To Save Species from Extinction, We Must Consider More than Just Numbers

H. Resit Akcakaya

Bison migrate out of the Gardiner Basin in Yellowstone National Park. Tens of millions of bison once roamed across much of North America, but today bison are extinct in all by 1% of their historic range.

Around the world, ani­mals and plants are dis­ap­pear­ing at alarm­ing rates. In May 2019, a major U.N. report warned that around one mil­lion species were at risk of extinc­tion — more than at any oth­er time in human history.

Con­ser­va­tion sci­en­tists like me focus on pre­dict­ing and pre­vent­ing extinc­tions. But we see that as an essen­tial first step, not a final goal. Ulti­mate­ly, we want species to recover.

The chal­lenge is that while extinc­tion is easy to define, recov­ery is not. Until recent­ly, there was no gen­er­al def­i­n­i­tion of a recov­ered” species. As a result, some species recov­ery plans are much less ambi­tious than oth­ers, and sci­en­tists don’t have a com­mon yard­stick for rec­og­niz­ing con­ser­va­tion successes.

To address this chal­lenge, the Inter­na­tion­al Union for the Con­ser­va­tion of Natures Species Sur­vival Com­mis­sion — the world’s largest net­work of con­ser­va­tion­ists — is devel­op­ing a Green List of Species to high­light species recov­ery. This tool will com­ple­ment the well-known Red List, which high­lights endan­gered species.

While the Red List focus­es on extinc­tion risk, the Green List will mea­sure recov­ery and con­ser­va­tion suc­cess. As a mem­ber of the team charged with mak­ing the Green List a prac­ti­cal con­ser­va­tion tool, I see it as a way of mea­sur­ing the impact of con­ser­va­tion and com­mu­ni­cat­ing con­ser­va­tion suc­cess sto­ries, as well as learn­ing from failures.

Defin­ing recovery

To know how much con­ser­va­tion has accom­plished, and to encour­age ambi­tious con­ser­va­tion goals, we need an objec­tive way to mea­sure progress toward a species’ recov­ery. Stud­ies of recov­ery plans devel­oped under the U.S. Endan­gered Species Act show that some plans con­sid­er a species recov­ered even if its pop­u­la­tion remains the same or shrinks dur­ing the recov­ery effort. A stan­dard def­i­n­i­tion of recov­ery would pre­vent such incon­sis­ten­cies and encour­age wildlife man­agers to aim higher.

Con­ser­va­tion sci­en­tists have long attempt­ed to iden­ti­fy dif­fer­ent facets of species recov­ery. Review­ing these efforts, our team came up with sev­er­al require­ments for con­sid­er­ing a species ful­ly recovered.

As I explain with an inter­na­tion­al group of col­leagues in a new study, one key idea is that pop­u­la­tions of the species should be func­tion­al.” By this we mean that they are able to per­form all the roles that the species is known to play in ecosys­tems where it exists. This may seem like an obvi­ous mea­sure­ment, but in fact, some species that are con­sid­ered to be recov­ered” in the U.S. fail this test.

The Kirtland’s War­bler was declared recov­ered in the U.S. in 2019, but will still rely on land man­agers to main­tain stands of jack pine where it nests and con­trol par­a­sitic cow­birds that prey on it. (Pho­to byJoel Trick/U.S. Fish and Wildlife Service)

What’s your function?

Each species has many kinds of eco­log­i­cal func­tions. For exam­ple, bees help plants repro­duce by pol­li­nat­ing them. When birds and bats eat fruits and lat­er excrete the seeds, they help forests regenerate.

Sim­i­lar­ly, when salmon swim upstream to spawn and then are con­sumed by bears and oth­er preda­tors, that process moves essen­tial nutri­ents from the oceans up into rivers and forests. And when flam­ma­ble grass­es burn in the U.S. South­east, they fuel fires that main­tain lon­gleaf pine forests.

All these crit­i­cal func­tions are pos­si­ble only when enough mem­bers of the key species are present. Put anoth­er way, keep­ing a species alive is not enough – it also is essen­tial to keep its func­tions from going extinct.

Rein­tro­duc­ing wolves to Yel­low­stone Nation­al Park has spot­light­ed the wolves’ eco­log­i­cal func­tions as reg­u­la­tors of graz­ing species. How­ev­er, wolves once influ­enced much larg­er areas than they cur­rent­ly occupy.

Func­tion­al extinction

Sci­en­tists have known for decades that species may per­sist at such low num­bers that they do not ful­fill the eco­log­i­cal roles they used to per­form. This can be true even if sig­nif­i­cant num­bers of ani­mals or plants are present.

One exam­ple is the Amer­i­can Bison, which is a great con­ser­va­tion suc­cess sto­ry in terms of pre­vent­ing its extinc­tion. Hunt­ing reduced bison to just a few hun­dred indi­vid­u­als in west­ern states at the end of the 19th cen­tu­ry, but con­ser­va­tion ini­tia­tives have restored them to pub­lic, pri­vate and Native Amer­i­can lands across the West.

Today bison do not appear to be at risk of extinc­tion. How­ev­er, they occu­py less than 1% of their his­tor­i­cal range, and most of the rough­ly 500,000 ani­mals that exist today are raised for com­mer­cial pur­pos­es. Few­er than 20,000 bison live in con­ser­va­tion herds – a small frac­tion of their pre-Columbian pop­u­la­tion, which totaled mil­lions or tens of mil­lions.

The chart shows the cur­rent IUCN clas­si­fi­ca­tion for Amer­i­can bison. (Graph­ic cour­tesy of the Inter­na­tion­al Union for the Con­ser­va­tion of Nature)

Before they were reduced to near-extinc­tion, bison shaped prairie habi­tats and land­scapes through wal­low­ing, pound­ing and graz­ing. They influ­enced ecosys­tems by con­vert­ing veg­e­ta­tion into pro­tein bio­mass for preda­tors, includ­ing peo­ple, and by redis­trib­ut­ing nutri­ents in these ecosystems.

Even though bison are not at risk of extinc­tion, for the pur­pos­es of their con­tri­bu­tions to the ecosys­tems and land­scapes they once inhab­it­ed, I believe the species should be con­sid­ered to be func­tion­al­ly extinct and not a ful­ly recov­ered species.

This does not mean its con­ser­va­tion is a fail­ure. To the con­trary, accord­ing to new con­ser­va­tion met­rics that I and oth­er sci­en­tists have pro­posed for the Green List, the bison would receive high scores on sev­er­al counts, includ­ing Con­ser­va­tion Lega­cy – mean­ing it has ben­e­fit­ed sig­nif­i­cant­ly from past pro­tec­tive efforts – and Con­ser­va­tion Gain, or poten­tial to respond pos­i­tive­ly to fur­ther initiatives.

Ospreys are effi­cient hunters that help to reg­u­late fish pop­u­la­tions. (Pho­to by Tra­cie Hall, CC BY-SA)

A full recovery

For con­trast, con­sid­er anoth­er species wide­ly viewed as a con­ser­va­tion suc­cess sto­ry: The osprey. Pop­u­la­tions of this fish-eat­ing bird of prey crashed across North Amer­i­ca in the 1950s to 1970s, pri­mar­i­ly due to poi­son­ing from the insec­ti­cide DDT and its derivatives.

Con­ser­va­tion efforts since then, includ­ing a fed­er­al ban on DDT and pro­vi­sion of nest­ing struc­tures, have result­ed in a dra­mat­ic recov­ery, back to pop­u­la­tion lev­els before the declines. Actu­al­ly, many U.S. and Cana­di­an pop­u­la­tions of Osprey now exceed his­tor­i­cal num­bers. Under the Green List cri­te­ria we are propos­ing, this species would now be con­sid­ered eco­log­i­cal­ly func­tion­al in most if not all parts of its range.

Ambi­tious goals

Con­ser­va­tion sci­en­tists have long con­sid­ered a species’ influ­ence on oth­ers and on the ecosys­tems it inhab­its to be a fun­da­men­tal aspect of its essence and its intrin­sic val­ue. The Green List of Species ini­tia­tive seeks to go beyond sim­ply pre­vent­ing extinc­tions to defin­ing recov­ered species as those that are eco­log­i­cal­ly func­tion­al across their nat­ur­al ranges. This new focus aims to encour­age con­ser­va­tion opti­mism by high­light­ing suc­cess sto­ries and show­ing that with help, species once at risk can reclaim their places in the web of life.

Edi­tor’s Note: This arti­cle is repub­lished from The Con­ver­sa­tion under a Cre­ative Com­mons license. Read the orig­i­nal arti­cle.

H. Resit Akcakaya is Pro­fes­sor of Ecol­o­gy and Evo­lu­tion at Stony Brook Uni­ver­si­ty of The State Uni­ver­si­ty of New York.
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