EPPO Global Database

EPPO Reporting Service no. 10 - 2011 Num. article: 2011/230

Pathways of introduction: unintentionally introduced species as a threat to natural areas

Understanding the dimensions of pathways of introduction of alien plants is important to regulate species invasions. Among 1007 neophyte species recorded in the Czech Republic, 93 (6.7%) were released into the wild (from horticulture), 599 (43.1%) are escapes from cultivation, 443 (31.9%) were introduced unintentionally as contaminants and 254 (18.1%) were introduced as stowaways. These numbers reflect the fact that some species were introduced by multiple pathways.
Species released into the wild were more likely to be trees and shrubs than other life forms. The probability of a species to be introduced by escape from the cultivation pathway was generally lower for grasses than other species. It was also higher for perennials than for annuals, but for both the probability increased with longer residence time. The probability for a species to be introduced as a contaminant was high for annuals.
The 4 pathways differ strikingly in the number of species introduced that became naturalized or invasive, and are presented in the table below:

Percentage of naturalized species
Percentage of invasive species
Deliberate releases
Escapes from cultivation

Deliberate releases thus yielded more invasive species.
It was also noted that the naturalized species introduced as contaminants were recorded in significantly more semi natural habitats than those introduced by other pathways.
As a consequence, invasions by unintentionally introduced plant species need to be considered by management as seriously as those introduced by horticulture, because they invade a wide range of semi natural habitats, hence representing an even greater threat to natural areas.


Pyšek P, Jarošik V, Pergl J (2011) Alien plants introduced by different pathways differ in invasion success: unintentional introductions as a threat to natural areas. Plos One 6, 24890.