Research & Innovation 2015-16 - Page 91



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Our results also suggest that this shift has been aided
by the presence of power-line infrastructure. This has
provided sites for crows to build their nests in what is
otherwise a virtually treeless landscape.
We conclude that while power lines have facilitated the
increase of pied crows in the Karoo, climate change
has driven their soaring numbers in these areas.
The combination of climate change and electrical
infrastructure has created the ‘perfect storm’ of
conditions to favour an explosion of pied crow numbers
in the shrublands of South Africa.
An unloved species
Crows are big and obvious. Pied crows, in particular,
are easy to identify, with their white tank-top plumage.
These birds have had a bad rap globally. This dates
back to medieval times, when they were reviled as
carrion birds on battlefields; and large, black birds are
superstitiously associated with ill omens or death.
Their collective noun – a murder of crows – doesn’t
exactly do much for their public image either. Given all
this history, it is perhaps understandable that people
react emotionally when they see crows doing what
crows do best: being predators.
In South Africa, pied crows are notorious, and viewed
with great suspicion by urbanites and rural farmers
alike. They are accused of such gory deeds as plucking
the eyes from newborn lambs, destroying the eggs
of ground-nesting birds and decimating populations
of tortoises.
They’re also vilified for harassing other much more
glamorous species, notably Verraux’s eagles. Indeed,
these perceptions have led to calls for the control
of pied crows by those who are worried about their
negative impacts.
But these observations are not sufficient evidence
to suggest that crows have an overwhelming
negative impact on ecosystems. A recent
scientific review suggests that in general, they don᾽t.
The truth is that their role within ecosystems is not
necessarily that straightforward. For example, they
also eat other predators, such as small snakes, which
can be a major cause of nest failure in Karoo birds.
Thus, increased numbers of pied crows certainly have
the potential to change the balance of predator-prey
interactions.
Given the situation, it may be that pied crows are an
example of the relatively new phenomenon of the native
invader. These are species that occur naturally in one
area, but whose numbers suddenly increase out of all
proportion with their surrounding ecosystems, shifting
the balance of nature in unpredictable ways.
For a species to be considered a native invader in the
truest sense, it needs to have a demonstrably negative
impact on other species. But while the pied crow clearly
has such potential, we do not yet have the evidence
to confirm they are causing declines of other species.
Therefore, in assessing the pied crow ‘problem’, we must
be careful not to jump to conclusions.
Climate change is ongoing
Throughout the world, animals and plants are
responding to the changing climate by shifting their
ranges, changing their behaviour, and changing
their abundance.
But climate change is ongoing, so these shifts may
continue to change over time. It is likely that we are not
stepping into a new stable state, but rather witnessing
one step in a continual transition, as species adjust (or
fail to adjust) to conditions that are in a state of flux.
What happens next is uncertain.
As warming continues, will pied crow numbers in
the south-west of South Africa subside again? Or
will they adapt to their new conditions? Either way,
it is still unclear what the legacy of the ‘pied crow
invasion’ will be.
By Susan Cunningham and Arjun Amar, lecturer and
senior lecturer at UCT’s Percy FitzPatrick Institute
of African Ornithology. First published in The
Conversation. Images by Peter Ryan.
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