[female] The Sierra Nevada yellow-legged frog was once the most abundant amphibian in the Sierra Nevada
and probably the most abundant vertebrate.
[male #1] Back into the early 1900s, we had researchers in the park -- and also just park visitors --
and we have reports of them going to all these different lakes
and they'd see hundreds or thousands of these frogs.
[female #1] And they'd talk about how when they were walking along
they had to watch where they were stepping
because there were so many frogs!
So Yosemite is known for this incredible landscape --
this incredible glacially carved landscape --
and as a result of this landscape we wound up with over 2,500 lakes in Yosemite.
And one of the interesting things about these lakes is that they evolved without fish
because we have these grand, cascading waterfalls
that prevented the fish from being able to populate those lakes
after the last glacial period.
And so you had these great ecosystems that evolved
with animals like the Sierra Nevada yellow-legged frogs
and this incredible invertebrate diversity.
And then, starting in the late 1800s, we started fish stocking.
We started stocking lakes with things like rainbow trout and brook trout and brown trout
that people love to fish.
And what happened is the fish started competing with the frogs,
and preying on the frogs, and we started losing frog populations very rapidly.
[male #2] By the 1960s and 1970s, we were seeing the endgame in that process,
which was the virtual elimination of mountain yellow-legged frogs
from most of their former habitat by fish introductions.
That's a dramatic change, and that dramatic change has meant
that not only is the amphibian disappearing
but all the pieces of the ecosystem that depend on that amphibian
or are affected by that amphibian are also being changed.
So we have garter snakes that were once a major predator on mountain yellow-legged frogs
now disappearing as their prey, the mountain yellow-legged frog, disappears.
[Heather McKenny] One of the reasons that frogs and fish can't coexist is because
for one, the fish feed and prey on the frogs -- but they also compete with them for food.
They compete with them for the same kind of invertebrates.
[Travis Espinoza ] By the 1990s, we started to notice really steep declines,
and so we were trying to figure out why this was happening.
[Roland Knapp] It became clear to me that something else was driving declines of this frog
in addition to fish.
[Heather McKenny] They would find hundreds and hundreds of dead frogs in the lakes
and people started asking what was going on.
Was it something in the environment? Was it pesticides? Was it UV radiation?
What was going on?
And it wasn't until 1999
that we finally identified amphibian chytrid fungus
as the primary factor that was causing these massive declines.
So the amphibian chytrid fungus is a non-native species
that we think evolved with the African clawed frog in sub-Sahara Africa.
And starting in the 1930s, this frog that evolved with this pathogen
started being transported globally throughout the world
for laboratory experiments and pregnancy tests and the pet trade.
And you can track the shipping of this frog with the spread of this disease
that's now blamed for the extinction of probably 200 species of amphibians throughout the world.
What this amphibian chytrid fungus does is it grows inside the frog's skin
and if there's enough of that fungus growing in the frog's skin
it'll eventually kill the frog.
But basically, the frogs can keep the infection low --
the amount of chytrid that's growing in their skin low enough --
that they're actually able to survive, even though they're infected.
And so in Yosemite today, we assume that every single living population in the park
is infected with this amphibian chytrid fungus.
At this stage we really can't control chytrid, but what we can do
is we can increase the amount of high-quality habitat that we have
for the remaining populations of Sierra Nevada yellow-legged frogs in Yosemite.
What we've done in Yosemite is we've identified the most advantageous places in the park
to do restoration to recover the frog.
And by restoration I mean eradicating the non-native fish.
[Roland Knapp] So the goal of the project is not to remove fish from the entire park
or to remove fishing. We realize that's a recreational activity
that people have been doing for decades, and it's really important to some people.
So we're not trying to get rid of that. The goal is to restore habitat for the frogs
by removing these non-native predators at some select sites.
[Heather McKenny] So what's exciting is we're actually seeing these chytrid-positive frogs
moving back in and recolonizing sites where we're doing habitat restoration.
So we're seeing success from our fish eradication efforts.
[Roland Knapp] So what we've seen over and over
is after we've done these fish removals and the frogs have recovered
we've watched all those functions that are related to the frog also recover.
So we've seen the recovery of these invertebrate populations --
which are now food for this increasingly abundant frog --
and we've even seen effects of that recovering invertebrate population
on terrestrial predators like birds.
So rosy finches are a really common bird in the alpine zone, where there isn't a whole lot of food,
and what we've seen is as the food resource from these lakes increases following fish removal
the rosy finches begin to target that food resource.
So they are very dependent on resources that are coming out of those lakes.
[Heather McKenny] So we know, even in the face of chytrid,
we know that if we restore these habitats -- these key habitats for the frogs --
that we are going to have success
and that we are going to be able to increase the number of healthy, viable populations in Yosemite
and hopefully keep the species from going extinct.
I would love to see a day where we have basins in the Sierra Nevada
that are teeming with frogs once again.