Shifting Sea Levels, Shifting Fortunes
As we discussed last post, penguins were very diverse in Africa in the past. We have two good geological time slices, one from the Miocene and one from the Pliocene. At both times, there were four different species inhabiting the southern coast. Today, there is only one. This raises the question: what happened to all those extinct species? It might seem natural to point the finger at humans, since we have been involved in the extinction of many flightless birds such as the Dodo, Moa, and Great Auk. In this case, the fossil record seems to exonerate us. By the time the first human settlers arrive in South Africa, all of the extinct species had already vanished and only the modern Black-footed Penguin was hanging around. Archeological sites show no evidence of early humans roasting up big piles of penguins at campsites, suggesting that overhunting had no role in the extinctions.
A more plausible explanation involves sea level change. Penguins love islands. This is because the land is actually a dangerous place for penguins. Predators like stoats, cats, dogs and even large birds pose a big danger, especially to hatchlings and eggs. This is one of the reasons why penguins tend to breed in isolated places, where there are likely to be few land mammals to harass them. Today, it is possible to see Black-footed Penguins nesting in places like Boulders Beach on the coast near Cape Town. However, the majority of the regional population nest on small offshore islands, where they are safer from land predators.
Over the past 12 million years, sea level along the African coast has ramped up and plunged down like a slow-motion roller coaster. During the time the fossil species lived, there were far more islands in the area. This is because higher sea levels swamped the coast, submerging many low-lying areas and turning hilltops into islands. These areas would have been prime penguin real estate. As sea level dropped towards the present, the islands would have been reconnected to the mainland, allowing terrestrial predators to invade. We suspect this could have removed much of the suitable nesting colony areas, resulting in a lower carrying capacity for penguins and the extinction of some species.
Roberts, D. L. et al. 2011 Regional and global context of the Late Cenozoic Langebaanweg (LBW) palaeontological site: west coast of South Africa. Earth Sci. Rev. 106, 191 – 214.