Invasion physiology : do physiological characteristics facilitate the spread of invasive fish species?

A considerable number of abiotic (e.g. temperature) and biotic factors (e.g. intra-interspecific interactions) contribute in shaping species’ distribution and invasiveness but knowledge is still lacking regarding the importance of physiological and behavioural traits in determining the distributions of ectotherms and especially the invasion success of non-native species into novel habitats. With rising temperatures, distribution shifts in many of fish species have been observed. Additionally, changing thermal conditions are facilitating the colonisation of invasive species. It is crucial that we gain an increased understanding of the mechanisms underlying the effects of environmental change on the distribution of aquatic species and the ecological damage caused by invasive species. Physiological traits are likely to present a fundamental constraint on the environments that are habitable to a given species. Whole animal traits associated with energy metabolism and locomotory performance are especially likely to be important in this regard. Nevertheless, the effects of traits such as metabolic rate and aerobic scope (AS) on the distributions of native and invasive species have not been thoroughly studied. In a first step toward understanding these effects, I performed a phylogenetically-informed analysis of links between AS and absolute latitudinal distribution range in 86 fish species. From the results obtained in Chapter 2, I found no evidence for the direct implication of AS in the currently observed distributions of fishes. Moreover, no association between AS and invasion success in 59 freshwater fish species (23 invasive and 36 native species) was found. These results suggested that peak AS is not a constraining or determining factor in the distribution range and invasion success in fish. Following on from these results, there is a possibility that there could be a trade-off between peak and breadth of performance for AS across temperatures in fishes. Species with a higher peak AS might only be able to function normally over a narrow range of temperatures. In Chapter 3, I collected AS data from literature and conducted phylogenetical-informed analysis to test the trade-off theory in AS across 28 fish species. No evidence could be found for a trade-off between peak and breadth performance in AS for fish. Interspecific competition between invasive and native fish species might cause changes in the structure of native fish communities. Furthermore, these interactions can vary over competitive context (e.g. for prey or cover), differ over a range of environmental factors (e.g. in response to temperature variation) and be linked directly or indirectly to species’ metabolic capacity (e.g. aerobic scope). With increasing temperatures, invasive species might gain a competitive advantage over the native species though shifts or changes in competitive behaviour and traits such as AS. After having investigated broad patterns among AS and geographical distributions in fishes, the remainder of my thesis focused on trade-offs in energy allocation and tolerance to environmental stressors in a pair of species to determine the role of aerobic capacity as a factor in competition between these two species. Specifically, I examined interactions between native stone loaches (Barbatula barbatula) and invasive bullheads (Cottus gobio), two species which occupy the same ecological niche and that are believed to compete for similar habitats. Physiological and behavioural traits could play an essential role in the spread of invasive species, particularly the internal underlying mechanisms that modulate an organism’s response to environmental changes. In Chapter 4, I examined physiological and behavioural responses of invasive bullheads and native stone loaches to acute and acclimated temperature shifts (13-21°). I found that invasive bullheads had a lower AS than stone loaches over all temperatures tested. Bullheads were also less active overall and preferred colder temperatures (17.5-19°C) than stone loaches (21-22.4°C). Therefore, changes in AS in response to thermal variation are unlikely to be a contributing factor in invasion success of bullheads in Scottish rivers. In Chapter 5, I investigated the direct competitive interactions between bullheads and stone loaches at three different temperatures (13°C, 17°C and 21°C). Overall, native stone loaches were better competitors for shelter use and in particular at colder temperatures. There was no clear causal effect of temperature or AS on competitive outcomes between these two species. Low competitive ability found in invasive bullheads suggests that bullheads may not be actively displacing stone loaches. It has been suggested that a successful invader should have a wide tolerance range for different environmental factors. For example, invasive species might be more tolerant to hypoxic events as compared to native species. In Chapter 6, I looked at the hypoxia tolerance and avoidance behaviour of bullheads and stone loaches over different dissolved oxygen (DO) concentrations (100%, 80%, 60%, 40%, 30%, 25% and 20% DO levels). Surprisingly, I found that bullheads were less tolerant to hypoxic conditions with a Pcrit value of 4.96 mg O2 l-1 at 14°C. Avoidance behaviour towards progressive hypoxia was similar between bullheads and stone loaches; both species spent most of their time utilising the shelter even in hypoxic conditions. Low tolerance towards hypoxia did not give an advantage to invasive bullheads over native stone loaches in particular during harsh environmental conditions. The results from this thesis suggest that metabolic traits may not play as strong a role in constraining species distributions as previously suspected, particularly in the specific case of interactions between native stone loaches and invasive bullheads in Scotland. Instead, other physiological factors, life history traits, and population demographics may play a primary role in affecting invasion success in this case.