The Why Files The Why Files --

Bugs mimic mice!
16 MARCH 2006

Assume you're a bug on an island without rats or mice. Do you start to fill the same ecological niche that these rodents occupy elsewhere? Specifically, do you eat seeds and spread them around the landscape -- a common role for small mammals?

3-d looking image of New ZealandIf you're a plant on an island where the cast of native small mammals includes nothing more than a couple of bats, do you start to rely on bugs to spread your seeds?

We're not asking these questions just because we like to sound smart (though we do...). We're asking because Kevin Burns, an assistant professor of biology at Victoria University of Wellington (New Zealand) was curious. He's a specialist in seed dispersal.

New Zealand image from original by NASA.

Nothing is more important to a plant than spreading to new territory, and they have plenty of mechanisms for moving seeds. Floating seeds spread in the water. Burr-like seeds spread on animal fur and feathers. Lightweight seeds move on the wind, or are transported when insects eat them. Mammals and birds eat large, fleshy fruits and distribute their seed in scats.

So how do heavy seeds get around in the absence of terrestrial mammals?

Grasshopper, with giant rear legs, looks at camera, perched on dry branch.
Many wetas live in trees, like this adult male Auckland tree weta (Hemideina thoracica). Courtesy G. Gibbs

Meet the weta
New Zealand is an oceanic island that broke away from the ancient super-continent Gondwanaland 80 million years ago, Burns wrote us by email. Before Polynesians and Europeans arrived, New Zealand had an impoverished cast of mammals.

Cropped image of two big grasshoppers are black, brown and yellow. Antennae are intertwined, wings are absent. click for larger photo Wetas are flightless grasshoppers native to New Zealand. This is an adult, male Wellington tree weta (Hemideina crassidens). Click photo for larger image. Courtesy G. Gibbs

Weta are flightless grasshoppers that are some of the biggest bugs in the world, Burns says. "They can reach over 70 millimeters in length and weigh over 70 grams," more than three times the average lab mouse. There are just over 100 species and they perform many of the same ecological services as rats and mice elsewhere. They are omnivorous, and feed on leaves, fruits, flowers and other insects."

You don't need a PhD to recognize that wetas are eating from the traditional menu of little rodents. "New Zealanders sometimes call weta 'invertebrate mice,'" he continued. "So we reasoned if weta really are invertebrate mice, they should disperse seeds, as mice and rats do elsewhere. "

To study the issue, graduate student Catherine Duthie fed wetas the fruits of 19 plant species. As might be expected, many seeds were destroyed in the process. Five species, however, actually germinated better after their gastrointestinal jaunt.

In other words, having their seeds devoured gave these plants a "fitness advantage."

A statistical comparison of those five surviving species against the 14 that perished revealed a suite of differences in "fruit length, pulp mass, water content, fruit pulp percentage, and seed length," the researchers wrote. This suggests that plants and wetas are co-evolving in the kind of symbiotic relationship that is often seen between plants and other fruit-eating animals. Some new-world ants, for example, colonize acacia trees and protect them. The trees, in return, have special glands that produce a sugary syrup for feeding the ants.

enlarged image of reddish brown ants on bright green stalkThese ferocious ants uses a nasty sting to protect its home, the acacia tree. The acacia returns the favor by feeding the ants. Courtesy © Scott Camazine

In co-evolution, two or more species change to improve their own survival in a cascading series of steps that often lock them closer, so they begin to rely on each other for survival.

Ants protecting Bullhorn Acacia. Courtesy © Tim Brown
closeup of 2 ants on large tree thorn
(original image from Brown's Corcovado page)

Graduate duties
And then came the nifty part. Burns sent Duthie on a weta-scat hunt (are graduate students created for such glamorous tasks?). Duthie found that some scats did contain intact seeds, and that they did germinate in the lab. If the plants actually survive better when their seeds are eaten by wetas, the researchers wrote, "our results provide the first example of a mutualistic association between fleshy fruits and an insect fruit consumer."

Elegant black and brown grasshopper, rear legs speckled white, raised in air An immature female giant bluff weta (Deinacrida elegans). Click photo for larger image. Courtesy B. Robertson

The study does show, Burns informed us, that "weta are indeed ecological replacements for small mammals as seed dispersers." But that could change. In New Zealand, he added, "Humans have introduced rats, mice, a weasel and a variety of other mammals who are now serious problems. Introduced mammalian predators consume many types of native animals and have driven many to the brink of extinction. Weta are a popular item in their diet, and as a result, many weta can now be found only in areas where introduced mammals have been eradicated."

And as the wetas disappear, so could the native plants they transport....

-- David Tenenbaum

Seed Dispersal by Weta, C. Duthie, G. Gibbs, and K. C. Burns, Science, March 24, 2006.

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