People and Plants on the Move

Social Factors and Exotic Plant Invasions in the United States

Chinese tallow tree, a fast-spreading nonnative exotic plant. Photo by Jim Miller, courtesy of

New USDA Forest Service research using improved data from previous studies on exotic plant species in the United States shows that social factors such as human population and time of settlement play a greater part in the spread of exotic species than the natural factors such as temperature and area that affect native plant populations. The study was published in the open access journal NeoBiota.

Ongoing research on biotic invasions around the world constantly increases data availability and improves data quality. The new study led by Qinfeng Guo, research ecologist with the Forest Service Eastern Forest Environmental Threat Assessment Center (EFETAC) shows how using improved data can change the understanding of patterns of species naturalization, biological invasions, and their underlying mechanisms.  

Over the centuries, people have brought uncounted numbers of nonnative or exotic plant species to the United States for a range of purposes. Many others were introduced accidentally. Usually cultivated for food or ornamental purposes, most of these plants are considered naturalized” when they reproduce and sustain populations over many generations without direct help from humans.

“About 10 percent of naturalized plant species usually become invasive, possibly more over time,” says  Guo. “This means that they begin to spread considerable distances from the parent plants, often crowding or shading out native plant populations.”

Guo and fellow researchers used newly added and improved data to look more closely at plant naturalization patterns across the United States. Focusing on the state level, they correlated previously used independent variables such as human population and land area with comparable-level data on native and exotic plant richness.

“When we looked at the additional variables for which data just became available, some different patterns emerged, some significant,” said Guo. “We found that though the number of native plant species is largely controlled by natural factors such as area and temperature, exotic species are predominantly influenced by social factors such as human population levels and dates when states were first settled.”

The researchers concluded that in the United States the positive effect of human population on the number of exotic species is probably associated with the primary sources and points of introduction, along with human-assisted dispersal. They also found a strong relationship between foreign and domestic exotic richness, which might indicate that exotic plants exhibit similar patterns and mechanisms of naturalization across the 48 United States despite their different origins.

Though this particular study was conducted primarily at the state level, different statistical methods produced remarkably similar results regardless of spatial correlation. “However, the greater challenge ahead is how to properly handle the greater numbers of variables that come with increased data availability,” said Guo. “Caution is needed when dealing with this data at other spatial scales such as county and county-level.”

Access the article on the SRS website.

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