What Should We Save? The Strange Puzzle at the Heart of Conservation Biology

Imagine you’re a scientist standing in a forest. Warblers are calling, a salamander is hiding under a log, and somewhere in the soil there are millions of bacteria you can’t even see. You have a limited amount of money and time, and you want to protect this place. What exactly should you be trying to save?

That’s the question conservation biology has been wrestling with since it became a real science in the 1980s. And it turns out to be much harder than it sounds.

The Word That Changed Everything

In September 1986, about 14,000 people gathered in Washington D.C. for something called the National Forum on BioDiversity. A biologist named Walter Rosen came up with the term “biodiversity” by taking the “logical” out of “biological diversity.” As he later explained, “To take the logical out of something that’s supposed to be science is a bit of a contradiction in terms. And yet, maybe so logical that there seems to be no room for emotion in there.”

That word—biodiversity—became the rallying cry of an entire scientific field. By 1993 it appeared in scientific papers 72 times. By 2025, it was in the titles of over 200,000 papers.

But here’s the weird thing: nobody can quite agree on what it means.

A standard definition from a textbook says biodiversity is “the sum total of all living things—the immense richness and variation of the living world.” That sounds nice, but think about what it actually asks us to do. Conserving all biological variation would mean conserving everything biological—every gene, every species, every ecosystem, every variant of everything. That’s not a goal; it’s just the entire subject of biology.

As philosopher and biologist Sahotra Sarkar put it: “Biodiversity in effect becomes all of biology. Conservation would be an impractical proposal if biodiversity were construed this way.”

So conservation biologists needed to get more specific. But how?

Three Ways of Thinking About What to Save

Philosophers and scientists have come up with three very different answers. They’re still arguing about which one is right.

Answer One: Biodiversity Is a Bundle of Different Things

Two philosophers, James Maclaurin and Kim Sterelny, argued in 2008 that biodiversity isn’t one simple thing—it’s several things that all matter.

First, there’s species richness: simply how many different species live in a place. But even that’s not straightforward. If you have two forests, each with 10 species, but in one forest the species are evenly spread (500 of each) and in the other one species has 900 individuals and the rest have tiny populations, those forests are very different. Biologists call this “evenness,” and it matters.

Second, there’s disparity: how different the species are from each other in their body shapes and features. A forest with 10 species of very similar beetles is different from a forest with 10 species that include beetles, birds, mammals, and ferns. The second forest has more “morphological diversity”—more variety in how life is put together.

Third, there are ecological interactions: who eats whom, who competes with whom, who helps whom. A group of species that are tightly connected through predator-prey relationships or mutualism is different from a collection of species that barely interact.

Fourth, there’s phylogenetic diversity: how much evolutionary history is represented. A set of species that are all closely related (like different species of finch) has less evolutionary diversity than a set that includes species from very different branches of the tree of life.

Maclaurin and Sterelny think we need to measure all of these things together. But this is extremely difficult to do in practice. How do you compare phenotypic variation across very different groups of animals? How do you measure ecological interactions in a way that’s consistent across different places? Even the scientists who like this approach admit it’s more of a goal than something we can actually do right now.

Answer Two: Biodiversity Is What Our Conservation Algorithms Optimize

Sahotra Sarkar takes a completely different approach. He says: don’t try to define biodiversity at all. Instead, define it by what we do.

His idea works like this. You start with a list of “surrogates”—things that stand in for biodiversity. These might be species, or habitats, or environmental features like soil type and rainfall patterns. You set targets: we want to include at least 10% of each type of forest, or 30 populations of each rare species, or whatever we decide matters.

Then you use computer algorithms to figure out the smallest possible area that meets all your targets, or the best possible set of areas given a fixed budget. The thing that these algorithms are trying to maximize? That’s biodiversity.

This is called an “operational” definition. You don’t need to know what biodiversity is in some deep sense. You just need to know how to identify it when you see it, through procedures that everyone can agree on.

Some philosophers worry about this approach. What if the algorithm is wrong? If biodiversity is defined by what the algorithm optimizes, then the algorithm can never make a mistake—which seems strange. And what if different algorithms give different answers? Wouldn’t that mean they’re really defining different things? It’s a bit like saying “whatever this thermometer measures is temperature”—but if you have a broken thermometer, it still measures something, just not what you wanted.

Answer Three: We Should Stop Using the Word “Biodiversity” Altogether

The most radical proposal comes from philosopher Carlos Santana. He argues that the concept of biodiversity is doing more harm than good and should be eliminated from conservation science.

Santana points out that the different kinds of diversity don’t correlate with each other very well. A place with high species richness might have low evolutionary diversity. A place with lots of phenotypic variation might not have many interesting ecological interactions. And not all species are equally valuable—most people would say it matters more to save a top predator or a pollinator than some obscure minnow. Some species, like Ebola, most people would happily see go extinct.

If biodiversity was a real, measurable thing, all these different measures would tend to move together. But they don’t. So what exactly are we talking about?

Santana suggests we skip the middleman. Instead of saying “we need to preserve biodiversity” and then arguing about what that means, we should just say “we care about species X, ecological process Y, and landscape feature Z, and here’s why.” This would force people to be honest about their values instead of hiding behind a fuzzy scientific-sounding word.

What Kind of Science Is This?

These debates about biodiversity aren’t just abstract philosophy. They connect to deeper questions about what conservation biology even is as a scientific field.

Most sciences aim for truth. Physicists want to know what’s really out there. Biologists want accurate descriptions of how organisms work. But conservation biology has a different goal: it wants to save things. That makes it what philosophers call “mission-oriented” or “normative”—it’s built around a value, not just a fact.

Philosophers distinguish between “epistemic” values (truth, accuracy, simplicity) and “non-epistemic” values (fairness, human welfare, the flourishing of other species). In most sciences, the epistemic values are supposed to come first. But in conservation biology, the non-epistemic values are right at the center.

Consider a concrete example. In the Pacific Northwest, the northern spotted owl is being pushed toward extinction by a larger, more aggressive invasive species called the barred owl. Wildlife officials have proposed killing nearly half a million barred owls over 30 years to save the spotted owl. A biologist studying this has to decide: is it worse to falsely conclude that removing barred owls will work (when it won’t) or to falsely conclude that it won’t work (when it would)? The risk of making the wrong decision isn’t just “oops, our theory was wrong”—it’s the extinction of a species, or the pointless killing of thousands of animals. You can’t avoid making a value judgment about which mistake is worse.

The Fight Over What Conservation Is For

In the 1980s, the founder of conservation biology, Michael Soulé, laid out what he called the “normative postulates” of the field. The most important one was that biodiversity has intrinsic value—value that doesn’t depend on what it does for humans. “Species have value in themselves,” Soulé wrote, “a value neither conferred nor revocable, but springing from a species’ long evolutionary heritage and potential or even from the mere fact of its existence.”

But not all conservation biologists agree. When interviewed, many of them gave very different answers. One said “I just believe it, I can’t prove it.” Another said “you can’t possibly defend that scientifically.” A third said the whole idea of value is “anthropocentric”—human-centered—and that talking about “intrinsic value” in nature is contradictory.

In the 2010s, a group called the “new conservationists” argued that conservation biology needed a total rethink. Peter Kareiva and Michelle Marvier claimed that focusing on intrinsic value had failed. People don’t care about abstract value—they care about what nature does for them. Ecosystem services like clean water, pollination, carbon storage, and flood protection are what motivate people to act. Conservation, they argued, should be about human well-being.

This sparked a huge fight. Critics said that if conservation is just about helping humans, then we should be spending our money on hospitals and schools, not on protecting forests. Others argued that the new conservationists ignored the fact that some ecosystems are still relatively undisturbed, and that focusing on human benefits would lead to sacrificing the most wild and precious places.

One of the most controversial proposals to come out of traditional conservation biology is “Half-Earth”—the idea that we should protect 50% of the planet’s land and oceans for nature. Right now, only about 15% of land and 3% of oceans are protected. Supporters say this is necessary to save 85% of Earth’s species. Critics say it would displace indigenous people, ignore the real drivers of environmental destruction (like corporate resource extraction and overconsumption), and offer no plan for the other half of the planet.

Why This Matters to You

You might be thinking: okay, this is an interesting debate, but does it actually change anything? The answer is yes, in very practical ways.

The way we define biodiversity affects where conservation money goes, which species get protected, and which places become national parks or nature reserves. If biodiversity means “species richness,” we protect areas with lots of species. If it means “evolutionary uniqueness,” we protect oddball species like the tuatara (a reptile that is the last survivor of an entire order). If it means “ecosystem services,” we protect forests that clean our water and store carbon.

When you hear people argue about whether we should protect a particular forest or develop it, or whether we should save pandas or mosquitoes, or whether a new highway should be rerouted to protect a rare salamander—those arguments are ultimately about what we value and why. Conservation biology can’t escape those questions, no matter how much it wants to be “just science.”

As the philosopher David Takacs wrote back in 1996: “Biodiversity is the rallying cry currently used by biologists to draw attention to this crisis and to encapsulate the Earth’s myriad species and biological processes, as well as a host of values ascribed to the natural world.”

The word works as a rallying cry precisely because it’s fuzzy. But that fuzziness creates real problems when scientists and policymakers have to make hard choices. Maybe we need to get comfortable with the idea that conservation is not just about facts—it’s also about values, and values are something we have to argue about, not something we can measure with a machine.

Appendix

Key Terms

Term	What it does in the debate
Biodiversity	The catch-all term for what conservation biology aims to protect, but nobody agrees on exactly what it means
Species richness	Simply the number of different species in a place—often used as a stand-in for biodiversity, but it misses a lot
Intrinsic value	The idea that something is valuable just for existing, not because it’s useful to humans
Epistemic value	Values related to truth and knowledge, like accuracy and simplicity
Non-epistemic value	Values related to what we care about as humans, like fairness, health, and the welfare of other species
Inductive risk	The risk of making the wrong decision when you have incomplete evidence—and the fact that you have to choose which kind of error is worse
Normative postulate	A basic assumption that carries a value judgment, not just a factual claim

Key People

Sahotra Sarkar — A philosopher and biologist who argues that biodiversity should be defined “operationally” by whatever conservation planning algorithms optimize
Maclaurin and Sterelny — Two philosophers who argue that biodiversity is a multidimensional concept including species, morphology, ecology, and evolutionary history
Carlos Santana — A philosopher who argues the concept of biodiversity is so confused it should be eliminated from conservation science
Michael Soulé — A founder of conservation biology who argued the field rests on “normative postulates,” including that biodiversity has intrinsic value
E.O. Wilson — A famous biologist who proposed the “Half-Earth” idea—that we should protect 50% of the planet for nature
David Takacs — A historian of science who studied how conservation biologists talk about biodiversity and value

Things to Think About

If you had to choose between saving 100 species of beetles in a small forest, or one species of eagle that needs a huge territory, which would you pick? What’s your reasoning? Does it depend on whether the beetles or the eagle are more “useful” to humans?
The “new conservationists” say we should focus on what nature does for humans. Critics say this is selfish and ignores the value of other living things for their own sake. Can you think of a situation where both approaches would lead to the same decision? A situation where they’d lead to very different decisions?
The northern spotted owl example shows that scientists have to decide which kind of mistake is worse. How would you decide whether to risk killing animals unnecessarily or risk letting a species go extinct? Is there a right answer, or does it depend on your values?
If you had to rewrite the definition of “biodiversity” to make it both accurate and useful, what would you include? Is it even possible to capture everything we care about in one definition?

Where This Shows Up

National park and nature reserve planning — When governments decide which areas to protect, they’re implicitly choosing a definition of what matters
Environmental lawsuits — Court cases about endangered species or habitat destruction often hinge on arguments about what counts as “biological diversity worth protecting”
Climate change policy — Debates about whether to focus on saving individual species or preserving ecosystem functions mirror the philosophical debates in conservation biology
School science projects and nature clubs — When you choose which local species or habitat to study or protect, you’re making the same kind of value judgment that conservation biologists face