Spawned from undersea volcanoes and thousands of miles
from any continent, the Hawaiian Islands harbor very few indigenous land
animals. Once in a while a bird or insect made its way here – perhaps blown in
by a storm – and over the eons a few survived and were able survive. From these
the entire Hawaiian terrestrial fauna evolved. Most mammals don’t do overseas
travel very well, as evidenced by the fact that the only native mammal in the entire
Hawaiian archipelago is the reclusive Hawaiian Hoary Bat
(‘ope‘ape‘a). After human technology developed to the point where people could
travel long distances by boat, the situation changed. Some of the earliest Polynesian explorers
that made their way the shores of these islands in seagoing canoes unwittingly
carried with them the Polynesian
rat (Rattus exulans). When
Europeans began to visit the islands they brought along their own rat: black rat (Rattus rattus; sometimes called
the “ship rat”). Also onboard were mice (Mus musculus).
Rats and mice have followed humans around for millennia,
inhabiting our homes, barns, fields, roaming the streets and back alleys of
cities and towns. They are well known thieves of food and have a catholic diet.
Sure, they like cheese and peanut butter, but contrary to Walt Disney, they are
not vegetarians and are known to a variety of animal prey and will even resort
to cannibalism when the situation demands. People have battled with rats and
mice for centuries and the success of the otherwise ornery and not always
appreciated house-cat can probably be better attributed to our distaste for
rodents than for our love of felines.
The immediate effects of rodents are pretty obvious: spilled
rice grains on the shelf of the pantry reveal, on closer inspection, a hole
chewed through the bag; small brown droppings foul the silverware drawer. Rats and mice are also
notorious for their role in spreading diseases such as hantavirus,rabies,
and bubonic plague. But because of their sheer number, rodents affect the world
in ways that can be difficult to fully appreciate. In the 1980s James Brown (the
ecologist, not the soul singer) of the University of Arizona conducted a series
of experiments which showed that certain seed-eating rodents could transform a
grassland into to a shrubland. Once Polynesian rats established themselves in Hawaii,
they took to eating the seeds of native fan palms eventually wiping out forests
in places like Ewa Beach on Oahu.
The uninhabited Northwestern Hawaiian Islands remained
rodent-free until fairly recently.
Lisianski Island may have been the first to have been colonized when in 1844
a ship visiting from Honolulu deposited some mice onshore. Lacking natural
predators, introduced rodents can increase in number at astonishing rates. Five
decades after their first introduction, John Cameron, captain of the ship Ebon, made shore at Lisianski and found
the island overrun.
We settled
ourselves in for an appetizing supper of fresh food when myriads of mice attacked
our meal ravenously and utterly without fear. Drive them away we could not; we
slaughtered them by the hundreds, yet they would not be denied (quote taken from M Rauzon, Isles of Refuge)
Rodents didn’t find their way to Midway until decades later.
Despite the establishment of the Pacific Commercial Cable Company Station on
Sand Island in 1903 and the frequent supply ships that brought provisions to
its residents, it was not until after WWII that rats and mice arrived, more or
less at the same time, presumably on a Navy ship. By 1943 the black rat and the
house mouse were noted as present. While the black rat eventually become
established on all three of the atoll’s islands, mice were limited to just one,
Sand Island.
The effects of rats on Midway’s wildlife were noticed pretty
quickly. Harvey Fisher and Paul Baldwin
of the University of Hawaii visited the atoll in 1945 to see how birds were
faring after the war. They were especially interested in two species: the Laysan rail and Laysan finch. Not indigenous
to Midway, these birds had been brought there decades earlier from their native
Laysan Island, where an overpopulation of rabbits had transformed their lush
habitat into a barren wasteland, in what must have been one of the earliest
wildlife translocation efforts ever attempted. Fisher and Baldwin were
extremely disappointed to find that neither the finch nor the rail, which had
thrived after their introductions, had made it through the war. The cause of
the bird’s demise, they concluded, wasn’t bombs or bullets, nor could it be
attributed to the habitat that had been lost to buildings, roads, and runways but
rather it was due to a major infestation of rats. Sadly for the Laysan rail –
and for us – its disappearance from Midway was tantamount to extinction; if you
want to see one you’ll have to settle for an old grainy photo or a stuffed
specimen in a museum. Laysan finches were able to pull through the tough times;
rabbits were exterminated on Laysan Island in 1923 after which populations
rebounded.
Rats had taken their toll on other bird species as well
especially burrow-nesting species such as Bonin petrel, Bulwer’s petrel, wedge-tailed
shearwater which are especially vulnerable. Numbers of Bonin petrels had
been estimated at about half a million prior to the arrival of rats on Midway; Fisher
and Baldwin counted only about 25,000, a 95% reduction. Rats not only preyed on
the birds’ eggs but also attacked chicks and adults. Capable climbers, rats also
impacted tree-nesting like the brown noddy.
As rats were a nuisance to people as well as wildlife, the
Navy made some efforts to control their populations, but impacts to many seabird
species persisted. Albatrosses, may have been less affected by rats than were
other species, perhaps because their chicks are large and are guarded by a
parent for some time after hatching. Which is not to say that it was smooth
sailing for them during those times. Roadbuilding, mowing of lawns, overhead
power lines, collision with aircraft, harassment by people and dogs, and the deliberate
destruction of nests in areas where they were not convenient resulted in steep
declines in their numbers, especially for Laysan albatross which inhabits the
central portions of the islands. Although no reports of rats killing
albatrosses had been made on Midway, there were highly credible accounts of rat
attacks on Laysan albatross similar islands.
While visiting Kure Atoll in the late 1960s, Cameron Kepler of Cornell
University observed Polynesian rats attacking and sometimes killing adult
Laysan albatross.
[We] frequently encountered
injured Laysan Albatrosses
and noticed dying
and dead adults
with large gaping
wounds in their
backs; 12 such
birds were found
in the 1963-64
breeding season… The open
wounds were always
found on the
birds' backs, either
slightly anterior to
the uropygial gland,
or forward between
the scapulae. Small
holes, one to
two inches in
diameter, were occasionally
found. At this
stage, wounds were
sometimes obscured by
feathers, and the
injury did not
cripple the bird …
More often, sores
five to seven
inches in diameter
were seen. The
thoracic cavity was
often exposed, and
ribs and scapulae,
or even lungs,
were visible through
the gaping hole.
The wings drooped
when the bird
stood or walked,
as a result
of severed muscles.
Birds that had
wounds on their
rumps often limped
or were unable
to walk. The
injuries were often
infested with the
eggs of flies,
and occasionally harbored
maggots. The feathers
surrounding the hole
were caked with
blood, and the
birds' bills were
stained from probing
into the wound.
Birds in these
advanced stages rarely
survived the night
following their discovery (Kempler, 1967)
Could it be that the Polynesian rat is more likely to attack
albatrosses than the black rat? Or,
perhaps, control of rats on Midway kept them numbers low enough such that they could
subsist on other prey? I’m not sure we’ll ever know, but the rodent situation at
Midway continued to harm seabird populations until the mid-1990s when, in
anticipation of transfer of the atoll to the US Fish and Wildlife Service, rats
were eradicated through a massive trapping and poisoning campaign. The last rat
to be seen at Midway was in October of 1997.
The birds approved. Bonin petrels, which
had nearly been wiped out during the age of rats, returned to the atoll to nest
once again. While numbers are hard
to come by, it is thought that over one million of these birds now breed on
Sand Island alone. Rats on Midway may have been exerting strong effects on
vegetation as well. After their eradication, the native shrub naupaka appeared
to greatly increase in abundance. Rats, apparently, were eating more than just
seabirds and as naupaka plays an important role in stabilizing dunes, rats may
even, indirectly, have been accelerating coastal erosion on the atoll.
With rats out of the picture one might think Midway’s rodent
problems had finally been solved. And so it seemed for a couple of decades.
Though Sand Island still hosted a significant mouse population, they were
thought to be primarily a nuisance to the human residents. During my first few days
at Midway last spring, I was a bit surprised at how many mice scurried
across my headlight beam as I rode my bike at night. It was also hard not
notice the dozens of little black bait boxes placed outside of buildings. While
there were certainly lots of conservation challenges to be addressed regarding
the birds of Midway – ingestion of plastic, lead poisoning, invasive species,
and entrapment hazards just to name a few – rodents weren’t high on the list.
During the fall and winter of 2015-6, exceptionally warm
water in the Pacific created a particularly strong El Niño which, for
this part of the world, means below average rainfall. Black-footed and Laysan albatrosses arrived
from the northern Pacific Ocean around Halloween to begin their long breeding
cycle. Things were going along pretty
much as usual when the team of bird counters arrived in mid-December to do
their annual albatross census. Then, two days before Christmas, several of the
bird counters reported finding five adult Laysan albatrosses with bloody wounds
on their necks and heads. The cause of
the wounds was a mystery. Albatross sometimes engage in fights that leave them
bloodied and occasionally a peregrine falcon or other avian predator finds its
way to the atoll and these possibilities were considered. Fish and Wildlife
Service staff investigated the situation through a variety of avenues. On
January 5th, motion-sensing cameras set up alongside nests captured
images that many had a hard time believing. Photos clearly showed mice crawling
onto the heads and backs of albatrosses while they were sitting on their nests disappearing
under their feathers. When those birds were examined, it was revealed that the
mice were chewing through the feathers and skin and then eating their victim’s
flesh. Both Laysan and black-footed albatross were affected though the former
made up the vast majority of the casualties.
Some of the birds succumbed to infection while others
abandoned their nests. Video footage revealed that albatross appeared to be annoyed
by the presence of the mice but did not seem to know how to respond to the
attacks. Perhaps this is not that surprising as animals such as albatrosses
that have evolved for millions of years in the absence of mammalian predators may
behavioral adaptations that would allow them to defend themselves. It is for
this same reason that the island
avifauna here and in places like the Galapagos Islands show little or no fear
of humans. Watching the videos is painful. You want the albatross to do something,
turn its head around and snap the mouse in half with its powerful, sharp bill.
Instead the bird is agitated and confused and the mouse returns again and
again, seemingly bolder each time, slowing chipping away at the life of a
magnificent bird that might otherwise have thrived for decades in some of
the harshest environments that our planet offers. It just seems so wrong.
The world’s largest colony of Laysan Albatross was under
attack the Fish
and Wildlife Service worked diligently with wildlife experts from other
agencies and organizations to formulate an appropriate response. The plan eventually hatched (OK, I know
that’s a blatant and not very clever pun but I thought you might need a little
levity at this point) was to try to reduce the mouse population in the areas
where the attacks were occurring using a combination of traps and rodenticide.
The strategy appeared to work and mouse numbers declined in the treated areas
and fewer new albatross casualties (birds bitten/injured, dead birds, and
abandoned nests) were reported by the field surveyors. Would the mice attack
the chicks once they hatched? No one
knew, so it was with great relief in late January when after checking hundreds of
nests with newborn chicks that no evidence of mice predation was revealed. When
all was said and done though, the impacts were significant: 480 albatross were injured,
57 abandoned their nests, and 52 died. In areas where albatross were seen
attacking mice, nests failed at twice the normal rate.
Scientists were dumbfounded as nothing like this had ever
been observed in any of the Pacific Islands. What had precipitated the sudden
change in the relations between albatrosses and mice on Midway? Maybe the El Niño-caused
drought had caused severe food limitations? Or maybe it was a consequence of successful
efforts in reducing the invasive weed golden crownbeard,
which produces abundant seeds? And most importantly of all, would the situation
repeat itself the following winter? There were many questions but few answers.
It turns out, though, that there was some precedent for mice
attacking albatrosses, but you had to go to the other side of the world to find
it. Gough Island, in the
South Atlantic Ocean, is a globally significant seabird colony and hosts the
largest colony of Tristan albatross in the world. During the nesting season of 2000/2001 a
substantial portion of albatross chicks died of unknown causes. Ross Wanless and colleagues from the University
of Capetown and the Royal Society for the Protection of Birds set out to find
the source of the nest failures. As islands go, Gough Island is unusual in
never having had populations of rats, cats, goats, or sheep. The only introduced mammal there is the house
mouse but few believed that they might be the cause. How could a tiny mouse
take down an albatross chick that weighs over 8 kg (17 lb)? Using a combination
of field surveys and motion-sensing cameras they discovered the cause.
Mice! In this case, however, the rodents
were not attacking the adult albatrosses but the chicks. Mice attacked Predation
on chicks by mice reduced halved the breeding success rate of these albatross
during that season. Subsequent monitoring of the Tristan albatross population
on Gough Island painted an even bleaker picture as attacks of mice on albatross
seemed to be increasing over time. A team led by Delia Davies from the
University of Capetown conducted a follow up study during the 2013/2014
breeding season and found that only 5 of 20 chicks they monitored fledged and
of the 15 failures, 14 were due to mice.
The situation on Midway remains unique. On Gough Island,
mice never were observed attacking adult albatrosses, only chicks. And at Midway
the reverse is true. Putting these two facts together probably makes everyone a
little nervous as it suggests that the situation at both of these seabird
colonies could probably substantially worse should mice at either location
discover there might be yet another source of food.
Given that mice and albatross had seemingly coexisted for
over half a century on Midway, no one was sure what the 2016/2017 nesting
season would bring. After the
albatrosses arrived in the fall and began nesting, we waited and watched
anxiously, hoping that the events of the previous year would prove to be an
anomaly. On December 4, while out checking birds in areas where mice had
attacked the previous year, Fish and Wildlife Service biologist Meg
Duhr-Schultz found several bitten birds, removing any possibility that the
events of 2015/2016 were some kind of El Niño-driven anomaly. Staff and
volunteers were deployed over the next few days to survey other parts of the
island and more attacks were discovered. In less than a week the area impacted
by mouse attacks had exceeded the total area affected during all of the
previous year. And the fact that the mouse attacks were noticed several weeks
earlier was of real concern. Again, the
Fish and Wildlife Service had to quickly figure out what to do and the decision
was made to take steps to reduce the mice populations in the affected areas. The
large area that had already been affected meant that staff and volunteers had
to log a lot of extra hours and additional resources were sent from Honolulu.
The bird counters pitched in too with some remaining an extra couple of weeks
to assist with surveys and treatment. So far the actions that the Fish and Wildlife have taken seem to be having a positive effect. The abundance of mice in the impacted area dropped sharply in areas where rodenticide was applied.
My primary role has been information gathering and analysis, including mapping out the areas affected, and the design of treatments using geographic information systems. This latter part is critical as the rodenticide that is being needs to be broadcast by hand within a predefined grid and must be done in strict accordance with procedures.
My primary role has been information gathering and analysis, including mapping out the areas affected, and the design of treatments using geographic information systems. This latter part is critical as the rodenticide that is being needs to be broadcast by hand within a predefined grid and must be done in strict accordance with procedures.
Caption: A dead albatross found during the attacks of 2016/2017 next to an abandoned nest (Photo: RV Taylor). |
As of today, 48 areas totaling 10.5 ha (26 acres) of mouse
affected area has been mapped and over 1,200 bitten birds have been discovered,
211 of which have died. Nearly one thousand abandoned nests have been
documented. Mice may also be having impacts on other seabirds here but it would
be more difficult to detect, especially for the burrow-nesting species. Sounds kind of depressing, huh? It is, but at least we’re not just sitting
back and letting it happen. And although the numbers may seem large they need to
be put into context. This year, the bird counters tallied nearly a million
breeding Laysan albatross across the atoll, so there are still hundreds of
thousands of birds going about their business, more or less, as usual. Add to
that all the non-breeders who liven up the atmosphere here nearly 24 hours a
day with their crazy
dancing and “singing” and a group of tight-knit community of good people
who recognize how important it is to laugh in the face of adversity.
Another bright spot is that there are plans underway to
address the underlying cause of the mouse problems here at Midway. As the only atoll within the Papahānaumokuākea
Marine National Monument that still harbors lingering populations of
invasive rodents, there have been plans to eradicate mice from Midway Atoll for
some time. The discovery that mice are harming the albatrosses should only strengthen
the case for their removal and expedite the project’s implementation. First
steps were, in fact, taken just last November when a team of biologists and
invasive species experts from the Fish and Wildlife Service and the non-profit
organization Island Conservation
visited Midway begin a study of the project’s feasibility. Although eradicating mice from a place as big
and complicated as Midway is challenging, similar projects have been
successfully completed on other, and in some cases even larger, islands. With
the first-hand knowledge I’ve gained on how damaging mice can be to wildlife, I
think it would be great to be able to play a role in the eradication effort
here. Who knows, maybe there will be an
opportunity? What I am sure of is that after this experience I’ll never look at
Mickey Mouse the same way again.
Further reading
Brown, JH, and EJ Heske. 1990. Control of a desert-grassland
transition by a keystone rodent guild. Science 250:1705-1707.
Davies, D., B. J. Dilley, A. L. Bond, R. J. Cuthbert, and P.
G. Ryan. 2015. Trends and tactics of mouse predation on Tristan Albatross
Diomedea dabbenena chicks at Gough Island, South Atlantic Ocean. Avian Conservation and Ecology 10(1):5.
Fisher, HI and PH Baldwin. 1946. War and the birds of Midway
Atoll. The Condor 48:3-15.
Hess, SC and J D Jacobi. 2011. The history of mammal
eradications in Hawai ‘i and the United States associated islands of the
central Pacific. Pp. 67-73 in Veitch et al. Island
invasives: eradication and management. IUCN, Gland, Switzerland.
Kepler, CB. 1967. Polynesian rat predation on nesting Laysan
Albatrosses and other Pacific seabirds. The
Auk 84: 426-430.
Rauzon, MJ. 2001. Isles
of Refuge: Wildlife and history of the Northwestern Hawaiian Islands. University
of Hawaii Press. Honolulu, HI.
Wanless, R. M., A. Angel, R. J. Cuthbert, G. M. Hilton, and
P. G. Ryan. 2007. Can predation by invasive mice drive seabird extinctions? Biology Letters 3:241-244.