Range limits, range shifts, and lower thermal tolerance in the tropical barnacle Megabalanus coccopoma.

摘要

Climate change and species introductions are leading to shifts in marine species' geographic distributions. However, few studies have examined range limits and range shifts in tropical marine species. The barnacle Megabalanus coccopoma is native to shorelines from Baja California to Peru and has been introduced to a number of locations worldwide including the Atlantic US SE coast, where its current introduced range extends from Ft. Pierce, FL to Cape Hatteras, NC with seasonal populations as far north as Kitty Hawk, NC. In order to understand the potential poleward range limit of M. coccopoma in the US SE under modern day climate regimes, the larval and adult lower thermal tolerances were examined and larval energy budgets were used to estimate the potential extent of larval dispersal. The distribution of M. coccopoma was then surveyed within the Atlantic US SE during the summers of 2010, 2011, and 2012. Changes in the range documented during these surveys were used to test the predictive accuracy of the species distribution model MaxEnt using data from the global range, the native range and the Atlantic US SE range. In contrast to work done with temperate species, which suggests larval thermal tolerances often set range limits, adult thermal tolerances corresponded most closely to current range limits. Larval energetic reserves may limit the extent of the seasonal range. Surveys revealed that, following the cold winter months of 2010, the range of M. coccopoma retracted down to northern Florida, rapidly re-expanded during 2011 and was re-established in its previous range by the summer of 2012. This range re-expansion is the fastest documented for any marine invertebrate and is one of the few documented for any tropical marine organism. When MaxEnt was used to predict the range retraction and range limits of M. coccopoma, only the MaxEnt model trained on the US SE data was able to accurately predict both the range retraction and range limits prior to the range retraction. Consideration of the range limiting mechanism(s) and spatial variation in the mechanism(s) is fundamentally important in accurately predicting range limits and even more so in predicting range shifts. In many marine species, and particularly in tropical species, lower limits to functionality and cumulative chill damage are likely the mechanisms responsible for setting poleward range limits at above freezing temperatures. Limited selection in tropical marine species for the ability to acclimate to cold temperatures may result in more ephemeral range expansions when compared to temperate species when both are moving poleward.