Hidden patterns of insect establishment risk revealed from two centuries of alien species discoveries

Abstract

Understanding the socioeconomic drivers of biological invasion informs policy development for curtailing future invasions. While early 20th-century plant trade expansions preceded increased establishments of plant pests in Northern America, increased establishments did not follow accelerating imports later that century. To explore this puzzle, we estimate the historical establishment of plant-feeding Hemiptera in Northern America as a function of historical U.S. imports of live plants from seven world regions. Delays between establishment and discovery are modeled using a previously unused proxy for dynamic discovery effort. By recovering the timing of pest arrivals from their historical discoveries, we disentangle the joint establishment-discovery process. We estimate long delays to discovery, which are partially attributable to the low detectability of less economically important insect species. We estimate that many introduced species remain undiscovered, ranging from around one-fifth for Eurasian regions to two-fifths for Central and South America.

Fig. 1.. Value of live plants imported into the United States and established non-native Hemiptera species discoveries in Northern America by biogeographic region of origin.

The map illustrates biogeographic regions that are the source for both annual plant material imports (A) and non-native plant-feeding Hemiptera species discovered (B) from 1854 to 2012. The widths of lines drawn from each region to the Nearctic region are scaled to cumulative live plant imports, and the map numbers in white indicate the cumulative non-native Hemiptera species from the region from 1800 to 2012. Values are disaggregated by source region shown in 5-year increments (except the first increment, 1854–1857). Data sources are described in section S1.

Fig. 2.. Schematic of model variables used to estimate the establishment and discovery rates of non-native Hemiptera species over time (years).

Fig. 3.. Non-native species discoveries and estimated establishments over time and by biogeographic region of origin.

Cumulative observed discoveries, fitted discoveries, and estimated establishments of non-native Hemiptera species aggregated across regions, 1800–2012 (A). The cumulative observed discoveries and gap between observed discoveries and estimated establishments (“establishment debt”) are reported as both levels and percentages for 2012. The observed discoveries and establishment debt (in levels and percentages) in 2012 are reported by region in (B).

Fig. 4.. Imports, estimated establishments, and estimated marginal risk over time in aggregate and for select biogeographic regions of origin.

Annual nursery product import values (log scale), estimated marginal (establishment) risk per additional $1 million ($2015) of imports, and estimated establishments, by exporting region (A to F) for years 1855–2012. All series are depicted as 10-year averages except for years at the boundaries (1855–1860 and 2008–2012).

Fig. 5.. The probability of discovering a non-native Hemiptera species, 1800–2012.

(A) The initial probability of discovery (in the year of establishment) changes with effort. (B) The probability of discovering a species after establishment defers by reference year (legend).