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Plants use many strategies to defend themselves against animals that eat plants, also called herbivores. One clever way that some plants, called myrmecophytes, do this is by teaming up with ants. Myrmecophytes are plants that have evolved close relationships with ants. In return for protection from herbivores, myrmecophytes provide ants with food and/or shelter. Sounds like a great deal, right? Yes, but sometimes working with ants can be tricky for a plant. Because myrmecophytes cannot choose the ant bodyguards that they get, they must balance their needs with the needs of their ant defenders. In this article, we introduce a classic example of plants that have beneficial relationships with ants, describe how ants and plants in these relationships depend on each other, and investigate how these ant-dependent plants interact with various ant species.

Myrmecophytes may adjust the investment on ant rewards, depending on tree size and ant defence level. In swollen-thorn acacias (Vachellia collinsii), we tested whether the level of protection provided by the resident ants (defending vs. non-defending) influenced the relation between tree size and ant rewards, or between types of ant rewards (housing and food). We quantified ant rewards in trees occupied by defending and by non-defending ants. We predicted: (1) a positive relation between plant diameter and ant reward investment, with a steeper slope for defending than for non-defending ant species; and (2) that if there is any tradeoff between ant rewards, it should be aggravated (steeper slope) when inhabited by non-defending ants. We found that most structures for ants grew according to plant diameter, but contrary to our first prediction it was independent of the level of ant defence. Most ant rewards did not show a tradeoff between them, besides a weak negative relation between spine length and number of pinnules, which contrary to the prediction occurred when occupied by defending ants. The evidence shows that the interacting ants had a weaker influence on the scaling of defence structures in myrmecophytes than the habitat (location).

Cleaning associations, which involve a "cleaner" species that removes and feeds on parasites, debris and other material from a "client" species, are a widespread type of interspecific interaction found in both aquatic and terrestrial ecosystems (Caves, 2021;Sazima et al., 2012).While largely cited as positive interactions, such associations can range in their specificity and/or outcome depending on the species involved, the client's parasite load, type of matter removed by cleaners, and environmental conditions (Caves, 2021;Cheney & Côté, 2005;Vaughan et al., 2017). Cleaning associations can obligate mutualisms if they represent a major food source for cleaners and result in a sizeable reduction in parasites that often inflict painful wounds, act as vectors of pathogens, and/or are a nuisance to clients (Poulin & Grutter, 1996). In other cases, cleaning associations can be facultative, with interactions being largely opportunistic and/ or commensal if parasite removal effects are negligible for clients (Caves, 2021;Sazima, 2011). Additionally, the way in which parasites or pests are removed by cleaners can further influence the outcome of these cleaning associations. For instance, clients may benefit from