Measure of Success
How do you know when you’re doing it right?
The mergansers have returned to Mountain Lake in San Francisco’s Presidio. Mergansers haven’t been seen here for at least 20 years. But in December of 2018, a male and a small group of Hooded Mergansers visited Mountain Lake for a week. What changed? Why does it matter?
Ducks are generally omnivorous, eating whatever they come across — snails, tadpoles, algae, seeds, berries. Mergansers are a variety of duck that has gone carnivore.* They eat small fish, crayfish and aquatic insects. Their beaks are narrow, ideal for slashing through water after prey. They are sight hunters, with eyes adapted for good underwater vision.
And Hooded Mergansers look cool. Both sexes have bright orange eyes, and sport a fan-shaped crest that can be raised or lowered, like an adjustable mohawk. The female crest is cinnamon colored, while the male’s crest is a dramatic black and white affair.
If you were a merganser, hunting for fish underwater by sight, one thing you would appreciate is clear water. And that’s why the mergansers haven’t been at Mountain Lake for 20 years. Before a massive restoration effort at Mountain Lake, the lake’s fish population was dominated by carp and catfish. Both fish are bottom feeders; happy to live in muddy water. Carp actually feed by sucking in mouthfuls of mud and spitting it back out. While those fish were present in Mountain Lake, no aquatic plants could take root, freshwater mussels couldn’t contend with the mud, and mergansers couldn’t see to hunt.
The restoration ended with the reintroduction of native species, like Pacific Chorus Frogs and freshwater mussels. Without the agitation by invasive fish species, the water is clearer. The dominant aquatic animal in the lake is the three-spined stickleback, a sturdy fish about three inches long. These fish have thrived in the lake since their reintroduction, now numbering in the 100,000s. An endless buffet for a small duck with a yen for seafood.
One of the goals of the habitat restoration at Mountain Lake and the broader Presidio is to increase biodiversity by reintroducing native species. Once the keystone native species are established, other native species should also return. And a duck that hasn’t been seen at Mountain Lake for a generation counts as a success, proof that the efforts pay off.
Read more: El Jefe Has a Posse
Get involved: Volunteer at the Presidio
*While watching Hooded Mergansers on the lakeside, a passerby asked what I was looking at. “Mergansers,” I said. “Mergansers are a sort of duck gone carnivore.” She paused, and asked, “they eat other ducks?”
