A handful of hippos imported to Colombia by drug lord Pablo Escobar have ballooned into a population of more than 80 animals — and they’re changing the aquatic ecosystem they’ve invaded. Escobar, who led the Medellin Cartel until his death in 1993, first imported four hippos (Hippopotamus amphibus), along with other exotic animals, to his personal family zoo four hours east of Medellin in the late 1980s. While most were removed, authorities left the rest of the hippos where they were, since they lacked a safe, practical way to relocate them. Now, new research published in Ecology shows that the growing population of hippos — the world’s largest invasive species — is changing the oxygen levels, water quality and chemical make-up of the waterways they live in. Researchers said that this could lead to increased toxic algae growth as the hippos provide increased fertilization through their waste.