Climate change and commercial fishing are luring endangered African penguins into an “ecological trap” that threatens their survival, a new study warns.
Warming sea surface temperatures and overfishing have made food scarce in the usual feeding areas for African penguins. But young penguins aren’t able to keep up with the shift in their preferred prey.
The results of the study, published in Current Biology, suggest that the number of breeding penguins is 50% lower than it would be if they were able to find the best spots for feeding.
The study is the first to identify an ecological trap in marine environments, other scientists tell Carbon Brief, but climate change is likely to cause more in future.
African penguins (Spheniscus demersus), also known as “Jackass penguins”, live around the coasts of South Africa and Namibia.
Between the late 1970s and 2015, the number of breeding pairs in South Africa dropped from about 70,000 to less than 20,000. In Namibia, the number of pairs declined from around 12,000 to less than 6,000 over the same period.
It’s no surprise, then, that African penguins are listed as an endangered species by the International Union for Conservation of Nature (IUCN). The IUCN say the fall in numbers has been caused by food shortages as a result of “shifts in the distributions of prey species” and competition with commercial fishing.
The new study pinpoints exactly why the penguins aren’t coping with these changes.
African penguins explore thousands of square kilometres of the ocean, seeking out areas with cool sea surface temperatures and high levels of nutrients. These “highly productive” areas of the ocean would usually be teeming with fish.
But they are being fooled by their own instincts, the new study says:
“These were once reliable cues for prey-rich waters, but climate change and industrial fishing have depleted forage fish stocks in this system.”
“An ecological trap is when the visual, scent and other cues that are usually associated with good habitats, suddenly become associated with bad habitats.”
Stocks of the penguins’ preferred prey – sardines and anchovies – are instead in areas where the penguins wouldn’t expect them to be – and wouldn’t usually look.
In South African waters, for example, warming temperatures and changes in saltiness – as well as pressure from fisheries – have seen sardines and anchovies shift eastwards and around Cape Agulhas, where the Atlantic and Indian Oceans meet.
To test what impact the shift in prey was having on the penguin feeding habits, the researchers tracked 54 young penguins from eight colonies between 2011 and 2013.
The penguins consistently ventured northward and westward in search of food, the researchers find. You can see their prefered feeding grounds in the map below.
Top of the list were Swakopmund in Namibia (see “SW” on map), north of St Helena’s Bay on South Africa’s west coast (“SH”), and Cape Agulhas.
In the past, these locations were all key spawning areas for sardines and anchovies. For example, adult sardines used to be abundant in St Helena’s Bay and off Namibia’s coast during spring. The fish are now scarce in these areas, the researchers find, yet young penguins still return there to feed.
So, while the penguins are heading north and west for food, the fish are actually shifting south and east.
The young penguins seem to be particularly susceptible to falling into the trap of looking in the wrong place for food, the paper says:
“Foraging decisions [are based on] environmental cues that juvenile penguins have evolved to guide themselves to reliable food sources, but [these] are no longer reliable.”
As penguins can take many years to adapt their feeding patterns, young penguins aren’t necessarily surviving long enough to change their behaviour. And they’re not guided to the best feeding sites by adult penguins, the paper says, as they “actively exclude juveniles from foraging groups”.
The study’s results show, for the first time, that this ecological trap is the key reason why so many African penguins aren’t surviving their first year.
The scarcity of sardines off South Africa’s western coast, in particular, is linked to low survival rates of both adult and young penguins from colonies on Dassen and Robben Islands, the researchers say.
And low survival of young penguins from Namibian colonies “seems to be limiting growth of the Namibian penguin population,” the paper says.
Using a computer model to simulate penguin population numbers, the researchers find the ecological trap causes the number of penguins to make it to breeding age to fall by half.
This “groundbreaking” study is “very concerning from a conservation point of view,” says Robertson:
“Birds, insects and all kinds of life forms are born with innate orientation and navigation programs that rely upon the use of environmental cues to guide them to profitable habitats at the correct time. This research illustrates a general mechanism by which these behaviors can become severely compromised by ecological traps.”
According to the IUCN, the recent population declines in African penguins show “no sign of reversing” and “immediate conservation action is required to prevent further declines.” In the short term, this could mean managing – and ultimately ending – the impact that commercial fishing adds to this ecological trap, the paper says.
The study is also the first where “climate change has been so clearly shown to create an ecological trap”, Robertson notes.
Climate change has been largely underappreciated as a cause of ecological traps, says Dr Robert Fletcher, an associate professor in the Wildlife Ecology & Conservation Department at the University of Florida, who also wasn’t involved in the study. He tells Carbon Brief:
“It is likely that we will see more and more examples of such effects in the future. Because of the potential for climate change to generate ‘mis-matches’ of organisms with their environment, there is great potential for ecological traps to emerge.”
Sherley, R. B. et al. (2017) Metapopulation tracking juvenile penguins reveals an ecosystem-wide ecological trap, Current Biology, doi:10.1016/j.cub.2016.12.054
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