Bullfrogs? You’ll need snakes too
Nature is gross
We dragged the net to shore. It was heavy with shanks of fibrous, dark green algae. Where strands of algae quivered, Presidio Trust biologists Jon Young, David Harelson and I (author/citizen scientist) reached in and grabbed at squirming, alien creatures. Then drop them into a white 5-gallon bucket. It’s an odd sensation.
Imagine holding a peeled hardboiled egg. Now imagine it’s slimy, writhing, and has a tail. That’s what holding a bullfrog tadpole is like. Along with Jon and David, I was grabbing as many of these oversized tadpoles as I could.
Other critters came up in the net. Threespine stickleback fish glinted like blue-silver coins. We picked them out and released them into the lake. Crawdads sloshed alongside tadpoles in the bucket, with one juvenile smallmouth bass.
After sorting through the algae, we lugged the heavy green mass up the shore and dumped it under some bushes.
“It’s good fertilizer,” said Jon with a shrug.
Then Jon and David waded back into the lake to sweep the shore again. Armfuls of algae. Dozens of tadpoles, buckets brimming with tadpoles. I’m grossed out by these beasties, but I have a grudge.
Bullfrogs, bass, crawfish and red-eared slider turtles are all invasive species in Northern California. In San Francisco’s Presidio, Mountain Lake underwent a restoration program, starting in 2014. All non-native turtles, fish and frogs were removed, and the lake was restocked with native species: Western pond turtles, Threespine sticklebacks, Pacific chorus frogs, and freshwater mussels.
Now, with Jon and David, I’m again removing invasive species from the lake.
Why? What happened?
In a word, people.
What were you thinking?
A lot of effort goes into educating the public about the harm that invasive species do. They tend to crowd out native species. They reduce the diversity in an ecosystem. Diversity in an ecosystem means redundancy. A bigger variety of species means a bad season or disease doesn’t become a crisis. Redundancy builds resiliency.
It’s hard to convey this message to disinterested people in a park. It takes a minute to explain that it’s bad to feed stale bread to seagulls, and bad to let your pet turtles loose in the lake. The Presidio Trust spent years setting up a folding table at the lakeside, with posters and coyote pelts and patient naturalists explaining habitat restoration to anyone who’d listen (hint: kids listen).
They collected 10,000 pledges from visitors, promising not to let animals loose in the lake. There’s a big, clearly marked box, where people can put their unwanted aquarium fish and slightly used turtles, right by the lakeshore. Makes it easy to do the right thing.
But jeez, bullfrogs? They’re not just a nuisance, they’re a hazard. They will eat anything they can push into their mouths. That’s not a metaphor: they literally push big meals into their mouths with their front legs. The could crush the local chorus frog population, and wipe out any baby turtles that hatch. Swallow unsuspecting sticklebacks. Why would anyone let them loose in the lake? Jon offers three motives:
- I’m doing it for the frog. It deserves to live a free frog life.
- I fully intend to harvest frog legs someday soon.
- I hate ecology. I will wreck your precious experiment.
To be fair, the bullfrogs are just being bullfrogs(1). They didn’t ask to move to California, and in their home range, they’re not a menace to their neighbors. Bullfrogs are classified as “invasive” in California because here, they don’t have a full range of natural predators. Fish will eat their eggs, herons and raccoons will take tadpoles and adults. What’s missing, predator-wise? Bullfrogs are native to most of the East coast. What do we lack here in California?
As far as I can tell, what we are missing are hard freezes, water snakes, and alligators(2). Solutions that are worse than the problem. This, people, is why we can’t have nice things.
Push out the jive, bring in the love
There is good news about Mountain Lake. The water quality is better than it has ever been. The stickleback fish number in the hundreds of thousands. And the freshwater mussels are showing signs of breeding and growing. Our local freshwater mussels, called ‘California floaters,’ spread their fertilized eggs by squirting them onto a passing stickleback. The baby mussels hitch a ride in the fish’s gills and eventually fall off.
The first mussels in the lake were introduced in submerged cages, so their health could be checked. These cages are like trees on a savanna. As David put it, “Fish love structure. It provides security and food.”
Jon and David harvest mature freshwater mussels from the wild, and house them in aquariums. They add sticklebacks to the tanks, and the fish are inoculated with mussel larvae. Then these fish are re-released into the lake. The larvae fall off the fish throughout the lake.
But because fish congregate around the submerged cages, the biologists can check these cages for evidence of growing, baby mussels. Happily, they’re seeing these growing mussels, which means they’re also dispersed throughout the lake.
Yes, but what about the turtles?
I survey the native Western Pond turtles at the lake. In 2019, the highest turtle count was 19. After consulting with turtle experts, 19 was considered a solid success. But in April of 2020, the numbers jumped to 25. The turtles continue to make their way, sharing their basking logs with ducks and herons. They’re big enough to breed, and that’s one more reason to clamp down on the bullfrogs. Baby turtles don’t stand a chance on their own.
And one of those 10,000 pledges paid off recently. Looking in on the “put your excess pets here” box is one of my volunteer chores. In June, I checked the box and was delighted to find two red-eared sliders. Right where they should be. They’ll go to a ranch pond in Sonoma, with other red-eared sliders, and away from native Western Pond turtles.
Habitat restoration isn’t a one-off. Carving out breathing room for native species takes sustained effort. Mountain Lake is a little pond in a crowded city, and if we want to have nice things, like species-rich habitats, we’ll have to work for it.
More about Mountain Lake and its critters
- What’s Up My Turtles? (2019)
- Measure of Success (2019)
- El Jefe Has a Posse (2018)
- The Mountain Lake Turtles (2018)
Footnotes:
- I don’t hate all frogs. Chorus frogs are awesome. But bullfrog tadpoles have all the charm of hagfish. They’re especially creepy when their skinny little legs start growing.
- There was an alligator loose in Mountain Lake, back in the dark days of the 20th century.
