Quantitative synthesis of context dependency in ant–plant protection mutualisms

Abstract

Context dependency, variation in the outcome of species interactions with biotic and abiotic conditions, is increasingly considered ubiquitous among mutualisms. Despite several qualitative reviews of many individual empirical studies, there has been little quantitative synthesis examining the generality of context dependency, or conditions that may promote it. We conducted a meta‐analysis of ant–plant protection mutualisms to examine the generality of context‐dependent effects of ants on herbivory and plant performance (growth, reproduction). Our results show that ant effects on plants are not generally context dependent, but instead are routinely positive and rarely neutral, as overall effect sizes of ants in reducing herbivory and increasing plant performance were positive and significantly greater than 0. The magnitude of these positive effects did vary, however. Variation in plant performance was not explained by the type of biotic or abiotic factor examined, including plant rewards (extrafloral nectar, food bodies, domatia), ant species richness, plant growth form, or latitude. With the exception of plant growth form, these factors did contribute to the effects of ants in reducing herbivory. Reductions in herbivory were greater for plants with than without domatia, and greatest for plants with both domatia and food bodies. Effect sizes of ants in reducing herbivory decreased, but remained positive, with latitude and ant species richness. Effect sizes in reducing herbivory were greater in tropical vs. temperate systems. Although ant–plant interactions have been pivotal in the study of context dependency of mutualisms, our results, along with other recent meta‐analyses, indicate that context dependency may not be a general feature of mutualistic interactions. Rather, ant–plant protection mutualisms appear to be routinely positive for plants, and only occasionally neutral.

The Direct and Ecological Costs of an Ant-Plant Symbiosis

“…If herbivores impose stronger selection on plant investment in defense in tropical areas than in temperate areas (Coley and Barone 1996;Rasmann and Agrawal 2011; but see Moles et al 2011), the benefits of resistance may be larger in the tropics and may overcome even substantial costs. For example, in their recent meta-analysis, Chamberlain and Holland (2009) found that ants reduced herbivory more on ant-associated plants in tropical than in temperate ecosystems. Unfortunately, because almost all previous research on the costs of resistance has focused on temperate herbaceous plants (Bergelson and Purrington 1996;Strauss et al 2002), it is presently not possible to evaluate whether the intensity of selection on resistance traits increases toward the equator and/or whether other factors come into play, such as changes in plant physiology, that might also affect the costbenefit ratios of myrmecophytism in different habitats (e.g., King and Caylor 2010).…”

“…These ants collect extrafloral nectar or food bodies from plants, and symbiotic species nest inside live plant cavities called domatia. These ant-plant relationships range from facultative interactions, in which plants typically associate with a diversity of free-living partners, to obligate interactions, which tend to be more species specific (Chamberlain and Holland 2009;Trager et al 2010). In obligate associations, the plants and ants rely upon each other during much of their life cycles and are rarely found without each other.…”

Ants defend coffee from berry borer colonization

“…Ants benefit plants (Styrsky and Eubanks 2007;Chamberlain and Holland 2009;Rosumek et al 2009;Trager et al 2010). Humans have known this for quite a long time.…”

“…herbivores (Chamberlain and Holland 2009;Rosumek et al 2009;Trager et al 2010). Nonetheless, in many agroecosystems, the benefits of pest control services by ants are not recognized.…”

Protection against herbivory in the mutualism between Pseudomyrmex dendroicus (Formicidae) and Triplaris americana (Polygonaceae)

“…1), suggesting that the ants play an important protective role against herbivores. This has also been demonstrated in numerous other myrmecophytes, proving that there is a benefit from the effectiveness of the ant defense against herbivores (e.g., Chamberlain and Holland 2009, Rosumek et al 2009, Trager et al 2010. Frequent observations of the experimental plants revealed that herbivory tends to occur in a few concentrated events.…”