Welcome

Author: Luke Strotz

Though they say “’tis impos­si­ble to be sure of any­thing but death and taxes,” a bit of finan­cial chi­canery may get you out of pay­ing the tax­man. But no amount of trick­ery will stop the inevitabil­ity of death. Death is the inescapable end­point of life.

And this is as true for species as it is for indi­vid­u­als. Esti­mates sug­gest 99.99 per­cent of all species that have ever lived are now extinct. All species that exist today — includ­ing human beings — will invari­ably go extinct at some point.

Pale­on­tol­o­gists like me know there are key moments in Earth’s his­tory when extinc­tion rates are high. For exam­ple, researchers have iden­ti­fied the Big Five mass extinc­tions: the five times over the past half bil­lion years or so when more than three-​quarters of the planet’s species have gone extinct in short order. Unfor­tu­nately, we are also now get­ting a good first­hand view of what extinc­tion looks like, with the rapid increase in extinc­tion rates over the last century.

But what fac­tors make any one species more or less vul­ner­a­ble to extinc­tion? The rate of extinc­tion varies between dif­fer­ent groups of ani­mals and over time, so clearly not all species are equally sus­cep­ti­ble. Sci­en­tists have done a great job of doc­u­ment­ing extinc­tion, but deter­min­ing the processes that cause extinc­tion has proved a bit more difficult.

Who’s more vul­ner­a­ble to extinc­tion?
Look­ing at mod­ern exam­ples, some tip­ping points that lead to the extinc­tion of a species become obvi­ous. Reduced pop­u­la­tion sizes is one such fac­tor. As the num­ber of indi­vid­u­als of a species dwin­dles, it can lead to reduced genetic diver­sity and greater sus­cep­ti­bil­ity to ran­dom cat­a­strophic events. If the remain­ing pop­u­la­tion of a species is small enough, a sin­gle for­est fire or even ran­dom vari­a­tions in sex ratios could ulti­mately lead to extinc­tion.

Extinc­tions that have occurred in the recent past receive a great deal of atten­tion — for exam­ple, the dodo, thy­lacine or pas­sen­ger pigeon. But the vast major­ity of extinc­tions hap­pened well before the appear­ance of humans. The fos­sil record is thus the pri­mary source of data on extinction.

When pale­on­tol­o­gists con­sider fos­sils in the con­text of what we know about past envi­ron­ments, a clearer pic­ture of what causes the extinc­tion of species starts to emerge. To date, the like­li­hood of extinc­tion of a species has been linked to a host of factors.

We cer­tainly know that changes in tem­per­a­ture are one impor­tant ele­ment. Almost every major rise or fall in global tem­per­a­tures in Earth his­tory has resulted in the extinc­tion of a swath of dif­fer­ent organ­isms.

The size of the geo­graphic area a species occu­pies is also cru­cial. Species that are broadly dis­trib­uted are less likely to go extinct than those that occupy a small area or whose habi­tat is disjointed.

There are also ran­dom phe­nom­ena that cause extinc­tion. The mete­orite respon­si­ble for the extinc­tion of about 75 per­cent of life at the end of the Cre­ta­ceous Period, includ­ing the non-​avian dinosaurs, is per­haps the best exam­ple of this. This ran­dom aspect to extinc­tion is why some have argued that “sur­vival of the luck­i­est” may be a bet­ter metaphor for the his­tory of life than “sur­vival of the fittest.”

Study­ing extinct mol­lusks’ fos­sils sug­gested phys­i­o­log­i­cal rea­sons one species might be more likely to dis­ap­pear.
Hen­dricks, J. R., Sti­gall, A. L., and Lieber­man, B. S. 2015. The Dig­i­tal Atlas of Ancient Life: deliv­er­ing infor­ma­tion on pale­on­tol­ogy and bio­geog­ra­phy via the web. Palaeon­tolo­gia Elec­tron­ica, Arti­cle 18.2.3E, CC BY-​NC-​SA

Most recently, my col­leagues and I iden­ti­fied a phys­i­o­log­i­cal com­po­nent to extinc­tion. We found that the rep­re­sen­ta­tive meta­bolic rate for both fos­sil and liv­ing mol­lusk species strongly pre­dicts the like­li­hood of extinc­tion. Meta­bolic rate is defined as the aver­age rate of energy uptake and allo­ca­tion by indi­vid­u­als of that species. Mol­lusk species with higher meta­bolic rates are more likely to go extinct than those with lower rates.

Return­ing to the metaphor of “sur­vival of the fittest/​luckiest,” this result sug­gests that “sur­vival of the lazi­est” may apply at times. Higher meta­bolic rates cor­re­late with higher mor­tal­ity rates for indi­vid­u­als in both mam­mals and fruit flies, so metab­o­lism may rep­re­sent an impor­tant con­trol on mor­tal­ity at mul­ti­ple bio­log­i­cal lev­els. Because meta­bolic rate is linked to a con­stel­la­tion of char­ac­ter­is­tics includ­ing growth rate, time to matu­rity, max­i­mum life span and max­i­mum pop­u­la­tion size, it seems likely that the nature of any or all of these traits play a role in how vul­ner­a­ble a species is to extinction.

Plenty more extinc­tion unknowns
As much as sci­en­tists know about extinc­tion dri­vers, there’s still a lot we don’t know.

For instance, some pro­por­tion of species go extinct regard­less of any major envi­ron­men­tal or bio­log­i­cal upheaval. This is called the back­ground extinc­tion rate. Because pale­on­tol­o­gists tend to focus on mass extinc­tions, back­ground extinc­tion rates are poorly defined. How much, or how lit­tle, this rate fluc­tu­ates isn’t well-​understood. And, in total, most extinc­tions prob­a­bly fall into this category.

Another prob­lem is deter­min­ing how impor­tant chang­ing bio­log­i­cal inter­ac­tions are in explain­ing extinc­tion. For instance, extinc­tion of a species may occur when the abun­dance of a preda­tor or a com­peti­tor increases, or when a cru­cial prey species goes extinct. The fos­sil record, how­ever, rarely cap­tures this kind of information.

Even the num­ber of species that have gone extinct can be an enigma. We know very lit­tle about the cur­rent or past bio­di­ver­sity of microor­gan­isms, such as bac­te­ria or archaea, let alone any­thing about pat­terns of extinc­tion for these groups.

Many ani­mals — includ­ing the Scimitar-​horned Oryx — are cur­rently extinct in the wild.
Image of Oryx dammah in San Diego Zoo. Pho­tog­ra­phy & copy­right MoosMood.

Per­haps the biggest mis­take we could make when it comes to assess­ing and explain­ing extinc­tion would be to take a one-​size-​fits-​all approach. The vul­ner­a­bil­ity of any one species to extinc­tion varies over time, and dif­fer­ent bio­log­i­cal groups respond dif­fer­ently to envi­ron­men­tal change. While major changes in global cli­mate have led to extinc­tion in some bio­log­i­cal groups, the same events have ulti­mately led to the appear­ance of many new species in others.

So how vul­ner­a­ble any one species is to extinc­tion due to human activ­i­ties or the asso­ci­ated cli­mate change remains some­times an open ques­tion. It is clear that the cur­rent rate of extinc­tion is ris­ing well above any­thing that could be called back­ground level, and is on track to be the Sixth Mass Extinc­tion. The ques­tion of how vul­ner­a­ble any one species — includ­ing our own — may be to extinc­tion is there­fore one sci­en­tists want to answer quickly, if we’re to have any chance of con­serv­ing future biodiversity.

This arti­cle was orig­i­nally pub­lished on The Con­ver­sa­tion. Read the orig­i­nal arti­cle.