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Nutrition, disease, biodiversity: do we need a new relationship with animals?

Defaunation: 25% of the world’s birds disappeared in 30 years

Jean Zeid, Journalist
On December 15th, 2021 |
3 mins reading time
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Defaunation: 25% of the world’s birds disappeared in 30 years
Denis Couvet
Denis Couvet
President of the Foundation for Research on Biodiversity and Professor at Muséum National d'Histoire Naturelle
Key takeaways
  • Ecosystems are extensively transformed by human activities. These changes usually have negative impacts on different wildlife species, starting with their decline. This major issue is called “defaunation”.
  • Current extinction rates are estimated to be 100 to 1,000 times faster than during so-called “normal” geological periods. They have become so intense that a species can disappear in just a few decades. European birds have thus lost 25% of their population in only 30 years, which represents a loss of 500 million birds.
  • Domestic mammals, ~20 species, account for over 90% of the total biomass of mammals, while the remaining 10% are represented by ~5,000 species of wild mammals. An observation assuredly correlated with human activities.
  • The collapse of wildlife should prompt us to question and rethink our relationship with non-humans, and their ecological, social, and cultural worth.

Dis­creet, some­times almost invis­i­ble, wild ani­mals vary accord­ing to coun­tries and lat­i­tudes. In France, this fau­na is com­posed of well-known species such a deer, field mice, bears and wolves — which came back recent­ly — but also salmon and seals. It also includes less­er-known species like acarids or nema­tode worms. The lat­ter make up most of the wildlife both in terms of pop­u­la­tion and species diversity.

Today, ecosys­tems are exten­sive­ly trans­formed by human activ­i­ties. And although some anthro­pophilic species like pigeons thrive in human envi­ron­ments, human impacts on wildlife are most often neg­a­tive, even dis­as­trous. One of the major reper­cus­sions is undoubt­ed­ly the alarm­ing decline in ani­mal pop­u­la­tions, a cat­a­stroph­ic col­lapse referred to as “defau­na­tion”. 

The signs of defaunation

To begin with, defau­na­tion is the decline in species abun­dance. Today, dwin­dling pop­u­la­tions are observed in all ter­res­tri­al ver­te­brates and in insects, and can reach 3% per year. Euro­pean bird pop­u­la­tions have dropped by 25% in only 30 years, rep­re­sent­ing a loss of 500 mil­lion birds. It is worth men­tion­ing the emblem­at­ic decline of a “com­mon” species, the house spar­row, which suf­fers from scarci­ty of prey, espe­cial­ly in urban areas.

Some­times, the decline in pop­u­la­tions is so intense that a species can dis­ap­pear in just a few decades. Cur­rent extinc­tion rates are esti­mat­ed to be a thou­sand times faster than dur­ing so-called “nor­mal” geo­log­i­cal peri­ods. Many caus­es of this decline have been doc­u­ment­ed. It is first­ly due to the degra­da­tion and frag­men­ta­tion of nat­ur­al habi­tats caused by urban growth, the expan­sion of agri­cul­tur­al land­scapes and live­stock farm­ing, the inten­si­fi­ca­tion of agri­cul­tur­al prac­tices, as well as the con­struc­tion of road net­works. How­ev­er, the rea­sons for this decline are also linked with the over­ex­ploita­tion of resources, cli­mate change and var­i­ous types of pol­lu­tion. Final­ly, the extinc­tion of a species can trig­ger the sec­ondary extinc­tion of oth­er ani­mal pop­u­la­tions. These are known as “extinc­tion cascades”. 

Human activ­i­ties also lead to very rapid changes in com­mu­ni­ty com­po­si­tion and bio­di­ver­si­ty loss. Local fau­na can become increas­ing­ly sim­i­lar over time when gen­er­al­ist species replace habi­tat spe­cial­ists, a process called “biot­ic homogeni­sa­tion”. For exam­ple, sky­larks, spe­cial­ist birds found in agri­cul­tur­al habi­tats, have been replaced by black­birds in fields. Anoth­er demon­stra­tion of biot­ic homogeni­sa­tion is “bio­log­i­cal inva­sion” which occurs when native species are replaced by exot­ic, inva­sive species. 

Cli­mate change is also to blame, too. Many ani­mals change their life habits, par­tic­u­lar­ly their breed­ing sea­son, to adapt to vari­a­tions in cli­mate. Final­ly, in the last 10,000 years, wildlife has giv­en way to domes­tic species. A mas­sive reor­gan­i­sa­tion has been observed in mam­mals. Today, domes­tic mam­mals, which only include about 20 species, account for over 90% of the total mam­malian bio­mass in the world. On the oth­er hand, wild mam­mals, which are rep­re­sent­ed by more than 5,000 species, make up less than 10% of this biomass.

The consequences of defaunation

These phe­nom­e­na result in a homogeni­sa­tion of wildlife, a loss of genet­ic diver­si­ty and func­tion­al orig­i­nal­i­ty, lead­ing to major eco­log­i­cal con­se­quences. The eco­log­i­cal and evo­lu­tion­ary poten­tial of this fau­na is becom­ing more lim­it­ed, since diver­si­ty makes it pos­si­ble for wildlife to adapt to dif­fer­ent, or new, envi­ron­ments. For instance, the diver­si­ty of wing colour pat­terns (black or white) allowed the pep­pered moth (Bis­ton betu­lar­ia) to adapt to black­ened birch tree trunks dur­ing the indus­tri­al revolution.

Accu­rate­ly pre­dict­ing the effects of these changes is still chal­leng­ing. How­ev­er, the col­lapse of wildlife should prompt us to ques­tion and to rethink our rela­tion­ship with non-humans, and their eco­log­i­cal, social, and cul­tur­al worth.

It would be par­tic­u­lar­ly inter­est­ing to allow time for nat­ur­al process­es and to sup­port wildlife by leav­ing them suit­able habi­tats. These rewil­d­ing mea­sures can take sev­er­al forms, depend­ing on the social, geo­graph­i­cal, and eco­log­i­cal con­text. The removal of dams to restore rivers allows some migra­to­ry fish species to return, like stur­geon or eel. It also ben­e­fits oth­er species liv­ing in these ecosys­tems like otters or the white-throat­ed dip­per. Some­times, it is bet­ter to do noth­ing and let nature run its course, even if the out­come can some­times take us by sur­prise. Anoth­er rewil­d­ing project for her­bi­vores is cur­rent­ly under­way in Europe. In this case, the emblem­at­ic species are bison, which reg­u­late ecosys­tems, their diver­si­ty, fau­na, and flo­ra. In the marine envi­ron­ment, rewil­d­ing par­tic­u­lar­ly involves whales because of their major role in the nutri­ent cycle.

With­in agroe­cosys­tems, on which our food depends, the field of agroe­col­o­gy and its tech­niques and process­es offer a promis­ing prospect for improv­ing our rela­tion­ship with nature, and in par­tic­u­lar wild ani­mals. By favour­ing crop diver­si­ty, by adding hedges and groves, and by leav­ing room for wild plants, agroe­col­o­gy seeks to replace hos­tile rela­tions with nature by a mutu­al­is­tic rela­tion­ship. It strives to include bio­di­ver­si­ty and all its prop­er­ties in agro­nom­ic ini­tia­tives. In urban ter­ri­to­ries, a more abun­dant and more diverse wildlife can also enrich the lives of city dwellers, both in terms of qual­i­ty of life and social rela­tion­ships. Which implies that wild flo­ra must be present as well.

Our vision of ani­mals must evolve by tak­ing into con­sid­er­a­tion the cul­tur­al, social, and eco­log­i­cal chal­lenges asso­ci­at­ed with their pres­ence. The vital impor­tance of wildlife is most often over­looked in socio-ecosystems.