Extinction is part of evolution, but the unnatural rapidity of current species losses forces us to address whether we are cutting off twigs or whole branches from the tree of life.
— Matt Davis, scholar of evolutionary history
There is more than one way to look at the current period of mass extinction — count the species blinking out, chart them by class or map their geographic distribution, track the pace of the demise and so on. A particularly intriguing one, new to me and maybe to you, is at the center of a fresh analysis quoted above, which examines the variable impact of ongoing mammal losses on Earth’s evolutionary history, and concludes that not all critters contribute equally.
Think for a moment of evolution not as a set of independent threads, each ending in an animal we know, but as a branching process. Maybe try seeing it as that splendid maple with the vermilion leaves that caught your eye the other day.
Maples lose their leaves each fall; that’s life. Species go extinct from natural causes and are replaced by new types; that’s evolution. But if someone prunes an entire small branch from the tree, perhaps to make the lawn-mowing easier, or if a windstorm tears a large limb off at the trunk — that’s a loss of wholly different magnitude.
This new analysis speaks of the “tree of life” on this planet, and its twigs don’t all end in the same kind of leaf. Some produce bats, still others give us beavers or boars or bulldogs — not to mention the bluebuck and a few types of bandicoot that are no longer with us. (Also bobolinks, broccoli and the Monarch butterfly, but this paper limits its consideration to mammals.)
Examining extinction in this branching way — looking beyond the disappearance of the black rhino here or the passenger pigeon there — brings into focus the loss of entire lineages of life, and it will not surprise you to hear that the picture is not pretty.
On the other hand, it raises a provocative notion that maybe not all species are equally worth saving, and the potentially useful suggestion that this perspective ought perhaps to shape conservation strategies:
Rather than try to save every extant species, or to set aside some large percentage of the earth’s habitat still hospitable to whatever wildlife happens to live there, the researchers think we maybe ought to emphasize the species that carry a long line of evolutionary history and, thus, contain an ability to extend it. Which may challenge our patterns of affection for some species over others.
For examples, they contrast the critically endangered pygmy sloth, about the size a cat and kind of cute as sloths go, with the aardvark, a personal favorite of mine since junior high school.
Save sloth, or aardvark?
The critically endangered pygmy sloth has walked the earth, sluggishly, for less than 10,000 years or so, and is just one species on the bountiful limb of sloth life. As for aardvarks, many kinds have come and gone over the last 75 million years; just one remains, but its odds of beating near-term extinction are looking pretty good.
If the pygmy sloth blinks out, the authors calculate, its contributions to “phylogenic diversity” or PD — essentially, evolutionary history — would be replaced in about 20 months. But if the aardvark disappears, so does the entire lineage of its order Orycteropus, along with 75,000 millennia of contribution to biodiversity.
For this analysis, published last week in the Proceedings of the National Academy of Sciences, the authors looked back 130,000 years to an interglacial period they feel fairly marks the beginning of the current mass extinction — the sixth in Earth’s history, and the first in which human activity is the driver.
Then they drew up a “birth-death tree framework” to measure losses of PD up to now, and forecast how long their recovery might take. They figure that humans erased about 2 billion years of PD up through the year 1500; since then, the pace has picked up and another 500 million years of diversity has been wiped.
This represents more than 300 mammal species, unevenly distributed throughout the tree branches. For reasons that probably began with hunting efficiency, and center today on habitat loss, the biggest animals have been hardest hit. Their emptied ecological niches — as predators, prey, seed distributors, manure factories, landscape remodelers and more — are also oversized, as are the blank spots they leave in evolutionary history:
Several unique mammal lineages (notably the endemic South American orders Litopterna and Notoungulata [of hoofed and usually heavy mammals]) were completely lost during the likely human-linked extinctions of the terminal Pleistocene and early Holocene. These extinctions also decimated the sloth and anteater, armadillo, odd-toed ungulate, and elephant lineages, all disproportionately rich in PD….
[P]rehistoric and historic extinctions were highly size-biased, devastating large mammals, a group shouldering a disproportionate share of PD. Evolutionary history has its own intrinsic value, but these lost years also represent a loss of instrumental value in the extinction of unique functional traits. Human-linked extinctions have already left the world in an atypical state: depauperate of large animals and the important ecosystem functions and services they provide.
No ‘Lilliput effect’ yet
Despite some hopeful speculation about a “Lilliput effect” in which “small, surviving species rapidly evolve into vacant niches,” the paper says there is no evidence to date that this can happen with mammals, especially since they evolve more slowly than other forms of animal life.
Climate change and its impacts are not accorded much of a role as extinction drivers to date, but those contributions are changing rapidly.
As of today, the paper says, it would take nearly 500,000 years for the planet’s 5,400 surviving mammal types to restore phylogenic diversity as it existed before people began species exterminations, even if the calculations assume neither losses of more mammal species nor gains of new ones. There will be both, of course, with the losses more readily predicted than the gains; the International Union for the Conservation of Nature predicts that two-thirds of species now listed as endangered — and virtually all listed as critically endangered — will be lost in the coming century.
The researchers calculate that if current patterns persist for just 50 years, it would take the Earth’s mammals 5 million to 7 million years to recover the level of PD that existed before human influence. So, they say, it might be best to take the next steps with highly targeted efforts in mind:
The only real option to speed PD recovery is to save unique evolutionary history before it is already lost. In addition to increasing overall conservation efforts, we should use available PD methods to prioritize action for evolutionarily distinct species. … If we could momentarily stop extinctions for mammals, we would save as much evolutionary history in the next 100 years as what our ancestors lost in the last 100,000.
But is that even possible? Matt Davis, a paleontologist at Denmark’s Aarhus University who is chief author of the paper, told National Geographic’s Christie Wilcox:
With the extinction of so many megafauna, we’ve lost both a whole chunk of functional space and some of the longest branches on the evolutionary tree. This kind of pattern isn’t common in the extinctions we know of from the fossil record, so we are entering uncharted territory.
The full paper, “Mammal diversity will take millions of years to recover from the current biodiversity crisis,” can be read here, but access is not free.