The many, many roles of ants

Ant closeup 5 dead bamboo
Ant closeup 5

The many, many roles of ants

The importance of ants within ecosystems

What roles do they play in the ecosystem? If, like a movie, there are credits for planet Earth, how many times do you think ants’ name will appear on the list? Just several times? Well, ants are likely a lot more important than you might think! While many ant species are hunters that prey on insects and other arthropods, ants themselves are also prey of other animals. So, ants are an important part of the food web. Ants that are not predators are often scavengers instead, and they consume both plants and decaying material. They help returning energy and nutrients back to the ecosystem. Many ant species have co-evolve with other animals, fungi and plants and developed close relationships with these species. When forming large colonies, ants are even able to change their habitats.

Hunter and prey

Ants are one of the major predators of invertebrates in many ecosystems. This is particularly true in tropical and subtropical regions, including Hong Kong, where the diversity of ants is the highest. Most predatory ants are generalist, opportunistic hunters that are not particularly picky about what they capture and eat, with their diet including various groups of insects and other invertebrates (e.g., snails, earthworms, spiders…) and even some live or dead vertebrates when they get a chance. But there are also many ant species that are specialized predators of a particular group, such as springtails, millipedes, certain group of insects and their brood, or even other ants! Hence, ants are able to keep the population size of their potential prey in check. Without their presence in the habitats, it is possible for these invertebrates (in particular caterpillars) to go out of control and lead to an explosive population growth! In fact, ants have been purposefully introduced in some agricultural area and used as a biological agent to control the population of agricultural pest due to their effectiveness as a predator (we have been doing this for nearly 2500 years!). Some species of ants are even able to dominate their habitats. They form huge colonies and their predation are able to influence the whole local fauna. For example, army ants in the New World and in Africa forage in mass and cooperatively attack any animal in contact with them, even small vertebrates! They have huge influence on the population size, species composition and behavior of the local community.

Many other animals also feed on ants. Ants constitute an important part of the diets of many arthropods, amphibians, fishes in forest streams, reptiles, birds and mammals. Ants’ ability to form huge colonies mean that the biomass (= dry weight) of ants in some habitats are quite remarkable. This means that some animals, known as myrmecophagous, are even able to almost solely depend on ants for their diets, such as pangolins and anteaters. If ants are gone, a lot of the animals that feed on ants will be in trouble.

Ants the gardener and recycler

Aside from being a predator, many ants are also herbivores or omnivores and have diverse diet of plant and decaying material. Ants that collect seeds help the dispersal of plants. By carrying seeds underground into their nests, ants provide them protection and nutrient-rich soil to germinate. In fact, no less than 11,000 species of plants rely on ants for their dispersal. Leafcutter ants, as the name suggests, collect leaves and other plant material and bring them back to the nests (which can be enormous and complex underground structure) to cultivate fungi for food. The leaves collected by those ants are used as the main substrate for the fungus which is patiently inseminated into the leaves and groomed to provide a nutritious source of food for the ant larvae. The waste generated from this agriculture represents an important fertilizer once buried into the soil. Similarly, omnivorous ants that scavenge decaying materials such as dead invertebrates, as well as ants that build nest inside decaying wood, also contribute to the decomposition process of material in their habitats. Last but not least, through their activity of nest excavation and gallery building, ants play a fundamental role in the cycling of energy and nutrients within the ecosystem, and to limit soil compaction and increase of air exchange favoring the growth of plants and the survival of thousands of soil invertebrates!

Ants and ecosystems

Most modern ant genera started appearing in early Eocene (around 55 million years ago). Over their long evolutionary history, ants are able to form close ties with many other organisms. Some plants offer nesting sites and food (fruit body and nectary) to ants, while ants, in return, protect their host plants from herbivorous insects and other competing plants (these ants would prune the leaves of any vine or competing plant that grow too close to their host, essentially like a gardener!). Ants also form mutualistic relationship with many Hemiptera (aphids, coccids, scale insects) and other sap-sucking insects, offering protection and receiving honeydew from them in return. The efficiency of ants in foraging food and feeding the whole colony means that some animals will try to take advantage and live as a parasite inside the ants’ nests. This is known as social parasitism, and these animals, such as insect larvae or other species of ants, will try to fool the host workers using chemical and visual signals that they are part of the colony and get fed by the host workers! The dominance of predatory ants in the ecosystem, in particular for species that hunt cooperatively, mean that they are often avoided by other arthropods and, thus, many insects and spiders will mimic ants to evade predation. All these diverse ways of interaction and complicated relationship between ants and other organisms is one of the major drivers in modern biodiversity.

Ants are an important part of the ecosystem. Without them, the modern ecosystem would not be as diverse as it is today. Even though they are tiny, they are undoubtedly one of the giant pillars that supported all the life on Earth.

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Glossary

A･B･C･D･E･F･G･H･I･J･K･L･M･N･O･P･Q･R･S･T･U･V･W･X･Y･Z

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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