Plants use macronutrients accumulated in leaf-cutting ant nests

Abstract

Leaf-cutting ants (Atta spp.) are known for their extensive defoliation in neo-tropical forests and savannahs. Debate about the costs and benefits of their activities has been largely dominated by their detrimental effects on agriculture and agroforestry. However, the large accumulation of nutrients and changes in soil properties near their nests might benefit plants growing near them. Here, we test whether trees use nutrients that accumulate in debris piles near, or refuse chambers within, leaf-cutting ant nests. At two tropical sites (a moist tropical forest site in Panama and a savannah site in Brazil), we fed leaves labelled with the stable isotope 15N to two species of leaf-cutting ants (Atta colombica and Atta laevigata) and traced the stable isotope label in plants surrounding the two nests. Thus, we show that plants in both sites access resources associated with Atta nests. In addition, leaf tissue of trees near the nests labelled with 15N had significantly higher calcium concentrations than those of distal, unlabelled conspecifics. It has been documented that calcium is a limiting macronutrient in tropical forests and savannahs. Atta may thus play an important ecological role through their long-distance transport, redistribution and concentration of critical macronutrients.

Figure 1

Leaf-cutting ants and their refuse deposits. (a) Atta colombica and (b) Atta laevigata in the process of cutting leaf fragments. Note that the ¹⁵N-labelled potassium nitrate crystals can be seen on the surface of the leaves being cut by (b) A. laevigata. Also shown is (c) the nest refuse being deposited on an outside pile by A. colombica and (d) a typical underground refuse chamber of Atta sexdens which is very similar to that observed for A. laevigata.

Figure 2

δ¹⁵N-values of ants periodically collected near the treated and control nests in the tropical forest and the savannah sites (single value from merged samples). Light grey bars represent δ¹⁵N-values of ants from the treated nest, and black bars represent the average values of ants collected in the control nest. The bars shown for the tropical forest site represent the average±s.e.m. The bars shown for the savannah site represent a single value from merged samples.

Figure 3

Average δ¹⁵N-values of leaf tissue collected periodically from trees and/or shrubs in the vicinity of the tropical forest and savannah ant nests. Samples were collected within 11 m in the tropical forest site and within 5 m in the savannah site. Filled circles represent the δ¹⁵N isotope ratios of leaves from plants near the treated nest in either the forest or savannah site. Open circles represent δ¹⁵N isotope ratios of leaves collected in the control areas. Error bars represent ±s.e.m.

Figure 4

Mapping of trees near treated nests in the tropical forest and savannah sites. Filled circles represent trees that had leaves with significantly higher δ¹⁵N-values compared with the average of the control trees, and open circles represent trees with leaf δ¹⁵N-values similar to those of the control plants. The large cross-hatched circle in each panel represents either the external refuse pile in the tropical forest site or the nest mound in the savannah site. Distance between grid lines for both sites represents 5 m. Only trees that were tagged and sampled are shown in the figure, whereas trees farther than 15 m in the savannah site are not shown.

Figure 5

Calcium concentration (% dry weight) of leaves from individuals close to the ant nests showing significant ¹⁵N enrichment versus the calcium concentration of leaves from conspecific individuals in a control area far from the treated nest. Triangles represent the following species in the savannah site: 1, Rapanea guianensis; 2, Aspidosperma macrocarpon; 3, Caryocar brasiliense; 4, Roupala montana; 5, Kielmeyera coriacea and 6, Sclerolobium paniculatum. Circles represent the following species in the tropical forest site: 7, Chrysophyllum argenteum; 8, Tocoyena pittieri; 9, Quararibea asterolepis and 10, Faramea occidentalis. Horizontal error bars for the savannah site represent ±s.e.m.