Global Warming Turns Rainforest Leaves into Junk Food

By Elizabeth Preston | March 24, 2015 9:11 am


Like those breakfast cereals that look healthy on the box but have even more sugar inside than Cocoa Puffs, some rainforest trees engage in false advertising. It’s not their fault—it’s ours. Climate change has made their leaves less nutritious than they used to be. And the animals who live off of those trees don’t exactly have another store to shop at.

Experiments in labs and greenhouses have given scientists mixed answers about what happens to plant tissues in a changing climate. So primate ecologist Jessica Rothman of Hunter College and her coauthors set out to get some facts from a real-world setting. To do this, they combined several decades’ worth of data from the rainforests of Uganda.

One team member, Duke University’s Thomas Struhsaker, had kicked things off way back in 1979. He’d sawed off mature leaves from various species of tree in Uganda’s Kibale National Park and let them dry in the sun. Later, researchers had analyzed the chemical makeup of those leaves. In 2007, Struhsaker returned to the park with Rothman and other researchers and retraced his steps. (Luckily, his three-decades-younger self had left a map of what he did.)

Over the next three years, the researchers gathered leaf samples from the rainforest trees. They studied 10 different tree species and sampled them at least once each year, so that a one-year fluke would be less likely to skew their results. As before, they dried the leaves in the hot equatorial sun, then crushed them and brought them back to the lab for analysis.

While they were at it, the scientists also recreated similar work that one of them, Colin Chapman of McGill University, had done in the mid-1990s. This meant gathering younger leaves from eight tree species that the local red colobus monkeys like to munch on.

The scientists analyzed all these leaves and discovered a clear pattern. Everywhere they looked, foliage had become less nutritious. All but one of the 10 tree species first studied in the 1970s now had more fiber and less protein making up its mature leaves. The young leaves from trees studied in the 1990s had also increased in fiber and decreased in protein content.

And these changes weren’t small. In the two tree species whose young leaves are most often eaten by red colobus monkeys, the ratio of protein to fiber had decreased by more than 30 percent. Overall, “Our results suggest that nutritional quality of foliage has declined…over the last 15–30 years,” the authors write.

They aren’t sure which factors in the environment are causing the change in leaf chemistry. Kibale has become substantially hotter and wetter over the last century. Earlier studies found that tropical trees receiving extra rainfall end up with lower nitrogen concentrations; nitrogen is a building block of protein. In forests that are facing drought, the results might be different.

Leaf-eating monkeys seem to choose the trees with the best ratio of protein to fiber in their leaves. If leaves are becoming less nutritious, these forests might not be able to support as many monkeys. But there’s some good news, Rothman writes. Colobus monkeys in Kibale haven’t started disappearing—not yet, at least. They may be making up for their junk food diets by eating extra leaves or switching to different preferred trees. Or there might be a lag time before we can see the effect on these animals.

Even if local monkeys are managing the changes for now, these tropical trees feed a host of other mammals, reptiles, birds, and insects. All of them are on a new food regimen they didn’t ask for, which could affect the health of the whole ecosystem. Maybe we should ship them some Cocoa Puffs.

CORRECTION: An earlier version of this post misstated Colin Chapman’s affiliation.

Image: by Marc Veraart (via Flickr)

Rothman, J., Chapman, C., Struhsaker, T., Raubenheimer, D., Twinomugisha, D., & Waterman, P. (2015). Long-term declines in nutritional quality of tropical leaves Ecology, 96 (3), 873-878 DOI: 10.1890/14-0391.1

  • David Antonio Zavala Gutiérrez

    Strickingly to notice there’s a change in the proportion protein-fiber content in tree leaves, young leaves included. Well, I’ve been studying about coffee, coffee harvest and didn’t recognize the importance of the quality of nitrogen soil. Maybe it’s time to look inside the ground, searching the symbiosis between plant roots, fungi, bacteria and other organisms.

    • Gus Bailey

      But, but they had that one control datum from way back in 1979 when science (reporters) told us the earth was going to freeze!!eleventy!!

  • Uncle Al

    Put the colobus monkeys on Welfare, have big charitable concerts: Kibble for Kibale! Build a million kilometers of drainage culverts in forests to dehydrate them. Air-disperse broccoli, kale, aruglia, and radicchio seeds. Quinoa! (no gluten). Air-drop combat vegans.

    Pay for it all with the Carbon Tax on Everything.

    • kems

      this is a rich idea.

  • ThuggishRuggishJose

    test the soil?


    IT’s time you removed “science” from your banner.

    • kems

      absolutely agree

  • Ernesto Franklin


  • Lorie Franceschi

    I am soooooo glad that we humans are that reason this is happening, NOT!!!! Everyone is going to try to pin this on man made global warming. YES the Globe is warming up…NO!!! it is not because man is the leading factor. Man does contribute, but not in the amount that everyone seems to think. It is NOT HUMANKIND’s fault there is global warming. Nature has its own way of correcting things. for proof, look at ice cores taken from all around the world. they will show many times when the earth got warm and when it got cold, well before man or our cousins showed up. We have known for a long time that we need to do crop rotation, we now know that it is because nitrogen needs to be “fixed” back into the system. these trees probably need the same thing. because the trees need some type of chemical, nature is doing it in a way that will allow the trees to “get better” without harming nature.

    • kems

      Yeah, God has a way of doing what He wants. This c-rap about carbon (which was 10X’s higher when the dinosaurs were around), and ‘warming’ and vortices, and … and … and … I need a valium.

      • nickshaw

        Leaving out “God’s will” (why give the pinheads any more ammo?) just the undeniable fact that CO2 is currently at the lowest it has been for millennia should shut them up.
        But no, when whole careers are based on this scam, they are not going away quietly.

  • OWilson

    What a juvenile waste of money. I bet they broke open the champagne when their biased results came in, and man was found guilty.

    I’m sure they’ll be going back to the same exotic locale, all expenses paid every year to follow up.

    In the real world, any gardener or farmer would first look at the soil, moisture content over time and pest activity.

    When a soil becomes depleted (all growing soils do) it affects the dependent plants, and makes way for other species that are more suited to the new conditions.

    Apparently the monkeys know this better than these juvenile idiots, and are doing just fine.

    I’m sorry, but junk science makes my blood boil.

    • kems

      I am so glad to see that absolutely everyone’s response to this asinine article was more educated than anything this magazine puts out.

  • MackDAD

    yeah, I’d like to see these assertions about the Easter bunny making leaves less nutritious collated with some satellite weather data for the region ( none of those “corrected” terrestrial stations for me).

    It’s crazy, 18 years of no observed easter bunny and “science lovers” are still making up crap to blame on the easter bunny.

    • OWilson

      Don’t forget the “boogyman”.

      He still there, he’s just hiding under the bed (I mean ocean) :)

      • MackDAD

        right, I hate science so much, I forgot that the lack of easter bunny was totes explained by the boogey man stealing all his heat and hiding it under the bed. Just like the science people said it would 20 years ago.

  • MackDAD

    ah. it’s always around 1983, the end of the last global cool down, the nadir of global temps, that these alarmists start their studies. In this case “Duke University’s Thomas Struhsaker, had kicked things off way back in 1979”

    FWIW, the earth …if you can believe this…went through a cooling trend up until 1983 and then warmed up until around 1996 where it stalled. What that means for nutritious leaves? You’d think warmer temps (and moar CO2) means more photosynthesis but hey … easter bunny.

    • George Nixon

      Lack of rainfall and
      foliage suffers to the point where plants including trees wither and
      carbon uptake ceases. Excess rainfall and foliage becomes lush and
      emerald green. Photosynthesis of CO^2 to form fibrous network of leaf
      increases in proportion, whilst leaching to subsoil of other
      essential nutriments increases in proportion. During the change of
      environment, there is always a period of ideal conditions.

      Given the fact that the
      Earth is populated by carbon based lifeforms rapidly increasing in
      number, then if that increase is to continue, perhaps there will
      be a need to rethink our attitude regarding CO^2. In actual fact,
      CO^2 is highly beneficial for our warmth and well-being, whilst the
      climate cycles on Earth are subject to compulsory changing by
      external precursors such as the GTE, gravitational thermodynamic
      effect of the Great planets.

      Unfortunate for humanity,
      the teaching of the part correct Theory of General Relativity, has
      assured that physics pertaining to the fundamental dynamic nature of
      gravity and gravitation has not advanced since the Cavendish

  • Jack Hagan

    Do you get a bigger car and house right away when you do this propaganda? Is it “understood” at the time like the mafia with “buffers”? I mean, can I just put together some sort of online thing and spout propaganda and then get my house? Do I have to know somebody? Just would like to get in on the con. It seems like a pretty safe way to just make up lies and get paid. Why don’t you guys do an article on that? Bet you get a lot of page views!

  • astonerii

    Global warming science says the warming effects the equatorial areas far less than any other part of the world. So, pray tell is the actual driver of this change?
    Oh wait, you are not scientists, but prophets of doom for your religion.

    • Jack Hagan

      This is just an old fashioned con job, just done on an industrial global scale.

    • nickshaw

      In truth, satellite measurements are showing CO2 concentrations much higher in the sub equatorial regions of the planet.
      Particularly over the Brazilian and African rain forest areas.
      Exactly as one would expect!
      Totally unexpected was the relative dearth of CO2 over the Northern Hemisphere (other than parts of China and hot spots of volcanic activity).
      Imagine that.
      Of course, the problem really is soil degradation and monkeys are smart enough to move.

  • astonerii

    If it were CO2, then explain how much higher concentrations of CO2 (8000 ppm compared to sub 400 today) at the time the monkeys “evolved” allowed them to survive.

  • Jonesie

    SNAP benefits for rain forest denizens.

  • nickshaw

    Jeezus! Are the monkeys shorter too?
    Maybe we just need to redistribute some wealth to them so they can have their food imported.
    Will this nonsense ever end?

  • JayWills

    This is the best unintentionally funny article I have read today. Thanks!

  • Tim David

    Is there anything climate change CAN’T do? Amazing. Desperately trying to ban a compound, CO2, that is necessary for life, while at the same time flying 1,700 private jets to their little echo chamber in Davos, Switzerland. One of the founders of Greenpeace came out and admitted that he no longer believes the human contribution to climate change is significant. And, he also stated that a couple of degrees warmer could make life on earth vastly better for lots of people. But there, my friends, is the crux of the problem. At the bottom of this sordid climate change scheme there is an anti-human agenda. You’ve heard them before, telling how much better things would be without all of these humans running around. They’ve even proposed ways to reduce populations, including adding sterilants to public water systems. That idea was advanced by none other than Obama’s Science Czar, John Holdren, along with his equally disturbed co-author Paul “Population Bomb” Ehrlich. You see, they want only the “right kind” of people populating the earth. And by that they mean “people who look like, and most importantly, agree with, us.”

    • Jack Hagan

      We used to use an old fashioned term for these people. Evil.

  • tmitsss

    Ted Cruz on Global Warming

    • Bindar Dunit

      It causes real harm when oil corporations can’t continue to make soaring Profit as they have since illegal invasion of Iraq ….loss of profit causes hardship on the 1% who fund these politicians….and it aint called global warming rather it is CLIMATE change.

  • Kurt S

    If all you above commenters are so disintrested in a possible link to anthropogenic global warming through CO2 emissions, why are you reading such an article?

    We (as in those of us who understand the science behind CO2) know the difference of skepticism and denial. We also can recognize psuedoscience and unfounded conjecture.



Like the wily and many-armed cephalopod, Inkfish reaches into the far corners of science news and brings you back surprises (and the occasional sea creature). The ink is virtual but the research is real.

About Elizabeth Preston

Elizabeth Preston is a science writer whose articles have appeared in publications including Slate, Nautilus, and National Geographic. She's also the former editor of the children's science magazine Muse, where she still writes in the voice of a know-it-all bovine. She lives in Massachusetts. Read more and see her other writing here.


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