Franklin D. Roosevelt famously said “the only thing we have to fear is
fear itself” and our results reveal that, at least when it comes to wildlife,
Roosevelt’s rhetorical flourish accurately reflects a fundamental biological
fact. Our results show for the first time in any wild bird or mammal that the
FEAR of predators is itself is powerful enough to affect wildlife population
dynamics - even when predators are
prevented from directly killing any prey - and there is thus nothing to
actually fear, but fear itself. The impact of predators on wildlife populations has been hotly debated
for decades because of the many public policy implications. For example, there are currently heated
debates concerning the reintroduction of wolves to Yellowstone National Park,
and the management of feral cats and the impact that this may be having on
songbird numbers. These debates concern
the TOTAL impact of predators on prey
populations. The traditional view of predator-prey interactions is that predators kill
prey and that’s that – the zebra, wildebeest, bunny or bird that got away, that
wasn’t killed, is completely unaffected.
But what we show is that fear of falling victim to a predator can
traumatize the escapees so much that they have fewer babies, and this effect
can be as important as direct killing in reducing prey numbers. In other words, predators affect the population
sizes of their prey not just by killing prey, but by scaring them as well. The bottom line for conservation and wildlife
management being that the total impact of predators on prey populations will be
significantly underestimated - if the effects of fear itself are not considered;
both killing AND scaring must be
considered to fully and accurately evaluate the TOTAL impact of predators on
wildlife populations We found that fear alone caused a 40 % reduction in the number of
offspring songbirds produced per year in an experiment on a population of
free-living song sparrows. We first isolated
the effects of fear itself by protecting
every nest with netting and electric fencing, making it impossible for
predators to attack and kill any offspring.
We thereby knew for CERTAIN that any reduction in the number of
offspring was definitely NOT due to direct killing and so MUST be due to fear
itself. To
MANIPULATE fear we hung speakers from trees every 0.4 ha over a 16 ha area,
some of which broadcast predator calls and sounds, like ravens cawing, while
speakers at other locations broadcast non-predator
calls and sounds, like geese honking.
All of the speakers broadcast every few minutes, 24 hours per day over the entire 130 day
breeding season. The 40 %
reduction in the number of offspring we saw thus resulted from just HEARING the
sound of predators, i.e. the SOUND of fear alone was enough to reduce the
number of offspring produced. Birds that heard predator calls and sounds were
so scared that they laid fewer eggs, fewer of their eggs hatched, and
more of their nestlings starved to death.
To be very clear it is THIS that makes our study unique, because this is
the first time in ANY wild bird or mammal, that FEAR ALONE has been UNAMBIGUOSLY
shown to affect BIRTH and SURVIVAL, and thus the NUMBER of individuals, in
wildlife populations. These effects on BIRTH and SURVIVAL resulted from effects on behaviour
that would be recognized as indicative of fear in any animal. Mothers that heard predator calls and sounds
tried to better protect their nests by building them in denser and thornier
plants, they were also more nervous since they flushed from their nest at the
first hint of danger, and they spent more time watching for predators and less
time looking for food meaning they were unable to adequately feed their
nestlings. Because anti-predator behaviours like these are seen in EVERY kind of
bird and mammal our results suggest fear effects on BIRTH and SURVIVAL are
almost certain to be pervasive in wildlife. We have conducted previous studies directly measuring the effects of
predator-induced fear – on the stress hormone levels in the sparrows – that
suggest our experiment likely had long-lasting effects that extended beyond the
breeding season. We showed that the
nestlings of frightened mothers were fed less, and we know from our previous
work that this likely had lasting adverse effects on their stress physiology,
and brain development, that further reduced their survival. Frightened mothers themselves likely also had
elevated stress hormone levels, based on our previous work, and this probably
also reduced their survival. In sum, the
TOTAL impact that fear alone had on the number of sparrows was almost certainly
larger than that indicated by the 40 % reduction in the number of offspring
produced. In other words, the 40 %
reduction we report represents a conservative estimate of the effect that fear
itself can have on wildlife populations The conservation and wildlife management implications of our results – are
that the DISTURBANCE to native ecosystems due to the loss of NATIVE predators
has probably been GREATER than we
previously thought – and the ADVERSE effects of INTRODUCED predators – are
likely WORSE than we previously imagined. Fear effects are at the centre of the debate concerning the
reintroduction of wolves to Yellowstone National Park. There are half as many elk now as there were
prior to the reintroduction of wolves in 1995.
Since there are now half as many elk – there are a lot fewer plants
being eaten – and this has meant that the – previously over-browsed trees and
shrubs – are recovering throughout the park – providing more habitat for lots
of different animals, like songbirds, beavers, badgers and even pronghorn
antelope. All of this suggests that
restoring native predators – like the wolf – can help restore ENTIRE
ecosystems. Critics of the wolf reintroduction have been able to point to 2 things to
suggest that the wolves are NOT responsible for the reduction in elk numbers
and thus the restoration of the ecosystem, the first being that wolves don’t
directly kill enough elk to account for the reduction in numbers, and the
second being that most of the reduction is due to a decrease in the pregnancy
rate. Both these objections are based on
the traditional view of predator-prey interactions – that the only way
predators can affect prey numbers is by directly killing them. Our results corroborate correlative studies
on the changes in elk behaviour that suggest wolves ARE helping to restore the
ecosystem of the Park – primarily by FRIGHTENING the elk – rather than directly
killing them. OUR results show that the
FEAR of predators CAN affect the number of offspring produced by wildlife – be
it the number of fledglings or calves. The
frightened birds in our study behave just the same way that frightened elk do –
they choose to spend their time in safer places that may be less hospitable,
they’re more skittish, and they consequently get less food to eat. Recognizing the importance of fear effects also necessitates
re-evaluating the management of introduced predators. For example, more and more jurisdictions throughout
North America and Europe are adopting programs to manage feral cats by
trap–neuter–return (TNR), in which cats are trapped and sterilized, and then
returned to the environment to be fed and cared for by volunteer caretakers. Advocates of trap–neuter–return argue that
because they are fed, the cats won’t directly kill any wildlife, and so there are
no adverse effects on native species. Our
results show that fear of predators alone adversely affects prey numbers,
meaning that the mere presence of this introduced predator is enough to
negatively impact native wildlife. Understanding first-hand about fear effects is as close as your own
backyard. As anyone who has watched a
flock of birds at their backyard feeder knows, one moment the birds will be busily
munching their seeds and the next they will be desperately diving for cover
from some real or imagined predator.
This is what fear looks like; you’ve seen it, and your kids have seen it
too. What OUR results show is that this
repeated interruption of feeding is not just an inconvenience but has real
consequences that affect the size of wildlife populations. Unfortunately, if you live in a city, there are probably many fewer birds
at your backyard feeder than ever, because our cities are teeming with
introduced predators like cats and rats and are often also replete with native
predators, like raccoons and crows – that feed on our trash. Our results indicate that population declines
of native wildlife, witnessed in many urban areas, are happening not just
because native species are being directly killed, but because this
superabundance of predators is instilling fear and terror in them. If you’ve had a chance to watch the video
concerning our paper and seen that rat leaping out at you,
you’ll have a better appreciation of just what this fear and terror feels
like. Again, if you’ve had a chance to watch the video, you’ll have a better appreciation of what we mean when we
say that, fortunately, for the most part, humans REALLY don’t know the MEANING
of fear – not compared to that bird at your backyard feeder, or a bunny in the
forest, both of which are in peril every moment of every day, of being torn
limb from limb, by any number of predators.
The closest humans likely come to experiencing anything akin to the daily
predator-induced fear and stress experienced by wild animals – is during times
of war. Nature really is “red in tooth
and claw” and “kill or be killed” combat stress is probably the closest we come
to the DAILY experience of most wild animals. Though this may sound like hyperbole, many neurobiologists, psychologists
and psychiatrists have begun using exposure to a predator – showing a lab rat a
cat – to try to better understand, and develop drugs to treat, post-traumatic
stress disorder, or PTSD, for short.
These researchers seized upon using predator exposure for practical
reasons, as this permits the researcher to utilize – a psychological stressor –
that is life-threatening – but does not involve pain – all of which is consistent
with the development of post-traumatic stress disorder in humans. Researchers studying the “predator model of PTSD” have increasingly begun
to suggest that predator exposure offers an additional advantage in attempting
to understand PTSD, because long-lasting predator-induced fear and stress, is
ecologically relevant – and represents a valid experience applicable to animals
in their natural environment. Indeed,
some psychiatrists have even begun to discuss the EVOLUTION of post-traumatic
stress disorder.
OUR results VALIDATE this argument about
ecological relevance because we have shown that, rather being acute and
transitory, as has traditionally been assumed; predator-induced fear and stress
in WILD animals in their natural environment CAN BE sufficiently long-lasting
to EVEN affect birth and survival. As already noted, these effects on birth and survival resulted from effects on
behaviour that would be recognized as indicative of fear in any animal, and our
previous work shows that fear-induced changes in stress hormone levels were
almost certainly also involved, both of which parallel JUST the kinds of
behavioural and hormonal responses seen in LAB animal studies of the “predator
model of post-traumatic stress disorder”.Media background: Zanette et al. 2011 Science paper
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