A highly aggregated geographical distribution of forest pest invasions in the USA

Abstract

Aim: Geographical variation in numbers of established non‐native species provides clues to the underlying processes driving biological invasions. Specifically, this variation reflects landscape characteristics that drive non‐native species arrival, establishment and spread. Here, we investigate spatial variation in damaging non‐native forest insect and pathogen species to draw inferences about the dominant processes influencing their arrival, establishment and spread. Location: The continental USA, including A laska (H awaii not included). Methods: We assembled the current geographical ranges (county‐level) of 79 species of damaging non‐indigenous forest insect and pathogen species currently established in the continental USA. We explored statistical associations of numbers of species per county with habitat characteristics associated with propagule pressure and with variables reflecting habitat invasibility. We also analysed relationships between the geographical area occupied by each pest species and the time since introduction and habitat characteristics. Results: The geographical pattern of non‐native forest pest species richness is highly focused, with vastly more species in the north‐eastern USA. Geographical variation in species richness is associated with habitat factors related to both propagule pressure and invasibility. Ranges of the non‐native species are related to historical spread; range areas are strongly correlated with time since establishment. The average (all species) radial rate of range expansion is 5.2 km yr−1, and surprisingly, this rate did not differ among foliage feeders, sap‐feeders, wood borers and plant pathogens. Main conclusions: Forest pest species are much more concentrated in the north‐eastern region of the USA compared with other parts of the country. This pattern most likely reflects the combined effects of propagule pressure (pest arrival), habitat invasibility (pest establishment) and invasion spread. The similarity in historical spread among different types of organisms indicates the importance of anthropogenic movement in spread.