Sometimes as a scientist, I notice instances where enthusiasm for things green gets ahead of the science on an issue, and the so-called exciting green solution may not be so good after all.
A few recent instances of things that people got really excited about and then realized, "well, maybe we've gotta be careful here and take a hard look at the underlying science and logistics to make sure we're doing this right," include carbon offsets and biofuels.
Use of goat-grazing to control brush, which Treehugger writes about here, is another one of those things. This author seems a little too excited about how wonderful goats are for the land, without acknowledging the cautions required in managing them -- and damage goats can do if the cautions aren't heeded.
Basically, as the author notes, goats can be very good -- for things like removing thick mats of weeds from roadsides and other disturbed areas. However, if not used properly to control weeds in natural areas, they can be very bad and can actually favor more weed invasions by disturbing and fertilizing soils.
In a simple example of how they can be bad, back in 1997, my landlord outside of Portland became pregnant and so stopped mowing our nice green lawn, bringing in 2 sheep and 2 goats to take care of it. The goats were extremely personable (and would come to the fence to say hello when we got home), but the sheep were so dumb they were like barrels with feet. She divided the lawn (two adjacent one-acre home plots) into 3 pastures.
Within a month or two, after she rotated the animals out of each "pasture", what had been a nice green lawn turned into a mess of thistles and other weeds that had never been there. What happened to our nice lawn??
As a conservation scientist who has since studied the science of grazing impacts on vegetation, you had two things going on there: hoof disturbances that create open space, and nitrogen additions from urine and dung that fertilize the soils, which tends to favor weedy species.
In the end, whether grazing will have a positive or negative long-term effect on areas depends on the weed species in question, the timing and duration of grazing, proper stocking density, and the desired outcome you want (do you just want a roadside or urban hillside lot cleared of fire-prone weeds until you graze it again next season, or are you shooting to begin a native habitat restoration in a natural area by eliminating a thick mat of weeds?).
If you want a natural area rich in native species in the long-term, you'll need to do some additional native restoration (re-seeding, substrate alterations) work after the grazing to make sure the species that come up after the goats leave are the ones you want -- and not a new slew of spiny weeds...
The response of native plant species to grazing differs considerably east of the Rockies (where native grasses and wildflowers co-evolved with buffalo, so have traits that allow them to tolerate that kind of heavy hoofed animal grazing) vs. west of the Rockies (where they didn't, and so many natives don't do so well with heavy livestock-type grazing, so grazing favors weedy species). There are, of course, cases where grazing west of the Rockies can be great for native species -- as is the case for favoring native annual wildflowers in many California grasslands, for example. Similarly, some species and habitats east of the Rockies may be highly sensitive, and grazing won't be a good idea.
The bottom line to Treehuggers who love goats -- to protect the health of your land, be sure you know what you're doing and what your goals are before you bring them in...