Opposing e � ects of floral visitors and soil conditions on 1 the determinants of competitive outcomes maintain 2 species diversity in heterogeneous landscapes

Theory argues that both soil conditions and aboveground trophic interactions are equally important for 14 determining plant species diversity. However, it remains unexplored how they modify the niche di erences 15 that stabilise species coexistence and the average fitness di erences driving competitive dominance. 16 We conducted a field study in Mediterranean annual grasslands to parameterise population models 17 of six competing plant species. Spatially explicit floral visitor assemblages and soil salinity variation 18 were characterized for each species. Both floral visitors and soil salinity modified species population 19 dynamics via direct changes in seed production and indirect changes in competitive responses. Although 20 the magnitude and sign of these changes were species specific, floral visitors promoted coexistence at 21 neighbourhood scales while soil salinity did so over larger scales by changing the superior competitor’s 22 identity. Our results show how below and aboveground interactions maintain diversity in heterogeneous 23 landscapes through their opposing e ects on the determinants of competitive outcomes. 24