POPULATION BIOLOGY
Case of the absent lemmings
Tim Coulson and Aurelio Malo
Changing weather patterns, producing the wrong kind of snow, have transformed the population dynamics
of lemmings in northern Scandinavia. The knock-on effects have been felt throughout the ecosystem.
A colleague from Oslo once told me that when
the Bible was translated into Norwegian,
mention of plagues of locusts was replaced
with plagues of lemmings. The logic behind
this change was that most Norwegians knew
nothing of locusts, but were all too familiar
with periodic explosions in lemming numbers.
The story is apocryphal, with references
to lemmings only scrawled by the trans lator
in the margin. Yet these scribbles suggest
that lemming outbreaks have been a feature
of northern ecosystems for the past millennium.
But now the outbreaks, at least in some
areas, have stopped. On page 93 of this issue,
Kausrud et al.1 explore the underlying reasons.
Norway lemmings (Lemmus lemmus) are
remarkable animals. These rodents can live
for three or four years, spending their winters
beneath the snow and feeding mostly on moss.
A female can produce up to three litters a year,
with as many as 12 young per litter. Lemmings
occasionally become super-abundant when
large numbers of young survive2. In northern
Norway in 1970, lemmings were so common
that snowploughs were used to clear the vast
numbers of squashed animals from roads. Outbreaks
don’t last long: food becomes scarce, and
lemmings will then often disperse en masse
in search of greener pastures. On occasion,
desperate to find food, they jump into water
and start swimming. This behaviour led to the
myth that lemmings commit suicide.In northern Scandinavia, lemming outbreaks
typically occur once every three to five years.
Or they used to. In the past 15 years, localized
outbreaks have either stopped or occur less frequently3.
The cause of this change is the subject
of debate, partly because the reason that rodent
populations often show periodic outbreaks is
itself controversial4–8. Fluctuating predation,
food availability or quality, and climate variability
have all been proposed as plausible mechanisms
generating these population cycles.
Whatever the cause, it is clear that in parts of
northern Europe something is now preventing
these rodents from periodically producing
large numbers of surviving young3.
Kausrud et al.1 analyse a 27-year time series
of lemming numbers from one site in Norway.
They first demonstrate statistically that climate
change means that Norway now gets a lot of the
wrong sort of snow. Lemmings do well when
warmth from the ground melts a small layer of
snow above it, leaving a gap between ground
and snow. This subnivean space provides
warmth and allows lemmings to feed in relative
safety from many of the animals that eat them.
Climate change now means that the subnivean
space does not exist for as much of each year
as it used to. Worse still, the space itself is less
likely to form: warmer temperatures mean that
snow melts and refreezes, producing a sheet
of ice that prevents lemmings from feeding on
the moss.The wrong sort of snow therefore means
that food is hard to come by, keeping warm
is challenging, and being eaten is more
likely. Kausrud et al. use their statistical associations
to construct a predictive model of
lemming dynamics. This model, fitted to data
from before the outbreaks stopped, predicts
the observed cessation, providing compelling
evidence that changing snow conditions
are a major factor in the change in lemming
population dynamics.
The researchers then go on to show that
the reduction in the frequency of lemming
outbreaks has knock-on consequences for the
wider ecosystem. They argue that the scarcity
of lemmings means that predators such as foxes
turn their attention to other species, including
willow grouse and ptarmigan, adversely affecting
their populations. Evidence for changes in
the numbers of species other than lemmings in
these northern ecosystems is convincing. But
although the mechanism that Kausrud et al.1
propose — a shift in predation patterns — is
plausible, it is speculative.
The critical reader will complain that the
story is based on correlations. Although this
is true, it is often the only way to study populations
and the consequences of changing
climate for ecosystems9. The collection of
detailed long-term data on the dynamics
of free-living populations of animals and
plants rarely attracts the same excitement asgenomics or particle physics, yet such data are
vital in characterizing the consequences of climate
change for the natural world on which we
depend. Describing and predicting such effects
of climate change will help us prepare for, and
possibly minimize, adverse affects. Kausrud
et al.1 elegantly show the value of detailed
long-term ecological data, and demonstrate
the benefits of maintaining existing studies
and instigating new ones.
By the time the Norwegian translator of
the Bible got to the book of Revelations, he
had stopped making references to lemmings,
so we do not know whether the cessation of
outbreaks foretells the imminent arrival of the
four horsemen of the apocalypse. However, we
do now understand that climate change has
made lemming outbreaks much less common,
which has in turn affected the fragile ecology of
northern ecosystems. This research1 providesa striking example of how climate change can
modify the workings of the natural world —
raising the question of what consequences such
change might have for us.