Big herbivores shape plant community response to global warming

ResearchBlogging.orgThe cover article from this week’s PNAS has important implications for how we plan for, and deal with, climate change. Post and Pedersen report that the way an arctic plant community changes in response to warming depends heavily on the presence of large herbivores [$-a], like muskoxen and caribou.

Photo by Giant Ginkgo.

Previously, it was thought that one effect of global climate change would be for woody shrubs and dwarf trees to become more common in arctic and subarctic plant communities. This increase in woody plants could trap more atmospheric carbon and increase the albedo of the land – meaning more heat could be reflected back into space. Both of which effects might help slow a warming global climate.

However, Post and Pedersen show that large herbivores can reduce this shift in community composition. In huge five-year study, they set up experimental plots from which caribou and muskoxen were either excluded by fencing, or not excluded. Within each class of plot, they also placed 1.5-m “open-topped chambers” (OTCs) made of fiberglass – basically, cylinders that help trap solar heat, warming the ground inside. In the fenced plots, the plant communities inside the OTCs shifted toward more woody species; but in the unfenced plots, where large herbivores could reach in and graze inside the warmed cylinders, plant communities didn’t develop greater cover by woody species.

Now, it’s not surprising that large herbivores can have a profound effect on the plant species that grow in their grazing land. Where I come from, in the northeast U.S., large swaths of forest have been dramatically altered [$-a] by a population explosion of white-tailed deer freed from their natural predators. But Post and Pedersen have drawn a connection between this effect and the ways in which natural communities may respond to the most dramatic environmental change in human history. It just goes to show what a massively complex system we humans are tinkering with, and how little we know about what that tinkering may ultimately do.


E. Post, C. Pedersen (2008). Opposing plant community responses to warming with and without herbivores PNAS, 105 (34), 12353-8 DOI: 10.1073/pnas.0802421105

F.L. Russell, D.B. Zippin, N.L. Fowler (2001). Effects of white-tailed deer (Odocoileus virginianus) on plants, plant populations and communities: A review The American Midland Naturalist, 146 (1), 1-26 DOI: 10.1674/0003-0031(2001)146[0001:EOWTDO]2.0.CO;2

More from Anthony Lane on the Olympics

And it’s just as good:

The first heroes to enter the stadium, that morning, had been the leaders of the twenty-kilometre walk, an event considered hilarious by everyone on planet Earth except the athletes themselves. Somehow, wordlessly, a deal has been agreed on: we will not giggle, for politeness’s sake, and they will continue to propel themselves, year in, year out, as if learning to moonwalk too soon after a hip replacement.

The whole thing is on the New Yorker website.

The evolution of offspring size

ResearchBlogging.orgThe question of offspring size – that is, how big a child is relative to its parent – can seem downright absurd. In fact, it was the subject of the only paper (to my knowledge) ever published in the journal Evolution that ends with a punch line. That piece, written by Ellstrand in 1983, pretended to seriously address the question of why juveniles are smaller than their parents [$-a]. It was basically pointing out the absurdity of assuming that every trait we observe in a living organism has evolved adaptively, or specifically because natural selection favors it. Clearly, not all traits are adaptive – juveniles are smaller than their parents because, universally, a child has to emerge from its mother. It’s a basic fact of the conservation of mass-energy.

Photo by TIO….

On the other hand, the size of a child as a proportion of its mother’s body size varies tremendously in the natural world. Mushrooms release nearly-invisible spores, while kiwis lay eggs equal to a quarter of their body mass. There seem to be clear benefits to making bigger offspring – they should be better competitors against their peers, they may be more likely to survive to reproduce, and they may reach reproductive maturity faster. But there are also costs, in terms of the energy a parent uses to either produce an egg with a bigger yolk, or to provision a bigger embryo in the uterus, or to feed a juvenile in the nest.

The classic description of this trade-off is a mathematical model developed by Smith and Fretwell in 1974 [$-a]. But the Smith-Fretwell model doesn’t explain the wide variety of offspring sizes we see in nature, especially among species that seem to have more or less the same ecological requirements. In the current issue of The American Naturalist, Falster et al. propose an extension of Smith-Fretwell to better capture this variation, which follows juveniles from the moment they leave their parents, through a phase of establishment and growth, and then through a period of competition with their peers. Which juveniles survive to adulthood is determined by body size – in the final period of competition, big individuals win.

Falster et al. then use the parameters of the model – adult body size, total adult lifespan, and energy used for reproduction – to predict juvenile body sizes for mammals and plants. The model seems to predict the relationship between parental size and offspring size pretty well for mammals, not so much for plants. Which is actually not all that surprising. In mammals, adult and juvenile body sizes probably have a lot less “wiggle room” relative to each other; but adult plants can be multiple orders of magnitude bigger than the seeds they produce. So there’s just a lot more variation to try and explain in plants than there is in mammals.


N.C. Ellstrand (1983). Why are juveniles smaller than their parents? Evolution, 37, 1091-4 URL:

D.S. Falster, A.T. Moles, M.Westoby (2008). A General Model for the Scaling of Offspring Size and Adult Size. The American Naturalist, 172 (3), 299-317 DOI: 10.1086/589889

C.C. Smith, S.D. Fretwell (1974). The optimal balance between size and number of offspring The American Naturalist, 108, 499-506 URL:

Stop trying to overturn the Modern Synthesis

There’s a great letter in this week’s Science, which points out the absurdity of talking about new “theories” of evolution [$$]. Writing in response to a recent “news focus” article on the subject [$-a], U. Kutschera argues that, because it is such an interdisciplinary field, modern evolutionary biology isn’t supplanted by new ideas of how life changes over time, but instead absorbs them into its broad synthesis.


U. Kutschera (2008). From darwinism to evolutionary biology Science, 321, 1157-8 DOI: 10.1126/science.321.5893.1157

Teaching evolution below the Bible Belt

Today’s New York Times has a fantastic piece that follows Florida high school science teacher David Campbell as, with the support of new state science education standards, he teaches a class of mostly conservative Christian students about evolution. I’m probably going to steal some of Campbells lines in my own future teaching:

“True or false?” [Campbell] barked the following week, wearing a tie emblazoned with the DNA double helix. “Humans evolved from chimpanzees.”

The students stared at him, unsure. “True,” some called out.

“False,” he said, correcting a common misconception. “But we do share a common ancestor.”

I attended a church-affiliated high school and university, and I know my teachers and professors put up with at least as much resistance as depicted here; possibly more, since a lot of parents sent their kids to my schools looking for safe havens from the “dangers” of biological fact. But in spite of all that, my biology teachers and professors taught evolution. And there’s something kind of heroic about that.

Christ Church as it is: Church and state

Here in Moscow, Idaho, it’s hard to avoid hearing about Christ Church, a local conservative mega-church with ministries encompassing a publishing house, a Bible college with dodgy accreditation, a private k-12 school, a ministerial training program, and its own denomination of conservative neo-Calvinists. All of which are headquartered in Moscow, ID, population circa 23,000.

Figure 1: State, and Church
Photo by eqqman.

Non-Christ Churcher Moscovites mostly know the group for its ties to Christian Reconstructionism, a movement that aims to return U.S. society resurrect the Confederate States of America as a “Christian” nation. Including, yes, slavery. Douglas Wilson, the pastor of Christ Church and éminence grise behind most of its ministries, earned the attention of the anti-racist Southern Poverty Law Center when he published a (substantially plagiarized) book explaining that slavery wasn’t so bad after all. Christ Church has since tried to downplay this fundamentally racist history, and succeeded in snowing a New York Times reporter who profiled the affiliated New Saint Andrew’s College.

Actually, reading that Times piece, you can sense that the reporter may have under-represented CC/NSA/Doug Wilson’s racist undercurrents not so much because Christ Churchers hide them very well (Doug Wilson on mainstream conservatives: “They voted for Bush; I’d vote for Jefferson Davis.”) but because locals who are creeped out by Christ Church can sound a bit, well, shrill. Which brings me, finally, to the point of this post: I’ve started looking into what Christ Church has to say about itself. It’s one thing for a bunch of Moscovite hippies (yours truly included) to be worried about a conservative megachurch in our midst; it’s another if that church actually, by its own admission, takes extreme and worrying positions.

Today’s topic: the relationship between church and state. This is a good starting point because one of the major accusations against Christ Church is that its aims are basically theocratic – that, politically, Christ Church (or Wilson, who might as well be synonymous with the church’s policy positions, as we’ll see below) wants to enforce Old Testament law on the entire population of the United States. Now, there are actually three theological components to that position, if Christ Church does indeed hold it:

  • Whether, for Christians, the Old Testament text is a standard for normative ethical behavior (as opposed to viewing the Old Testament through the lens of the Gospel, as we Mennonites do);
  • Whether Christians should ask non-Christians to accept Christian ethical views (whatever those are); and
  • How Christians should go about doing that.

You might take all sorts of positions on these three questions and still fall somewhere on the spectrum of theology Christians have built up over the last two thousand years. The question is, where does Christ Church fall on that spectrum? Well, fortunately for the curious among us, Christ Church’s newsletter, “Credenda/Agenda”, has just run a cover story on religion and politics. Let’s see what it (and, by it, I mean Doug Wilson, who is the editor of “Credenda/Agenda”, natch) has to say.

(First, a complaint/warning to those clicking through to the source text: it’s long and kinda rambling, much like this post. This is probably because Wilson’s primary employment is writing sermons, which are not usually structured as five-paragraph essays. That’s fine for speaking aloud, but less so in written form.)

Anyway. Wilson starts out by strongly affirming the political relevance of the Gospel:

Jesus was crucified in a public way, and His death necessarily has public ramifications. There is no way to be fully faithful to the message of His death and resurrection in private.

I could actually fully agree with Wilson on this point; my thinking about the relationship of the Gospel to politics is basically lifted wholesale from the writings of John Howard Yoder (no relation), who wrote things that sound a lot like the above quote. For example, in his masterwork The Politics of Jesus:

The kingdom of God is a social order and not a hidden one. … it is that concrete jubilary obedience, in pardon and repentance, the possibility of which is proclaimed beginning right now, opening up the real accessibility of a new order in which grace and justice are linked, which men have only to accept.

But it’s also possible that Wilson means something completely different from Yoder when he says the Gospel is politically relevant. Yoder held that Christian ethics are political because they necessarily challenge the authority of the state, but he didn’t mean that Christians should take over the state – he meant that Christianity challenges the entire concept of human government:

Preaching and incorporating a vision of an order of social human relations more universal than the Pax Romana … [Jesus] permitted the Romans to deny their vaunted respect for law as they proceeded illegally against him. This they did in order to avoid the threat to their dominion represented by the very fact that he existed in their midst so morally independent of their pretensions.

Figure 2: Jesus conquers
Photo by RATAEDL.

(That’s more from The Politics of Jesus.) Yoder was unsure whether Christians should vote, much less run the country. What political meaning does Wilson see in the Gospel, then? Explicating a passage from Paul’s first letter to the Corinthians, Wilson writes,

… Christ and Him crucified is theonomic postmodernism — when the nations are discipled in accordance with His Word and are taught to obey all that He required, this means the necessary exclusion of secular democracy (which is the political expression of modernity). … Christ is the Lord of the new polis that has been planted right in the middle of the old polis. God did this so that the new way of being human in Christ would … gradually transform all the nations of men. [Italics all Wilson’s]

That sounds to me like Wilson wants the Church (presumably, his) to be more than present in the world – he wants it to rule the world. A quick dig in Wikipedia for the definition of “theonomy” suggests this is the case: the term literally translates as “God-law,” and in modern times, it’s apparently associated with Christian Reconstructionist (theocratic) thinkers.

In theological response, I’d point to the words of Jesus to his disciples in the Gospel according to Matthew:

You know that the rulers of the Gentiles lord it over them, and their high officials exercise authority over them. Not so with you. Instead, whoever wants to become great among you must be your servant, and whoever wants to be first must be your slave — just as the Son of Man did not come to be served, but to serve, and to give his life as a ransom for many.

Jesus isn’t just rejecting the current model of government under which he and his disciples live (i.e., Roman dictatorship), but the entire concept of government, of one human being exercising power over another. Note also the reference to the Crucifixion – Jesus is setting up his basically passive submission to Roman authority as a model for his disciples. It’s hard to govern in any meaningful sense of the word if you’re submitting yourself to death on the cross.

And but so Wilson thinks that the Church should rule the world. Eventually, anyway – he’s vague on the topic of how it gets to rule. That seems to answer both the second and third question above – what about the first? Where is Wilson getting the ethical standards by which his Church will rule? Here there is much verbiage about “cruciform politics” and “sacrificially” testifying to the lordship of Christ, but not a lot of actual policy positions, or the specific sources from which they spring. Except for this one:

The idols of the age are always decked out in respectable clothing, and people who attack them are always dismissed (initially) as crazed nutjobs and disturbers of the peace. In our day, one of the central idols is the the swollen state. In other words, the political aparatus [sic] over which Jesus will be Lord needs to be about 100 times smaller than it currently is. [Italics Wilson’s]

And this one:

Issues like abortion and homosexual marriage are far more important than are issues like minimum wage laws. But the lordship of Christ does apply to minimum wage laws. It is just that we might not want to start there—especially if we think they are a good idea.

So, according to Wilson, Jesus is for limited government, outlawing abortion, and preventing legalized gay marriage. He’s interested in minimum wage improvements, but they’re not a top-tier priority precisely because they sound like a good idea. Apart from this last weird, counter-intuitive assertion, this is a pretty tired outline of Christian Right talking points. And it’s maddening, and sad, and completely disconnected from the Gospel text.

Jesus never said a word about the “size” of government (whatever that means), abortion, or homosexuality. Never. Nothing. If you dig around in the Old Testament and the Epistles, you’ll find a handful of statements against homosexuality (depending on how you define the term), and also against women leaving their heads bare in church, men shaving their sideburns, and anybody eating owls. Meanwhile, Jesus spends a lot of time talking about being generous to the poor. So does the Old Testament. Which suggests to me, just looking at, you know, the Bible, that God might actually care about minimum wage laws. Quite a lot. And way more than he (or she) cares about the latest skirmish in the U.S. culture wars. In other words, Doug Wilson and Christ Church might be interested in setting up a theocracy, but it sure won’t be a Christian one.

Orwell’s diary as a ‘blog

For about a week, now, I’ve been following a new release of George Orwell’s diary, in the form of a weblog, published exactly 70 years after the dates of the original entries. The contents are awfully appropriate for the format – brief, first-person notes on Orwell’s day-to-day life, with a surprising (to me) amount of emphasis on the developments in the natural world. As in the entry for 17 August, 1938:

Catmint, peppermint & tansies full out. Ragwort & willow-herb going to seed. A few ripe blackberries. Elder-berries beginning to turn purple.

Oak planks etc. made from the boughs instead of the trunk is known as bastard oak & is somewhat cheaper.

The presenters have added hyperlinks to specific useful information, like pages about the plant species mentioned above. And, as if it weren’t blog-like enough already, there are frequently clipped newspaper stories.

How the chili got its hots

In this week’s PNAS: capsaicin, the stuff that makes chili peppers hot, may have originally evolved as an anti-fungal agent [$-a].

Photo by bleu celt.

Tewksbury et al. examine variation in “pungency” (that is, concentration of capsaicin) in wild populations of the chili Capsicum chacoense and compare it to rates of fungal infection in the fruit. The result is interesting, and not necessarily clear-cut: more-pungent fruits are less frequently attacked by an assortment of true bugs, and when these bugs attack, they can introduce fungal spores into the fruit, which ultimately destroys the seeds inside. So more pungency means less bug damage, and lower rates of fungal infection, and potentially more seeds.

But the story of chili pungency is more complicated than that. Back in 2001, Tewksbury and Nabhan showed that capsaicin helps ensure that chilis are eaten by birds instead of mammals [$-a]. Birds make good seed dispersers – they eat a fruit, then, um, pass the seeds on undigested; mammals, on the other hand, like to eat the seeds specifically. Capsaicin irritates mammals, but doesn’t bother birds.

To complicate things still further, there’s a downside to producing capsaicin. In this January’s issue of the journal Ecology, Tewksbury and his coauthors showed that Capsicum chacoense seeds from more-pungent fruits also had thinner seed coats, which meant they were more likely to suffer damage in birds’ digestive tracts [$-a].

So why are chilies spicy? The answer is, probably for all these reasons, and maybe more that haven’t been discovered yet. This is a common situation in evolutionary biology – in many organisms, the traits that scientists find interesting may be useful in several different ways, and unhelpful in others. Very few traits experience natural selection in only one direction, as it turns out. The traits that we observe in nature are usually compromises between many different, sometimes directly conflicting, sources of natural selection.


J. J. Tewksbury, D. J. Levey, M. Huizinga, D. C. Haak, A. Traveset (2008). Costs and benefits of capsaicin-mediated control of gut retention in dispersers of wild chilies Ecology, 89 (1), 107-17 DOI: 10.1890/07-0445.1

J. J. Tewksbury, G. P. Nabhan (2001). Seed dispersal: Directed deterrence by capsaicin in chilies. Nature, 412 (6845), 403-4 DOI: 10.1038/35086653

J. J. Tewksbury, K. M. Reagan, N. J. Machnicki, T. A. Carlo, D. C. Haak, A. L. C. Penaloza, D. J. Levey (2008). Evolutionary ecology of pungency in wild chilies PNAS, 105 (33), 11808-11 DOI: 10.1073/pnas.0802691105