“Invasive” grey squirrels, feral pigs, & rats: leave ’em alone, say scientists

Studies suggest grey squirrel, feral pig,  & rat control are barking up the wrong tree

BELFAST;  GUAM;  VANCOUVER––“Invasive” grey squirrels are allegedly extirpating red squirrels in the British Isles by transmitting squirrel pox.

Feral pigs allegedly run amok to the detriment of rare native plants on Guam,  a remote Pacific island.

Rats present a perennial purported risk of spreading disease to humans.

Yet all three situations are ecological problems that essentially solve themselves,  recent research suggests––if humans leave the animals alone.

Red & grey squirrels battle over British Isles

Summarized Queen’s University Belfast Ph.D. researcher Joshua P. Twining in the February 26,  2020 edition of The Conversation,  of studies he did recently in partnership with National Museums Northern Ireland research coordinator David Tosh,  “The North American grey squirrel was brought to Britain and Ireland during the 19th and 20th centuries,”  beginning in 1876.

“Since their introduction,”  Twining wrote,  “grey squirrels have replaced the native red squirrel across much of its former range,  mainly by transmitting squirrel pox to reds,  a deadly virus to which greys are immune.”

Ecological nativists in general and red squirrel enthusiasts in particular have been howling for the extermination of grey squirrels at least since 1930,  as ostensibly the only way to save red squirrels.

Red squirrels are in trouble––but are grey squirrels really why?

Red squirrels,  meanwhile,  have indeed been in trouble,  down to as few as 25,000 according to a 2007 survey,  from an estimated high of 3.5 million.

Seven years later,  in 2014,  after intensive efforts to kill grey squirrels and reintroduce red squirrels,  the red squirrel population was reportedly back up to 120,000-140,000,  about three-quarters of them in Scotland,  while the grey squirrel population appeared to be steady at approximately five million.

But red squirrels were in 2016 discovered to be harboring a form of leprosy,  a disease last known to afflict a human in the United Kingdom in 1798,  but possibly also helping to account for the red squirrel population crash.  The finding called into question whether red squirrels even could be recovered to their former abundance,  and if so,  what the consequences might be.

Pine martens crashed too

Pine martens,  meanwhile,  an obscure arboreal member of the weasel family that eats both red and grey squirrels,  were believed to be almost extinct in the British Isles as recently as 2002,  with about 6,400 in Scotland,  1,600 in England,  and an estimated 760 in Wales,  although none had been seen in Wales since one roadkilled carcass was recovered in 1971.

Pine martens are also known to remain along the west coast of Ireland.

Efforts to boost the pine marten population with selective reintroductions began in Scotland in 2009,  and accelerated after the discovery of a living pine marten in 2012.

Observed Twining and Tosh,  “In parts of Ireland and Scotland where this native predator has recovered,  there have been subsequent declines in grey squirrel populations,  allowing reds to recover.  Could the grey squirrel,  an animal not native to Britain or Ireland, be naive to the risk posed by the pine marten?

Red squirrels “showed a clear fear response”

“Armed with camera traps,  squirrel feeders and a solution of pine marten poo,  we set out to find out if this was the case.  We repeated our experiment at 20 different locations across Northern Ireland,  amassing more than 8,000 minutes of squirrel footage,”  Twining recounted.

“Emerging red-eyed from our lab, we realized that red squirrels showed a clear fear response to pine marten scent,  while greys didn’t.  Reds visited feeders less,  fed for shorter periods of time,  and were more vigilant––standing on their hind legs with their heads upright and tails twitching from side to side.  Meanwhile,  the greys continued as if nothing had changed.  In some cases,  grey squirrel visits to feeding stations actually increased while their vigilance decreased around pine marten scent.  Failing to recognize the scent of a predator as a threat leaves the grey squirrels vulnerable.

“The results are published in Royal Society Open Science,”  Twining concluded.

Ecological logic of predation

What the Twining/Tosh study confirms is exactly what ecological logic would suggest:  that introduced species,  as a general rule,  will be more vulnerable to predation in a new habitat than native species.  But predation will only control the introduced species if the native predators are allowed to do the job.

Intensively hunted during most of the 20th century for fur and sport,  pine martens had no chance to control grey squirrels,  until wearing fur passed from vogue and the Hunting Act 2004,  often flouted though it is,  put pack hunting under at least some restraints.

Brown tree snakes & feral pigs

The peer-reviewed journal Royal Society Open Science also published a study of feral pigs on Guam by Iowa State University researcher Ann Marie Gawel and colleagues,  noted by Anthropocene science writer Brandon Keim in May 2018 for findings significantly contradicting conventional belief about the ecological effects of feral pigs in general and on remote islands in particular.

Among the most often repeated scare stories about “invasive species,”  as adaptive species are often called,  is the outcome of the arrival of brown tree snakes on Guam,  brought as accidental stowaways on military transport aircraft soon after the end of World War II.

“The snakes,  who had no natural predators on the island,  proliferated at extraordinary rates. Within a few decades they had almost entirely extirpated the island’s birds,”  recounted Keim.  “Those birds played vital roles in spreading seeds and regenerating forests — a role now performed, unexpectedly,  by feral pigs,”  residing on Guam for more than 200 years,  who also eat any brown tree snakes they find on the ground.

Pigs needed but not wanted

Wrote Gawel and colleagues,  “While non-native species may be neutral or detrimental in pristine ecosystems,  it is possible that even notorious invaders could play beneficial or mixed roles in novel ecosystems.  Pigs appear to be one of the last vertebrate seed-dispersers on an island that has lost its native dispersers.”

“From 31 pig scats,  the researchers germinated no fewer than 1,658 seedlings,”  Keim summarized.  “Were pigs eradicated from Guam,  the island’s already damaged ecology would be further impoverished.”

Concluded Gawel et al,  “The role of non-native species must be evaluated on the basis of each habitat and ecological situation.”

Notwithstanding the value of feral pigs to maintaining tree cover on Guam,  USDA Wildlife Services kills about 450 pigs per year there,  in addition to those hunted for food and sport by residents and U.S. military personnel stationed on Guam.

“The Case For Leaving City Rats Alone”

Science journalist Becca Cudmore in “The Case For Leaving City Rats Alone,”  originally published by Nautilus in 2016,  reviewed the work of University of British Columbia School of Population & Public Health veterinary pathologist Chelsea Himsworth.

“Her research has made her reconsider the age-old labeling of rats as invaders that need to be completely fought back,”  wrote Cudmore.  “They may, instead, be just as much a part of our city as sidewalks and lampposts.  We would all be better off if,  under most circumstances,  we simply left them alone.”

While rats favor disrupted environments,  whether in the wild or in human cities,  Cudmore explained,  “Rats live in tight-knit family groups that are confined to single city blocks,  and which rarely interact.”

Rats on the run are most dangerous

Therefore,  Himsworth hypothesized,   “When a rat is ousted from its family by pest control, its family might flee its single-block territory,  spreading diseases that are usually effectively quarantined to that family.  In other words,  the current pest control approach of killing one rat per concerned homeowner call could be backfiring,  and spreading disease rather than preventing it.”

Because rats live chiefly on refuse,  scavenge carcasses,  and prey upon mice,  especially newborns still in the nest,  they tend to pick up whatever viruses and bacteria are shed in their human habitats.

“So it’s not like the presence of harmful bacteria are characteristic of the rats themselves,”  Himsworth said.  “They get that bacteria from their environment, and when they move,  they take these place-specific pathogens with them.”

Threat is not the rats but the family

“While one block might be wholly infected with a given bacteria,  adjacent blocks were often completely disease free,”  Cudmore summarized of Himsworth’s findings.

Said Health Canada National Microbiology Lab researcher Robbin Lindsay,  who helped Himsworth,  “Disease risk doesn’t really relate to the number of rats you’re exposed to as much as it does which family you interact with.”

Concluded Cudmore,  “Rather than focusing on killing rats,  we need to try to keep their populations stable and in place.”

Black Death,  leptospirosis,  & hantavirus

This is not to suggest that rats have not made significant contributions to transmitting diseases including the Black Death in the Middle Ages, actually carried by the fleas infesting black rats, and leptospirosis and hantaviruses in our own time, carried by bacteria living in rats’ urine.

But a closer look at each of those situations shows that rats brought the Black Death when transported to Europe and China along newly opened routes of commerce; spread leptospirosis chiefly when displaced by flooding; and do not really spread hantaviruses much at all.

Rather, human hantavirus victims typically become infected by inadvertently invading habitats which have for several rat generations been left undisturbed, for instance by cleaning a long neglected shed or attic, or entering a poorly ventilated crawl space.

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