Getting a handle on ants’ activities
Seed dispersal by ants (myrmecochory) is a
feature of sclerophyll ecosystems in different parts of the world, but, as
discussed by Leal et al.
(Recife, Brazil and Kaiserslauten, Germany, pp. 885–894), little is known
of its importance in Brazil’s
Caatinga ecosystem. This consists of patches of seasonally dry forest and
sclerophyll vegetation. Although some seeds may be fortuitously dispersed by
ants, it is generally held that seeds adapted for myrmecochory possess an
appendage, the elaiosome. This is believed to serve two functions. First, it
may be used as a handle enabling the ants to carry the seed to the nest.
Secondly, it provides food for the ants. After consumption of the elaiosome,
the ‘cleaned’ seed is discarded. The authors’ initial observations revealed
that seeds of approx. 25 % of the woody species in this region were
‘manipulated’ by ants but only about half of these, nearly all members of the
Euphorbiaceae, exhibited true myrmecochory. In some of these species,
myrmecochory was preceded by ballistic discharge. For further study of ant–seed
interactions, the authors set up a 100-m transect and placed seeds of seven of
the myrmecochorous euphorbs along it at 10 m intervals. Subsamples of seeds had
their elaiosome removed prior to being placed. Seeds were set out at 0700 h and
ant behaviour was observed at intervals until 1800 h. Ants collected and
transported eliaosome-bearing seeds at twice the frequency of elaiosome-less
seeds. Of the seeds picked up by ants, over 80 % were eventually discarded on
nests; dispersal distances ranged from a few centimetres to over 11 m. Perhaps
surprisingly, the ants only removed elaiosomes from about one-third of the
seeds transported to the nest. This is important because in greenhouse
experiments, removal of the elaiosome led to a 30 % increase in germination.
Further, in all the plant species tested, percentage germination was higher in
soil from ants’ nests than in soil from random sites.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
