Beggars can't be choosers: Competitive dominance and size‐dependent resource investment jointly constrain partner associations in a multispecies mutualism

Competition theory predicts that dominant species should gain preferential access to more valuable resources, whereas optimal foraging theory predicts that organisms should invest more heavily in higher-value resources. Together, these theories predict that dominant species should exhibit stronger value-dependent investment than subordinate species, consistent with the adage that 'beggars can't be choosers'. In mutualisms, where partners differ greatly in the rewards they provide, it remains unknown how competitive hierarchies influence associations with partners that vary in value. We tested whether competitive dominance and value-dependent investment jointly determine partner associations in an ant-aphid mutualism, focusing on resource quantity as a key axis of value. We measured ant responses to variation in resource quantity using experimental sugar baits and repeated censuses of wild aphid colonies spanning a range of sizes. The dominant ant preferentially recruited to larger baits and tended the largest aphid colonies, whereas subordinate ants increased recruitment with bait size only when the dominant ant was excluded. Aphid colony size also influenced behavioural investment in tending: the dominant ant showed strong increases in tending initiation, continuation and forager allocation with colony size, while subordinate ant taxa responded more weakly. Together, these results demonstrate that aphid colonies function as a graded resource whose size shapes competition among ants, leading dominant taxa to monopolize larger aphid colonies while constraining subordinate taxa to smaller ones. More broadly, these findings show that partner associations emerge from the interaction between competitive dominance and value-dependent investment, integrating classical competition and optimal foraging theories to explain partner associations in a multispecies mutualism.