The introduced wild pigs – feral pigs, wild boars, and their mixes – were associated with lower abundance and lower species richness of invasive plants in forest understories. In contrast, native white-tailed deer were associated with reduced abundance of native tree seedlings, although their effects on seedling richness were more complex.
The study has just been published in Proceedings of the Royal Society B and was led by Ming Ni, at the time a postdoctoral researcher at the Center for Ecological Dynamics in a Novel Biosphere (ECONOVO) at Aarhus Universitet.
“We need to move beyond the idea that animals and forests are automatically maladapted to each other simply because they do not share the same origin. Our findings support the idea that ecological impact cannot be predicted from origin alone; feeding behavior, functional traits and environmental context also matter greatly,” says Ming Ni.
Selective deer and omnivorous pigs
The study is based on analyses of more than 68,000 forest plots (for deer; hereof more than 32,000 plots for pigs) across the eastern United States, combined with data from Snapshot USA 2021, the largest annual national mammal camera-trap survey in the United States to date.
The extensive dataset reveals how two major herbivores – the native white-tailed deer and the introduced wild pig – shape forests in very different ways.
Deer are selective feeders. They prefer foliage and seedlings of certain species, while wild pigs forage differently. As omnivores, they root through the soil in search of roots, plants and other food sources.
The animals’ contrasting behaviors are clearly reflected in the vegetation: areas with many deer contained more invasive plants and greater invasive plant diversity, while areas with many wild pigs generally had fewer invasive species.
According to Ming Ni, one possible explanation is that pigs’ rooting behavior and broad diet may disadvantage some invasive plants, but this mechanism requires further testing.
“Our results challenge the widespread assumption that introduced large herbivores are necessarily more harmful than native species. Their ecological impact depends strongly on feeding behavior, functional traits, and the environments they live in,” says Ming Ni.
He also stresses that the effects were highly context dependent. Climate, topography and human influence all played important roles in determining how strongly the animals affected forest understories.
Adding nuance to the debate
In the United States, feral pigs are widely regarded as a major invasive species problem, while in Europe, their native conspecifics wild boars are at the center of intense debates over growing populations, agricultural damage, and disease-management concerns.
Jens-Christian Svenning, senior author of the study and director of ECONOVO, believes the findings are also highly relevant outside the North American context.
“This study shows that we need a more nuanced discussion about biodiversity and ecosystem management. A species’ impact on ecosystems does not mainly depend on whether it is native or non-native, but on what it actually does in nature. That insight is important when discussing the future of forests, biodiversity, and large animals in our present-day human-changed landscapes worldwide,” says Jens-Christian Svenning.
Hereby, the research contributes to the growing international debate about non-native species and their ecological roles. The researchers argue that their results support a nuanced, evidence-based approach to non-native species, recognizing that their roles may span from negative over neutral to positive for resident biodiversity as well as for ecosystem services to society.
Journal
Proceedings of the Royal Society B Biological Sciences
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