Corresponding author: Jonathan M. Jeschke (
Academic editor: Sven Bacher
The worldwide transport of species beyond their native range is an increasing problem, e.g. for global biodiversity. Many introduced species are able to establish in new environments and some even become invasive. However, we do not know which traits enable them to survive and reproduce in new environments. This study aims to identify the characteristics of exotic ants, and to quantitatively test previously postulated but insufficiently tested assumptions. We collected data on nine traits of 93 exotic ant species (42 of them being invasive) and 323 native ant species in North America. The dataset includes 2536 entries from over 300 different sources; data on worker head width were mostly measured ourselves. We analyzed the data with three complementary analyses: univariate and multivariate analyses of the raw data, and multivariate analyses of phylogenetically independent contrasts. These analyses revealed significant differences between the traits of native and exotic ant species. In the multivariate analyses, only one trait was consistently included in the best models, estimated with AIC
Due to globalization, more and more species are being transported across the globe and introduced to regions where they did not occur before. Such species have taken step 1 of the invasion process (
A central question of invasion biology has been which species with which characteristics are the ones that establish themselves and become invasive. What separates such species from those that have not established populations in exotic ranges? Most studies focusing on this question investigated plants and vertebrates (reviewed in
Most countries and regions of the world are now populated by numerous exotic ant species. Hawaii, for instance, has no native ants but 51 exotic established ant species (
Despite this lack of formal quantitative analyses, exotic established and invasive ants are often assumed to have the following characteristics (
Using a dataset with more than 400 species, we quantitatively tested the six assumptions about the characteristics of exotic ants by comparing traits of exotic and native ant species in North America. These comparisons were done twice: once between exotic established ants and native ants, and once between exotic invasive ants and native ants. We expected that differences will be more pronounced for the latter comparison, as exotic invasive ants are the subset of exotic established ants that have successfully completed the full invasion process. If our analysis identifies traits that are related to the success of ant species in new environments, these traits should be more pronounced in the subset of invasive species.
We focused on regions that are particularly well investigated with respect to exotic ants: 14 states of the U.S. (Alabama, Arizona, Arkansas, California, Florida, Georgia, Illinois, Louisiana, Mississippi, Missouri, North Carolina, Ohio, South Carolina, and Tennessee) and one state of Mexico (Baja California). This study area has definite and naturally given borders (e.g. coastlines) and is part of the Nearctic and Neotropic bioregion, with mean annual temperatures from 10° C in northern Illinois and northern California, to 25° C in southern Baja California and southern Florida (
Our species list is based on the database
In a literature search until July 2009, we collected data on traits of the 416 ant species in our species list. The sources included scientific papers, books, websites, and personal communications with researchers (Appendix 1 and Acknowledgements). Synonyms and antiquated names of species were noticed. Tools for searching were Google, Google Scholar, Google Books, and the ISI Web of Science. The complete dataset with references for all data points is provided as Appendix 3. It includes 2536 data points from over 300 different sources; data on worker head width of 414 ant species were measured ourselves. The nine specific traits we analyzed are as follows:
A) Gyny – the degree of gyny, i.e. the number of reproductive queens (corresponding to assumption 1 mentioned in the Introduction;
B) Colony size – the mean colony size, defined as the average number of workers in a colony (corresponding to assumption 2;
C) Founding – how new nests are founded (
D) Parasitism (
The remaining traits only concern the workers of each species. As for colony size, we again excluded the two species
E) Morphs – the morphology of the workers (
F) HW (head width;
G) TL (total body length) – an alternative measure of body size in ants is total body length (
H) Reproduction – the reproductive ability of workers (
I) Stinger (n = 388) – we discriminated workers with a functional stinger (
For all metric literature data (colony size, HW1, and TL), we adopted means reported in the literature for a given species. If no mean but only an interval was reported (minimal and maximal limits for di- or polymorphic species; colony sizes for different colony ages), we calculated the mean by averaging the minimal and maximal value of each interval. If data for a given trait and species were available from more than one source, we calculated the mean by averaging across sources.
Comparisons were done between native (Nat) and exotic established (Exo) ants, and between native and exotic invasive (Inv) ants. We applied univariate analyses, multivariate analyses of the raw data, and multivariate analyses of phylogenetically corrected data. For the univariate analyses, we performed two different two-sample tests for each of our nine traits. In these tests, one sample consisted of Nat species; the other sample consisted ofExo species for the first test and of Inv species for the second test. In the multivariate analyses, the independent variables were the traits, and the dependent variable was the species category: Nat/Exo for the first comparison and Nat/Inv for the second comparison. In our analyses, we followed the approach taken by
A) For the univariate analyses, we ran two-tailed binomial tests for the binary variables parasitic, reproduction, and stinger;
B) As mentioned above, multivariate analyses can only consider those species for which data on all included variables are available. As our dataset includes empty cells, reducing the number of variables in the multivariate analysis increases the number of species in the analysis and thus the sample size. This is one reason why it is necessary to consider
C) As our data quantify traits of species that are phylogenetically related to each other, they are not independent of each other. To correct for this phylogenetic dependence, we calculatedphylogenetically independent contrasts (
The above mentioned existing assumptions on differences between exotic and native ant species are supported by the univariate analyses. As expected, ant species that are exotic in North America (Exo and Inv) are more often polygynous than native (Nat) species (assumption 1;
Results of univariate analyses, categorical traits. Differences among native (Nat), exotic established (Exo), and exotic invasive (Inv) ants in North America. Illustrated are means ± SE. Asterisks indicate significant differences between native and exotic established species, and between native and exotic invasive species: ***
Results of univariate analyses, metric traits. Differences among native (Nat), exotic established (Exo), and exotic invasive (Inv) ants in North America. Asterisks indicate significant differences between native and exotic established species, and between native and exotic invasive species: ***
Only multivariate analyses can reveal the relative importance of traits to differentiate between exotic and native ant species. In the four types of multivariate analyses that we performed, the only variable that is included in all models with substantial empirical support (Akaike weight ω
Results of multivariate analyses. Shown are multivariate regression models with substantial empirical support, i.e. with Akaike weights ω
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Colony size, founding, reproduction, morphs | 0 | 0.203 |
Colony size, founding, reproduction, morphs, gyny | 0.267 | 0.178 |
Colony size, founding, reproduction, morphs, HW | 0.686 | 0.144 |
Colony size, founding, reproduction, morphs, gyny, HW | 1.879 | 0.079 |
Colony size, founding, reproduction, morphs, stinger | 2.467 | 0.059 |
Colony size, founding, reproduction, morphs, gyny, stinger | 2.568 | 0.056 |
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Colony size, founding | 0 | 0.089 |
Colony size, founding, reproduction | 0.983 | 0.055 |
Colony size, founding, stinger | 1.011 | 0.054 |
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Colony size, founding, reproduction, morphs, gyny | 0 | 0.428 |
Colony size, founding, reproduction, morphs, gyny, stinger | 1.268 | 0.227 |
Colony size, founding, reproduction, morphs, gyny, HW | 2.350 | 0.132 |
Colony size, founding, reproduction, morphs, gyny, HW, stinger | 3.752 | 0.066 |
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Colony size, reproduction | 0 | 0.202 |
Colony size, reproduction, HW | 1.264 | 0.107 |
Colony size, reproduction, founding | 1.762 | 0.084 |
Colony size, reproduction, morphs | 2.267 | 0.065 |
Colony size, reproduction, gyny | 2.295 | 0.064 |
Colony size, reproduction, stinger | 2.300 | 0.064 |
We examined traits of exotic and native ant species in North America in order to test previously postulated but insufficiently tested assumptions on the characteristics of exotic ant species. Our results support all investigated assumptions. Thus indeed, colonies of exotic ant species have more reproducing queens (polygyny; assumption 1) and more workers (assumption 2) than colonies of native species; they form new nests more frequently in a dependent way than native species (assumption 3a); parasites are found less frequently among exotic than among native species (assumption 3b); and the workers of exotic species are more frequently monomorphic (assumption 4), smaller (assumption 5), and more frequently sterile than the workers of native species (assumption 6). As expected, differences between exotic invasive and native species are more pronounced and in the same direction than differences between exotic established and native species.
Of the six assumptions, only assumptions 2 and 5 were, to our knowledge, previously tested (see Introduction). Assumption 2, which says that exotic ant species tend to form larger colonies than native species, has been previously tested by
Our results also support assumption 5, which says that the workers of exotic ant species are smaller than those of native species, and are in line with previous tests of this assumption by
Our results show that exotic ants have a suite of characteristics that separate them from the native ant fauna of North America (
Summary of the combined results. Listed is the suite of traits that characterize exotic as compared to native ants, based on this study’s results.
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Large colony size |
New nests founded by queen with workers† rather than queen alone |
Not socially parasitic on other ants |
Sterile workers |
Monomorphic workers |
More than one queen per colony (polygyny) |
Small body size |
Equipped with a functional stinger |
†Dependent nest founding, either via budding, splitting, sociotomy, or fission.
In addition to colony size, the sterility of workers is an important characteristic of exotic ant species in North America, which is in line with the literature (
Besides testing previously postulated assumptions, we also investigated if workers of exotic ant species differ from those of native ant species in the presence of a functional stinger. Our analyses revealed such a difference, showing that workers of exotic ant species are more frequently equipped with a functional stinger than workers of native ant species. Within exotic species, a functional stinger is more frequent in exotic invasive than exotic established species. These findings support the line of thought mentioned above that a functional stinger is a weapon that helps to survive in an exotic environment. Our dataset also showed a significant relationship of stinger presence with worker body size: workers of species with a functional stinger are smaller (mean head width in mm = 0.788, SE = 0.026,
A weakness of our dataset is that it does not include information on ant species that were introduced to North America but did not establish there. Even although sub-samples of ant species introduced to North America exist (
Another weakness of this study is that it can only provide correlative patterns rather than causative findings. For example, our finding that exotic ants have larger colonies than native ants does not necessarily imply that they successfully established
We thank S. Foitzik, K. Ivanov, R.A. Johnson, L. Keller, J.T. Longino, T. McGlynn, J. Ness, G.C. Snelling, P.S. Ward, and V. Witte for providing us with important advice and information. J.D. Olden, V. Witte, and two anonymous reviewers provided comments on the manuscript. Financial support was received from the Deutsche Forschungsgemeinschaft (JE 288/2-1, JE 288/4-1).
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