Mechanistic understanding of climate-driven range shifts: using thermal tolerances of rock lobster to predict future change

Climate change and ocean warming are driving changes in marine ecosystems globally. One of the most observed alterations are species' geographical re-distributions, or range shifts. Many of these shifts have been poleward extensions of species' ranges, however there is high variation in many aspects of these shifts. What causes these differences in species responses is likely complex and we currently do not have a strong understanding of the cause of this variation. Therefore, there is a pressing need to understand the mechanisms behind species' range shifts to be able to better predict future changes. One way to begin to understand what may cause a species to shift their range is to investigate their physiological and behavioural thermal tolerances. Here, the thermal tolerances of a range of performance measures of a local and a range-shifting species of spiny lobster in south-east Australia were explored, and potential responses of both species to future ocean warming in the region were modelled. Eastern rock lobster, Sagmariasus verreauxi, are a large species of spiny lobster found along the east coast of Australia and are suspected of undergoing a range shift further into Tasmanian waters, currently occupied by the resident species, southern rock lobster, Jasus edwardsii. The effect of this range-shifting species and its potential interaction with the local species has implications for the valuable local commercial J. edwardsii fishery, as well as for the marine ecosystem in Tasmania. Understanding potential mechanisms behind species' range shifts is key to being able to predict future changes and proactively manage resources sustainably. Thermal performance curves allow exploration and visualisation of how a species' measure of performance changes with temperature. Thermal performance curves of multiple measures of performance for the two species of lobster, J. edwardsii and S. verreauxi, at two different life stages, puerulus (final larval stage) and juvenile, were investigated. Using intermittent-flow respirometry, the effects of temperature on aerobic metabolism and aerobic scope across the temperature gradient were developed into thermal performance curves. The effect of temperatures on escape speed, an important survival performance measure, was also measured. It was found that the two species have different thermal tolerances between multiple measures of performance as well as between life stages. This suggests that a single measure of performance may not accurately be able to predict whole-organism changes under ocean warming scenarios. To further determine whether individual species' thermal tolerances can be used to predict changes to species' interactions under ocean warming scenarios, competitive trials for food resources between the two lobster species over a range of temperatures were conducted. Single adult individuals of both species were placed in a tank with a single food item and the resulting interaction filmed for analysis. Jasus edwardsii was successful in obtaining the food item before S. verreauxi in the majority of trials. Jasus edwardsii also exhibited significantly more aggressive behaviours than S. verreauxi, who exhibited significantly more submissive behaviours over the temperatures range tested. These results indicate that Jasus edwardsii is competitive at and above temperatures determined optimal for other performance measures in previous studies. It also suggests that while individual thermal tolerances are valuable measures for individual performance under changing climate conditions, they may not be sufficient in predicting changes to the outcomes of interactions due to unforeseen indirect effects in terms of organism behaviour. To gain a more comprehensive understanding of how populations may react to changing ocean conditions, modelling approaches can be used to investigate aspects of species performance not readily obvious from experimental results. Models of Intermediate Complexity for Ecosystem assessments (MICE) are flexible, targeted models that can be used to answer specific questions such as; how will a range shifting lobster affect a resident species in Tasmania? The two-part model first explores how individual physiological thermal tolerances affect projected biomass under a range of scenarios, and secondly incorporates competitive interactions into the framework to project changes as a result of indirect effects of ocean warming. From part one of the model, population trajectories that incorporated physiological data into the model greatly changed the projected biomass. Jasus edwardsii projected biomass remained stable or increased slightly over the 50-year projection, while S. verreauxi biomass is projected to greatly increase, a result consistent with their thermal tolerances and future projected ocean warming in the region. Incorporating competitive interactions in the second part of the model changes the dynamics due to the two populations sharing resources. In these projections, J. edwardsii projected biomass decreased, while S. verreauxi projected biomass increased. Here, it is assumed that the effects of thermal tolerances of the two species outweighs the increased competitive ability of J. edwardsii over S. verreauxi. These results show that incorporating thermal tolerance data into models can greatly affect the outcome of the projections, and that indirect effects of climate changes such as those to species interactions may have considerable effects on populations with potential knock on effects for marine ecosystems. These results all indicate that the mechanisms facilitating or hindering species range shifts are complex and not always easily apparent. While predictions about individual species performance under climate change may be relatively simple to develop, the ability to predict changes to marine communities or ecosystems is much more complicated. Here it is shown that a wide range of techniques can be used to aid in our pursuit for greater understanding of species' range shifts and their potential effects on marine communities. Ultimately, a comprehensive framework is required that incorporates both experimental and modelling techniques to best grasp current and future changes to both individual species and marine ecosystems.