Invasive species can dramatically reshape
environments and cause extinction, even when they don't prey on their
newfound neighbours, according to new research that highlights the
dangers of altering habitats.
The six-year study of lizards on a string of remote islands found that
the arrival of a predator can prompt species that previously co-existed
peacefully to cluster in a shared refuge, creating competition that can
be devastating.
It challenges a long-standing theory that predators feeding on animals
lower down the food chain prevent any one prey species from dominating a
habitat, and so promote ecological diversity.
"Our results suggest that we need to update conventional wisdom in a few
ways," said lead author Robert Pringle, associate professor of ecology
and evolutionary biology at Princeton University.
"We need to think about unexpected indirect impacts -- the species at
greatest risk of extinction might not necessarily be those with the
greatest risk of being eaten," he told AFP.
- A blank canvas -
Understanding how species co-exist is one of the biggest challenges in
biology, not least because it is hard to find or create control
environments for experimentation.
But Pringle and his team found a solution in the form of a string of 16
islands in the Caribbean, home to an unassuming lizard known as the
brown anole, which largely hunts on the ground and in low branches.
The islands provided the researchers with the perfect blank canvas to
observe how the native brown anole population would react when different
invasive species were introduced.
They examined four scenarios: a control group of islands where the
native population was left alone, a group where the competitor
tree-dwelling green anole was introduced, a group where a top predator
called a curly-tailed lizard was introduced, and a group where both new
species joined the native brown anoles.
The first test was to see if the ground-hunting brown and tree-dwelling green anoles could co-exist successfully.
The study found that was possible: the green anoles rapidly multiplied,
and while the brown anole population didn't grow as much as on the
control islands, it still expanded.
The introduction of the curly-tailed predator lizard -- a ground-dweller
that can eat both the unfortunate brown anole and its prey -- saw the
native population change behaviour.
Brown anoles took to the trees, leaving behind much of their usual
hunting grounds to stay beyond the reach of the new arrivals. This meant
the brown anole population did not grow, but the native lizard at least
remained static.
- 'Landscape of fear' -
But the most interesting results of the research, published Thursday in
the journal Nature, came from the final scenario, where all three
lizards were forced to share the same habitat.
Under the existing theory, the curly-tailed lizard should have prevented
either of the anoles from dominating and detente would have prevailed.
But in fact, brown anoles moved up into the trees to escape the
predators and found themselves in competition with the green anoles,
with dire results.
The brown anole population shrank more than 40 percent, and on two of
the islands the green anoles went extinct. On a third their population
was static and only on one did they grow moderately.
The population collapses came despite the fact that the curly-tailed
lizards were rarely eating the anoles -- they were effectively driving
themselves into extinction.
"The 'landscape of fear' created by the predator forces prey species into intense competition," Pringle said.
"Introducing a predatory species can cause the extinction of prey species that never even encounter the predator."
The research leaves some questions unanswered, including why the green
anoles suffered more than the brown anoles in their refuge from the
curly tailed predators.
And the islands have more to offer: having documented how brown anoles
altered their habitat and diet to escape predators, the researchers now
want to examine what evolutionary shifts these changes may have caused
in the species.
https://www.geezgo.com/sps/58027
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