The Tangled Web of Montana Spiders
Air Date: Week of September 30, 2011
As invasive plants take root and spread in the grasslands of Missoula, Montana, a tiny native spider is flourishing on the plants and helping wreak havoc on the ecosystem. Producer Ari Daniel Shapiro met with a scientist who is monitoring the transformation and trying to keep the cascade of events in check.
GELLERMAN: It’s Living on Earth, I'm Bruce Gellerman. In E.B. White's classic children’s book - Charlotte the barnyard spider saves Wilbur the pig from becoming roast pork for Christmas dinner. Charlotte does it by weaving a web with words designed to discourage the farmer intent on his feast. Well, it turns out there are spiders in Montana spelling out a very different kind of message. To see it - you just have to look closely. Reporter Ari Daniel Shapiro tells our story.
[SOFT, RUSTLING SOUND]
PEARSON: We can just go and look at some of these webs.
SHAPIRO: It’s early morning in the hills of Missoula, Montana, and Dean Pearson – an ecologist with the U.S. Forest Service – is on a mission. He’s looking for Dictyna spiders in the plants out here.
PEARSON: Dictyna spiders are really quite small. If you’re afraid of spiders, these are probably not ones that are gonna be too frightening to you – they’re maybe half of your fingernail. And that’s the females, which are larger.
SHAPIRO: The Dictyna major and Dictyna coloradensis spiders are light gray with a dark stripe down their backs.
PEARSON: Um, I see a few spiders that are adding a little bit to their webs. And uh, this one’s just hunkered down at the base of the web waiting for the action to come, I guess – waiting for a capture. I have a male and female in this web that are making new spiders. We’ll leave them alone.
SHAPIRO: At this early hour, the webs are lit by the sun from behind.
PEARSON: They’re quite beautiful webs, sort of shaped like an upside down pyramid, often.
SHAPIRO: It all seems so picturesque – these backlit webs, spiders making love in the morning light. But when Pearson looks out across the hills, something’s wrong.
PEARSON: If you squat down on your knee, it puts you about at vegetation height. And you just see webs everywhere.
SHAPIRO: In the last 10, 15 years, these rolling hills have been transformed, and that transformation’s ideal for building all these webs.
PEARSON: We’re standing here in the middle of what was once a beautiful, beautiful grassland. We’ve got things like blue lupins in flower. But mostly what we’re seeing here is a stand of yellow-green leafy spurge that’s gotten pretty heavy – scientific name is Euphorbia esula. There’s Potentilla – sulfur cinquefoil, it’s called, which is also a nasty weed. Spotted knapweed, and that one is Centaurea stoebe. So what we’re looking at is a lot of exotic, broad-leaf plants that have come in and really started taking over this system, and they’re, they’re changing the architecture of the grassland.
SHAPIRO: And the little spiders?
PEARSON: They really love this transformation that’s gone on - this invasion of these exotic plants.
SHAPIRO: The invasive plants don’t die back at the end of the growing season like the native plant species. So the spiders have many more places to build their webs. And not just that – because the plants are taller and wider too, the spiders can make even bigger webs – four times larger than before.
In other words, these exotic plants that’ve taken root and flourished here – like the sulfur cinquefoil and the spotted knapweed – they’ve catapulted the indigenous Dictyna spiders to the top of the heap.
PEARSON: The webs are passive prey capture devices – the bigger your web, the more prey you catch. We can look at this one right here in front of me – so this one’s on Potentilla, and there are probably about 5 or 6 prey items caught in this web. And another one over here, and we can see a couple of flies in this web and a pretty good size wasp. Despite their small size, they’re pretty vicious spiders in terms of prey capture ability. Some of the wasps that they catch are probably almost 20 times larger than the spiders.
SHAPIRO: Oh my goodness, how the heck do they incapacitate something like that?
PEARSON: Well, the web helps a lot. But being a spider and being venomous, they can go up and subdue ’em by making a quick bite and then wrapping them up.
SHAPIRO: The more the spiders eat, the quicker they mature. Twice as many spiders are reproducing here compared to the native grassland. And more of them are then surviving on the invasive plants. So the Dictyna spiders are reigning supreme at the moment.
PEARSON: And that’s pretty much the perspective for a fly or a wasp that’s buzzing through here trying to visit these plants. You’re running through this gauntlet of web after web after web out there.
SHAPIRO: It’s not just the webs decorating all the plants here – there’s a more metaphorical web at work. The native spiders are thriving because of the new exotic plants. They can eat more insects, and these insects can then no longer keep the growth of certain plants in check, and on and on. You alter one piece of the ecosystem, and the whole web changes.
PEARSON: I don’t think we’ve fully understood to what extent these changes are cascading up and down through the system in all sorts of crazy ways.
SHAPIRO: The changes have made their mark on Pearson as well. He grew up here, in the hills of Montana, and he misses the way things were when he was a boy.
PEARSON: As somebody who’s fallen in love with native grasslands, it’s, it’s pretty sad to watch these things unfold, and yeah, I’ve just tried to hold back the tide.
SHAPIRO: Hold back the tide, that is, of the invasive plants that are altering this ecosystem. He’s exploring whether certain herbicides or insects can be used to manage the weeds and fix what’s happened. Try as he might, though, Pearson thinks this ecosystem will ultimately fall into a new balance. But it might take a thousand years. And in the meantime, at least for now, the Dictyna spiders have it made.
For Living on Earth, I’m Ari Daniel Shapiro.
GELLERMAN: Our spider story comes to us from the series "One Species at a Time." It's produced by Atlantic Public Media with support from the Encyclopedia of Life. There's more at our website: LOE dot org.
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