. They concluded that the star is orbited by two concentric disks of debris, a phenomenon that had never been observed before. They posit that these disks formed from two fragmented planets that had separate orbits around the star. The disks contain elements that are also found in the solar system, such as oxygen and iron, suggesting that Earth and other rocky planets could one day disintegrate into similar structures.

15:03Read that earth-shattering news in full in Nature Astronomy.

15:12So there's a paper out in Nature this week where a team of researchers are testing an updated way to diagnose obesity and covering the story for Nature is Mariana Lunaro who joins me down the line from New York. Mariana, thank you so much for being here. Hello, Ben. It's great to be here. So, of course, obesity is something that affects millions of people around the world, and it's associated with a range of health impacts. Yeah, that's correct. Obesity is associated with cardiovascular disease, diabetes, and many other health problems in several organs of the body. These researchers are testing a new way to measure, to define obesity.

15:51So this is proposed as an evolution from measuring obesity just by the BMI, which is the body mass index. Right, because BMI has been a key way for medical professionals to assess if someone may be obese. Now, BMI is a number based on someone's height and weight, and it has some advantages, but it also has several disadvantages too. Yes, the big advantage of BMI is that it's very simple to calculate. It's someone's weight in kilograms divided by the square of their height in meters. And it has been used by clinicians all over the world to classify someone as obese, overweight, or regular weight, or underweight.

16:37So at a population level, someone's BMI correlates with higher risks of several diseases, but at an individual level, the BMI doesn't tell us much about a person's body composition. So within obesity, what's really harmful is the excess fat, especially when it's concentrated around the abdominal area. So just by looking at the BMI, you don't know where the fat is stored. You don't even know if that person has excess fat. For example, a person who's very muscular might have an elevated BMI, but it all may be lean mass and not fat mass. So in practice, clinicians often see that patients with an elevated BMI are pretty healthy and some people with a BMI that's considered normal might have already like health problems arising from an excess fat.

17:40So it doesn't really match people's health. Right, because I measured my BMI today in advance of us chatting, and the actual number puts me in the kind of overweight category. But as you say there, people who do a lot of sports, such as I do, would fall into that number. So there is a nuance to it. And clinicians have debated for years whether BMI is the best metric to define obesity, which leads us on to an idea for a new system that was put forward last year. That's correct. So a group of more than 50 experts came together to think about what would be a better way to define obesity. So they came up with this new metric that involves not only BMI, but also considers someone's waist circumference and another measure that is waist to height ratio.

18:32So based on previous knowledge and previous studies, they came up with this new definition and they're calling it combined anthropometric definition of obesity or CAD obesity. And now what they're doing is that they're putting this new definition to test. So they're analyzing these big data sets of patients from four different countries that include overall 800 ,000 patients and they're applying both definitions of obesity. So the BMI definition, the old definition, and the new definition that includes waist circumference and the other measures. And what they're doing is they're looking at these patients' history and they want to learn whether the new definition is better at predicting the risk of death.

19:25And this, I'm sure, would be useful information. Yeah, because they want to see whether this new definition of obesity is useful in clinics. and it is useful if it can predict the risk of the person developing health problems and eventually dying. So that way they know that if a clinician sees that this person is considered obese according to the new definition then they are at increased risk and they need special treatment and they need special attention. And so what did the results of this study show then? What they found is that indeed the new definition is better than BMI alone at predicting these health outcomes.

20:05And what they also found is that some people who were considered not obese according to the old BMI definition, they are flagged as obese according to the new definition. So this means that the old BMI classification was missing out on identifying people that could be receiving better health care to prevent these health issues. Right. But what was unusual as well is that there was this underestimation, but also BMI could lead to an overestimation as well. Exactly. On the flip side, they found that some people who were considered to be obese under the old definition, with the new definition, they are considered normal weight.

20:52So what they found was that the old BMI definition both overestimates and underestimates obesity. So I'm imagining that when the researchers put forward this new idea for categorization a year ago, they were hoping it would be more accurate. What are they saying about the results now that they're being published? Yeah, they are saying that indeed this new category is more accurate than BMI alone. And they hope that this will be a push to incentivize clinicians and health systems to adopt this new classification. And they think that that will help to identify people that need care and they hope that this will lead to a cultural shift in which clinicians will now start to measure waist circumference in addition to height and weight.

21:48And potentially then to help people who may have been missed by the old method but also prevent unnecessary treatment as well. Yeah that's exactly right. I spoke with one author of this study he's a bariatric surgeon and he said that in his practice he has seen patients who he knows could benefit a lot from a bariatric surgery to lose weight but based solely on BMI they weren't eligible for the treatment and conversely he has also seen patients who were eligible but did not really require such an invasive procedure to manage their weight. So he hopes that this will help correct that mismatch. And the results are out now.

22:33But of course, BMI has its advantages because it is such a simple thing to do, right? If you have a calculator to hand, you can do it in five seconds flat. What have the researchers you've spoken to said about the chances of these new criteria being taken up? The authors of the paper hope that this publication will push clinicians to adopt this new measure now that we can see that it is more accurate than BMI alone. But some people that I've spoke to think that measuring waist circumference is not that simple. It can be kind of an invasive process for someone with obesity. And for people with obesity, sometimes even stepping on a scale can be an uncomfortable thing to do.

23:18So some people think that this is too complex and too complicated and that clinicians can actually evaluate patients by just looking at them. And so it's unclear how this will be received in practice by clinicians. It's something, yeah, we still have to see how this will be adopted. Well, Mariana, thank you so much for coming on the show. We'll put links to your story in the show notes. Mariana Lonaro, thanks for being here. Thank you so much, Ben. And that's all for this week. But before I go, just time to say that I'm joined by the one and only Nick Petrich-Howe. Nick, thank you so much for being here.

23:57Well, happy to be here then. Well, you've been off this week putting the finishing touches to a new series that you've been working on for quite some time, and it's due out very, very soon. That's right. I've got a new mini-series coming out this Friday, and it's all about climate change. and the reason that I've been doing this and I've been spending a year more or less working on this is because I feel like a lot of people I have felt a little bit hopeless on the climate front recently but like a good science journalist I've gone up there I've found some data and looked to see what the situation is actually like and well spoiler alert things are bad but they're not hopeless and so I learned about some recent world events that could really move the needle on renewable energy.

24:40That has the potential to move this energy transition over the edge. The AI, even though it gets a lot of criticism on the environmental front, its impacts are quite unpredictable. There's like 10 crisscrossing vectors of uncertainty that make it really, really hard to say what's going to happen. And that ultimately, even if things feel like they're hopeless, some things are changing for the better. The point is not that these problems are solved in any way. There's still huge problems, but we have started to make progress and there's lots of opportunity for us to make more progress. I've spent a long time putting this together and I'm quite proud of it.

25:17So if it sounds like something that would be up people's streets, then, you know, do give it a listen. The first episode again is coming out on Friday and it'll be live here on this feed that you're listening to this podcast on now. So yeah, give it a listen. Tell us what you think. Yeah, I would thoroughly recommend you give that a listen. It really is terrific. there'll be no briefing show this Friday but yes the first episode of Nick's new series will be up then Nick given that you're here why don't you sign off the show with me listeners of course you can reach out to us on social media at nature podcast or you can send an email to podcast at nature.com I'm Benjamin Thompson and I've been Nick Petrich Howell thanks for listening

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