There's a conservation-biology rule of
thumb that, for a species to survive for another century, it must have a stable
population of about 5,000. Want to settle Mars? Bring 4,999 other
unrelated humans with you and populate it.
And yet a recent study finds that the bog
turtle--the rarest turtle in the continental US--could survive with only 40
members, including 15 breeding females. The study's authors want this to change
how legislators protect flora and fauna. But will it be for the best?
The bog turtle is North
America's smallest and rarest native turtle. It's primarily found
in New York, New Jersey,
and Pennsylvania, though there are isolated
populations in North Carolina,
and estimates of its population range from about 2,000 to 10,000, a loss in
population of 80 percent in the past 30 years. George Amato, a conservation
biologist at the American
Museum of Natural
History, told me that the turtle lives in a very specific and disappearing
habitat called a fen. Fens are a particular kind of wetland, sort of but not
quite like a marsh or a bog (yes, these are all different, ecologically
speaking). The predominant theory about these turtles is that they used to be
very widespread during the later part of the Late Glacial Maximum, about 10,000
years ago, when the ice sheets that covered North America
began to melt. As they melted, they turned the land incredibly soggy, forming
lots and lots of wetlands, including fens--a happy time for the bog turtle,
which spends most of its time submerged in fen mud. But as the continent dried
out and temperatures continued to rise, there were fewer and fewer fens, so now
the bog turtle is patchily distributed in increasingly rare and specific
environments. "As the temperature changed, that environment has become
kind of a relic," says Amato. "They probably weren't isolated before,
but they are now."
It's also a victim of the pet trade--Amato
says that when the species was written about, biologists "wouldn't
actually put the exact location of studied populations" in their articles,
to keep collectors in the dark about where to find them. And, of course,
there's encroaching human development and invasive species of plants which
choke out its food sources.
It's estimated that there are about 570
populations of bog turtle in the world. The turtle is hardy and long-lived, but
it doesn't travel, which makes it hard for populations to interact, and it's
estimated that half of those populations aren't exactly "populations"
in the way you might imagine--they consist of, well, one turtle. Only 15 of
those populations consist of at least 50 turtles.
There's a significant effort on the part
of legislators, conservationists, and zoos to boost the turtle's population.
But this study, published in this month's issue of Conservation Biology,
finds that as few as 15 breeding females (and about 25 males) could be enough to
sustain the bog turtle's population for another 100 years. The bog turtle lives
for decades--up to 50 years--and gives birth to one offspring per year with a
nearly 33 percent chance of survival, which is quite high. That's a major
shakeup to the idea that a given population needs thousands to survive.
When biologists want to get more precise
than the 5,000-individuals rule, they can calculate a number called the Minimum
Viable Population, or MVP, the result of computer simulations to figure out the
lowest possible number of members of a species that would result in a 90-95
percent probability of survival after 100 years. The game is to balance the
number of lost specimens (due to death or leaving the group) with the number of
new specimens (being born or coming in from another group).
Those simulations take into account how
long the species generally lives, how often it breeds, the size of the litter,
and the probability of young surviving into breeding age. Then it adds in
external sources of stress, which could include inbreeding, natural disasters,
climate change, encroaching human development, poaching, or any other event
that could have an impact on a population. You come up with a few hundred or
thousand possibilities--what if the temperature rises 2 degrees, what if
poachers snag one female from a population per year for the pet trade, what if
it loses 7 percent of its habitat due to construction of some awful beige
McMansion--and see how often the species will survive to see the year 2113.
Weight them and average them and you've got your number. But these simulations
are basically possible futures for the species, like a sci-fi writer discussing
the near future. They're well-researched guesses, but they're guesses, and
they're extremely important for deciding what kind of protection to give each
species. According to George Amato, "there's been a lot of discussion
[about MVPs]; it's a little controversial."
The bog turtle is highly protected, and
any statement about its long-term survival will necessarily have an impact on
how it's protected. Kevin Shoemaker, the author of the paper, told me that
"the point I'd like to make is that in a larger context, these very small
units are very, very important for conservation. We don't want to discount very
small populations just because they're so small." Populations in the
double-digits are sometimes seen as lost causes--but this study also indicates
that maybe that's not the case. It takes issue with the problem that
conservation resources are often shifted away from populations with numbers as
small as the bog turtle's, when in fact, as Amato says, "protection of
small and fragmented populations may constitute a viable conservation option
for such species."
I spoke to Dr. Alison Whitlock of the U.S.
Fish and Wildlife Service to see about the legislative effects of the study.
Under the Endangered Species Act, each threatened species is given a recovery
coordinator--a kind of czar who's in charge of that species' well-being. Dr.
Whitlock is that person for the bog turtle, and she was thrilled about this
study. "This actually gives me more hope," she said, as soon as I
asked her about the paper. "For those of us who've been working with bog
turtles for years, we've had a gut feeling that they're holding on in these small
populations, but they're still there and still worth saving."
The bog turtle is protected federally. If
you're a developer in Pennsylvania or New York or wherever
else, you have to take your prospective project to your individual state agency
so they can check with Dr. Whitlock (or another expert) and make sure the
project won't violate the federal protection. Each state can make their
laws stricter than the federal law, but the federal law is the
minimum. I was concerned that this study would have the reverse of its intended
effect--that legislators would see that the MVP is only 40, and think, well,
why are we wasting money on protecting this turtle when there are a couple
thousand of them? "If people think that you can have stable populations of
40 organisms, I'd be really concerned if managers were thinking 'oh, then we
shouldn't worry so much,'" says Amato. Shoemaker echoed the concern;
"I definitely don't want to make it sound like a population of 40
individuals is all you need," he said.
But Dr. Whitlock thought that an unlikely
scenario for the bog turtle. It's not up to local legislators; it's up to her,
and people like her. It's a perfect argument for why top-down, federal mandates
are so important. This is a very wonky situation, requiring quite a bit of
knowledge of conservation biology and this species in particular, and local
governments can't possibly be expected to have bog turtle experts on staff.
Hell, I spoke to just about every bog turtle expert in the country while trying
to figure out what's going on here, and discovered that there are like...four
of them. And they're in charge, just the way they should be, even if they may
not have as many resources as they'd like.
In fact, Dr. Whitlock believes that this
study is going to have only positive effects on conservation efforts for the
bog turtle. The federally mandated minimum conservation is not always ideal;
Dr. Whitlock works for the government, and was not entirely willing to
criticize her employer, but made it clear that the federal laws are only the
bare minimum. With this extra data, she and other conservationists can go to
the state agencies and make a stronger case than ever before. "Sometimes
it's hard to go to the next step and argue for land preservation or resource
protection," she said. "This gives us a little extra backing to not
just protect but also restore."
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