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1. Competition-defence trade-offs have long been thought to promote plant coexistence and increase species diversity. However, insights from modern coexistence theory clarify that such trade-offs alone cannot sustain coexistence. Coexistence also requires stabilising niche differences and the ability of competitor populations to persist under consumer pressure. Despite the breadth of potential consequences of competition-defence trade-offs, we have little mechanistic understanding of how they affect diversity in natural communities.

2. We investigated the effects of seed harvesting by ants on coexistence in an annual plant community. We parameterised a model of plant competitive population dynamics with data from two field experiments: (a) plant demographic rates and competition coefficients determined by growing plants alone and against intra- and interspecific competitor density gradients; (b) plant fitness losses to ant consumers determined by measuring seed removal from experimental depots. We tested for a trade-off between a plant species’ demographic potential and its susceptibility to consumption and then determined the bounds of ant effects on pairwise and multispecies coexistence by comparing mod& projections with and without seed consumption.

3. Ants harvested seeds of all plant species, but they strongly preferred the competitively superior large seeded species, inducing a competition-defence trade-off. Unequal seed loss to ants changed competition-based fitness hierarchies, affecting both the number and identities of plant species pairs that were predicted to coexist compared to a scenario without ants. The trade-off most often prevented coexistence by severely disadvantaging the superior competitor or excluding species directly through overconsumption, and a simulated reduction in the overall consumption rate opened few additional opportunities for coexistence. Ant exacerbation of average fitness differences was particularly disruptive to multispecies coexistence, where niche differences were insufficient to stabilise the coexistence of plant triplets and quadruplets.

4. Synthesis. Our results show that the presence of a competition-defence trade-off in a community with stabilising niche differences does not always increase diversity. Instead, the full range of diversity outcomes-positive and negative changes in species number and changes in the identity of the dominant-are possible. Taken together, our results support the emerging paradigm that consumers have wide ranging impacts on plant diversity and suggest that variation in consumer pressure may be an important driver of large-scale diversity patterns.