Background/Question/Methods Matings between co-occurring species can result in fitness costs that disproportionately affect rarer species, and thus may limit ecological coexistence. While patterns of spatial segregation are common in closely related species, this is often attributed to differences in competitive ability or niche differences, and rarely are reproductive interactions invoked in explaining these patterns. In studies on two spatially segregated California monkeyflower species, Mimulus guttatus and M. nudatus, we previously found evidence for costly hybridization when species were transplanted in experimental sympatry. We therefore hypothesized that reproductive interference contributes to the repeated patterns of spatial segregation through immigrant rarity disadvantage. To test whether hybridization and fecundity were dependent on the frequency of the immigrant species, we performed two separate field experiments. In each study, we manipulated frequency and abundance of transplanted immigrant species into zones dominated by heterospecific plants. Results/Conclusions As hypothesized, hybridization decreased with increasing conspecific abundance and frequency of immigrant M. guttatus, but M. nudatus immigrants suffered from high hybridization rates regardless of conspecific frequency. Despite this asymmetry in frequency dependence, hybridization reduced the fecundity of rare immigrants relative to common residents of both species. However, fecundity also decreased with increasing frequency of immigrant Mimulus nudatus, possibly indicative of above-ground resource competition and/or pollen limitation. Collectively, our results suggest that frequency dependent hybridization is in part responsible for the maintenance of habitat segregation between these two monkeyflower species. More broadly, reproductive interference has the potential to underly patterns of coexistence and segregation in many other systems where spatial segregation patterns are ascribed to other factors.
