Thermal reaction norms of a subtropical and a tropical species of Diaphanosoma (cladocera) explain their distribution

Abstract

Species distribution boundaries are often associated with aspects of climate and modified by competitive interactions. In the tropical region, Diaphanosoma is the most diverse genus of Cladocera. Diaphanosoma dubium is widely distributed in China, whereas the tropical Diaphanosoma excisum is restricted to the islands off the south China coast. The mean temperature of the coldest month at the northern limit of its distribution is 15°C. We studied the effect of temperature (10–40°C) on life history traits of clones from 16 populations of D. dubium and D. excisum in life-table experiments in order to determine if their thermal performance curves (TPCs) explain their distribution and to estimate the extent and mode of genetic variation in their TPCs. Our results show that both species have higher optimum temperatures (To ∼ 33°C), critical thermal minimum (CTmin = 10–15°C), and maximum (CTmax = 39–40°C) than temperate cladocerans. Clones of D. excisum had narrower TPCs and higher To than D. dubium clones, in accordance with predictions based on changes of temperature distributions with latitude. Significant levels of genetic variation in TPCs were present in both species. D. excisum had higher fitness than D. dubium at all temperatures above 15°C. However, at 15°C and below, D. dubium fitness was higher. Negative r at 15°C and below in combination with competition with the more cold-adapted D. dubium exclude D. excisum from mainland China. TPCs of interacting species may be the basis of predicting future species distributions as temperature increases change the balance of biotic interactions.