Résumé
Species engaged in multiple, simultaneous mutualisms are subject to
trade-offs in their mutualistic investment if the traits involved in each
interaction are overlapping, which can lead to conflicts and affect the
longevity of these associations. We investigate this issue via a
tripartite mutualism involving an ant plant, two competing ant species and
a fungus the ants cultivate to build galleries under the stems of their
host plant to capture insect prey. The use of the galleries represents an
innovative prey capture strategy compared with the more typical strategy
of foraging on leaves. However, because of a limited worker force in their
colonies, the prey capture behaviour of the ants results in a trade-off
between plant protection (i.e. the ants patrol the foliage and attack
intruders including herbivores) and ambushing prey in the galleries, which
has a cascading effect on the fitness of all of the partners. The
quantification of partners' traits and effects showed that the two
ant species differed in their mutualistic investment. Less investment in
the galleries (i.e. in fungal cultivation) translated into more benefits
for the plant in terms of less herbivory and higher growth rates and vice
versa. However, the greater vegetative growth of the plants did not
produce a positive fitness effect for the better mutualistic ant species
in terms of colony size and production of sexuals nor was the mutualist
compensated by the wider dispersal of its queens. As a consequence,
although the better ant mutualist is the one that provides more benefits
to its host plant, its lower host–plant exploitation does not give this
ant species a competitive advantage. The local coexistence of the ant
species is thus fleeting and should eventually lead to the exclusion of
the less competitive species.