DNA study ties the aurochs' disappearance from Scandinavia to several overlapping pressures

Aurochs began colonising Scandinavia 11,700 years ago, once the last ice age ended ^[4]. On the new genetic evidence, they stayed in contact with the mainland population until the land bridge flooded about 8,000 years ago ^[8]. The connection lasted roughly 3,700 years ^[1]. Earlier work had put the isolation at around 10,000 years ago, so the revision moves it about 2,000 years later ^[7][3]. The remnant in southern Sweden then held on in low numbers until at least 6,500 years ago, some 1,500 years or more after the sea cut it off ^[10][2].

Scandinavia was one of the first places the species disappeared ^[1]. Elsewhere it lasted far longer. The last aurochs died out in Poland in the 17th century, ending a decline that had run for 8,000 years, and human activity was the main reason for the global extinction ^[3]. Why the Scandinavian animals went so much earlier had been unclear ^[3].

Two findings come straight from the genetic data: the founding bottleneck and the later date of isolation ^[5][7]. "I found it incredibly interesting to see how the genetic diversity of the aurochs decreased as you moved north from mainland Europe up through Jutland and then dropped further across Zealand and Scania. It's a textbook example of a bottleneck in evolutionary genetics," said Mikkel-Holger S. Sinding, an associate professor at the University of Copenhagen and a senior author of the study ^[6].

The evidence for hunting and forest change is of a different kind. The press account calls the hunting that coincided with the straits forming "seemingly intense," and links it to a sharp fall in Swedish numbers and the aurochs' complete loss from Zealand ^[9]. The forest explanation is hedged as well: the animals "likely disappeared" as open land gave way to dense forest, a change "thought to have been detrimental" ^[11]. Both are arguments from timing. Intense hunting and a new sea barrier came in the same window, so the Zealand collapse fits either cause, or both ^[9].

The team concludes that the extinction came from the combined effects of genetic depletion, human pressure, habitat fragmentation and degradation ^[12]. The press account does not estimate how much each contributed, nor does it say how many animals were sequenced. In my view the multiple-pressure reading is still the best one this record supports, because each later pressure lands on a population already thinned by its founding bottleneck ^[5][9][11].

Erika Rosengren, a doctoral student in historical osteology at Lund and the paper's lead author, draws a general rule from this single regional case ^[2]. "They confirm that species rarely disappear because of a single factor. Conservation plans must therefore be comprehensive and consider multiple stressors at the same time," she said ^[13]. I would want more populations than one before calling that rule confirmed.

Her corridor recommendation sits closer to the data. "Our results support the need to establish ecological corridors for threatened wildlife, rather than isolated nature reserves," she said ^[13]. In Scandinavia the barrier was a flooded land bridge ^[8]. The lesson for reserve design carries over through what isolation does to a small population that is already short of genetic diversity ^[5][12].