Spatial segregation of closely related species is usually attributed to differences in stress tolerance and competitive ability. For both animals and plants, reproductive interactions between close relatives can impose a fitness cost that is more detrimental to the rarer species. Frequency-dependent mating interactions may thus prevent the establishment of immigrants within heterospecific populations, maintaining spatial segregation of species. Despite strong spatial segregation in natural populations, two sympatric California monkeyflowers (Mimulus nudatus and M. guttatus) survive and reproduce in the other’s habitat when transplanted reciprocally. We hypothesized that a frequency-dependent mating disadvantage maintains spatial segregation of these monkeyflowers during natural immigration. To evaluate this hypothesis, we performed two field experiments. First, we experimentally added immigrants in varying numbers to sites dominated by heterospecifics. Second, we reciprocally transplanted arrays of varying resident and immigrant frequency. Immigrant seed viability decreased with conspecific rarity for M. guttatus, but not M. nudatus. We observed immigrant minority disadvantage for both species, but driven by different factors– frequency-dependent hybridization for M. guttatus, and competition for resources and/or pollinators for M. nudatus. Overall, our results suggest a major role for reproductive interference in spatial segregation that should be evaluated along with stress tolerance and competitive ability.

Three data sets and one R code file are included. The 2019 transect data was collected at one site where both M. nudatus and M. guttatus were present in close proximity at the UC McLaughlin Reserve. The 2018 M. guttatus immigration experiement data was collected at a single transplant site dominated by M. nudatus at the UC McLaughlin Reserve. The 2019 frequency-manipulation reciprocal transplant data was collected at two transplant sites dominated by either M. guttatus or M. nudatus at the UC McLaughlin reserve.  Fruit were collected at the end of the season, then seeds were counted from every transplant and categorized as viable or inviable based on morphology. The statistical analysis R code tests whether the number of conspecific flowers was associated with seed viability in the transect, whether the number of conspecific neighbors was associated with fitness components in the immigration experiment, whether immigrant frequency treatments affected fitness components and lifetime fecundity in the reciprocal transplant.

README.txt describes the R code and data files, including column descriptions for each data file.