Swollen-thorn acacias increased their investments in resident ants during an El Niño event.

Abstract

Species interactions are frequently context-specific. Climate fluctuations can inflict stress on individuals, inducing plasticity in their behavior or changes in resource allocation. Obligate mutualisms may be particularly sensitive to such fluctuations if they affect the quantity, quality, or timing of investments between partners. In the obligate mutualism between Pseudomyrmex ants and swollen-thorn acacias (Vachellia collinsii), the ants defend their host plant in exchange for domatia, extrafloral nectar, and protein-rich Beltian bodies. Ant reward size is known to vary temporally and according to the identity of the associated ant partner, with better-quality defenders (P. spinicola and P. simulans) receiving relatively more plant resources than non-defending ants. We studied how an El Niño-related drought impacted the investments of acacias in their ant colonies. Hence, we resampled acacias that were previously measured for ant rewards during a neutral El Niño Southern Oscillation period again during El Niño 2023. We predicted that water limitations would reduce the resources available for allocation toward ant colonies and that acacias hosting obligate-nesting Pseudomyrmex would show a greater decrease in ant rewards compared to plants associated with non-defending ants (Crematogaster sp.). Unexpectedly, we found that acacias allocated more resources to ant rewards in 2023, irrespective of the ant partner. Heightened herbivore pressure, increased light availability, decreased precipitation, or a combination of these factors could explain why acacias invested more in mutualism with ants. Though the long-term consequences of these climate fluctuations remain unknown, this work highlights the plasticity of coevolved mutualistic interactions in the face of global change.