Lack of temporary pools could cause some desert animals to croak

August 28, 2012

When the summer monsoons come to the southwest, abundant runoff leads to major problems for cities — streets flood and temporary ponds appear that serve as breeding grounds for mosquitoes. Engineers try to contain and eliminate the water, and environmentalists try to recycle it, but some desert animals need these temporary pools to stick around. From Tucson, Sarah Bromer shares this report.

full pond empty pond A temporary drainage pond next to a Tucson shipping warehouse became a toad habitat, but it dried up quickly. (Photos courtesy of Sarah Bromer)

I was taking a walk after a summer storm in downtown Tucson one evening when I heard a sound I’d never heard before. I thought, is that a flock of sheep? So I followed it to the edge of my neighborhood, where I found people gathered with flashlights.

There, in a temporary drainage pond next to a shipping warehouse, were dozens of toads. Amphibians are pretty much the last animals you expect to find in the desert, and nobody seemed to know much about them. So I called Dr. Phil Rosen.

"I’m Phil Rosen, and I’m a research scientist in natural resources and the environment at the University of Arizona," he said.

I took Dr. Rosen to the pond the next night, but all we heard was silence. The pond looked a lot smaller. We found tadpoles in the water, but no toads, so I tried to explain the sound to him. Dr. Rosen was pretty sure that they were Couch’s Spadefoot toads. 

"I fully expected it to be nothing but Couch’s Spadefoot because of their ability to use such short-lived water," Rosen said. "They’re uniquely explosive breeders with very short tadpole stages."

The toads spend most of their lives underground, emerging each summer for maybe only a night or two of frantic feeding and breeding. Their eggs hatch within hours, and only seven or eight days later the tadpoles become toads.

"It’s fastest metamorphosing species here, probably one of the fastest in the world," Rosen said.

I thought Dr. Rosen would be really excited to find prime toad breeding habitat in my neighborhood, but he wasn’t very excited at all. He told me that the water in the pond probably wouldn’t last long enough for the toads to complete their lifecycle.

"This basin that we’re standing next to is designed to dry itself within three days," he said. "It’s an attractive nuisance for them, basically. It brings them in to breed in a place they can’t survive. 

Tucson’s building codes require all businesses to capture their run-off so it won’t contribute to flooding, and then sink the water into the ground quickly, so it won’t breed mosquitos. This is good news for the people in the neighborhood, but, obviously, bad news for the toads.

"How do you feel when you see sites like this?" I asked him.

"Well, you know, this sort of thing worries me, because it exemplifies the ability and the tendency of people to just keep modifying the environment and making it more and more perfect for people," he said.

"And, you know, unfortunately, there’s less tendency for there to be little corners where water lasts for two or three or four weeks. So I suspect there’s an ongoing, very steady decline of these species throughout most the city," he added. 

So what are we supposed to do if we want toads in our neighborhoods, but not mosquitos? According to Dr. Rosen, we may not need to make that choice.

"If a pond, let’s say, like this one, lasted on average for two to three months, it would be so full of predatory crustaceans as well as insects, that it wouldn’t really produce any mosquitos," Rosen said.

He’s been working with city officials to engineer longer-standing pools of water that use biodiversity to control mosquito populations.

"I’ve been working to develop that it into kind of an art form, because I completely sympathize with people not wanting to live around lots of mosquitos," Rosen said. 

He believes that his solution, though more complicated, is the best one.  

"You know, if we simplify our environment, we’re going to end up with just the things that we can’t deal with: diseases and vectors and things like that," Rosen said.

"We’ll either have to reengineer everything so there’s zero standing water, or we’ll have to try to live with the biodiversity and turn it to our ends and let the kids growing up have something interesting to grow up with," he added.

I went back to the pond a few days later, and, as Dr. Rosen had predicted, all the water was gone. Birds were pecking at the muck. I guess it wasn’t a good year for the Couch’s Spadefoots in my neighborhood. But maybe someday this pond will be re-engineered and stocked with a lot of different species that will allow us to have the best of both worlds: a neighborhood rich with biodiversity and singing toads, and not too many mosquitos.

Sarah Bromer is a high school teacher and independent radio producer in Tucson, Arizona.


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