Magellanic penguins colonized the Patagonian mainland because the land predators were gone. Cattle ranching had displaced pumas from the region for decades, and in the predators' absence, penguins — evolved on islands with no terrestrial threats — moved ashore. It worked. The colony grew to over 90,000 adults. A stable population on land that looked safe.
Then the pumas came back. When ranching was abandoned in southern Argentina in the 1990s, pumas began recolonizing their historical range. The two populations met. Over four years, pumas killed more than 7,000 adult penguins — roughly 7.6% of the colony — and left many of the carcasses uneaten (Bingham et al., Journal for Nature Conservation 2026).
The surplus killing is the telling detail. Pumas normally invest significant energy in hunting. Stalking, ambush, pursuit — each carries a real cost that limits how many kills a predator makes beyond what it can eat. But these penguins have no evolved escape response to terrestrial predators. They don't flee at the right cues. They don't recognize the approach patterns. The energy cost of each kill drops so low that the usual economic brake on predation doesn't engage. The pumas aren't being wasteful in any biological sense. They're responding rationally to prey that is astonishingly easy to catch.
The naivety is symmetrical. The penguins have no defense because they never needed one. The pumas have no restraint because they've never encountered prey this vulnerable. Both species are behaving exactly as their evolutionary histories predict. The pathology isn't in either population — it's in the combination, which has no evolutionary history at all.
This is what happens when two populations that evolved in separate contexts meet for the first time. There is no co-adaptation, no arms race, no calibrated response. The predator overshoots because its hunting behavior was tuned against prey that resisted. The prey is decimated because its vigilance was tuned against predators that didn't exist. The missing ingredient isn't intelligence or adaptation — it's the coevolutionary feedback that normally tunes predator and prey to each other over generations.
The modelers note that pumas alone are unlikely to eliminate the colony. The greater threats are poor breeding success and low juvenile survival — the slow background pressures that a naive population is poorly equipped to handle when they combine with a sudden predator. But the deeper lesson is about the fragility of populations that appear stable only because the stressor they're vulnerable to hasn't arrived yet. Stability in the absence of a test is not the same as resilience.