Figure 1. Flows of non-native insects between North America (NA), Europe (EU), and Australasia (AU). Numbers indicate the total count of species established from donor to recipient, with flow widths being proportional to these counts. Overlapping flows on the donor side indicate the fraction of species that established in both recipient regions.
It's an observable, and often regrettable fact of biology that the majority of invasive species tend to be from Europe into North America and Australia and not vice versa. That’s not to say there are no migrations the other way, but the distribution is highly asymmetric, so is unlikely to be due to chance alone.
There have been several attempts to explain this in terms of the North American and Australian environment being inherently more amenable to invasion, and the European environment being less so. However, historically, Europe has been repeatedly invaded by insect species from Asia, so the latter explanation is unlikely.
It is generally assumed that insects are spread inadvertently by trade in goods, when insects 'stow away' in packages, the holds of ships, etc., but research led by Dr. Rylee Isitt of the University of New Brunswick, and published in the journal NeoBiota, shows that after accounting for patterns of international trade, the number of insects that have spread from Europe into North America, Australia, and New Zealand far exceeds expectations.
The question is then, is there something different about European insects that makes them better at invading new territories; are they more numerous? Or is there something else at work here?
The same research found no evidence for these ideas:
Figure 2. Cumulative discoveries (observed and modelled) and establishments (modelled) of non-native insects exchanged between Europe (EU), North America (NA), and Australasia (AU) versus cumulative import value (inflation-corrected to 2020 British pounds sterling, billions), 1827–2014. Alternating background shading indicates decadal increments, with shading omitted prior to the 1940s for clarity.
So, what's going on?
Dr. Isitt and collaborators have proposed that the abundance of European insect invaders may be a result of deliberate introductions of non-native plants into Europe's colonies. Plants introduced into European colonies could have promoted the spread of European insects into North America and Australia by two different means.
This is, of course, consistent with a basic prediction of the Theory of Evolution by Natural Selection. If an environmental change creates an opportunity for a species to expand its territory, then it will expand into that new territory, where if there is any competition, natural selection will determine the outcome. However, if the change in the environment is the introduction of a food resource that was previously absent, there is unlikely to be much competition, so the invasive species gets a free ride.
And of course, as Europeans have colonised new lands such as North America, Australia and New Zealand, so they have taken their traditional staple food plants with them, created a perfect environment for European insects to tag along and become established just as the human colonists and their plants have.
The research team have published their findings in NeoBiota:
AbstractIt's good to see how the basic principles of environment-led evolution (which often starts with migration into new territory) underpins the observable phenomenon of invasive insect species and what governs their movements. Earth is, of course, a dynamic and changing place where these sorts of migrations in response to environmental change have been going on for hundreds of millions of years. Recently however, Man has become a major factor in this process, the consequences of which are still being played out. One thing we can be sure of though is that there is no place for supernatural magic in the explanation for it.
The geographical exchange of non-native species can be highly asymmetrical, with some world regions donating or receiving more species than others. Several hypotheses have been proposed to explain such asymmetries, including differences in propagule pressure, source species (invader) pools, environmental features in recipient regions, or biological traits of invaders. We quantified spatiotemporal patterns in the exchange of non-native insects between Europe, North America, and Australasia, and then tested possible explanations for these patterns based on regional trade (import values) and model estimates of invader pool sizes. Europe was the dominant donor of non-native insect species between the three regions, with most of this asymmetry arising prior to 1950. This could not be explained by differences in import values (1827–2014), nor were there substantial differences in the sizes of modelled invader pools. Based on additional evidence from literature, we propose that patterns of historical plant introductions may explain these asymmetries, but this possibility requires further study.
Introduction
Non-native insects have been implicated in displacing native species, altering the composition of ecological communities, damaging economically important trees and food crops, vectoring diseases, and more (Kenis et al. 2009; Bradshaw et al. 2016). An intriguing aspect of insect invasions is that some regions appear to have donated disproportionately more non-native insects during biotic exchange than others. For example, considerably more phytophagous forest insects have invaded North America from Europe than the reverse (Niemelä and Mattson 1996.1), and Europe has contributed a large fraction of New Zealand’s non-native insect fauna (Edney-Browne et al. 2018). Consequently, the question of why such asymmetries may occur has fascinated ecologists for decades, with several mutually compatible hypotheses offered: (1) differences in the magnitude of invasion vectors, such as international trade, may lead to differences in the arrival and establishment rates of non-native species; (2) differences in the size of potential invader pools may drive differences in the numbers of species being donated to other regions; (3) environmental differences (e.g., climate and availability of host plants) in recipient regions may promote or inhibit invasion; and (4) biological traits of insects native to some regions may make them better at invading or competing than insects native to other regions (Vermeij 1991, 1996; Niemelä and Mattson 1996; Visser et al. 2016.1).
The latter two hypotheses are often tested on a single insect order or guild and at smaller spatial scales (e.g., Rigot et al. 2014; Guyot et al. 2015; Rassati et al. 2016.2), but less commonly on multiple insect orders and multiple geographical regions. Testing them requires regional knowledge of the nature of recipient environments and their ecological communities, and of the biological traits of the invaders, information that is often available only for certain regions or certain insect groups/guilds. The former two hypotheses are more approachable, given the availability of datasets on international trade, regional insect richness, and modelling approaches that can estimate invader pool sizes.
Our research goals were firstly to test for the existence of asymmetries in the cumulative numbers of insect invaders, across all taxa, exchanged between three world regions of interest: North America, Europe, and Australasia (limited to Australia and New Zealand). These regions were chosen due to their histories of anthropogenic interactions and exchange of species, existing literature suggesting asymmetrical exchange of insects between them (see above), and the availability of data. Secondly, if clear asymmetries were found, we aimed to determine if they could be explained by differences in propagule pressure (using the value of international trade as a proxy) or by differences in estimates of invader pool sizes. We did not statistically test hypotheses (3) and (4), above, but considered them as possible explanations for asymmetries that could not be explained by hypotheses (1) and (2).
Isitt R, Liebhold AM, Turner RM, Battisti A, Bertelsmeier C, Blake R, Brockerhoff EG, Heard SB, Krokene P, Økland B, Nahrung HF, Rassati D, Roques A, Yamanaka T, Pureswaran DS (2024)
Asymmetrical insect invasions between three world regions. NeoBiota 90: 35-51. https://doi.org/10.3897/neobiota.90.110942
This is an open access article distributed under the terms of the CC0 Public Domain Dedication.
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