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.

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.

Is blogging effective at communicating biodiversity research?

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

My original intent in writing this post was to compare the 5 most-cited papers on biodiversity to the 5 most blogged-about papers on biodiversity to address the differences between what we value as researchers versus what we value as general science communicators. However, I was shocked by the results of my search.  Over 49,000 papers have been published containing the word “biodiversity”.  In contrast, only 448 blog entries on researchblogging.org contain the word “biodiversity”, citing just 427 papers. What does this mean about the state of biodiversity research versus the state of biodiversity-research communication? Do bloggers and their readers value fundamentally different research than that being conducted and cited? Or are researchers simply falling short of effectively communicating the most important research in biodiversity science? Here at biodiverse perspectives, we aim to bridge the gap between research blogging and scientific research, by focusing on the research that we consider to be truly transformative in the field of biodiversity regardless of publication date.

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In this blog, we present the papers that we think are the foundational and frontier papers in biodiversity science. But we are just a subset of a vast scientific community. I recently began to wonder, if I were able to poll all biodiversity researchers, what would they say are the five most important papers for biodiversity science?

Well, I don’t have the eyes and ears of all biodiversity researchers, but I do have some tools for garnering their opinions. If journals are the primary method for sharing scientific material, then to some extent, the number of citations should tell us something about how we value research.

So, I sent a letter to every published biodiversity researcher:

 Dear ISI Web of Science,

What are the five most-cited papers that contain the word “biodiversity”?

Sincerely,

Fletcher Halliday

And, believe it or not, I got a reply! Here’s a list of the top five cited articles from Web of Science:

Myers et al. (2000) Biodiversity hotspots for conservation priorities. Nature. 4,790 citations

Vitousek et al. (1997) Human domination of Earth’s ecosystems. Science. 2,410 citations

Sala et al. (2000) Biodiversity – Global biodiversity scenarios for the year 2100. Science. 1,764 citation

Thomas et a.l (2004) Extinction risk from climate change. Nature. 1,690 citations

Hewitt, GM (1996) Some genetic consequences of ice ages, and their role in divergence and speciation. Biological Journal of the Linnean Society. 1,667 citations

But this website wasn’t designed just to be a compendium of articles that may or may-not be well cited in the literature. As the name suggests, the purpose of this website is to present the perspectives of a diverse community of graduate students. In many ways, this blog is unique. We have over 15 contributing authors, representing 5 countries, and we focus primarily on sharing our opinions on primary literature in biodiversity science. However, in many ways we aren’t so unique. In fact, there are over 2,500 blogs on the web registered as “research blogs” with the website researchblogging.org. And it seems to me that my letter to every published biodiversity researcher may have missed a large segment of the scientific community.

So I sent out another letter:

Dear Researchblogging.org,

What are the 5 most blogged-about papers containing the word “biodiversity”?

Sincerely,

Fletcher Halliday

And researchblogging.org replied too- and what a surprising reply it was! Of 448 blog posts, only 19 papers were written about more than once and only 2 papers were written about more than twice:

Evans, et al. (2011) The spatial distribution of threats to species in Australia. BioScience. 3 blog posts

Strassburg et al. (2010) Global congruence of carbon storage and biodiversity in terrestrial ecosystems. Conservation Letters. 3 blog posts

17 papers tied for the remaining three slots in the top 5, each with 2 posts apiece. They ranged in subject from the use of DNA barcoding to infer patterns of avian diversification all the way to role of the Chicxulub Asteroid in mass extinctions.

I wasn’t surprised that the top-five papers were different between ISI and researchblogging.org, but I was surprised to find that the top-five most-cited articles on ISI have never been written about on researchblogging.org. What does it mean that the 5 most important papers to biodiversity researchers don’t even register with biodiversity bloggers?

I’ll pose 3 possible explanations and then leave it up to you to decide what we need to do.

First, perhaps researchblogging.org isn’t a representative sample of science blogs on the web.  But then, if researchblogging.org is not, then what is? If blogs about research aren’t easy to find, then are they really doing their job?

Second, by focusing on very recently published articles, researchbloggers are missing some of the more foundational research in the field. Researchblogging.org launched in 2008 and consequently, the vast majority of papers that have been blogged about were published after that time. Maybe the purpose of researchblogging isn’t to share the most important research, but rather the new and exciting research. However, by only focusing on recent publications, do we run the risk of perpetuating the trend in ecology to reinvent the wheel?

Finally, perhaps not enough researchers are contributing to researchblogging. There are WAY more publications than there are blogs about them! In fact, ISI gave me more than 49,000 results for my query of papers using the word “biodiversity”, whereas researchblogging.org gave me 448 blog entries, citing 427 papers. Just to put this in perspective, the most cited paper containing the word “biodiversity” has been cited 10 times more than the total number of blogs on researchblogging.org that contain the word “biodiversity”. Maybe research blogging and the literature don’t agree because not enough scientists write in blogs.

If science blogging is intended to disseminate science to the public, then it seems important that science blogs aren’t limited to presenting just the newest and most exciting research out there. Here at Biodiverse Perspectives, we are trying to put biodiversity research into some kind of context by writing about papers that are new and exciting as well as papers that have contributed to our foundational knowledge of the field. We hope that this endeavor will allow us to share our excitement about the future direction of biodiversity research in light of its past. What do you think?