Ecological Archives E095-179-A2

Julian Resasco, Nick M. Haddad, John L. Orrock, DeWayne Shoemaker, Lars A. Brudvig, Ellen I. Damschen, Joshua J. Tewksbury, Douglas J. Levey. 2014. Landscape corridors can increase invasion by an exotic species and reduce diversity of native species. Ecology 95:2033–2039.

Appendix B. Information on nest counts, social form determination, land use, and block orientation.

Nest counts: As a second measure of fire ant abundance, we counted the number of active, mature fire ant nests in one of each patch type per block in the summer of 2008. One of each duplicate patch was randomly selected to be surveyed in each block. We collected these data by systematically walking and inspecting the entire patch. Nest counts provide a good metric of ant density because colonies (rather than individuals) are the reproductive units of social insects (Gotelli et al. 2011). However, because fire ant nests are often cryptic and do not represent reproductive units in the case of polygyne fire ants, we used pitfall incidence as our primary measure to quantify abundance.

Social form determination: Fire ant social form is genetically determined (Ross and Keller 1998). To determine which social form was present in each patch, we collected and stored in 95% alcohol approximately 30 workers from four fire ant nests per patch, one from each quarter of each patch. DNA extractions were performed on pooled samples of 10–15 workers per nest. All extractions were done using the Puregene DNA extraction kit (Gentra Systems Inc., USA) following the suggested protocol for extracting DNA from animal tissues. Bulk-extracted DNA samples were used as the template for determination of social form, first using a diagnostic Gp-9 polymerase chain reaction (PCR) assay developed by Valles and Porter (2003) and then using a second series of PCR assays that more reliably distinguish variation within the class of alleles associated with the expression of polygyny (b-like alleles) not detected by the first assay (see Shoemaker and Ascunce 2010, Yang et al. 2012 for details regarding social form determination using the informative gene Gp-9).

Land-use history: Soil disturbance is important for the establishment of fire ants (Tschinkel 2006). We tested whether the polygyne form was more likely to appear in landscapes that were historically (prior to forest planting in 1951) in agriculture or in forest, using aerial photograph covering our sites prior to conversion to pine plantation forest. We found that most patches had an agricultural history (80% of patches from monogyne blocks and 67% of patches from polygyne blocks had over half of the patch area in agriculture in 1951). We found both social forms in patches with historical forest and agriculture cover. The association between presence of agricultural history and social form was not statistically significant (Fisher's exact test, P = 0.65).

Experimental landscape orientation: Orientation with respect to wind direction may be a factor, as we have found previously for wind-dispersed plants (Damschen et al. 2008), that could affect the establishment of fire ans. Stiles and Jones (1998) found that fire ants nests were more abundant along linear habitats oriented east/west. Although the orientation of our experimental blocks were randomly assigned, we tested whether orientation of linear habitats (i.e., corridors and wings) systematically differed by chance between polygyne and monogyne blocks. We measured the direction of the corridors and wings in each of our blocks using the Google Earth ruler tool, and transformed these values as the absolute value of the cosine of the corridor direction in radians. We found corridor and wing directions did not differ between monogyne blocks and polygyne blocks (two-sample t test, P's ≥ 0.68).

Literature cited

Damschen, E. I., et al. 2008. The movement ecology and dynamics of plant communities in fragmented landscapes. Proceedings of the National Academy of Sciences USA 105:19078–19083.

Gotelli, N. J., A. M. Ellison, R. R. Dunn, and N. J. Sanders. 2011. Counting ants (Hymenoptera: Formicidae): biodiversity sampling and statistical analysis for myrmecologists. Myrmecological News 15:13–19.

Ross, K. G. and L. Keller. 1998. Genetic control of social organization in an ant. Proceedings of the National Academy of Sciences USA 95:14232–14237.

Shoemaker, D. and M. S. Ascunce. 2010. A new method for distinguishing colony social forms of the fire ant, Solenopsis invicta. Journal of Insect Science 10:73.

Stiles, J. H., and R. H. Jones. 1998. Distribution of the red imported fire ant, Solenopsis invicta, in road and powerline habitats. Landscape Ecology 13:335–346.

Tschinkel, W. R. 2006. The Fire Ants. The Belknap Press of Harvard University Press, Cambridge, Massachusetts, USA.

Valles, S. M., and S. D. Porter. 2003. Identification of polygyne and monogyne fire ant colonies (Solenopsis invicta) by multiplex PCR of Gp-9 alleles. Insectes Sociaux 50:199–200.

Yang, C. C., et al. 2012. Propagule pressure and colony social organization are associated with the successful invasion and rapid range expansion of fire ants in China. Molecular Ecology 21:817–833.

[Back to E095-179]