Impact of ants on native arthropods of Hawaii
The greatest loss in biodiversity resulting from ant invasion occurs among the arthropods. Ants can prey directly upon native arthropods, exclude them through interference or exploitation or competition for food resources, or displace them by monopolizing nesting or shelter sites. The striking pattern of presence and absence of a range of endemic arthropod groups, and in particular beetles, within and without habitat invaded by Pheidole megacephala led to some of the earliest conclusions about the devastating effects of this ant in Hawaii. Although already widespread, Perkins (1913, in D. Sharp [ed.], Fauna Hawaiiensis) reported that Pheidole megacephala was still invading new areas and concurrently eliminating native arthropods around the turn of the 19th century. Similarly, at Puaaluu and Oheo streams on Maui, the invasion of Anoplolepis gracilipes into low elevation riparian corridors in the 1970s is the most likely explanation for an apparently synchronized decrease of native aquatic insects. More recently, invasion of Pheidole megacephala into some forested areas of eastern Kauai appears to have heavily impacted endemic ground crickets in the genus Laupala. The dramatic increase in ant numbers observed at these Kauai sites undoubtedly affected other ground active arthropods as well; however, the degree to which this effect has penetrated forest distant from roadsides and trailsides is unclear.
Perhaps the greatest concern stemming from ant invasions in natural areas is the potential for endemic species extinction. Many Hawaiian arthropod species have very limited distributions, and the expansive unicolonial populations of the most dominant invasive ants can sometimes occupy an endemic species' entire natural range.
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Impact of ants on native ground-nesting birds of Hawaii
Ants can potentially reduce hatching success, growth rates and overall reproductive success of ground-nesting birds in Hawaii. Injuries from the aggressive, stinging tropical fire ant (Solenopsis geminata) are observed on the feet and undersides of wedge-tailed shearwater (Puffinus pacificus) chicks on some of Hawaii's offshore islands. Some injuries to the feet of chicks are severe and can result in the loss of more than 20% of the affected tissue. Pheidole megacephala also attacks the feet of shearwater chicks, although observations are limited to Kure Atoll. The Argentine ant can recruit heavily to pipped Nene (Branta sandvicensis) eggs, attacking the emerging chicks. This ant may reduce suitable habitat for the tree-nesting Palila (Loxioides bailleui) on Mauna Kea. As an exception, the Uau (or Hawaiian petrel, Pterodroma sandwichensis) appears to be unaffected by Argentine ant invasion of its nesting habitat in the cliffs high in Haleakala crater. In this case, cold temperatures within the petrel burrows likely discourage heavy ant foraging to the nest chambers.
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Impact of ants on native plants of Hawaii
Hawaii's native plants can be damaged by elevated populations of hemipterans (honey-dew collectors) in the presence of ants. In comparison to agricultural systems, however, this facultative relationship between ants and honeydew-producing insects typically appears
less dominant in natural areas, particularly in higher elevation habitats. The reason for this is unclear, because ants can occasionally be found heavily tending scales on native plants in natural areas, and on some plant species, aphid densities are higher on individuals located within ant populations as compared to those outside ant populations.
Another mechanism by which ants can indirectly affect native plants is through their impacts on pollinators. The severe reduction in larval numbers of both Hylaeus bees and Agrotis moths resulting from Argentine ant invasion at Haleakala National Park was proposed as a potential threat to the Haleakala silversword (Argyroxyphium
sandwicense macrocephalum) and other obligate outcrossing species. In addition to preying upon pollinators,
ants may exclude pollinators from flowers or exploit their resources through nectar
thieving, both of which could also reduce seed set. Anoplolepis gracilipes,
Pheidole megacephala and Linepithema humile were found to all recruit to a high proportion of ohia (Metrosideros polymorpha) flowers, with Anoplolepis gracilipes and Linepithema humile actively defending these nectar sources. Native Hylaeus bees were less likely to land on flowers occupied by Pheidole megacephala, and seed set was slightly lower in flowers visited by Linepithema humile relative to those in which ants
were excluded. While these effects were moderate in ohia, it is possible that suppressed pollinator visitation due to ant presence may be more detrimental in rare, or less fecund, native plant species.
Finally, ants may in some cases impact native plants, as well as the animals that depend on them, by interfering with biological control of invasive plants. Reimer, for example, found that Pheidole megacephala can reduce the number of thrips biocontrol agents on Clidemia hirta, and may thereby increase the plant's vigor.
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Impact of ants on agriculture in Hawaii
The most consistent and detrimental effect of ants in agriculture is an indirect one. By tending and protecting honey-dew producing Homoptera such as aphids, scales and mealybugs, ants cause great increases in the abundances of these pest insects. In pineapple, this leads to the transmission of wilt disease by a pair of mealybug species. In coffee, the presence of Pheidole megacephala, Anoplolepis gracilipes, or occasionally Linepithema humile often results in outbreaks of green scale (Coccus viridis). The large amounts of honeydew excreted by this scale promotes the growth of sooty mold, which reduces fruit production and may sometimes cause tree death. Ants also tend mealybugs in sugar cane, but the principal damage caused by ants in this crop has been their destruction of drip irrigation equipment. Other crops are also affected by ants, including cut flowers, dryland taro, beans and various orchard
fruits. In all of the cases where ants tend pestiferous insects, the primary control method involves control of the ants, not the pest hemipteran; in the absence of ants, hemipterans rarely reach pest proportions. Experimental studies in both agricultural and non-crop systems have demonstrated that ants boost hemipteran numbers by removing predators and often by interfering with parasitoids, and that in the absence of ants hemipteran densities decrease.
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1. Krushelnycky, Loope, and Reimer. 2005. The ecology, policy, and management of ants in Hawaii. Proc. Hawaiian Entomol. Soc. (2005) 37:1-25. (URL: http://www.hear.org/articles/pdfs/krushelnyckyetal2005.pdf)
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