Biologists Laura Carolina Leal and Felipe Passos performed a series of experiments in Brazil’s Northeast region — specifically in the interior of Bahia State, where the semiarid Caatinga biome predominates — to determine how plants with extrafloral nectaries interact with ants.
Extrafloral nectaries are nectar-secreting glands not involved in pollination which supply insects with carbohydrates in exchange for defense against herbivores. The nectar attracts predatory insects that consume both the nectar and plant-eating arthropods, functioning as bodyguards.
“In contrast with the previous belief, we discovered that carbohydrate is only one of the forms of payment offered by plants to the ants that protect them. Another is protein, which ants obtain by consuming the herbivorous arthropods available on or around the plants they visit,” said Leal, a professor in the Federal University of São Paulo’s Institute of Environmental, Chemical and Pharmaceutical Sciences (ICAQF-UNIFESP) in Brazil.
“This finding contradicts the idea that payment is in sugar only,” Leal told. “It shows that what ants gain from herbivores also matters. We discovered that ants may be more aggressive in environments where arthropods and other sources of protein are scarce, defending their food sources and hence protecting plants.”
The study was supported by São Paulo Research Foundation — FAPESP and by the Brazilian education ministry’s graduate research council (CAPES). A paper on it has recently been published in Biological Journal of the Linnean Society.
The research interests pursued by Leal and Passos focus on the various forms of insect-plant mutualism. “Mutualism is a form of interaction between two species in which each benefits from the interaction in some way. If it isn’t advantageous for both species, but only for one, it’s parasitism,” Leal said.
“Several studies have shown that nectarivorous ants expel herbivores and enhance the reproductive success of plants with extrafloral nectaries. The greater the importance of extrafloral nectar to the ants, the better for the plants, as this increases the ants’ aggressiveness toward herbivores. We decided to find out whether nectar is the only payment by plants for the ants’ protection or whether eating herbivores might also be advantageous to the ants.”
Leal and Passos confirmed the hypothesis that plant attendance by more aggressive ants and the efficiency of their defense increase when the availability of carbohydrates and/or proteins to the ants is low, enhancing the relative value of both extrafloral nectaries and protein-rich herbivores to these insects.
The study was conducted on the campus of the University of Feira de Santana in Bahia. The region has a semiarid climate with an annual average temperature of 25.2 °C and rainfall averaging 848 mm per year. The vegetation in the Caatinga is xerophytic (adapted to life in a dry habitat), consisting of a mosaic of thorny shrubs and seasonally dry forests.
The researchers established 19 study plots measuring 16 square meters each, in…