Impact of a mutualism between an invasive ant and honeydew-producing insects on a functionally important tree on a tropical island

Abstract

Mutualisms between invasive ants and honeydew-producing Hemiptera have the potential to result in unusually high population levels of both partners, with subsequent major changes to ecosystem composition and dynamics. We assessed the relationship between the invasive ant, Pheidole megacephala, and its hemipteran mutualists, Dysmicoccus sp. and Pulvinaria urbicola, on Cousine Island, Seychelles. We also assessed the impacts of the mutualism on the condition of the hemipteran host plant, Pisonia grandis, a native and functionally important tree species. There was a strong positive relationship between Ph. megacephala activity and hemipteran abundance, and the exclusion of ants from Pi. grandis resulted in a significant decline in Pu. urbicola abundance. High abundance of the mutualists was strongly associated with damage to the Pi. grandis forest. This indicates that the mutualism is contributing to the massive increase in the population levels of the mutualist species, and is intensifying their impacts on the island. The widespread trophobiosis and its associated high densities of mutualists pose serious threats to the ecosystem, highlighting the need to control the ant and associated hemipteran populations.

Effect of an invasive ant and its chemical control on a threatened endemic Seychelles millipede

“…This has been shown to be the case on several Pisonia grandis forested Australian islands , and on Palmyra Atoll, south of Hawaii (Krushelnycky et al 2005), which have undergone rapid forest declines. A similar situation is found on Cousine Island, with Gaigher et al (2011) showing that P. megacephala hemipteran mutualisms have caused significant damage to the P. grandis forests. As P. grandis forest makes up an important habitat type on Cousine, the loss of such forest would result in the decline Data range in parenthesis of many valuable endemic and keystone species including the Seychelles giant millipede.…”

“…commun.). Recently, very high densities of this ant have been observed on certain parts of Cousine Island, corresponding with hemipteran mutualisms, resulting in foliage damage, tree dieback and displacement of certain native invertebrates (Gaigher et al 2011). All these points strongly suggest that this ant is invasive.…”

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

“…To secure large quantities of carbohydrates, A. gracilipes establishes a mutualism with hemipterans, which provide a reliable source of honeydew as food (Delabie 2001;Abbott and Green 2007;Blüthgen and Feldhaar 2010). In the Vallée de Mai, hemipterans, such as introduced Pulvinaria urbicola and Dysmicoccus sp., which are invasive elsewhere in Seychelles (Gaigher et al 2011), were rarely observed on palms (CKB, pers. obs.).…”

The Ecological and Evolutionary Consequences of Alien Invasive Species

“…Further west, high densities of Big-headed ant, Pheidole megacephala , occur on Cousine Island, Seychelles, where the Hemiptera (themselves alien cosmopolitan scale insects, notably Pulvinaria urbicola and Dysmicoccus sp.) cause severe damage to indigenous trees, notably Pisonia grandis (Gaigher et al 2011 ). Increased scale insect abundance associated with increased ant abundance intensifi ed damage to the host trees, implying gradual forest decline.…”

“…The same broad Pheidole -scale insect mutualism has similar impacts on P. grandis forests elsewhere. These invasive insects and the mutualisms they then form are thus a clear threat to some key island ecosystems, and their control is an urgent conservation need (Gaigher et al 2011 ). Even such apparently clearcut associations, however, are not universal.…”