Immigrant wild boars can benefit the forest, while native deer do the opposite
When wild boars root through the soil, invasive plant species struggle to spread, benefiting the forest ecosystem. This is the finding of a research project from Aarhus University based on data from more than 60,000 forest plots and an extensive network of wildlife cameras across the eastern United States. At the same time, the study shows that native deer are associated with a greater abundance of invasive plants and fewer naturally regenerating tree seedlings.
Wild boars are generally considered among the world's most problematic invasive mammals, having spread far beyond their native range in Europe and Asia. However, a new study from Aarhus University suggests that these immigrant animals may actually provide ecological benefits in American forests.
The large-scale study shows that immigrant wild boars are associated with fewer invasive plant species in forests across the eastern United States. At the same time, the findings indicate that native white-tailed deer hinder tree regeneration and promote the spread of the very invasive plants that many American forest managers struggle to control.
The study was published in Proceedings of the Royal Society B and led by Ming Ni, a former postdoctoral researcher at the Center for Ecological Dynamics in a Novel Biosphere (ECONOVO) at Aarhus University.
"We need to move beyond the assumption that animals and vegetation are finely tuned to one another simply because they share the same origin. It is not an animal’s birthplace that determines its role in an ecosystem, but rather its feeding habits and behavior,” says Ming Ni.
Selective deer and omnivorous wild boars
The study is based on analyses of more than 68,000 forest plots across the eastern United States, combined with data from one of the country's largest wildlife camera networks.
These extensive datasets reveal how two large herbivores, the native white-tailed deer and the immigrant wild boar, influence forest ecosystems through their feeding behavior.
Deer are selective feeders. They prefer fresh shoots and tree seedlings, while wild boars take a very different approach. As omnivores, they root through the soil in search of roots, tubers, and other plant material.
The contrasting behaviors are clearly reflected in the vegetation. Areas with high deer abundance tend to have more invasive plants and greater invasive plant diversity, whereas areas with many wild boars typically contain fewer invasive plants.
According to Ming Ni, one likely explanation is that wild boars’ rooting behavior and broad diet suppress certain invasive plants that deer generally avoid eating, either because the plants are toxic or because they offer little nutritional value.
"Our findings challenge the widespread assumption that non-native large herbivores are necessarily more harmful than native species. Their ecological effects depend to a great extent on what they eat, how they behave, and the environments in which they live,” says Ming Ni.
He emphasizes that these effects are highly context-dependent. Climate, topography, and human influence all play major roles in determining how strongly these animals affect forest ecosystems.
Adding nuance to the debate at home
In the United States, feral pigs are widely regarded as a serious invasive-species problem. In Europe, their native relatives, wild boars, are at the center of ongoing debates about growing populations, agricultural damage, and the management of disease risks.
Jens-Christian Svenning, senior author of the study and director of ECONOVO, believes that the findings from the American data are also relevant in a Danish context.
"This study shows that we need a more nuanced discussion of biodiversity and nature management. A species’ importance to an ecosystem depends not primarily on whether it is native or non-native, but on what it actually does in nature. This knowledge is important when discussing the ecosystems, biodiversity, and large animals of the future in Denmark,” says Jens-Christian Svenning.
This article was translated by Microsoft Copilot.
About the study:
Study type: Empirical macroecological study based on existing large-scale datasets.
Funding: Danish National Research Foundation; Independent Research Fund Denmark.
Link to the scientific article: https://royalsocietypublishing.org/rspb/article/293/2071/20252461/481901/Divergent-effects-of-native-deer-and-alien-wild
Contact:
Ming Ni
The study was conducted while Ming Ni was employed at the Danish National Research Foundation's Center for Ecological Dynamics in a Novel Biosphere (ECONOVO). He is currently a postdoctoral researcher at the University of Wisconsin-Madison.
E-mail: niming.anhui@gmail.com
Phone: +1 6086923110
Jens-Christian Svenning
Professor, Centre Director
Danish National Research Foundation, Center for Ecological Dynamics in a Novel Biosphere (ECONOVO), Department of Biology
Aarhus University
E-mail: svenning@bio.au.dk
Phone: +45 2899 2304