Launched in the United Kingdom, a book organized by a professor at Unicamp brings together international studies on the mutualism between ants and plants
Late spring afternoon. Artificial light in the work room of biologist Paulo Oliveira, professor at the Biology Institute (IB) at Unicamp. On the walls, books, maps, photos of ants, newspaper clippings, articles, scientific publications and souvenirs from students, teachers and scientists. The half-open window alleviates the heat and dry weather. The sun sets outside. Birdsong invades the small enclosure.
“Many people think that birds and other large animals are exclusively responsible for dispersing seeds in the forest. Vertebrates are essential, but once the seed falls to the ground, ants are the dispersers”, says the scientist.
Oliveira, at Unicamp since 1983, is a specialist in interactions between ants and plants. There are around 15 thousand species of ants identified by science. Ants, social insects, are responsible for the so-called secondary dispersal of seeds. The work they perform — transporting the seed to the nest and ingesting the energy contained in the pulp of the fruit that falls to the ground, leaving it clean and freeing it from fungal infestation — facilitates seed germination.
The primary dispersal is the responsibility of birds, monkeys, bats and other larger animals, such as the maned wolf, which are attracted by the colorful fruits with tasty pulp. These animals travel long distances and release fruit seeds into the soil, increasing the chances of plants colonizing other territories and ensuring the genetic diversity of an ecosystem.
The importance of these small invertebrates, the most numerous among insects, in a terrestrial ecosystem increases as human action produces its effects on the planet. In the current geological era, which is called the Anthropocene, one of these effects is the wave of biodiversity loss, like the one scientists project in the Brazilian Cerrado, characterized by the disappearance of part of the large animals that carry out primary dispersal.
Ants suffer less interference from human activity. They interact with other species of insects, plants and vertebrates. Furthermore, ants reign in the soil of most terrestrial ecosystems on the planet, with the animals — along with termites — being more abundant. “Imagine a square in the Amazon measuring 100 meters by 100 meters — or 1 hectare. We will count 8 million ants. Now expand this to other tropical regions of the planet and you will have an idea of how common the ant is”, comments Paulo.
Recently released by Cambridge University Press, the book Ant-Plant Interactions: Impacts of Humans on Terrestrial Ecosystems (not yet translated into Portuguese) highlights the strategic ecological role of ants for the survival of biomes around the world and focuses on the effects of human changes on the planet in the interaction between these social insects and plants.
Biologist Paulo Oliveira and researcher Suzanne Koptur, from Florida International University, organized the work in which 52 researchers from around the world demonstrate, in 452 pages, that interactions between ants and plants can be extremely beneficial. The studies were carried out in temperate and tropical ecosystems in countries in the Americas and Africa, in Japan, Polynesia, Indonesia and Australia.
The evolutionary paths of ants and plants have been crossing for 100 million years. As representatives of different kingdoms, the animal and the plant, ants and plants are involved in several cases of mutualism, generating benefits for the species involved. “Nine out of 10 invertebrates that are on tree leaves are ants”, recalls Oliveira.
One of the cases of defense mutualism described in the book occurs between ants of the genus Crematogaster and species of a shrub dominant in African savannas, of the genus Acacia. Ants, which are aggressive, have a high potential to defend plants against herbivores, such as elephants.
“To increase the presence of ants on the branches and leaves, the acacia offers in exchange drops of nectar, which are food resources, or housing — that is, space for the formation of anthills”, points out the Unicamp biologist. Every time the elephant approaches its trunk to tear off the leaves, the ants aggressively move forward to bite it, driving the large mammal away and protecting the anthill and the bush.
The book also presents other common cases of interaction between ants and plants: the transport of seeds in the soil. Biology calls this dispersal mutualism. In the Cerrado and Atlantic Forest, for example, the ground is full of ants. Oliveira observes that a process of secondary seed dispersal takes place in areas where nature is well preserved in these biomes.
The fruit naturally falls from the tree when it ripens or because a larger animal drops it or discards it. Once in the soil, there are species of ants that interact with the fruit, taking an interest in the fleshy and nutritious part. The worker ants of these species take the remains of pulp to those that are in the larval stage inside the anthill.
“Even carnivorous ants, which hunt other ants and caterpillars, are interested in fruits that have fat and protein in their pulp”, explains the biologist. “In the Atlantic Forest, there is a genus of ants, the Odontomachus (carnivorous), which take the seed that falls under the tree and help to colonize another piece of land. This has a very important effect on the early stage of the plant’s life,” he points out.
The behavior of Odontomachus and 15 other species in the dispersal of the Clusia criuva tree was described for the first time in a study carried out on Cardoso Island, on the south coast of São Paulo. When they remove the pulp, the ants clean the seed and discard it near the anthill. The seed germinates more easily and the plants have a better chance of developing and reaching their first year of life. The surroundings of the nest are rich in nutrients, such as nitrogen and phosphorus, due to the organic material accumulated by the ants.
The future of ecosystems
Man's intervention on the planet has caused changes in the configuration of global ecosystems, such as the destruction or degradation of native areas. The scientific community is also studying the relationship between the increase in the planet's average temperature and greenhouse gas (GHG) emissions in the Anthropocene. Scientists do not know precisely what the impact of the changes will be on the lives of all species. The imbalance of ecosystems is one of the signs.
“There are records of specific areas in African savannas where the presence of invasive ants in the ecosystem has created imbalance. One of the consequences is that acacia trees, previously protected by interaction with ants of the Crematogaster genus, are being devastated by the action of elephants, which eat the defenseless bush”, says Oliveira.
Ants are indicators of the conservation status of an ecosystem. If the small native animals disappeared, there would no longer be animals that aerated the soil, which would become more compacted. Fertilization would decrease considerably. Many of the plants that ants protect would be left unprotected. Others would be more common, being dominant. Many of the herbivores that ants fight would be preponderant.
“It’s a snowball. When there is an imbalance, the little ants are affected as much as other animals”, highlights the biologist.