Cascades

In Ecology we talk about a sterile yet captivating concept called the Trophic Cascade. It is really just an imposing term that describes fancy food webs. Energy enters our atmosphere as sunlight and gets converted into many different forms by the time it winds up as a handful of wet dirt. In a textbook, this ultimately gets boiled down into calories of energy being fractionally converted from one stage of life to the next (using words like primary autotrophs, first-order heterotrophs, second-order heterotrophs, etc.). At best, you are already bored reading this paragraph. At worst, you were affronted by the scientific jargon and dismissed the whole thing as abstract and heady. 

The reality is that things happen in our world that cannot be fully absorbed by the naked eye, and scientists happen to be in the business of figuring out these things. Trouble is, scientists generally describe findings in exacting, sterile, and valueless terminology. This jargon is necessary to properly document their studies, but it generally hides what that scientist wants to come right out and say: “WOW, check this out! This is AMAZING! Can you believe this?!”

I want to share with you a jargon-free story about the Trophic Cascade that shows just how awesome and complex our planet is. It’s a story about a bird and a tree, and how their relationship affects everything else living around them. 
It’s also happens to be a story about the bird and the tree that I am currently employed to study.

This is a Whitebark Pine:


This is a Clark’s Nutcracker. It’s related to ravens, crows, and jays. And like ravens, crows and jays, it’s pretty smart:
Photo courtesy of US Fish & Wildlife Service


And this is the ecosystem in which they both exist:
As it so happens, this is the ecosystem in which I also currently exist on work days.


The Clark’s Nutcracker is virtually unstudied in the wild. Almost everything we know about it is either based on lab studies or general observations from a couple guys in the 1950s. But here’s what we do know: They spend their autumn caching whitebark pine seeds in holes in the ground. Every year, each bird makes over 7,000 caches, and stores anywhere from 30,000 to 98,000 seeds over an area at least the size of Manhattan! At the end of the winter when food becomes scarce, it remembers, revisits, and digs up about 70% of these caches! And it’s not just guessing where it put its seeds 6 months ago: It can find them under several feet of snow, and on hillsides that have no outstanding features to cache next to. Nutcrackers have been seen digging diagonally through several feet of snow to hit their cache dead-on... and I can barely remember where I put my chapstick 5 minutes ago.

But 30% of the seeds it never digs up. It probably forgets about them, which is understandable when you’re trying to keep track of 60,000 other seeds across the landscape. This act of forgetting or neglecting turns the Nutcracker into a little, feathery gardener-- It is the only animal tasked through evolution to plant whitebark pine. Without nutcrackers, very few whitebark seeds wind up underground and ready for germination.
And here’s something pretty cool: Sometimes Nutcrackers don’t “plant” those whitebark seeds in the best places for germination. Enter squirrels, mice, and all sorts of other rodents: Though not very adept at harvesting whitebark seeds right from the cones, rodents will often raid Nutcracker caches and re-cache them elsewhere for themselves. These critters have comparatively terrible memories, so they forget where they put a lot of them. AND, for some reason, the places they prefer to re-cache the seeds are often much better for germination!






But for every hero there is a villain. This is the mountain pine beetle:
Photo courtesy of OregonLive.com
It bores holes into trees, lays eggs, and its larvae eat the tree from the inside out. The beetle also carries a little blue fungus that blocks up the tree’s tubing, preventing water from flowing up the trunk, and killing it from dehydration.

Now here’s where it gets really interesting. This beetle is NATIVE to the Rockies, and as of 2009, has made over half of the whitebark stands around Greater Yellowstone Ecosystem look like this:


And with few exceptions, any tree not already dead is infected. In some places in Montana and Colorado, over 90% of the whitebarks are dead. While you process that sad statistic, also keep in mind that these trees grow very slowly due to the short growing season and harsh climate where they live. The living whitebark in the first picture above (which is, by the way, infected), is probably about 500 years old. To make matters worse, another separate, introduced disease called blister rust plays cleanup batter, sickening or killing anything that has managed to evade the pine beetle devastation.

But let’s stick with the beetle, because, like mosquitoes and poison ivy, it has evolved as a native yet pesky member of our community. The beetle has not always caused so much damage. In fact, up until the mid-90s, it only infected a small percentage of the sickly trees in the landscape. But our climate is changing, and North America has been experiencing some undeniably and unusually mild winters. Historically, Mother Nature’s winter cold snaps killed off all sorts of overwintering insects. Our beetles can withstand some pretty cold nights, but it only takes one night of extreme cold (like -25°F to -40°F) to kill off almost all the beetle larvae hunkering down inside the whitebarks. You can imagine what happens after winters when the coldest snaps weren’t quite cold enough. This winter was, at some points, kind of chilly. I’m sure the beetles loved it.

We now have 5 characters: Nutcracker, Whitebark, Pine Beetle, and the supporting roles of Blue Fungus and Blister Rust. Let’s add some more.

Whitebark are excellent at germinating in bare soils in full sunlight, much like you would find after a fire passes through, after an avalanche wipes out a swath of forest, or after a bunch of clear-cut logging. Other trees in the Rockies—most of them, in fact—can’t germinate in such exposed conditions, and rely on “nurse trees”  — a.k.a. young whitebarks —  to provide a shady and climate-controlled patch of earth to get started. Englemann spruce, douglasfir, subalpine fir, limber pine, and a whole bunch of other forbs and shrubs may eventually outcompete the whitebark for that little patch of earth, but without the whitebark to get those plants started, our forests wouldn’t be nearly as diverse.

Each blueberry-sized seed contains about 1.2 calories of nutrition. The next closest conifer seeds contain, in order, 0.4, 0.2, 0.05, and 0.02 calories. As you can see, the whitebark seed is by far the best food around for lots of animals. Grizzly bears, in years of high whitebark cone crops, will spend most of their fall in the whitebark forests gobbling up these extremely calorie-dense seeds before hibernation. Nutcrackers don’t even breed in years of low whitebark cone crops, because there simply isn’t enough other food around to for their chicks to survive!
To string this circle back together: Whitebark dieoff means fewer whitebark seeds for Nutcrackers to eat, which means that many of these birds may leave for greener pastures or look elsewhere for things to eat. Without Nutcrackers, the Whitebarks have lost their main steward, so fewer seeds will be planted and germinate.

This even affects the nearby trout! With all the old trees dead and needle-free, and no young whitebarks germinating, the shade-free snow in these forests melts a lot faster, dirtying the streams long before the normal spring runoff season, and making it much harder for the already-near-endangered cutthroat trout to feed and spawn.

Why can’t the beetles and whitebarks just mind their own business, and not drag the rest of the ecosystem into their brawl? But no, thanks to some unseasonably warm winters and hungry larvae, suddenly a lot of creatures have to start thinking about other ways to get by: nutcrackers, squirrels, rodents, bears, trout, spruces, firs, and a whole lot more. Things that depend on or are eaten by these animals are also affected, and so on. The altered interaction between a bird and its tree, a tree and its beetle, has cascading impacts on everything else around.

Comments

  1. Wow. That is really cool! :D
    I mean, the way you explained it was really cool too.

    ReplyDelete

Post a Comment