The Role of Humans in U.S. Plant Invasions

Disturbance hypothesis for plant hybridization supported

Lonicera x bella, also called Bell's honeysuckle, is a hybrid between two exotics, Lonicera morrowii and Lonicera tatarica. Unlike many exotic hybrids, it is included on the invasive plants lists of 14 states. Photo by Joseph Berger, courtesy of Bugwood.org.
Lonicera x bella, also called Bell’s honeysuckle, is a hybrid between two exotics, Lonicera morrowii and Lonicera tatarica. Unlike many exotic hybrids, it is included on the invasive plants lists of 14 states. Photo by Joseph Berger, courtesy of Bugwood.org.

Plant breeders have produced hybrids for centuries, maybe even millennia, crossing genetically different varieties or species to accentuate desired traits. Plants continuously hybridize on their own, either within populations of their own species or across species, families, and even genera.

As exotic introduced plants began aggressively spreading into areas where they weren’t wanted, plant biologists and others started looking more closely at the effects of human activities on plant hybridization. Over half a century ago, two scientists came up with the “disturbance hypothesis,” which proposes that disturbances from human activities promote hybridization by creating habitats hybrids can persist in.

Though the hypothesis is widely accepted and proven in small-scale studies, the connection between human disturbance and hybridization hasn’t been satisfactorily corroborated at regional or national scales.

Until now, that is.

U.S. Forest Service scientist Qinfeng Guo analyzed huge datasets from plant, population, weather, and other sources to reveal that hotspots of hybrid plants occur in areas with large human populations and with many years of European settlement, supporting the disturbance hypothesis.

In an article recently published in the journal Biodiversity Research, Guo, research ecologist with the Forest Service Eastern Forest Environmental Threat Center, reports findings from his study, which is the first to analyze the richness and distribution of hybrid plants at the county level across the contiguous U.S.

“Most previous studies on hybridization focused on individual hybrids, often in comparison with their parent species, but not on overall richness or the distribution of hybrids,” said Guo. “I examined reported occurrences of three types of hybrid plants—native to native, native to exotic, and exotic to exotic—in 3107 counties across the U.S.”

Hybridization can take place both within (intra-) and between (inter-) species. Guo focused on interspecies hybridization, that between species. He defined the native or exotic status of a species relative to the boundaries of each surveyed county, rather than those of the state or entire U.S., thus considering a finer resolution of native and exotic plants than typically used.

To look at possible causes of plant hybridization, he looked at hybrid richness (number of different species present) in relation to climate variables, mean elevation, human population density, years in the Union, and presence or absence of the most recent glaciation. (Short-distance advances and retreats of the last glacier resulted in numerous native to native hybridizations.) He used human population as the indicator of intensity and magnitude of human disturbance.

Guo found a total of 1126 named hybrids in the U.S., 941 formed by two native parent species, 138 by two exotic parents, and 47 by one native and one exotic parent species. The hybrids formed from introduced species made up over 16 percent of overall hybrid plants in the U.S.

“There was a great deal of variation in hybrid richness at the county level,” said Guo. “Over half the counties had only two to six hybrids, while 57 counties had 30 or more. The three hotspots, areas that contained the greatest proportion of hybrids, were the northeast corner of New England, the Great Lakes region, and the coast of California, all areas with large human populations and with a long period of European settlement.”

“In contrast, many states in the southeastern U.S. and some border states have high exotic plant richness, but don’t necessarily have more hybrids, either of exotic plants or natives, probably because of a shorter history of colonization and a lesser degree of human disturbance.”

The three regions with high hybrid richness—the Northeast, Great Lakes, and coastal California—also have the highest population densities, largest travel and trading ports, and longest histories of European settlement associated with the most intense developments.

Guo’s study not only establishes the connection between humans and hybridization, supporting the disturbance hypothesis, but also provides important baseline data for future studies and conservation efforts.

Read the full text of the article.

For more information, email Qinfeng Guo at qguo@fs.fed.us.

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