- Juveniles of the Western Cape population of African penguins, an IUCN-listed endangered species, still frequent a subpar hunting ground, even though other options are within reach.
- This population of penguins has declined by 80 percent in recent decades.
- The current research projects that Western Cape penguin numbers are half of what they would be without this ecological trap.
Evolution can be an exquisite hammer, tapping out finely honed strategies for the survival of life on Earth against the blunt force of the environment. But mess with those surroundings too much or change them too quickly, and suddenly those adaptations might point organisms in the wrong direction – in effect, leading them into a costly trap that could lead to their demise.
On the southern tip of Africa, scientists recently discovered that African penguins (Spheniscus demersus), an Endangered species according to the IUCN, seem to be falling into this sort of “ecological trap”– the first ever found in the ocean environment. And it’s one that we humans have caused through overfishing and climate change.
Led by ecologist Richard Sherley of the University of Exeter in Britain and the University of Cape Town in South Africa, the team, which included researchers from the governments of Namibia and South Africa, published their research Thursday in the journal Current Biology.
Penguin numbers in South Africa’s Western Cape have plummeted by around 80 percent in recent decades. Biologists chalked this decline up to the disappearance of their favorite prey species, anchovies and sardines, from the Western Cape penguins’ preferred feeding grounds to the west of their nesting sites. Overfishing and changes in the water chemistry – knock-on effects of climate change – pushed breeding shoals of these fish eastward or wiped them out entirely.
For ages, ocean waters west of the Western Cape rookeries have been remarkably fertile. Northerly winds run along the coasts of South Africa and Namibia and pull cold, nutrient-soaked waters from a couple of hundred meters down in a process known as upwelling. That fuels hordes of plankton, and historically millions of anchovies and sardines have been quick to follow.
In turn, the aggregation of prey draws in larger predators, such as African penguins, which crisscross thousands of kilometers of ocean to find just such a buffet. Penguins have evolved to pick up on cues, such as the chemicals that plankton release and the temperature of the water, to tell them they’re on the right track, the researchers write.
But now, when they head to that formerly reliable feeding spot, they’re more likely to find jellyfish and low-calorie gobies, rather than energy-dense sardines and anchovies.
Adult penguins seem to have figured out that they need to head east toward their preferred prey, a lesson gleaned from years of hunting. But Sherley and his colleagues wondered if the younger ones might be stuck with a broken strategy – that is, a trap in which the senses they have evolved to point them toward prey are leading them to an environment that’s changed too quickly for them.
Sherley and his colleagues used satellites to follow the movements of young penguins as they left their breeding grounds in Namibia and South Africa and headed into the open ocean to feed.
“I was actually hoping when we did this tracking that we would see them moving eastward,” Sherley told Mongabay. But the data revealed that the fledgling penguins’ behavior wasn’t flexible enough to accommodate the change to their environment and instead led them to a subpar food source.
To quantify the impact of this upheaval, the team ran a series of models to approximate the size of the penguin population if juveniles instead swam toward the (currently) prey-rich waters to the east.
“The modeling we did suggests that the Western Cape population would be roughly double what it is now,” Sherley said. “That’s quite a clear demographic impact.”
But whether these western colonies will indeed survive remains an open question, he added.
“It may be that they’re on their way to becoming unsuitable habitat,” he said. “Or it may mean that they bumble along at [this] low level.”
Until now, most research into ecological traps has centered on sussing out the traps themselves, with less of an emphasis on how they affect populations, said Robin Hale, an aquatic ecologist at the University of Melbourne in Australia, who was not involved in the study. Hale and fellow ecologist Stephen Swearer compiled a review of research on ecological traps in 2016.
“This paper is really interesting in that takes the next step to start thinking about what they might mean at larger scales for population persistence,” Hale said.
Those revelations could inform conservation decisions. Sherley and his colleagues suggest that it might be possible to move chicks to rookeries in South Africa’s Eastern Cape, where they would be closer to spots where anchovies and sardines hang out. They also advocate dynamic protection measures, such as marine protected areas with boundaries that reflect current fish populations and shutting down fishing when population numbers of important prey species dip.
Still, uncovering the existence of the trap was a critical step, Hale told Mongabay. “This is the first empirical example of a trap in the ocean,” he added, which isn’t trivial.
It requires figuring out why an animal might be choosing a sub-standard habitat and how that affects its chances of survival, Hale said, all in the marine environment where we humans are just visitors.
“It’s quite a hard thing to do,” he added.
CITATIONS:
- Hale, R., & Swearer, S. E. (2016). Ecological traps: current evidence and future directions. Proceedings of the Royal Society of London B: Biological Sciences, 283(1824). https://doi.org/10.1098/rspb.2015.2647
- Sherley, R. B., Ludynia, K., Dyer, B. M., Lamont, T., Makhado, A. B., Roux, J.-P., … Votier, S. C. (2017). Metapopulation Tracking Juvenile Penguins Reveals an Ecosystem-wide Ecological Trap. Current Biology. https://doi.org/10.1016/j.cub.2016.12.054
Banner image: young African penguins (Spheniscus demersus) courtesy of SANCCOB
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