Invasion

The sun beat down on the small crew of volunteers armed with six-foot segments of Rebar. When I say the sun beat down, I mean the Arizona sun. It beat the volunteers as if with Tasers and billy clubs. It was seven o’clock in the morning.

So why the volunteers for this torment? They were fighting – and apparently winning – a small but significant skirmish in a war with an illegal invader that is threatening the very homeland they have inherited, or for the most part, adopted. And yada yada yada about the war and menacing alien metaphors. The popular literature on buffelgrass is already pregnant with them. I doubt I can add much, either to that or to the raft of comedians using Arizona crazy as their favorite subject of jokes these days.

Buffelgrass is a bunch grass originally from the African savannah, as far as I can find out. It was – and is still – widely planted in North America, especially in Texas and Sonora, primarily for cattle forage. But in Arizona, it has taken on a life of its own and been declared a noxious weed. Biologists fear the choking, fire-prone grass that spreads here on its own could wipe out the iconic species and forever change the ecosystems. Nothing against African savannas, but picture Arizona without the branched saguaro cactus, or tentacle-like ocotillo or broccoli-green trunked palo verde. People here are understandably concerned.

The Sonoran Desert Weedwackers are a volunteer group that has taken matters into its own hands – in coordination with the Desert Museum, Pima County, and other relevant authorities. Marilyn Hansen, who has been organizing and compiling a really remarkable data set on what the Tucson Mountain Park chapter has accomplished, beamed as she told me about the reaction she had gotten from local officials before clearing so much area, and such dense stands.

“They said this was impossible. Well, to Weedwackers, those are fighting words.”

That may be a point of pride to someone with the time and drive to spend at giving it a shot, but others have to calculate if the resources are worth the risk. Upon mentioned of the project my Conservation Bio students will do on biodiversity in the wake of buffelgrass removal, an outdoors trip leader in the area recently asked me,

“Do you think we can get ever get rid of it? Or is it a lost cause by now?”

I gave him my best scientific answer: I shrugged.

But it is something worth giving more thoughtful consideration to: should we spend the tens of thousands of dollars to spray Roundup from helicopters, ask the scores of volunteers to labor under the vicious Arizona sun, bulldoze acres and acres of buffelgrass covered land to replant natives?

Katriona Shea and Peter Chesson published a paper in 2002 titled “Community ecology as a framework for understanding biological invasions.” In it, they provide a brief run-down of the models and parameters that matter most to determine local coexistence, exclusion, and invasion processes. They claim you can divide an invasion into two parts: arrival and spread. While Mexico’s government continues to pay ranchers to plant buffelgrass, there is little we can do to slow seeds blowing in over the border. But what happens when they arrive? This is the community ecology part, where we look at the grass’s response to the environment, to competitors, and how it shakes up the predation pyramid.

I may spend some more field time in the near future examining the trophic (fancy word for “food web”) interactions, and the way it competes with natives for water and other resources. But a quick and dirty equation can provide at least the trivial answer the trip leader’s question: Under what circumstances can the Weedwackers win?

Population growth = birth rate – death rate.

When the Weedwackers, along with the county, private landowners, and others can drive the death rate (pulling, spraying, etc.) of buffelgrass higher than its recruitment rate (germinating seeds from populations in Arizona), no matter how many seeds blow over the border, they will not find a “niche opportunity,” as Shea and Chesson (2002) put it.

Impossible?

Tell that to the Weedwackers.

[Note: I had a bunch of great photos of a small desert tortoise we found, rebar use, before-and-after shots, etc. But… in my sleep-deprived haze this week I apparently cleared my camera card memory after only examining them on my laptop, not saving the photos. Note to self: sleep more.]

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