As he was walking through the woods in the Mata Atlântica, Atlantic Rainforest, and noticed that the pulp of the fruit of a jatobá tree (Hymenaea courbari) was being devoured by ants, biologist Paulo Oliveira, from the State University of Campinas/Unicamp, began to question the widespread notion that these social insects play a despicable role in seed ecology. Nearly 15 years later, the research group, immersed in the intimacy of the relationship between plants and seeds, has shown that these tiny insects not only drag seeds to more suitable places, but also clean them, thus facilitating germination. “The spreading of seeds in the tropics is much more complex than had been previously believed”, says Oliveira.
Most of the spotlights on studies on the ecology of seed dispersal are focused on birds, monkeys and other vertebrates attracted by the colorful fruit and tasty pulp of nine out of ten species of large trees and shrubs. These animals carry the fruit for long distances and throw the seeds on the ground. If the fruit drops accidentally, it is sometimes nearly intact; even after it goes through the animals’ digestive system, a good part of the pulp is still left over. What happens on the ground, however, had gone virtually unnoticed until Oliveira established the bases of his research study. One of the most recent findings in this respect come from the doctorate studies of Alexander Christianini, who is now a professor at the Sorocaba campus of the Federal University of São Carlos/UFScar. He and Oliveira showed that in the tropical savanna region of the Cerrado de Itirapina, in the state of São Paulo, five species of ants gather the seeds that drop to the ground. In an article published this month in Oecologia journal, the biologists suggest that an important role is played by ants after the birds transport the seeds far away from the parent tree: the ants do the more detailed gardening work.
Birds and monkeys usually deposit the seeds under a tree. The leftover pulps attract the ants, which then carry bits of the pulp to the anthill. “The clean seed is left on the forest ground”, says Oliveira, “preventing the fungi from installing themselves inside and killing the plant embryo”. In addition, some ants carry the seeds to the anthill, which the researcher describes as being an “island of nutrients”, as the anthill contains discarded parts of plants and the remains of dead ants and other insects.
The jatobá pulp (Hymenaea courbaril) aroused the researcher’s curiosity. An experiment he conducted with colleagues from São Paulo State University in Rio Claro and from the Federal University of Mato Grosso, showed that 70% of the seeds that had been cleaned by the ants had already sprung seedlings, in comparison to the 20% of the seeds that had not been cleaned by the tiny gardeners. From 1995 onwards, this line of research has resulted in four doctorate degrees which revealed that this relationship is quite common in the Mata Atlântica and the Cerrado.
Biologist Inara Leal showed that the leafcutter ants, which include the saúvas, (attas) also act as gardeners. Anyone looking at the long line – that resembles a miniature highway – of ants carrying pieces of leaves and flowers on their backs fears for the destiny of the destroyed plant. It is no wonder that a single ant colony can collect 30 kilograms of plants a day to be used as fertilizer for the fungi that they raise and feed on. These ants are able, in a matter of hours, to reduce a leafy shrub into a dry stub; but the important point for the biologist is that the leafcutters also carry fruits and seeds. After receiving her doctorate degree from Unicamp, Inara was hired as a professor by the Federal University of Pernambuco. She noticed that the Atta sexdens saúva ants are attracted by the yellow aril, an appendage of the seeds of the copaíba (Copaifera langsdorffii) tree, a tree that is commonly found in the Cerrado and Mata Atlântica. The rufous-bellied thrush and the guan are the main carriers of the copaiba seeds. The saúva ants carry the seeds a distance of up to ten meters, remove the nutritious aril and often break open the seed’s hard outer shell, which also helps germination, according to an article published in Biotropica in 1998. The same thing happens with the Cerrado’s native plants.
Marco Pizo, who was Inara’s colleague at Oliveira’s lab during the same period, concentrated on the interaction between plants and ants in the Mata Atlântica and showed that the nutritious red aril found around the seeds of the canjerana tree (Cabralea canjerana) attracts carnivorous ants. “For the carnivorous ants, protein-rich, fatty fruits are like insects that don’t fight, don’t bite and don’t run away”, Oliveira says. Pizo, who is currently at the Vale do Rio dos Sinos University/Unisinos in the State of Rio Grande do Sul, spread seeds with and without pulp on the forest grounds; the seeds were encased in small cages to prevent them from being eaten by bigger animals. It became clear that the ants prefer the seeds with pulps (70% of the red part is fat) and that these seeds germinate much faster after they are sown by small insects, as highlighted in a cover story published in the American Journal of Botany in 1998.
After having proved that ants transport seeds, the researchers had to investigate whether this dispersal is guided or random. During her doctorate studies under Oliveira, Luciana Passos investigated the relationship between plants and ants in the restinga (coastal tropical and sub-tropical moist broadleaf forest) on Ilha do Cardoso island, off the south coast of São Paulo State. Part of the Mata Atlântica, Atlantic Rainforest, this forest is less exuberant because it grows on sandy, nutrient-poor soil. She spread pieces of sardines around the island to attract the carnivorous ants; the ants led her back to their nests – she found 21 nests.
In an article published in the Journal of Ecology in 2002, Luciana describes what happens with the oil-rich fruit of the Clusia criuva, or clúsia, tree, which in one season produces approximately 5,800 fruit with a total of 25 thousand seeds. Most of the seeds (83%) end up in the feces of 14 different bird species. The researcher noticed that the seeds that drop to the ground are transported to up to a distance of ten meters by the Odontomachus and Pachycondyla carnivorous ants of the ponerinae subfamily, “whose sting is painful as that of wasps”, says Oliveira.
But this is not the end of the story. Luciana did more research and observed that these ants remove 98% of the not totally digested seeds that end up in the bird feces. The biologist counted the number of clusia seedlings and noticed a disproportionate number of them next to the anthills – double the number of seeds she had noticed in the forest. In addition, she maintained a census of the seedlings for one year and noticed that they had a much higher chance of survival around the anthills. Luciana sent samples of this soil for analysis to the Instituto Agronômico de Campinas, Agronomy Institute, and found that it contained more nitrogen and potassium than the rest of the forest soil, thanks to the detritus accumulated by the ants.
The same holds true for the maria-faceira (Guapira opposita) tree, whose black, red-stemmed fruits attract birds such as araçaripoca (Selenidera maculirostris) and the saíra-sete-cores (Tangarta seledon). This fruit has a high (28%) protein content, according to an article published in Oecologia journal in 2004. The Odontomachus ants carry the seeds to up to 4 meters and seedlings grow around their anthills. The group from Unicamp developed a system which measures how deep a pole penetrates into the soil and showed that the ants’ digging results in loose soil, rich in potassium, phosphorus and calcium.
Alexander Christianini went further and showed how the deforestation of the Cerrado invalidates the ants’ positive effect on plant ecology. Scientists already know that the center of the forest is cooler and more humid than the forest that borders the deforested regions. The researcher showed that big ants are more common in the inner region of the Cerrado, where the soil is nutrient-rich and softer. He monitored this region for one year and during this time he noticed that 92% of the ant colonies deep inside the forest persist for longer period, in comparison to 30% of the ant colonies that persist at the edge of the forest. As in the Mata Atlântica, plants in the Cerrado germinate better next to anthills, while seedlings on the borders have a 0.2% chance of surviving the first year of life. These results clearly show that deforesting has a significantly harmful effect on ants and on plants. Thanks to their gardening skills, which contribute to the germination of seeds, ants can help recover damaged forests.
The group from Unicamp has made many other discoveries on the ecological functions of these miniature soldiers and workers that live in groups comprised of millions of ants. Some plants produce substances to attract ants, and the ants, in turn, act as defense troops. This is the case of the pequi (Caryocar brasiliense), a plant native to the Cerrado, whose fruits are widely used and appreciated in regional cuisine. The ants enjoy the nectar that comes from the glands of the pequi flower buds and attack other insects, such as caterpillars. Sebastián Sendoya, one of Oliveira’s students, and André Freitas showed that the Eunica bechina butterflies, which lay their eggs on the pequi leaves, fly over the plants and detect predator ants. The paper, published in this month’s issue of American Naturalist, describes how the visual sophistication of butterflies allows them to lay eggs on safe leaves and even recognize harmless ants.
All of this information is found in what Oliveira considers to be his life project: the book The ecology and evolution of ant-plant interactions, which he wrote in partnership with his Mexican colleague, Victor Rico-Gray. Published by the Chicago University Press in 2007, the book is an extensive review of all the known ecological interactions between ants and plants. “People think that the vertebrate animals are more important, because they are easily visible”, says the biologist from Unicamp, “but in the Amazon Region, the dry weight of the invertebrate creatures is four times higher than that of the vertebrate animals”. Ants, whose colonies can comprise up to millions of workers, are among the most numerous invertebrate creatures.
Ecology and the behavior of neotropical ants (nº 08/54058-1); Modality Regular Funding of Research Project; Coordinator Paulo de Oliveira; Investment R$ 113.080,54
CHRISTIANINI, A. V. and OLIVEIRA, P. S. The relevance of ants as seed rescuers of a primarily bird-dispersed tree in the Neotropical cerrado savanna. Oecologia. v. 160, n. 4, p. 735-745. Jul. 2009.
SENDOYA, S.F. et al. Egg-laying butterflies distinguish predaceous ants by sight. The American Naturalist. v. 174, n. 1, p. 134-139. Jul. 2009.