AOBPreview originally published online on April 12, 2007
Annals of Botany 2007 99(5):885-894; doi:10.1093/aob/mcm017
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Seed Dispersal by Ants in the Semi-arid Caatinga of North-east Brazil
1 Departamento de Botânica, Universidade Federal de Pernambuco, 50670-901, Recife, PE, Brazil
2 Department of Plant Ecology and Systematics, University of Kaiserslautern, Postfach 3049, 67653, Kaiserslautern, Germany
* For correspondence. E-mail irleal{at}ufpe.br
Received: 7 November 2006 Returned for revision: 12 December 2006 Accepted: 3 January 2007 Published electronically: 12 April 2007
Background and Aims: Myrmecochory is a conspicuous feature of several sclerophyll ecosystems around the world but it has received little attention in the semi-arid areas of South America. This study addresses the importance of seed dispersal by ants in a 2500-km2 area of the Caatinga ecosystem (north-east Brazil) and investigates ant-derived benefits to the plant through myrmecochory.
Methods: Seed manipulation and dispersal by ants was investigated during a 3-year period in the Xingó region. Both plant and ant assemblages involved in seed dispersal were described and ant behaviour was characterized. True myrmecochorous seeds of seven Euphorbiaceae species (i.e. elaiosome-bearing seeds) were used in experiments designed to: (1) quantify the rates of seed cleaning/removal and the influence of both seed size and elaiosome presence on seed removal; (2) identify the fate of seeds dispersed by ants; and (3) document the benefits of seed dispersal by ants in terms of seed germination and seedling growth.
Key Results: Seed dispersal by ants involved one-quarter of the woody flora inhabiting the Xingó region, but true myrmecochory was restricted to 12·8 % of the woody plant species. Myrmecochorous seeds manipulated by ants faced high levels of seed removal (38–84 %) and 83 % of removed seeds were discarded on ant nests. Moreover, seed removal positively correlated with the presence of elaiosome, and elaiosome removal increased germination success by at least 30 %. Finally, some Euphorbiaceae species presented both increased germination and seedling growth on ant-nest soils.
Conclusions: Myrmecochory is a relevant seed dispersal mode in the Caatinga ecosystem, and is particularly frequent among Euphorbiaceae trees and shrubs. The fact that seeds reach micro-sites suitable for establishment (ant nests) supports the directed dispersal hypothesis as a possible force favouring myrmecochory in this ecosystem. Ecosystems with a high frequency of myrmecochorous plants appear not to be restricted to regions of nutrient-impoverished soil or to fire-prone regions.
Key words: Caatinga, Euphorbiaceae, directed dispersal hypothesis, myrmecochory, north-east Brazil, seed dispersal, seed germination, seedling growth