Impacts
Argentine ants frequently displace other ant species, which can have flow-on effects on native systems. Strong competitive ability, together with a broad diet, mean that through direct predation , competition, interference, and egg predation, Argentine ants interact with many invertebrate species. They feed extensively on the honeydew produced by homopterans and actively disperse the homopterans and protect them from predation. L. humile ranks highly as a domestic nuisance species. They invade houses and are capable of penetrating food containers. They infest gardens, making outdoor dining difficult
Native systems
Summary of the main documented impacts of L. humile in native systems
| Modification | Impact on | Mechanism | Selected References |
I. Community structure |
Ant diversity |
Interference competition and food monopolisation |
Human & Gordon 1996; Holway 1999 |
Abundance and diversity of other invertebrates |
Interference and resource competition; predation on eggs, larvae and adults |
Cole et al. 1992; Way et al. 1992; Human & Gordon 1997 |
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Abundance of vertebrates |
Interference and resource competition |
Suarez et al. 2000 |
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II. Community processes |
Pollination |
Competition for nectar with effective pollinators |
Buys 1987; Visser et al. 1996 |
Seed dispersal /Regeneration |
Displacement of specialist ants that have co-evolved to assist seed dispersal and seedling germination |
Bond & Slingsby 1984; Giliomee 1986 |
|
Decomposition/ nutrient cycling |
Changing the guild structure of the invertebrate community |
Ward 1987; de Kock 1990, Folgarait 1998 |
A wide range of detrimental effects of Argentine ants on native systems has been documented. Argentine ants frequently displace most ant species (e.g., Ward 1987; de Kock 1990; Cammell et al. 1996; Human & Gordon 1996; Suarez et al. 1998; Sanders et al. 2001). Changes in the ant community composition can have flow-on effects on the ecosystem as ants not only constitute a large component of the total animal biomass, but can also act as engineers affecting soil processes (Folgarait 1998).
Argentine ants are consistently better than the native ants at exploiting food resources in terms of speed of locating food, recruiting large numbers of workers to the food, and the length of foraging period (Human & Gordon 1996). Both direct interference and exploitation of food resources appear important in displacement (Human & Gordon 1996; Holway 1999). They overrun nests of larger species through physical aggression and numerical dominance, with displacement occurring within several hours (de Kock 1990). Epigaeic (above ground foraging) species are more affected than hypogaeic (below ground foraging). One epigaeic species that has been documented to survive in the presence of Argentine ants is most active in winter months (Suarez et al. 1998), another produces a defence secretion that keeps Argentine ants away from feeding sites (de Kock 1990). Those that do remain in the presence of L. humile are often “cryptic rarities” that are seldom encountered (Haskins & Haskins 1988).
The strong competitive ability of the Argentine ant, together with its broad diet, means that it will interact with many invertebrate species in any habitat where it establishes through direct predation (Human & Gordon 1997), competition, interference, and egg predation (e.g., cerambycids – Way et al. 1992). The conclusion from impact studies is that when the total ant biomass is increased following the invasion of Argentine ants, the invertebrate community is negatively affected (Human & Gordon 1997; Cole et al. 1992). However, if total ant biomass after invasion is similar to pre-invasion levels, the abundance and diversity of invertebrates (other than ants) is similar across the invasion front (Holway 1998b).
Detrimental impacts of Argetine ant invasion are documented from sites in Hawaii and California (Cole et al. 1992; Human & Gordon 1997; Bolger et al. 2000). In all these studies the invertebrate fauna was more diverse in the absence of L. humile, and the non-ant invertebrate abundance was higher. Many groups of invertebrates were entirely absent or rare in the presence of Argentine ants (e.g., muscid flies, springtails, cunipid wasps, ticks, or mites). Some taxa, mostly scavengers, were relatively more abundant in the invaded areas. Many of these were adventive species, such as some carabid beetles, non-native Isopods, Dermaptera, and Blattaria.
Ants are considered poor pollinators, so a reduction in other pollinators is likely to be detrimental to flowering plants. Buys (1987) demonstrated that Argentine ants collected large amounts of nectar from Eucalyptus spp. before native bees began foraging. Visser et al. (1996) found that the abundance and richness of pollinators on Protea were reduced when there were greater than 200 Argentine ants per infested inflorescence. They can also disrupt dispersal of seeds by native ants, resulting, for example, in lower survival of seeds after fire in the South African fynbos (Giliomee 1986; Bond & Slingsby 1984).
Argentine ants feed extensively on of the honeydew produced by homopterans. They actively disperse the homopterans and protect them from predation to maintain the food source. This may increase adventive homopterans in native habitats, interfere with native predators of homopterans, and aid transmission of diseases between plants.
Direct impacts on vertebrates are also possible. Newell and Barber (1913 in Wetterer 1998) described Argentine ants attacking and killing nesting birds: “workers swarm over young chicks in such numbers as to cause their death”. Competition for food is also highly likely and may cause the decline of some species (e.g., Suarez et al. 2000).
Human and Economic Systems
Argentine ants rank highly as a domestic nuisance species. They invade houses and are capable of penetrating food containers (Davis & Van Schagen 1993). They infest gardens, making outdoor dining difficult. When nests are disturbed, foragers will run up legs and arms, and some people are sensitive to their bite. They have the potential to carry and hence spread disease (e.g., Staphylococcus, Candida, and Enterococcus) around buildings, including hospitals (Fowler et al. 1993).
L. humile has the potential to cause economic losses through:
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the dispersal of homopterans (e.g., scales, aphids) and their protection, which reduced homopteran losses from predation. This reduces the quality of crops and disrupts biological control (e.g., citrus – Davis & Van Schagen 1993; Vineyards – Vega & Rust 2001);
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holes chewed in plastic drip irrigation pipes that have caused losses in orchards (Chang & Ota 1990);
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contamination of food products (Van Schagen et al. 1993);
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the robbing of bee hives and predation of bees, which affect honey production and pollination industries (Davis & Van Schagen 1993; Vega & Rust 2001);
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disruption of the poultry industry through stress on chickens and killing of hatchlings (Davis & Van Schagen 1993);
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trade restrictions as a result of contamination of exports to countries that do not currently have Argentine ants e.g., China, and Korea (Davis & Van Schagen 1993);
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their potential role in the transmission of pathogens from one plant to another through their feeding on sugary exudate (El-Hamalawi & Menge 1996) and the transfer of sap-feeding Homoptera.
Predicted impacts in New Zealand
We predict the total ant biomass is likely to increase at sites where Argentine ants establish, and community assemblages will be modified. Many endemic species will be adversely affected and localized extinctions are likely, placing species with restricted distribution at risk. Particularly at risk are those that occur in coastal/scrub vegetation in northern New Zealand (Harris et al. 2002).
There are several cryptic hypogaeic species in New Zealand (all adventive) that potentially could coexist, but these are forest species and consequently are likely to have little overlap in distribution with Argentine ants. The adventive species, Cardiocondyla minutior, newly established in Mt Maunganui may well coexist with Argentine ants (Harris & Berry 2001). No native ants are at risk of extinction, because of the wide distribution of the native ant species and their occurrence in indigenous forest.
Argentine ants will compete strongly with other species for carbohydrate resources, which could have flow-on effects for plant pollination. Seed dispersal is unlikely to be affected directly, as seeds are only a small component of the diet of L. humile, and plants with seed adapted for ant dispersal are absent.
Attacks on nesting birds have been observed in New Zealand (V. Van Dyk pers. comm.) and impacts on other vertebrates, such as geckos, are likely.
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