Plant Killing by Mutualistic Ants Increases the Density of Host Species Seedlings in the Dry Forest of Costa Rica

Sabrina Amador-Vargas

doi:10.1155/2012/491592

Abstract

Some species of plant-mutualistic ants kill the vegetation growing in the vicinities of their host plant, creating an area of bare ground (clearing). The reduced competition in the clearing may facilitate the establishment of host species sprouts (clones and seedlings), which in turn benefits the ants with additional food and shelter (“sprout-establishment hypothesis”). To test this hypothesis, the density and origin ofAcacia collinsiisprouts growing inside clearings and in the vicinities of acacia plants without clearings were compared. Also, to assess the pruning selectivity of acacia ants (Pseudomyrmex spinicola), seedlings were transplanted into clearings. The reaction of ants towards unrewarding acacia seedlings (without food and shelter) was also tested. The density of acacia sprouts growing inside clearings was almost twice that in the vicinities of host plants without clearings, and sprouts were inhabited by nestmates of the colony that made the clearing. Clones and seedlings were found in similar proportions in the clearings, and ants did not kill unrewarding acacia seedlings or seedlings unrelated to their host. The benefit reported here for the ants could be in conflict with the host plant, especially when the plant has rhizomal reproduction.

Highlights

In obligatory ant-plant mutualisms, the ants obtain food and shelter from their host, and, in exchange, they defend the plant against herbivores [1, 2]
The majority (88%) of the A. collinsii sprouts growing inside clearings were occupied by ants from the same colony of the experimental acacia; the remaining 12% were unoccupied (Figure 2)
The sprouts near acacias without clearings showed different proportions of sprouts on each category (χ2 = 11.8, d.f. = 2, P = 0.003): 65% were occupied by ants that were not attacked by the resident colony, 33% were unoccupied, and 2% were occupied by another species of ant (Crematogaster brevispinosa ants inhabited the main acacia, but the sprouts had P. spinicola ants)

Summary

Introduction

In obligatory ant-plant mutualisms, the ants obtain food and shelter from their host, and, in exchange, they defend the plant against herbivores [1, 2]. In addition to attacking insect or vertebrate herbivores, some ants (resident ants) kill plants in the vicinity of their host tree by biting or poisoning [1, 3,4,5,6,7,8,9,10], functioning as allelopathic agents [4]. The ants leave an area of bare ground around the host plant (“clearing”; Figure 1). Two main functions have been attributed to the clearings: (1) isolating the colony and (2) reducing competition for the host plant. The isolation that the clearings provide to the host tree may prevent invasions from other ants by reducing or eliminating access venues to the colony that other invasive ants might use to enter the tree. Crematogaster ants living in nonwaxy species of Macaranga trees pruned more intensively than those living on trees with waxy barriers [9], presumably because ants in waxy trees are already isolated from potential intruder ants, which have difficulty walking on waxy surfaces

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Results

Conclusion