Polish primeval forest affected by boar decline: paper

A group of Polish scientists have studied the effects of a deadly disease that disseminated the number of wild boars in the Białowieża primeval forest in the northeast of the country and concluded that the effects on the local biodiversity of the area – particularly oak tree regeneration – could be affected for many years to come.

A decline in the feral pig population has a strong effect on the dynamics of oak regeneration in the Białowieża Forest, argue the authors of a new paper published in “Proceedings of the Royal Society of London B”.

As a result of the African swine fever outbreak – which killed tens of thousands of wild boars in 2015 – fewer feral pigs are eating acorns and thus oaks can cope with regeneration on their own, producing more seeds during the so-called seed years.

As a result of African swine fever (ASF), the wild boar population in Europe has fallen dramatically, by as much as 90 percent in some regions. As the scientists led by Prof. Michał Bogdziewicz from the Adam Mickiewicz University in Poznań point out, the ecological consequences of this phenomenon are largely unknown.

“Data from ungulate exclosures demonstrated that ASF led to a reduction in acorn predation,” the scientists wrote. “Tree seedling data indicated that oak recruitment increased twofold relative to the pre-epidemic period. Our results showed that perturbations caused by wildlife disease travel through food webs and influence forest dynamics.

“The outbreak of ASF acted synergistically with masting and removed herbivore top-down control of oaks by mobile consumers. This illustrates that the ASF epidemic that currently occurs across Europe can have broad effects on forest dynamics.”

Prof Rafal Zwolak was quoted by PAP as saying that this strategy is effective for specialised seed consumers, such as many insect species that cannot move far in search of food in non-seed years. “For more mobile seed consumers, such as wild boar, seed years are a less effective escape strategy because large animals forage over large areas, finding alternative food sources when seeds are scarce. Then they return.”

Scientists have shown that the large wild boar population strongly limited the regeneration of the pedunculate oak (Quercus robur) in the period before the ASF outbreak.

"In 2015, an ASF outbreak occurred in Bialowieza, which led to a severe decline in wild boar numbers. The densities of these animals dropped from 4-5 individuals per square kilometre to less than one," says Dr Tomasz Podgórski, an ecologist from the Bialowieza-based Institute of Mammal Biology of the Polish Academy of Sciences and the Czech University of Life Sciences.

He added that after the virus hit the forest and led to the extermination of most wild boars in the region, seed years were able to saturate consumers. The seed year, which took place in 2018, came at the right time, allowing abundant oak regeneration.