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An outbreak of African swine fever led to the growth of more oak trees in a Polish forest.

African swine fever is a highly contagious and deadly viral disease that affects domestic and wild pigs. It has been present in Poland since 2014, and reached Białowieża primeval forest in March 2015.

at Adam Mickiewicz University in Poznań, Poland, and his colleagues analysed what effect the had on oak trees in the forest.

Acorns, the nuts produced by oak trees, can form up to 70 per cent of a wild boar’s diet, says Bogdziewicz. The more acorns are eaten, the fewer are left to grow into trees.

The researchers monitored the acorn production of 29 oak trees in the forest between 2009 and 2020. They also counted the number of acorns littered on the floor near the trees.

Oak trees produce acorns during distinct periods every few years, a strategy called masting. In a mast year, all the trees in a community produce lots of the nuts at the same time. As a result, animals cannot eat them all.

In the intervening years, when oak trees don’t produce acorns, there is less food for seed-eating animals and their populations decline.

This strategy works well for oak trees when it comes to smaller rodents, but is less effective with boars. “Boars can move further to look for other sources of food and they also have wider diets that allow them to switch to other stuff in years of famine,” says Bogdziewicz.

The 2015 outbreak of swine fever in the forest led to a 90 per cent drop in boar numbers in the following year. After the outbreak, twice the number of oak trees were successfully established compared with before 2015, the study found. “What the virus did today will have its mark on the forest for centuries to come,” says Bogdziewicz.

Disease outbreaks in animals have led to more trees in other places. “One famous example is from anthrax epidemics in Tanzania that drastically reduced impala populations and created a window for tree establishment,” he says.

“The paper expands on the classic disease triangle of disease, host and environment to consider the knock-on impacts of the disease on other organisms in the environment,” says at the University of York in the UK. “No doubt, the cascading effects of reduction in wild boar numbers and increases in oak recruitment will continue to play out for many years.”