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Ecology

Life amongst dry leaves

Biologists identify behavioral standards of ants from the Atlantic Rain Forest

LARA GUIMARÃESPheidole flavens: a soldier, fit for heavier work LARA GUIMARÃES

Usually only remembered by the majority of people when they infest the sugar bowl or the audio system, ants have been occupying the planet for at least 100 million years, according to the oldest fossils. Something that may sound even more surprising: they are essential components of the ecosystems and have a greater ecological importance than one might expect, besides offering a great richness and high diversity of species, all of them social.

The biggest study about these insects ever carried out in the Brazilian Atlantic Rain Forest, which brought together specialists from 11 institutions in the country and collaborators from abroad, proves that ants are one of the main indicators of biological diversity of a region: the more species of ants, the more species there will probably be of other animals and plants.

The team coordinated by Carlos Roberto Brandão, a biologist from the Zoology Museum of the University of São Paulo (USP), has up until now identified 410 species of ants from the Atlantic Rain Forest, but it is estimated that this coastal forest may shelter up to a thousand species – worldwide, of a total estimated at 20 thousand species, about 12 thousand have now been described. “Based on this data,” Brandão explains  “the Atlantic Rain Forest can be seen as one of the environments richest in ant species in the world”. There are regions with much fewer: in Great Britain, for example, only 36 species of ants live.

“Ants live in colonies that can house from a few to millions of individuals, which puts them as one of the most abundant terrestrial animals in tropical and subtropical regions”, he says.  Studies carried out in the Amazon indicate that ants and termites, another group of social insects, account for almost 70% of the terrestrial animal biomass, measured from their dry weight.

In other terms, the populations of these insects that measure from 1 millimeter to 4 centimeters and individually do not weigh more than a tenth of a gram, could be gathered together and weighed, they would show a heavier mass of organic matter than that of all the other invertebrates and vertebrates together. According to Brandão, some animal groups, in particular beetles and mites, are even richer in species, but they are usually solitary and, accordingly, each species is represented by fewer individuals than the social species.

Over two years, from 1999 to 2001, the biologists covered 26 areas of the preserved Atlantic Rain Forest in ten states – Santa Catarina, Paraná, São Paulo, Rio de Janeiro, Espírito Santo, Bahia, Sergipe, Pernambuco, Alagoas and Paraíba. They collected 1,400 samples from 1 square meter of the most superficial layer of the soil and from the cover of dry leaves, the forest floor, where 60% of the known species of ants. In general, straying no more than two meters from their nests, these insects inhabit the spaces between the leaves that fall on the ground, protected against the attack of other animals and, at the same time, finding there their favorite foods, like the mites.

Two of the most common species of ants in the Atlantic Rain Forest are the Pheidole flavens, with workers only 1 millimeter long, found in 842 of the 1,400 samples – that is to say, almost in 2 out of every 3 meters studied -, and Pyramica denticulata, also measured in millimeters, with workers endowed with very long jaws and a heart-shaped head, present in 780 samples. “Probably”, says Brandão, “these two species are among  the most common animals in the Atlantic Rain Forest”.

Analyzing the information that resulted from this lengthy field work, besides tens of probable new species, in particular in very rare genera like Asphinctanilloides and Cryptomyrmex, the researchers found refined forms of organization of the ant’s fauna, normally seen as members of a simple society, with males, which act only in reproduction, and females, in turn divided into queens, workers and soldiers, which are modified workers that do the heavier work. The study of the workers, more abundant and more easily found outside the nests, showed an unexpected wealth of behaviors.

Nine distinct standards of behaviors and habits were identified. Researchers usually recognize these behavioral standards from prior information on the habits of each species. Rogério Rosa da Silva, one of the biologists of the team, examined the species that lived in four of the 26 localities studied and developed another approach. That gave birth to a proposal for classifying the behaviors of the ground ants, which may be valid in the whole of the Atlantic Rain Forest and represent in a more precise manner what the other specialists were doing in a subjective manner.

Even though the composition of species varies from one locality to another, the structure of the set of communities is constant: the ants always organize themselves according to the same standards of behavior, called guilds, which show how each species acts in the environment. Where there are ants, there are nine guilds, made up of five basic categories, one of them with four subsets.

The basic groups are: the generalist predators, which hunt any kind of prey; the specialized predators, which collect specific preys, like eggs of other insects or even other ants; the fungus cultivators, which take to the nest leaves, pieces of plants and carcasses of other insects, which are used to feed the colony of fungus that grows at the bottom of the nest and supplies the ants with sugar and proteins; and, finally, the generalists, which collect plant sap and small animals, on which the ants feed.

The generalist predators are those grouped into four sets: those that collect only what is on the ground, called epigaeic; those that also visit the superficial layers of the soil, or hypogaeic, and the species with relatively large workers and relatively small ones, the distinction being, in this case, the size of the prey they collect. There are also, though they have not been collected, six other guilds: two of nomad species, which move about under the ground, three arboricoles, and the exclusively subterranean ones, which live in fixed nests.

Competition
“This classification allows a finer analysis of the structure of ant communities”, says biologist Rogério Silva, from USP’s Zoology Museum. Each place can have room for only a limited number of species in each category of behavior or guild: in a region where only four or five species of predator ants can live, 20 predator species will never be found. “This limit is derived from competition between species, since large predator ants contend only with other large predators for a finite number of preys”, Brandão says.

“Guilds, in this case, represent the scenarios for the competition.” As it was demonstrated that the ant fauna of the Atlantic Rain Forest must always be made up of the same 15 guilds, the state of conservation of a forest can now be evaluated more precisely, something that used to be done only by means of comparative lists of names of species.

The regularity with which these standards of behavior are found leads to the conclusion that the alterations imposed by human activities, such as the deforestation of a stretch of the forest, can cause imbalances amongst these groups and the consequent overpopulation of some of them, bringing losses to the communities themselves and to the animals and plants that depend on them to survive. “They maintain so many mutualistic relationships that it is possible to conclude that if in one place there are more ants, there are also more of other species”, says Brandão.

In the Cerrado, 70% of the plants have nectar-producing glands, the so-called nectarines, which attract the ants. By collecting the nectar, the ants protect the plants, preventing other insects from coming to feed on the plant itself. They also control the population of other insects and of other small invertebrates, since many species are predators, while others spread seeds. The relationships between ants and plants can be positive, when they eliminate herbivorous animals, in exchange for nectar, or negative, when they implant colonies of insects capable of getting sap, the excess of which they collect, in exchange for protection for these insects, like cochineals, aphids and other relatives of cicadas.

Indicators
The first study to show that ants could act as an indicator of the diversity of other animal species was done by English and American researchers, following a comparison of eight groups of animals in the Mbalmayo forest reserve, in Cameroon, Africa, published in 1998 in Nature. Other studies were born of this that may help to orientate the choice of areas to be preserved and to dimension the minimum size of new areas of native vegetation to be preserved.

This possibility is now being put into practice. According to Brandão, the Secretariat for Planning and Environment of the State of Tocantins intends to use the data from a survey about the diversity of ants in the state to select priority areas for conserving the Cerrado. Last year, in the municipality of Craolândia, in Tocantins, Rogério Silva found a new genus of ant, still without an official name.

Also taking part in the survey about ants of the Atlantic Rain Forest were researchers from the Ribeirão Preto Biology Institute, the University of Mogi das Cruzes, the State University of Santa Cruz and from the Executive Commission of the Cacao Plantation Plan, in Ilhéus, in the state of Bahia, and the Federal University of São Carlos, in São Paulo, the Federal Rural University of Rio de Janeiro, and the Federal Universities of Viçosa (Minas Gerais), Espírito Santo, Paraíba and Pernambuco. Working together, they also helped to change some very deep-rooted ideas.

Four decades ago, it used to be believed that the number of species of animals and plants varied according to the latitude: the closer to the equator, the greater the biological diversity would be. That was not what was seen. The greatest diversity of species was found in stretches of the Atlantic Rain Forest in the north of Rio de Janeiro to the south of Espírito Santo, with about 10% more species than in localities further to the north, which, it was believed, ought to house a greater diversity. In this stretch between Rio and Espírito Santo, Brandão reports, as many as 140 species were collected – just of those that live on the ground, in an area of 1 square kilometer.

In parallel to the demonstration of the diversity of species of the Atlantic Rain Forest and of the importance of these insects in supporting the definition of environmental preservation strategies, one more peculiar characteristic of these insects with such complex habits came to light. In an article published in January in Nature, a team coordinated by Nigel Franks and Tom Richardson, from Bristol University, in the United Kingdom, showed that ants are capable of teaching others from the colony how to look for food. It is perhaps the first formal demonstration of teaching in animals, a capability hitherto attributed only to human beings.

The Project
Richness and diversity of Hymenoptera and Isoptera along a latitudinal gradient in the Atlantic Rain Forest
Modality
Thematic Project linked to the Biota-FAPESP Program
Coordinator
Carlos Roberto Ferreira Brandão – USP’s Zoology Museum
Investment
R$ 925,901.82 (FAPESP)
R$ 30,000.00 (CNPq)

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