Sterilization and canopy modification of a swollen thorn acacia tree by a plant-ant

Abstract

Obligate symbioses between specialized arboreal ants and plants have evolved independently in many lineages1,2. Ant-plants (myrmecophytes) typically provide hollow nest cavities and nutrition to the occupying ant colony1,3,4,5,6. In turn, resident plant-ants often protect their hosts from herbivory7,8,9,10,11 and/or overgrowth by surrounding vegetation12,13. As individual plants are rarely occupied by more than one ant colony14,15,16,17, co-occurring plant-ant species compete intensely for hosts13,14,18,19. In such multi-species systems, ecological interactions among potential partners may lead to the evolution of cheating20,21. Previous studies have revealed that some specialized plant-ants are effectively parasites of their host-plants8,18,22,23, but the selection pressures favouring such behaviours are poorly understood. Here we describe host parasitism in an east African plant-ant that prunes and sterilizes its host-tree canopies, apparently to minimize contact with competitively dominant ants occupying neighbouring trees. We propose that the high density of ant-trees and low diversity of tree species in this savanna habitat have selected for induced, parasitic pruning of host trees by this competitively subordinate ant species.

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Figure 1: Close-up of A. drepanolobium branches showing axillary leaves, paired slender thorns and a node occupied by a fused swollen thorn pair.
Figure 2: Canopy attributes of A. drepanolobium trees occupied by four different species of acacia ants.
Figure 3: Outcomes of staged conflicts between colonies of different ant species inhabitating neighbouring A. drepanolobium trees.
Figure 4: Growth asymmetry of A. drepanolobium canopies, as a function of ant occupant and the presence of hostile ant colonies on neighbouring trees.

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Acknowledgements

We thank J. Lemboi, L. King, F. Lorugoi and F. Frei for field assistance, and the staff of Mpala Research Centre and Segera Ranch for logistical support. Mpala Ranch and Segera Ranch granted access to field sites. This research was supported by the National Science Foundation and by the Bridge Grant program at the University of California at Davis.

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Correspondence to Maureen L. Stanton.

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Stanton, M., Palmer, T., Young, T. et al. Sterilization and canopy modification of a swollen thorn acacia tree by a plant-ant. Nature 401, 578–581 (1999). https://doi.org/10.1038/44119

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