AboutZoos, Since 2008


Inva­sive preda­tors are eat­ing the world’s ani­mals to extinc­tion – and the worst is close to home

pub­lished 24 Sep­tem­ber 2016 | mod­i­fied 24 Sep­tem­ber 2016

by Tim Doherty, Chris Dick­man, Dale Nimmo and Euan Ritchie

Inva­sive species are a threat to wildlife across the globe – and inva­sive, preda­tory mam­mals are par­tic­u­larly damaging.

Our research, recently pub­lished in Pro­ceed­ings of the National Acad­emy of Sci­ences, shows that these preda­tors – cats, rats and foxes, but also house mice, pos­sums and many oth­ers – have con­tributed to around 60% of bird, mam­mal and rep­tile extinc­tions. The worst offend­ers are feral cats, con­tribut­ing to over 60 extinctions.

So how can we stop these mam­mals eat­ing away at our threat­ened wildlife?

Count­ing the cost
Our study revealed that inva­sive preda­tors are impli­cated in 87 bird, 45 mam­mal and 10 rep­tile extinc­tions – 58% of these groups’ con­tem­po­rary extinc­tions worldwide.

Inva­sive preda­tors also threaten 596 species classed as vul­ner­a­ble, endan­gered or crit­i­cally endan­gered on the Inter­na­tional Union for the Con­ser­va­tion of Nature Red List. Com­bined, the affected species include 400 birds, 189 mam­mals and 149 reptiles.

Twenty-​three of the crit­i­cally endan­gered species are classed as “pos­si­bly extinct”, so the num­ber of extinc­tions above is likely to be an underestimate.

Until now, these shock­ing sta­tis­tics have been unknown, and the heavy toll of inva­sive preda­tors on native bio­di­ver­sity grossly under­ap­pre­ci­ated. Species extinc­tions attrib­uted to inva­sive preda­tors include the Hawai­ian rail (Zapor­nia sand­wichen­sis) and Australia’s lesser bilby (Macro­tis leu­cura).

Australia’s lesser bilby (Macrotis leucura)Australia’s lesser bilby (Macro­tis leu­cura), now extinct.

Who are the worst offend­ers?
We found that three canids (includ­ing the red fox and feral dogs), seven mem­bers of the weasel fam­ily or mustelids (such as stoats), five rodents, two pri­mates, two mon­gooses, two mar­su­pi­als and nine species from other fam­i­lies neg­a­tively impact threat­ened species. Some of these species, such as hedge­hogs and brush­tail pos­sums, don’t imme­di­ately spring to mind as preda­tors, yet they are known to prey on many threat­ened species.

Feral cats threaten the most species over­all (430), includ­ing 63 that have become extinct. This equates to one-​quarter of all bird, mam­mal and rep­tile extinc­tions – mak­ing the feral cat arguably the most dam­ag­ing inva­sive species for ani­mal bio­di­ver­sity worldwide.

Five species of intro­duced rodent col­lec­tively threaten 420 species, includ­ing 75 extinc­tions. While we didn’t sep­a­rate out the impacts of indi­vid­ual rodent species, pre­vi­ous work shows that black rats (Rat­tus rat­tus) threaten the great­est num­ber of species, fol­lowed by brown rats (R. norvegi­cus) and Pacific rats (R. exu­lans).

The hum­ble house mouse (Mus mus­cu­lus) is another inter­est­ing case. Despite their small size, house mice have been recorded eat­ing live chicks of alba­trosses, petrels and shearwaters.

Other preda­tors that threaten large num­bers of species are the domes­tic dog (Canis famil­iaris), pig (Sus scrofa), small Indian mon­goose (Her­pestes aurop­unc­ta­tus), red fox (Vulpes vulpes) and stoat (Mustela erminea).

Invasive mammalian predatorsInva­sive mam­malian preda­tors (clock­wise from top left): feral dog, house mouse, stoat, feral pig, feral cat, brush­tail pos­sum, black rat, small Indian mon­goose and red fox (cen­tre).
Clock­wise from top-​left: Andrey flickr CC BY 2.0 https://​flic​.kr/​p​/​4​M​2​E​7​y; Richard Adams flickr CC BY 2.0 https://​flic​.kr/​p​/​7​U​19​v​9; Mark Kil­ner flickr CC BY-​NC-​SA 2.0 https://​flic​.kr/​p​/​4​D​6​L​P​e; CSIRO CC BY 3.0 http://​www​.sci​en​ceim​age​.csiro​.au/​i​m​a​g​e​/​1515; T. Doherty; Toby Hud­son CC BY-​SA 3.0 https://​com​mons​.wiki​me​dia​.org/​w​i​k​i​/​F​i​l​e​:​B​r​u​s​h​t​a​i​l​P​o​s​s​u​m​.​j​p​g; CSIRO CC BY 3.0 http://​www​.sci​en​ceim​age​.csiro​.au/​i​m​a​g​e​/​10564; J.M.Garg CC BY-​SA 3.0 https://​com​mons​.wiki​me​dia​.org/​w​i​k​i​/​F​i​l​e​:​H​e​r​p​e​s​t​e​s​_​e​d​w​a​r​d​s​i​i​_​a​t​_​H​y​d​e​r​a​b​a​.​j​p​g; Harley Kingston CC BY 2.0 https://​flic​.kr/​p​/​c​e​W​F​r​7 (centre)

Island species most at risk
Species found only on islands (insu­lar endemics) account for 81% of the threat­ened species at risk from predators.

The iso­la­tion of many islands and a lack of nat­ural preda­tors mean that insu­lar species are often naïve about new preda­tors and lack appro­pri­ate defen­sive responses. This makes them highly vul­ner­a­ble to being eaten and in turn suf­fer­ing rapid pop­u­la­tion decline or, worse, extinc­tion. The high extinc­tion rates of ground-​dwelling birds in Hawaii and New Zealand – both of which lack native mam­malian preda­tors – are well-​known examples.

Accord­ingly, the regions where the preda­tors threat­ened the great­est num­ber of species were all dom­i­nated by islands – Cen­tral Amer­ica and the Caribbean, islands of the Pacific, the Mada­gas­car region, New Zealand and Hawaii.

Con­versely, the con­ti­nen­tal regions of North and South Amer­ica, Europe, Africa and Asia con­tain com­par­a­tively few species threat­ened by inva­sive preda­tors. While Aus­tralia is a con­ti­nent, it is also an island, where large num­bers of native birds and mam­mals are threat­ened by cats and foxes.

Red foxAlong with feral cats, red foxes have dev­as­tated native mam­mals in Aus­tralia.
Image credit: Tom Rayner

Man­ag­ing men­ac­ing mam­mals
Under­stand­ing and mit­i­gat­ing the impact of inva­sive mam­mal preda­tors is essen­tial for reduc­ing the rate of global bio­di­ver­sity loss.

Because most of the threat­ened species stud­ied here live on islands, man­ag­ing inva­sive preda­tors on islands should be a global con­ser­va­tion pri­or­ity. Inva­sive preda­tors occur on hun­dreds of islands and preda­tor con­trol and erad­i­ca­tion are costly exer­cises. Thus, it is impor­tant to pri­ori­tise island erad­i­ca­tions based on fea­si­bil­ity, cost, like­li­hood of suc­cess and poten­tial benefits.

On con­ti­nents or large islands where erad­i­ca­tions are dif­fi­cult, other approaches are needed. This includes predator-​proof fenc­ing, top-​predator restora­tion and con­ser­va­tion, lethal con­trol, and main­te­nance of habi­tat structure.

Despite the shock­ing sta­tis­tics we have revealed, there remain many unknowns. For exam­ple, only around 40% of rep­tile species have been assessed for the Red List, com­pared to 99% for birds and mam­mals. Very lit­tle is known about the impact of inva­sive preda­tors on inver­te­brate species.

We expect that the num­ber of species affected by inva­sive preda­tors will climb as more knowl­edge becomes available.

(Source: The Con­ver­sa­tion, 19.09.2016)

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