The undesired ant introductions

Ant closeup 6 charred wood
Ant closeup 6

The undesired ant introductions

How some ants become introduced species

Every day, millions of tons of merchandise are being transported around the world by planes, ships, trains, trucks or even in your luggage. But, sometimes, they may contain some unexpected hitchhikers inside, that unknowing, started an emigration journey.

Perhaps you have already heard of the term ‘introduced species’. Introduced species (or alien species, exotic species) refer to species which have been introduced to places beyond their native living range. It may be deliberate (e.g., as food, working animals or pet, or used as a biological control agent against pest in agriculture), or accidental through human activity, particularly through trade and commerce. For ants, partial or even full colony of ants being transported to new places through cargo is the most common mean of accidental introduction. Newly-mated queen may have founded their colony at the corner of the cargo, or inside a tree that happened to be logged, or inside the soil loaded onto a truck together with some harvested crops or potted plants someone bought online. In most cases, these undesired hitchhikers won’t be able to survive and establish in the new territory. But, as these events happen frequently enough, unfortunately, some will eventually succeed.

Successful introduced ant species often have one or more of the following traits: small size, polygyny (more than one egg-laying queen per colony) and colony reproduction by budding (a group of workers and queens with brood leaving their original colony and forming a new colony nearby). Small size means that they are less conspicuous and able to nest in small space, leading to higher chance to be transported undetected. Being polygynous means that the ant colony has higher chance to survive and to quickly increase colony size after being introduced into a new area. Colony reproduction by budding means that the ants have better chance to spread in the introduced range. Some species even show what is known as unicoloniality, which means workers from different colonies are not aggressive against each other, as long as they are from the same species. This would allow them to form massive supercolonies with numerous interconnected nests and millions to billions of individuals, spanning dozens to even thousands of kilometers. One extreme example would be the global supercolony of Argentine ant (Linepithema humile) ranging across Japan, Europe and America. Yes, that is right, some of the Argentine ants from the opposite sides of the globe are part of a single supercolony!

Tramp? Pest? Invasive?

Below are a few terms which also refer to species that have been found associated with human activities and may be used loosely interchangeably with ‘exotic species’ or ‘introduced species’. However, strictly speaking, their meanings do not fully overlap:

  • Tramp species. This refers to species that have been found associated with frequently disturbed habitats, such as urban parks, forest edge, managed forest or plantations, urban environment and households. A species can be a tramp species but not an exotic species, for it may already be commonly found in disturbed habitats or urban environment in its native range but has not been introduced to anywhere else (though it may just be a matter of time).
  • Pest species. This refers to species that have become a source of nuisance to people. They may be either household or agricultural pests. Household pest can be found nesting and foraging in buildings and household, and may partially or fully dependent on human for food. Besides their presence within houses which can be perceived as an annoyance, sometimes they can nest around electric wires and create short circuit, or even carry pathogens around. Agricultural pests damage crops directly, or indirectly by tending and protecting other sap-sucking insects (such as aphids) in exchange for honeydew. Like tramp species, species can already be a pest species without being introduced to a new region beyond its native range.
  • Invasive species. This only refers to species that have a profound ecological impact on the native fauna and flora following their introduction. Their impacts may be direct on native species, through competition or predation, or indirect by forming mutualistic relationship with other invasive species. Fortunately, the majority of introduced ant species do not become invasive (only about 16 out of ~500), for they may only form small colonies or have a relatively cryptic lifestyle, and thus have little impact on local ecosystems. However, for introduced ants with high level of aggressiveness, or in an environment where introduced ants face little to no competition (such as on small islands), they may become invasive and drastically alter the local ecosystem.
Introduction tramp
Common tramp ant species in Hong Kong: Anoplolepis gracilipes (top), Paratrechina longicornis (middle) and Tapinoma melanocephalum (bottom) under the same magnification. Although they can be found in disturbed environment, none of them is invasive species here in HK.

Hong Kong, as a major trade hub for over a century, has become a hotspot for the establishment of introduced species and their spreading into nearby regions. There are currently over 40 reported introduced ant species in Hong Kong, with the earliest record of an introduced species being in the early 20th century. Due to the long history of introduction, many of the introduced ants are well-established in Hong Kong and can be found throughout the territory, including on numerous islands. While some species may be known as household pest, most of them do not have a profound impact on the local fauna. One exception is the infamous red imported fire ants (Solenopsis invicta) detected for the first time in Hong Kong in the early 2000s.

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Glossary

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A

Alate queens / alate males. Queens or males that are born with wings and are capable of flying. After mating, queens would shed their wings and are now known as dealate queens.

Antennal club. Apical segments of the antenna that are distinctly enlarged.

Antennal scape. The basal-most segment of the antenna that is elongated.

Arboreal. Nesting and/or active on trees.

Army ants. Ants that do not build permanent nest and are constantly on the move. They often forage collectively in mass.

B

Brood. Non-adults, including eggs, larvae and pupae.

Budding. New queens leaving with some workers and brood from the original colony and found a new colony nearby. In contrast with alate (winged) queens founding a new colony alone after mating.

C

Castes. Roles of ants in the colony, usually predetermined before they become adults. Defined by sex, morphology and reproduction capability of the ants. Further subdivisions based on minor differences in morphology and role are known as subcastes.

Clypeus / clypeal margin. The anterior part of the head, in-between the insertion site of antennae and the mandibles of the ant. Clypeal margin refers to the anterior margin of the clypeus.

Coccids. A family of insects (Coccidae) belonging to the infraorder Coccomorpha (scale insects) of order Hemiptera, also known as soft scales or wax scales. They may form mutualistic relationship with ants, offering honeydew in return for protection.

Colony. The social structure of ant, constituted by workers and reproductive individuals that cooperate with one another, as well as their brood. Members of the same colony are able to recognize and are not aggressive with each other. Number of individuals can range from a few to several millions.

Cosmopolitan distribution. Global or near global distribution.

Cryptic. Obscure lifestyle, often refers to subterranean species and species that are active within leaf litter.

D

Dimorphic. There are two distinct subcastes for workers. There is no intermediate form in-between the two subcastes.

Disturbance / disturbed environment. Human activities that change the environment and impact ecosystem, which can be momentary or prolong. Agriculture and environmental management (e.g., cutting or trimming trees, use of pesticides) are two typical examples of the disturbance that may affect ants.

Dorsal view. Top-down view on an ant.

E

Epigaeic. Nesting and/or active above the ground and on lower vegetations.

Ergatoids. Queens or males that are born wingless. Ergatoid queens may look very similar to workers.

Exotic species. Species that has been introduced to place outside of where it is originated from (=introduced species).

F

Forage. Hunting or gathering food.

Full-face view. Frontal view on the head of an ant.

G

Gamergate. Workers that have mated with males and are able to lay fertilized eggs for reproduction. Depending on the ant species, gamergates may coexist with queens, or entirely replace queens.

Gaster. The posterior part of an ant’s body, connected with mesosoma through waist segment(s). In some ant species a sting is present at the end of the gaster for hunting and/or self-defense.

Gastral tergites. The dorsal part of the gaster, can be subdivided into segments.

H

Habitat. Where the organism live, characterize by the landscape and dominant plants (e.g., tree, shrub, grass, mangrove tree) present.

Hemiptera. An order of insects that includes aphids, cicadas, leafhoppers, scale insects and shield bugs. Most of them feed on plant sap, and sometimes form mutualistic relationship with ants, offering honeydew in return for protection.

Hypogaeic. Nesting and/or active within or below leaf litter, inside rotten woods, beneath rocks and within the soil.

I

Intraspecific variation. Variation between individuals of the same species, in contrast with interspecific variation, which describe variation between species.

Introduced species. Species that has been introduced to place outside of where it is originated from (=exotic species).

Invasive species. Species that has been introduced to place outside of where it is originated from and has a significant impact on the local ecosystem.

L

Leaf litter. The layer of plant debris above the soil, which is made up of fallen leaves, branches, fruits, seeds and flowers.

M

Mandibles. The pair of hard mouthparts of insects for biting, cutting and holding food. It is a part of the exoskeleton.

Mesosoma. The anterior part of an ant’s body, connected with gaster through waist segment(s). It is also where the legs and wings (when present) of the ant attached.

Microhabitat. Where the organism live, at a finer scale than habitat. It is further characterized by the local temperature, humidity of the environment. For example, beneath a rock and on the surface of tree branches are two different microhabitats of the same habitat (forest).

Monogyny. Colony with only one egg-laying reproductive individual.

Monomorphic. There is only one size and form for workers, and cannot be further subdivided into subcastes.

Morphology. The form and structure of an organism, both internal and external. For external morphology, it includes the shape, size, color, pattern and external structure of the various parts of the organism.

Morphospecies. A species defined solely base on its morphological differences with respect to other species. It may not have been formally described and named.

Mutualism. Mutually-beneficial relationship between organisms.

Myrmecophily plants. Plants that are positively associated with ants.

Myrmecophyte plants. Plants that have formed mutualistic relationship with ants, and possess adaptations for such relationship, such as food bodies and extrafloral nectaries dedicated to ants and/or specialized structure for ants to nest (e.g., hollowed structures in tree branches).

N

Native species. Species that that naturally live in a particular place without human intervention. ‘Native fauna’ further specifies animal species, while ‘native flora’ specifies plant species.

Nest. The physical structure that ant colony lives in. It may be permanent and extensively built, or temporary and has only been minimally modified by its residents.

Niche. It can broadly refer to the role an organism plays within its habitat, or specifically refer to where an organism lives within its habitat. Here niche means the latter.

O

Oligogyny. A special case of polygyny, where there is more than one egg-laying reproductive individual in an ant colony, but these individuals do not tolerate one another and stay far apart from each other in the nest. Workers still cooperate with one another like in a normal polygynous colony.

Open habitats. Habitats such as open shrubland and grassland, in oppose to habitats with dense vegetation cover, such as forest.

P

Parthenogenesis. Asexual reproduction through unfertilized eggs. For ants, it means reproductive females are able to reproduce partial or complete clones of themselves without mating (i.e., theylotoky).

Pest. Species that has been a source of nuisance to people. Common categories include household pest and agricultural pest (which causes damage to crops).

Petiole. The first segment of waist segments (or the only segment, when there is only one waist segment) of an ant, connecting mesosoma and gaster. It may be reduced and difficult to observe.

Polygyny. Colony with, on average, more than one egg-laying reproductive individual. Facultative polygyny (=secondary polygyny) means that colonies were originally monogynous when founded, but would later adopt other queens (usually daughter queens of the colony) and become polygynous.

Polymorphic. There is more than one size and/or form for workers, hence, can be further subdivided into subcastes. When there are two distinct subcastes but no intermediate form, it is known as dimorphic.

Postpetiole. The second segment of waist segments, connecting mesosoma and gaster. Some species of ants do not have a postpetiole.

Profile view. Side view of the ant, usually with its head facing left.

S

Scale Insects. An infraorder of insects (Coccomorpha) belonging to the order Hemiptera, also includes coccids. They may form mutualistic relationship with ants, offering honeydew in return for protection.

Scavenger. Ants that predominantly gather food instead of through other means, e.g., hunting. Depends on the ants’ preference, it may include fruit, seed and other food bodies of plants, carcasses of invertebrates and small vertebrates, honeydew produced by other insects, etc.

Sculpture. Pit- or grove-like structure on the surface of the exoskeleton of the ant, in contrast with smooth and shining surface. Its pattern can be used to differentiate between species of ants.

Secondary forest. A forest that has been regrown after timber harvest or agricultural activities, in contrast with primary forest, which has not been deforested by human. There are only secondary forests in Hong Kong.

Soil arthropods. Small invertebrates from the phylum Arthropoda that are active within and above the soil, as well as within leaf litter.

Spine. Hard and pointed structure on the back of the ant. It may be long and sharp like a thorn, or low and inconspicuous. It most often appears at the posterior part of mesosoma in one pair (propodeal spine), with other possible locations include pronotal spine and petiole spine.

Subcastes. Roles of ants in the colony, a further subdivision based on minor differences in morphology and role under their respective castes.

Supercolony. The unusual case where multiple colonies unite and are not aggressive with one another, often occur in invasive species in introduced regions. Individuals range from millions up to billions.

T

Tramp species. Species that are associated with habitats frequently-disturbed by human activities. They may further be dispersed by human to places beyond its native range.

Trophic eggs. Unfertilized eggs that function as food. In ants, they are usually produced by workers and offered to queens and larvae.

W

Waist segment(s). The segment(s) connecting mesosoma and gaster of an ant. The first segment is called ‘petiole’ and the second segment (when present) is called ‘postpetiole’.

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