Species distribution across environmental gradients on tropical rocky shores: how well does physiology explain distribution patterns?Broad-scale Spatial Patterns

Tommy Tin Yan Hui¹, Amy Ching Hei Leung², Nutcha Buasakaew², Phoebe Pui Ching Leung¹

¹Lingnan University, China - ²The University of Hong Kong, China

Understanding mechanisms driving species distribution is key to predict ecological responses under global changes. One such mechanism is species’ physiological tolerance which defines their potential niche and thus distribution along environmental gradients. On tropical rocky shores, temperature and salinity are two major physical factors controlling species distribution, where extreme variations in these conditions (> 50 °C in rock temperature or < 10 psu in salinity) can lead to substantial performance loss and even mortality. Using an island in Hong Kong with spatially contrasting hydrological regime as a model, the synergistic impacts of hyposaline and thermal stress were investigated by exposing ten rocky shore species to various durations of these stress in the laboratory. Species were differentially susceptible to combined salinity and thermal stress, with mid shore species (e.g. topshells, limpets, mussels) being less tolerant than the high shore ones (e.g. periwinkles and nerites). Habitat suitability estimated based on these laboratory measurements and on-shore recordings of physical conditions, however, showed that species’ acute physiological tolerance could only partially explain their distribution on the island (e.g. species were absent in localities where predicted habitat suitability was high). Such a mismatch between predictions derived from acute physiological tolerance and on-shore patterns highlights the hierarchical relationship among various processes in driving species distribution, including the regional species pool as shaped by larval supply and physiological tolerance over longer, more sustained time periods. These processes, which operate over larger and longer spatio-temporal scales than short-term physiological responses, may undermine the power of physiological tolerance alone to explain distribution patterns and, as such, should be considered carefully when interpreting physiology-informed species distribution models.