Yellow crazy ant (Anoplolepis gracilipes) invasions within undisturbed mainland Australian habitats: no support for biotic resistance hypothesis

Abstract

Ants are highly successful invaders, especially on islands, yet undisturbed mainland environments often do not contain invasive ants, and this observation is largely attributed to biotic resistance. An exception is the incursion of Yellow crazy ant Anoplolepis gracilipes within northeast Arnhem Land. The existence of A. gracilipes within this landscape's intact environments containing highly competitive ant communities indicates that biotic resistance is not a terminally inhibitory factor mediating this ant's distribution at the regional scale. We test whether biotic resistance may still operate at a more local scale by assessing whether ecological impacts are proportional to habitat suitability for A. gracilipes, as well as to the competitiveness of the invaded ant community. The abundance and species richness of native ants were consistently greater in uninfested than infested plots but the magnitude of the impacts did not differ between habitats. The abundance and ordinal richness of other macro-invertebrates were consistently lower in infested plots in all habitats. A significant negative relationship was found for native ant abundance and A. gracilipes abundance. No relationships were found between A. gracilipes abundance and any measure of other macro-invertebrates. The relative contribution of small ants (\2.5 mm) to total abundance and relative species richness was always greater in infested sites coinciding with a reduction of the contribution of the larger size classes. Differences in the relative abundance of ant functional groups between infested and uninfested sites reflected impacts according to ant size classes and ecology. The widespread scale of these incursions and non-differential level of impacts among the habitats, irrespective of native ant community competitiveness and abiotic suitability to A. gracilipes, does not support the biotic resistance hypothesis.

Invasion of yellow crazy ant Anoplolepis gracilipes in a Seychelles UNESCO palm forest

“…There is little and ambiguous information on the role of native ant species in conferring biotic resistance to the invasion of exotic ant species. Both Way (1953) and Hoffmann and Saul (2010) concluded that habitat suitability, and not the presence of native ants, is the strongest predictor of invasive A. gracilipes distribution. The invasion of other ants, such as the Argentine ant L. humile in Portugal, however, was prevented by dominant native ants in cork oak plantations and pasture habitat (Way et al 1997).…”

“…Hoffmann and Saul (2010) showed that habitat suitability is the primary determinant of A. gracilipes incursions, and climatic variables, especially temperature and humidity, are important factors determining ant distribution (Torres 1984;Porter 1988;Hölldobler and Wilson 1990;Krushelnycky et al 2005). In the Vallée de Mai, higher humidity and temperature and lower canopy cover were correlated with A. gracilipes presence.…”

“…Their invasion is of concern to conservationists due to their broad range of impacts (e.g., Holway et al 2002;O'Dowd et al 2003;Lach and Hooper-Bui 2010) and the difficulties of eradicating them once established (Silverman and Brightwell 2008;Hoffmann et al 2011). Impacts of invasive ants can include direct effects such as displaced vertebrate (Feare 1999;Holway et al 2002 and references therein), nonant invertebrate (Lubin 1984;Hill et al 2003;Causton et al 2006), and ant species (Walker 2006;Hoffmann and Saul 2010;Roura-Pascual et al 2010), and indirect effects on key ecological functions such as frugivory (Davis et al 2010), pollination and seed dispersal (Hansen and Müller 2009), and seedling recruitment and litter breakdown in a forest ecosystem (O'Dowd et al 2003). The yellow crazy ant Anoplolepis gracilipes is ranked amongst the top 100 worst global invasive species (Lowe et al 2000) and is responsible for catastrophic ecological impacts on islands (O'Dowd et al 1999;O'Dowd et al 2003).…”

Targeted Research to Improve Invasive Species Management: Yellow Crazy Ant Anoplolepis gracilipes in Samoa

“…In northern Australia, 62% of species collected co-existed with A. gracilipes and the relative contribution of ants smaller than A. gracilipes to total abundance and species richness was always greater in infested sites [26]. All quantitative studies within the Seychelles have found many ant species smaller than A. gracilipes coexisting with the invader, but not the considerably larger species O. simillimus [40], [48], [49].…”

Recent behavioural and population genetic divergence of an invasive ant in a novel environment

“…Unlike A. gracilipes invasions in Samoa (Savage et al, 2009) and on Christmas Island (O'Dowd et al, 2003), carbohydrate resources are not an obvious driver of differences in abundance (authors' unpublished data), nor are there clear associations with anthropogenic disturbance (Hoffmann & Saul, 2010). Unlike A. gracilipes invasions in Samoa (Savage et al, 2009) and on Christmas Island (O'Dowd et al, 2003), carbohydrate resources are not an obvious driver of differences in abundance (authors' unpublished data), nor are there clear associations with anthropogenic disturbance (Hoffmann & Saul, 2010).…”