Coexistence and intertidal zonation of chthamalid barnacles along central Chile: Interference competition or a lottery for space?

In systems with open populations, both local (e.g. hierarchical interference competition) and regional processes (e.g. recruitment limitation and source-sink dynamics) can be important in determining patterns of abundance and coexistence of species. In this study, we define patterns of local distribution and recruitment of two common intertidal chthamalid barnacles (Jehlius cirratus and Notochthamalus scabrosus) at 15 sites along the shore of central Chile. Surveys revealed patterns of adult distribution to be consistent across the study region; Jehlius was numerically dominant at the highest intertidal levels and Notochthamalus was more abundant at lower shore levels. Despite slight spatial segregation, our results show that there is ample overlap in the distribution of these species, with inter-specific mixing over scales of few centimeters, occurring over 60–80% of the entire barnacle zone. Recruitment rates of the two species were highly correlated among sites (separated by kms), suggesting similar determinants of onshore larval transport between these species. Additionally, we found evidence that Notochthamalus was recruitment limited while Jehlius experienced higher post-settlement mortality. Densities of established individuals were generally positively correlated at scales of just 25 cm2 and largely uncorrelated over scales of 2500 cm2 across sites and tidal levels, indicating that asymmetric inter-specific competition is unlikely to affect adult populations. After monitoring individuals under various crowding conditions (single individuals, surrounded by conspecifics, and surrounded by heterospecifics) and at different tide elevations for more than a year, we found no evidence of negative intra- or inter-specific effects on individual growth rates. We conclude that lottery-type interactions and species-specific post-settlement mortality rather than an inter-specific competitive hierarchy likely determine patterns of coexistence of Jehlius and Notochthamalus in central Chile.