Let the best one stay: screening of ant defenders by A cacia host plants functions independently of partner choice or host sanctions

Summary Multiple plant species are engaged in defensive mutualisms with members of the third trophic level. However, mutualisms are prone to exploitation by low‐quality symbionts that do not provide the adequate service to their host. Can mutualisms proceed only when hosts identify their symbionts in advance or continuously monitor their activity, or are there other mechanisms to avoid the invasion of mutualisms by exploiters? High‐reward species amongst Mesoamerican A cacia myrmecophytes are dominantly colonized by defending mutualistic ants, whereas about 50% of the low‐reward hosts are inhabited by non‐defending exploiters. I followed the development of recently founded ant colonies on a high‐reward and a low‐reward A cacia host species over 7 months, to investigate whether reward production correlates with a preferred maintenance of defending ants on the respective hosts. Ant diversity decreased sooner on high‐reward than on low‐reward hosts, and mutualistic ants were more likely to finally dominate the high‐reward hosts. I observed an increased frequency of mutualists replacing parasites at high initial rates of reward production. Apparently, higher nectar provisioning by the host plants shifted the competitive balance between mutualistic and parasitic ants. Independently of the causal reason for the different secretion rates, producing more nectar thereby favours the maintenance of defending mutualists on high‐reward hosts. Synthesis . The aggressiveness that enables ants to outcompete other ants also underlies their defensive effect against herbivores. I conclude that hosts can preferably associate with high‐quality mutualists without measuring their effectiveness. Mutualisms remain stable when partner screening is based on traits that are relevant for the mutualistic interaction, with no need for the host to have information on the quality or identity of the symbiont.