Supercolonies of the invasive yellow crazy ant, Anoplolepis gracilipes, on an oceanic island: Forager activity patterns, density and biomass

Abstract

Ants have the capacity to reach unusually high densities, mostly in their introduced ranges. Numerical dominance is often cited as key to the ability of exotic ants to displace native ant species, reduce the abundance of invertebrates and negatively impact upon bird, land crab and other vertebrate populations. On Christmas Island, Indian Ocean, the yellow crazy ant, Anoplolepis gracilipes (Jerdon), forms supercolonies, where extremely high densities of foraging ants have contributed to 'invasional meltdown' in rainforest areas. Densities of up to 2254 foraging ants per m 2 and a biomass of 1.85 g per m 2 were recorded, and nest densities reached 10.5 nest entrances per m 2 . Populations of A. gracilipes can overcome and kill red endemic land crabs (Gecarcoidea natalis) over 100 times their own biomass. This is the highest recorded density of foraging ants, and adds another element to the definition of 'supercolony' of unicolonial ants. This paper documents one extreme in a continuum of densities of unicolonial, invasive ant species and highlights the need to incorporate forager densities into invasive ant research.

Foraging activity and diet of the ant, Anoplolepis tenella Santschi (Hymenoptera: Formicidae), in southern Cameroon

“…These results corroborate those of Abbott (2005) on the congeneric tramp species, A. gracilipes, on Christmas Island, which also maintained continu- Table 1. Correlation between Anoplolepis tenella activity and air temperature, air humidity (in the dry season) and soil temperature (in the wet season).…”

“…Invasive potential in their native ranges was also demonstrated for M. opaciventris (Kenne & Dejean 1999) and A. custodiens (Addison & Samways 2006). Since invasive features are also found in A. custodiens (Addison & Samways 2006) and A. gracilipes (Abbott 2005), they probably occur throughout the genus.…”

“…Many studies of ant activity rhythms have been conducted at the entrance of the nest (Gordon 2002) or on trails or trenches (Lévieux 1983;Kenne & Dejean 1999), but as in A. gracilipes on Christmas Island, A. tenella rarely used observable trails (Fotso Kuate et al 2006). Thus, a plot-based counting method was used and was more appropriate for the estimation of densities of ant foraging on the ground (Abbott 2005). Based on the highest mean number (157.7) of ants counted in 1 m 2 during this study, A. tenella density in cassava fields could reach a maximum of ≈1.6 million ants/ha, which is low compared to reported densities of A. gracilipes of 5 million ants/ha in the Seychelles (Haines & Haines 1978) and 20 million on Christmas Island (Abbott 2005), but is higher than that of M. opaciventris, which reached a density of 72 400 foraging individuals per hectare (Kenne & Dejean 1999).…”

Endemic predators, invasive prey and native diversity

“…On other islands, A. gracilipes affects litter decomposition and nutrient cycling [26], trophic cascades [7] and even the tourism industry (via displacement of a bird species [27]). These negative impacts may, however, take several decades to unfold fully; on Christmas Island, yellow crazy ant populations exploded 60 years after its introduction [28].…”

Mechanism for rapid passive-dynamic prey capture in a pitcher plant

“…This makes it difficult for insects to avoid rain altogether. Insect activity tends to temporarily decrease during heavy rain (43)(44)(45)(46); however, this might not affect capture rates if pitcher visitors are already present at the onset of heavy rain. In addition, N. gracilis might benefit from the tendency of insects to seek shelter from rain on the underside of leaves.…”