Biological invasions represent serious threats to sustainable agriculture, forestry and public health throughout the world. Insects are the most diverse class in the animal kingdom and similarly non-native insect species outnumber all other invading animal species. The probability that a species may invade a new region is also strongly affected by the extent to which it may become linked to invasion pathways.
Many global insect invasion pathways have been identified and these include trade in agricultural products, movement of plant parts by international travellers, shipments of stored grain, trade in living plants, hitchhiking (e.g. on the outside of shipping containers) and wood packing material. Here we propose to analyse historical data on insect invasions from around the world and use it to:
- deduce what type of species are the “best” invaders,
- identify the major pathways by which species have historically been transported and
- reconstruct major patterns of global invasion spread.
Ultimately we anticipate that these analyses will result in a new understanding of the drivers of insect invasions.
These results are likely to have profound implications for management of invasions. Specifically, this work will identify the pathways that are responsible for the largest number of invasions and this could ultimately lead to new regulations (e.g. Quarantines) that limit these pathways. It also will provide useful insight on the traits that make certain species more likely to invade and this could be of great use in risk analyses performed by plant quarantine agencies. Finally, the analysis of habitat traits that promote invasions will provide useful information that potentially can guide forest managers and other land managers make sound decisions on how to increase habitat resistance to future invasions.
Current Research Topics:
Comparison of numbers of alien insects among world regions
During the last two centuries, thousands of insect species have been transported (largely inadvertently) and established outside of their native ranges worldwide, some with catastrophic ecological and economic impacts. Global variation in numbers of invading species depends on geographic variation in propagule pressure and heterogeneity of environmental resistance to invasions. Elton’s diversity invasibility hypothesis, proposed over sixty years ago, has been widely explored for plants but little is known on how biodiversity affects insect invasions. Here we use species inventories from 44 land areas, ranging from small oceanic islands to entire continents in various world regions, to show that numbers of established insect species are primarily driven by diversity of plants, with both native and non-native plant species richness being the strongest predictor of insect invasions. We find that at large spatial scales, plant diversity directly explains variation in non-native insect species richness among world regions, while geographic factors such as land area, climate and insularity largely affect insect invasions indirectly via their effects on local plant richness.
Drivers of global invasions by bark and ambrosia beetles
Here we assemble a global database of historical invasions of Scolytinae (bark and ambrosia beetles) species and explore factors explaining geographical variation in numbers of species invading different regions. This insect group includes several pest species with massive economic and ecological impacts and these beetles are known to be accidentally moved with wood packaging in global trade. Candidate explanatory characteristics included in this analysis are cumulative trade among world regions, size of source species pools, forest area, and climatic similarity of the invaded region with source regions. Species capable of sib-mating comprised the highest proportion on nonnative Scolytines, and these species colonized a higher number of regions than outbreeders. The size of source species pools offered little power in explaining variation in numbers of invasions among world regions nor did climate or forest area. In contrast, cumulative trade had a strong and consistent positive relationship with numbers of Scolytinae species moving from one region to another, and this effect was highest for bark beetles, followed by ambrosia beetles, and was low for seed and twig feeders. We conclude that global variation in Scolytine invasions is primarily driven by variation in trade levels among world regions. Results stress the importance of global trade as the primary driver of historical Scolytinae invasions and we anticipate other hitchhiking species would exhibit similar patterns. One implication of these results is that invasions between certain world regions may be historically low because of past low levels of trade but future economic shifts could result in large numbers of new invasions as a result of increased trade among previously isolated portions of the world.
Spatial dynamics of emerald ash borer spread in North America
We investigated the historical spread of the emerald ash borer in North America and identified correlates of county-level invasion risk. We estimated spread rates and frequency/length of long-distance dispersal across the contiguous USA from 1997-2018 and compared observed patterns with model predictions. A time-to-event model was then developed at the county-level to assess the influence of habitat characteristics and propagule pressure on invasion risk. The final model was used to forecast invasion risk across the contiguous USA.
Following the initial establishment phase, range expansion rates were biphasic, shifting to a faster, linear pattern around 2002-2003 and then declining from 2015 onwards. Populations invaded 6 – 134 new counties per year, including a mean of 14 discrete jumps per year averaging 84 ± 7 SE km. Risk of spread was positively associated with proximity to previously invaded areas, human population density, and densities of ash and non-ash forest trees but was negatively associated with temperature.