Even deleting the chestnut blight won’t necessarily bring the chestnut tree back*

This post was originally published on BioDiverse Perspectives – a research blog aimed at fostering communication about biodiversity.

100 years ago, the Eastern United States was a lot different than it is today.  Yeah, there was less urban development, and there were fewer paved roads, dams, and railroads, but by far the biggest difference (at least to an ecologist) was the makeup of the forests.  100 years ago, there stood a huge and dominant tree that is now a mere shrub.  Prior to 1904, the mighty chestnut was one of the most dominant trees in the entire northeast, comprising as much as 40% of the canopy.  Chestnuts grew up to five feet in diameter, and up to 100 feet tall, provided some of the best lumber, and produced some of the most valuable nuts to people and wildlife. But in 1904 in New York City, some chestnuts began to die.  The blight, caused by the introduced fungus, Cryphonectria parasitica, quickly invaded North American forests as it encountered American trees with little natural resistance, girdling and eventually killing them.  By the 1920’s, Cryphonectria had spread to the Appalachian forests and was quickly heading south, and by the 1930’s, the blight had entirely removed chestnuts from the southern Appalachian forests (McCormick and Platt 1980).

Twenty years after the demise of of the chestnut, researchers saw no evidence that it could ever recover, and until now they have been right. But what if the chestnut got a second chance?  What if suddenly and unexpectedly, Cryphonectria disappeared in the temperate deciduous forests of North America?  Because Cryphonectria appears to have been the only initial factor limiting the growth and abundance of chestnut, it is reasonable to believe that if it were to disappear, chestnut populations could recover back to their pre-blight status. And had humans contributed little else to change the forests since the early 20th century, disappearance of Cryphonectria could have as great an effect on Eastern US forests it had twenty years after its arrival to North America.   However, due to current and past land-use, chestnut interacts with a different suite of co-occurring species and must contend with a shifting climate. Therefore, while deleting Cryphonectria may change deciduous forests, it would do little to restore the chestnut as a dominant species.

To illustrate my point, lets delve a little more into the biology of the pathogen and the host.

What is Cryphonectria?

Crhyphonectria, a member of the phylum Ascomycota, grows on the shoots of Castanea.  The mycelium produces a canker inside the bark of the tree, and once the canker has encircled the entire stem, it girdles and kills it.  This leads to the death of the majority of the tree. But Cryphonectria never enters the roots, leaving them intact to produce shoots known as stump-sprouts.  As a result, chestnut is reduced to an understory shrub rather than being completely eked out.

What does this mean for the chestnut?  It means that with the disappearance of Cryphonectria, Chestnuts would not have to rely solely on seed dispersal to begin returning back into the forests.

A Closer Look at the Chestnut Tree, Castanea

The chestnut is a highly efficient seed disperser. And this may be one of the primary factors that contributed to the past dominance of the Chestnut tree in North American forests.  Chestnut seeds are nuts, which are protected by a thick husk that allows the seeds to survive past their most vulnerable stage to one that is more conducive to dispersal.  Chestnuts also produce massive quantities of seeds, and they produce them mid-summer, which protects the nuts from the potential impacts of frost.  The nuts are also highly palatable and rapidly harvested by animals, which aids in dispersal. This means that if chestnuts could grow large enough to produce seeds and become abundant enough to overcome seed predation, they could potentially proliferate quickly towards forest dominance. And because Castanea can produce shoots from already existing (and rather abundant) root stock, they should be able to produce seeds quickly, relative to reestablishing from seed.

What happened to North American deciduous forests after the Chestnuts died?

We don’t know a lot about chestnut ecology before the blight, but thanks to one plot of land on Beanfield Mountain, we know something about how forests responded to the chestnut’s demise. Prior to the blight, Castanea was a co-dominant species in all the sloped forests of Beanfield Mountain.  In 1939, about twenty years after introduction of Chestnut Blight to the mountain, the only perceptible difference in composition was the absence of chestnut. After about 50 years, openings in the forest canopy were eventually filled by hickory, as Eastern US forests shifted from chestnut/oak to oak-hickory dominated. Then, largely due to fire-exclusion during the early 20th century, populations of red maple invaded forests, and now red maple is one of the most abundant trees in eastern forests (Lord 2004).

How did chestnuts get to North America last time?

Long before they were deciduous, North America was home to boreal forests.  Then, around 16,000 years ago in the south, and 10,000 years ago further to the north, deciduous forest began to take over the landscape. But Castanea was the slowest species of tree to establish, expanding at roughly 100 meters per year, and reaching sites near Connecticut only as recently as 2,000 years ago, even though it was present near Memphis 13,000 years prior.  Margaret Davis (1983) suggested that that despite high seed production and dispersal, the fact that Castanea is self-sterile could be a leading factor for such a slow rate of establishment, and still account for the huge proliferation once established.

What does this mean for the chestnut tree?

What does this tell us about reestablishment of chestnut following the disappearance of Chryphonectria?  First, though chestnuts are not currently producing seed, they do so rapidly, yet they distribute very slowly as a species.  Second, despite their slow dispersal, they still exist in many areas as small shrubs awaiting an opportunity to grow.  And third red maple is their primary competitor in a world without chestnut blight.

 Factors Limiting Reestablishment of Castanea

Ok, you’re saying, the chestnut’s chances don’t sound so dire. They are already in forests, and they produce massive quantities of seed when mature. What’s the deal?

Plant competition for light is asymmetrical.  With a large enough canopy, trees can effectively block sunlight from reaching the branches of lower plants, slowing growth and seed production.  The chestnut has been reduced to an understory shrub. And although plants can survive for decades in the shade, and despite being widely considered one of the fastest growing canopy trees in the Appalachian forest when under direct sunlight, the chestnut’s growth is heavily stunted by shade (Bass 2002).  As a result, despite release from Chryphonectria, it would take chestnuts decades to grow large enough to produce seed and widely disperse throughout the forests.

Red maple, a current forest dominant, is one of the most sensitive forest trees to burning, and periodic fires resulting from Native American activities, lightning strikes and European settling practices were likely key factors in suppressing red maple populations in the past.  Perhaps with the re-introduction of fire into Eastern US Forests, canopy openings could allow new chestnut stump-shoots to grow and become dominant.  Sadly, current fire practices would need to change on a scale beyond any we’ve seen before, and so it appears that despite an absence of Cryphonectria as a shoot limiting factor, the red maple will still limit chestnut growth in its absence.

And human fire practices are not the only factors keeping chestnut trees in check.  As a result of the excessive killing of Wolves in the past, deer populations have increased. And because deer tend to choose other shoots over red maple (Abrams 1998), there could be a problem of excessive deer browsing of chestnut shoots were they able to grow to substantial size and health.  This could further deter them from recolonizing the forests and give the red maple another advantage.

Last, tree reestablishment would have to contend with shifting climate regimes.  Suppose chestnut trees were able to overcome the shading from red maple and preferential selection by browsers. They might be able at maximum growth, to reach half the height of the canopy in twenty years, and reach the full height in 80 (Bass 2002).  But in 80 years will current North American forests still be the ideal location for Chestnut trees?  It was once predicted that range extensions due to climate would require a dispersal rate over 200km per century, that’s over 2km per year (Davis, 1989).  With a dispersal rate of 100m per year, it seems unlikely that Castanea could keep up with increasing temperatures.

It’s sad to think that our introduction of Cryphonectria over 100 years ago, failed to serve as an effective warning, and that through historical resource use, we managed to prevent the forests from being successfully reinhabited by a once dominant and majestic tree.

 

Want more information about Chestnut blight and the chestnut tree? Check out these resources:

Bass Q. 2002. Talking Trees: The Appalachian Forest Ecoysystem and the American Chestnut. The Journal of The American Chestnut Foundation 16:42-55.

Davis M. B. 1983. Quaternary History of Deciduous Forests of Eastern North America and Europe. Annals of the Missouri Botanical Garden 70:550-563.

Delcourt H. R. 1979. Late Quaternary Vegetation History of the Eastern Highland Rim and Adjacent Cumberland Plateau of Tennessee. Ecological Monographs 49:255-280.

Keever C. 1953. Present Composition of Some Stands of the Former Oak-Chestnut Forest in the Southern Blue Ridge Mountains. Ecology 34:44-54.

Lord B. 2004. The Red Maple, An Important Rival of the Chestnut. The Journal of The American Chestnut Foundation 18:42-47.

McCormick J. F., R. B. Platt. 1980. Recovery of an Appalachian Forest Following the Chestnut Blight or Catherine Keever-You Were Right! American Midland Naturalist 104:264-273.

Paillet F. L. 2002. Chestnut: history and ecology of a transformed species. Journal of Biogeography 29:1517-1530.

Steele, M.A., McCarthy, B.C. & C. H. Keiffer. 2005. Seed Dispersal, Seed Predation, and the American Chestnut. The Journal of The American Chestnut Foundation 19:47-55.

VANDER WALL S. B. 2001. The Botanical Review; The Evolutionary Ecology of Nut Dispersal. 67:74.

Woods F. W., R. E. Shanks. 1959. Natural Replacement of Chestnut by Other Species in the Great Smoky Mountains National Park. Ecology 40:349-361.

 

*Disclaimer: This post is largely plagiarized from a paper that I wrote as an undergraduate, which is one reason why it’s so long, and also may explain why the citations are so old.

Diverse Introspectives with Tony Ives

This post was originally published on BioDiverse Perspectives – a research blog aimed at fostering communication about biodiversity.

On October 29, 2013, graduate students Rob Heckman, Claire Fieseler, and I sat down with Dr. Tony Ives, Plaenert-Bascom Professor of Zoology at the University of Wisconsin–Madison. Dr. Ives may be best known for developing theoretical models to explain complex population dynamics in lake midges and predator-prey dynamics of pea aphids and their parasitoids, but his research interests are broad. He has published over 120 articles on a huge variety of topics in population and community ecology from coexistence in carrion fly communities to phylogenetic methodology. He has won many awards and honors for his research, most notably the 2012 Robert H. MacArthur award from the Ecological Society of America for his work on population dynamics of midges in Myvatn, Iceland. Dr. Ives was elected to the American Academy of Arts and Sciences in April 2013. We were fortunate to sit down with Dr. Ives when he visited the University of North Carolina for the distinguished lecture in Ecology.

Midges swarm over Lake Myvatn, Iceland. Photo taken by Arni Einarsson
Midges swarm over Lake Myvatn, Iceland. Photo taken by Arni Einarsson

Congratulations on your recent MacArthur award. What was it like to win such a prestigious award from your peers?

It was a huge honor, especially because it was from my peers. But honestly, it was very much a joint award. It was the result of the work of a lot of people. I’ve been incredibly lucky to get great scientists through the lab, and it is really them who deserve the award as much as me.

In the lecture, you started with a question about whether ecology should be about general laws. Why did you feel that it was important to begin your lecture with such a provocative question?

To get people thinking. Most of my talk was about midges in a weird lake, in a weird place – Iceland. How do I justify working on such a peculiar system? How do I justify this in a broader context? Ecologists often try to work only on problems that are inherently broad and apply to a lot of different systems. I couldn’t justify our weird study in this weird lake on those grounds. So I needed to start with a justification for working on something that’s strange, but nonetheless can give broad answers or conceptual understanding.

The midges of Myvatn. Photo taken by Arni Einarsson
The midges of Myvatn. Photo taken by Arni Einarsson

On the issue of universal laws: I don’t think there’s anything universally true about what you’d find in a particular lake. It’s the differences among lakes that I think are interesting. As a theoretical ecologist, you might think that I’m motivated by general laws. But I don’t find general laws very interesting. I really like solving problems. If I’m using theory and not looking for universal patterns or universal laws, then people ask what the hell am I doing? My answer is that I think of ecology as a library of well-developed case studies. If you’ve come across something in your own system, you can go to the bookshelf, pull out a book – a case study – and read it. And maybe there’s some insight that inspires you to look at your system differently. This makes case studies very useful for ecology.

I wouldn’t argue that all case studies are singular and have nothing to do with each other. Clearly there are a lot of themes that cut across different systems. But I wouldn’t necessarily uphold them as laws that you’d expect to see in every system.

If you think about ecology as a collection of case studies, then theory aimed at problem solving fits in as a type of case study. You can take a model and analyze it to death. What’s nice about a model is that you know absolutely everything about it. That’s not like any ecological system, where there’s always going to be stuff that you don’t know. A model is very well defined — you can analyze it and really understand it. The model then becomes a case study — a book on the ecological bookshelf. If you’re looking at your new ecological system – your real system – and you find something that doesn’t make sense, then you can look back at a model, and maybe it will spur intuition about your system. Using models as case studies puts theory on par with empirical studies. I think conceptually, intellectually, logically, they are on par, if you think about them as providing inspiration.

To give an example, I like to use Bob Paine’s work on Tatoosh island. Bob Paine’s classic studies in the 60’s defined keystone species. This is a broad concept that people find very, very useful across a whole variety of systems. But think about what Bob did: He studied the intertidal systems on one island, and he collected all the data himself. From a statistical point of view, all of his conclusions about keystone species have to be confined to what goes on in these intertidal communities on Tatoosh Island. Yet, ecologists are quite happy taking the inference from that single study on that single island, and applying the idea of keystone species to a whole variety of ecosystems. That is an incredibly abstract thing to do. I don’t view doing the same thing with theory any more abstract than doing what people are already comfortable doing with Bob Paine’s work.

So, I use theory to try to solve specific problems, to find general laws. That is what is fun for me. But I hope that the specific problems can at least spark ideas for other people studying other systems.

Can you share with us a paper that was particularly influential to you when you were a grad student or early career scientist?

I have to confess that when I was a graduate student, I didn’t like reading “old” papers. But I should have. One of the things that I notice now is that I read ideas in papers that are being published today, and I think ‘Oh gosh, there was a paper that was published in the 70’s that was pretty much the same.’ I’m always amazed at how the memory of science is fairly short. But that’s not answering your question.

Maybe the most interesting paper that I read as a graduate student was Nicholson and Bailey, 1935. I worked on carrion flies as a graduate student. And Nicholson worked on carrion flies. You go back to this paper published in 1935, and all of the basic questions that they were asking were the same as people were asking when I was a graduate student. To a large extent, they are the same questions that people are asking now. On the one hand you could get depressed and ask “Have we really not come very far in the field?” But I think a more sensible interpretation is that Nicholson and Bailey were asking really good questions that don’t have simple answers, and we’re still asking the same questions. So these are rich questions, and therefore they’re good ecological questions. Nicholson and Bailey got me to recognize that the good questions are hard questions which are not going to be easily answered, at least not in my lifetime.

Do you have recommendations for how grad students or people starting out should read papers?

Oh, I don’t know. I think people should just read what they want to read. And people typically will do that.

I would encourage people to read a lot. I’ve gotten unbelievably bad at reading. I do batch reading, so if I need to know stuff for a particular project, I’ll just download 200 citations and read all of the abstracts, and from that pick papers that I want to read. But that’s not a very good way of keeping up with the broader literature. So I wouldn’t recommend doing what I do.

When I was a grad student, I was pretty good at reading broadly. I had a key to the library, and every Sunday morning I would go into the library – it actually used to be Robert MacArthur’s office, which is cool – and I’d go through whatever journals had popped up on the shelves in the last week. I’d read anything that had an interesting title. I guess now, nobody reads paper journals anymore, but there are easier ways to see what’s being published. I’d recommend people keep their reading broad.

Are there particular skills that you wish you had cultivated in graduate school? How do you think they differ from skills that scientists should be especially keen to cultivate now?

I think that’s the wrong question. I don’t think graduate school should be about cultivating skills. This makes it sound as if your brain ossifies when you get your PhD and you can’t learn anything after that. For me, I learned almost everything that I know about statistics after finishing a PhD. So, rather than cultivate skills, I would say you should come up with questions that you’re interested in and then learn the skills that you need to answer the questions. It has to be question driven skill development. I get students coming into my office sometimes, saying, ‘I want to do more theory in my work. What kind of theory do I need to know?’ And I say, ‘Come back when you have a biological question, and then we can talk about it.’ I think the biology should drive things, not skill development.

If you could go back in time and tell a graduate student version of yourself one thing, what would it be? And is that different than the advice that you would give a grad student now?

The year I entered grad school, I think there were two ecology faculty positions that opened up in the entire country. It was early-mid 80’s, there was an economic slump, and things were bad. I dealt with that by telling myself, I’m going to be in grad school not as a means to an end. I decided to stay in grad school because I couldn’t think of anything else that I wanted to do more. I couldn’t help myself. So, I stayed in grad school, and by the time I finished, jobs had opened up.

I guess my advice would be to live for the now, to study what you want to study, to be comfortable in what you are doing at this moment. Don’t try to do science by making sacrifices now for something that you expect in the future. If you make sacrifices now in a way that you think could help in the future, there’s no guarantee you will be right. Much better to simply do what you want to do now, because that’s going to make you more successful in whatever you do, and that’s going to lead to success however you measure success in the future.

Success in graduate school, at least of the grad students I’ve seen, comes from finding out pretty quickly what you’re good at and then pursuing it. Allowing yourself to be engrossed by it. Ultimately, that’s going to make you the most successful, whether that involves  teaching, whether it means working on very applied problems, or whether it means doing very, very basic research. People who are most successful seem to be those who figure out what they really enjoy doing and then just do it.

Can you tell us about a particularly memorable experience that you had doing fieldwork?

Well, okay, maybe I shouldn’t tell this, but it is the story that popped to mind most quickly. I discovered that I have a very difficult time doing research that involves simply sitting and watching something. I need to be doing something. I tried to do an experiment that involved sitting, watching carrion flies coming to dead carcasses. It was just unbelievably boring. I’d been doing it for a few afternoons, and I thought maybe it would be a nicer experience if I took a few beers with me. I finished the first beer, maybe 2, and after about an hour of sitting and watching, I thought, well, it’s a nice afternoon, maybe I should just like lie down for a little bit. I woke up at sunset. I concluded that, no, you should not mix alcohol with field work, and also that I should give up watching carrion flies at carcasses. So I did both.

Are there any other epic failures that have been important to your career?

I have had some epic failures that have just simply been epic failures, that have not led to anything good. One of the things that I’ve learned is to allow myself to be epically wrong. My graduate students will testify to that. I’ve said some things in lab meetings that made other lab members ask whether it is possible for me to say something sufficiently stupid to have my PhD revoked. But I think to be a good scientist, you have to be prepared to be wrong, and wrong in not a  ‘Oh I was wrong but I learned so much that good came out of it’ way, but just good plain wrong.

The freedom to be wrong is important. I’m probably more wrong than anybody else in the lab, and I think people need to know that, because you can’t live life as a scientist always being scared of being wrong or failing. It’s going to happen, and you have to get comfortable with it.

It seems like you’ve done a great job of overcoming your own imposter syndrome and setting a stage to help prevent it in your lab.

Honestly, sometimes I still have the oh-my-god-how-am-I-going-to-find-a-job nightmare. Okay, maybe its only once or twice a year now, and when I wake up I do have a job. Actually, I have tenure — cool. But I still have an imposter syndrome. I certainly did with the McArthur award. I don’t think I’ll ever get over the imposter syndrome. I’d like to say that it doesn’t affect me — that it doesn’t mean that I intentionally avoid things. But it does, and I do. I don’t think I can change that, though, and I don’t really want to.

What opportunities in your career have been most unexpectedly valuable? Are there any opportunities you wish you had taken?

I don’t know whether this answers the question, but a lot of the work that I do is collaborative. And I have stumbled into collaborations in all kinds of strange ways. I stumbled into the project in Iceland simply because I had family connections in Iceland. I wanted an excuse to go see family friends, and so for the first and only time in my life, I invited myself to give a talk at the University of Iceland. That turned into a 15-year collaboration with Árni Einarsson. Most of the fun collaborations that I’ve had were stumbled into. But this has lead to meeting great friends and colleagues and scientists. This has really been one of the most fun things about my job.

Midges darken the sky over Lake Myvatn, Iceland. Photo taken by Arni Einarsson.
Midges darken the sky over Lake Myvatn, Iceland. Photo taken by Arni Einarsson.

When does stumbling on an interesting opportunity become a good collaboration, specifically for the work that you’re doing in Iceland?

One of the most important things about a collaboration is enjoying the people that you are collaborating with. Sometimes there’s a bit of a dating process as the collaboration evolves. I would never underestimate the importance of getting along with people.

For example, Árni Einarsson is an incredibly nice person who I get along with very well. He is also an unbelievable naturalist. At the beginning of the season in Iceland, he’s doing the bird count at Lake Myvatn, and I always try and go along with him. Just walking through the landscape of Iceland, where he’s at home, is magical. He has all kinds of stories: ecological stories and archaeological stories. It’s really fun. Having nice and interesting collaborators makes the collaboration work. Collaborations have to do with science, but they’re also very personal.

What do you think is the appropriate balance between empiricism and theory given the renewed interest in the role of the two in ecology?

Oh, that’s easy. It’s whatever you want. There are some people who think in numbers, and there are some people who don’t. It’s totally individualistic. I don’t think there’s a blanket answer to your question. I think people should do what they want to do and if that involves theory, fine, and if it doesn’t, that’s fine too.

Is there anything else you’d like to share?

Just a reiteration that I think to be successful, the best strategy is simply deciding what you like to do. If you like to do it, you’re probably good at it, so you should just do it. That’s probably the best ticket to success.

 

11 March 2014

Biodiversity Challenge: Biodiversity hidden in plain view

This post was originally published on BioDiverse Perspectives – a research blog aimed at fostering communication about biodiversity.

I’m trapped in a barren wasteland. It smells of PineSol and fast food. There are people milling about all around me. They are coming and going in and out of little hallways, and there are so many iphones! Everyone is looking at their iphone! I am at the Dallas Fort Worth International Airport. There’s nothing here. Everything’s so sterile (which I guess is an improvement over the way it used to be). There can’t possibly be biodiversity here.

photo (2)

But wait! What’s that, over there by the power outlet? That filth! Those crumbs! There! That’s biodiversity! And there’s more! There’s biodiversity all over that bag of chips, and on the drinking fountain handles. There’s biodiversity on all of those iphones!. And there’s biodiversity on all of the people. All of them are teeming with germs! PineSol, you don’t stand a chance. There’s so much diversity, and it should stay that way!

Traditionally, when people think about biodiversity, they focus on the things that they can see: Tropical rainforests, African savannahs, wildflower meadows, even diversified agriculture systems. For me, it started with mangrove forests and grasslands, but recently, I’ve become more and more interested in the biodiversity that’s all around us that you can’t see. And I’m not the only one thinking about biodiversity hidden in plain view. Researchers are studying biodiversity in our houses, on our bodies, in hospital air, even inside the leaves of plants!

And those researchers are finding incredible things! The microscopic creatures inhabiting houses with dogs differ from houses without dogs, and might even contribute to healthier people in those houses. The microbes on our bodies vary by who we are, where we look, and even how much roller-derby we play.

Ok, Fletcher, so there’s biodiversity all around us, and some of it is pretty cool. But why on earth would you recommend that we conserve biodiversity in an airport? You’re talking about germs! People get sick in airports!

Well, although there’s a lot that we don’t know about the hidden biodiversity around us, we do know a few things. There are microscopic bacteria and fungi everywhere. And they’re important. We know that some of them cause diseases, but it looks like some of them might protect us from diseases, too.  We know that they can interact with each other, that they can form communities, and that those communities can be really different from each other. And I’m not talking apples and oranges different. I’m talking sea cucumber and redwood tree different, all in one square inch! And for the most part, we don’t know how they came to be different, but when we eliminate these communities from the face of the earth with PineSol or antibiotics, the communities that replace them are unlike those that were there before.

So we should conserve biodiversity at the airport because we don’t understand it. We should conserve biodiversity at the airport because it might just protect us from some of the diseases that we’re trying to prevent**. We should conserve biodiversity in the airport for the same reasons that we should conserve biodiversity in the Amazon and in the oceans and in our backyards. We should conserve it because it’s there.

30 October, 2013

*I should confess. I don’t study the microbial ecology of airports. I know virtually nothing about them other than what I’ve inferred from the references above.

**Alright, that might be a bit of a stretch. I am an advocate of sanitation at airports

Parasite biodiversity – a missing dimension?

This post was originally published on BioDiverse Perspectives – a research blog aimed at fostering communication about biodiversity.

Here are a few statistics:

  • Forty-percent of all species are parasites, and more than 75% of links in natural food webs are likely to involve them.
  • As many as 10,000 parasitic helminth species are threatened with extinction.
  • Decreases in avian diversity due to habitat loss and climate change will contribute to even greater parasite species loss in the future.

Parasites are everywhere and outnumber what we can see by a huge margin. So what does this mean when we start losing all these parasite species?

In their 2008 paper in PNAS, Dobson et al. had three objectives: 1) show exactly how abundant parasites are compared to other organisms, 2) estimate how many parasites are threatened with extinction, and 3) evaluate the potential impacts of parasite extinction.

They started by looking at previous estimates of parasite diversity, concluding that there could be over 300,000 parasitic helminth species that use vertebrates as hosts. They then took an alternate approach – evaluating all organisms in single habitat – and asked, how many parasites are there here? Their conclusion: at least 40% of all species in marshes along the California Coast are parasites. Not only that, but the structure of food webs changes dramatically depending on whether or not you include parasites in it.

Next, they looked at expected host extinction rates, and asked at what rate are parasites going to go extinct? Then they used theory on host-parasite interactions to ask, what can we expect as consequences of parasite extinctions?

This paper is a really cool example of what a talented group of researchers* can do when they really dig in to three simple questions about biodiversity and biodiversity loss, but here’s the reason that this paper should be considered a frontier in biodiversity research:

The authors fundamentally and meaningfully argued for a change in the way scientists describe biodiversity by showing that including parasites dramatically changes our understanding of global patterns of diversity, food-web structure, and the consequences of environmental change.  And in doing so, they challenged my notions about generality in ecological research.

What does it mean that 90% of biodiversity research addresses about half of biodiversity? Maybe nothing – Hechinger et al. (2011) argue that parasites obey similar ecological rules as free-living organisms when it comes to abundance, energetics, and production. And if parasites obey the same ecological rules as their free-living counterparts, then maybe it’s not that big of a deal that most ecological research ignores them. However, a recent meta-analysis by Kamiya et al. from the University of Otago in New Zealand suggests that parasite biodiversity may be structured by entirely different processes than those controlling the diversity of free-living organisms. If this is the case, then maybe we do have a problem.

I am aware of ongoing debates on the value of model systems vs. purely empirical, system-specific work in gleaning ecologically relevant information. Generally, I’m of the opinion that there is value in all of these approaches. Yes, while many specific ecological systems can be context dependent, theoretical models and microcosm experiments can tell us a lot about generality despite context dependence. But what if it’s not the context-dependence that we’re getting wrong. What if general biodiversity research is only targeting half of biodiversity? How general are even the most general of theories then?

Ok, so parasites are a diverse group, and much of biodiversity research has historically failed to address them. Are there other systems that biodiversity research has failed to do justice to?

*And a courteous airline staff? A note from the acknowledgements of this paper: ”The first draft of the article was written in Kilimanjaro, Nairobi, and Heathrow Airports; A.D. thanks British Airways and Precision Air for the patience, care, and attention of their ground staff.”

On biodiversity and disease risk

This post was originally published on BioDiverse Perspectives – a research blog aimed at fostering communication about biodiversity.

Few studies have had as large of an impact on me as Charles Mitchell’s study of the impacts of plant species diversity on fungal diseases at the Cedar Creek grassland in Minnesota, USA.

Ok; quick caveat, Charles Mitchell is my advisor. But I’m not saying this to put my advisor on a pedestal.  This study is in large part the reason that I study what I do, and that I am a graduate student where I am.  By evaluating disease impact in an experiment that directly manipulated host species diversity, Mitchell was able to provide empirical evidence that decreased host diversity should increase the abundance of many diseases. Not only did it key in on the link between biodiversity loss and health risk, but the study showed me that such a complicated question could be approached in a way that was experimentally tractable.

But I don’t want to focus on Mitchell’s research here.  See, although his study provided evidence to support the diversity-disease hypothesis, I am highlighting it here because it led to the search for general mechanisms behind that phenomenon.  Instead I want to focus on a paper that I consider a true frontier in biodiversity science. This is a paper that took an often disjointed and complicated field, grounded it in a very simple theoretical model, and then generated some clear, testable hypotheses to move the field forward.

In their 2006 paper, Effects of species diversity on disease risk, Keesing, Holt, and Ostfeld provided a synthesis that would address the key question that underlies the diversity-disease hypothesis: What is the mechanism by which biodiversity influences disease risk?  By generating 5 discrete mechanisms from a litany of previous research, they provided what would hopefully become a roadmap for future research aiming to understand and possibly mitigate for the relationship between biodiversity loss and increased disease risk.

I’m not going to get into the nitty-gritty details of this paper. Rather, I want to highlight one really cool aspect of it that I think was truly innovative and inspirational: that they take something almost immeasurably complicated (the ecology of plant and animal hosts, and the epidemiology of specialist, generalist, and vector born pathogens) and reduce it to the simplest system possible (a simple epidemiological susceptible-infected model) to identify the specific mechanisms by which diversity can influence disease risk. From this simplified model, they are then able to scale up in complexity to explain patterns observed in far more complicated systems.

So obviously, this paper is important to disease ecologists and conservationists aiming to prevent the spread and emergence of infectious diseases (not a trivial thing in and of itself). But I think this paper has value to all biodiversity researchers.  It’s so easy to get bogged down in our own subfields and forget that we can often look to other disciplines or simple theory to synthesize our own research. Keesing, Holt, and Ostfeld used a simple epidemiological model to decompose nearly 100 years of research into 5 testable hypotheses. Biodiversity, with it’s multiple dimensions, drivers, results, and feedbacks, can often seem immeasurably complicated. Is there a simple, ecological theory that can unify this field as well?

Update: The PEGE Journal Club just posted a review of a recent empirical study of biodiversity and disease risk in a trematode parasite of amphibians that was published in Nature. Pieter Johnson’s lab at CU Boulder is doing a lot of really cool research in disease ecology, and this recent paper is a great example! Here, they argue that there’s an emergent property of host diversity that can decrease disease risk that acts independent of host density.