A fundamental challenge of marine ecology is to understand climate change induced range shifts. At the level of individual ecosystems, the impacts of these range shifts will result from the complex changes in community composition. This forces us to consider not simply individual species, but the species that may replace them and play similar roles in an ecosystem. Here we consider the important zooplankton species Calanus finmarchicus and C. helgolandicus. Recent climate change has resulted in the gradual replacement of C. finmarchicus by C. helgolandicus in the North Sea. However, the ability of C. helgolandicus to fully replace C. finmarchicus has been questioned by some researchers. We therefore sought to fill a key knowledge gap. The comparative differences between the two species have never been critically reviewed. Further, the relative geographic distributions of both species have never been related to inter-species differences in biology or ecology. This thesis has two key elements. First, we critically review and synthesize existing knowledge of inter-species differences between C. finmarchicus and C. helgolandicus, overturning many assumptions common in the literature. Second, we produce a unified population model of both species across the North Atlantic, which relates differences in geographic distribution to inter-species differences in biology. This model is then used to highlight the limits of current understanding of mortality, and of the vital importance of improved quantitative knowledge of overwintering if we are to understand the future evolution of both species' distributions.