Host performance varies between ranges (iii)
We then hypothesized that at the elevated temperature (22 °C), non-native algae would show superior performance. This implies that, independent of a general effect of range, the interaction between range and temperature is significant. If non-native algae are indeed more tolerant to thermal stress, performance should decline specifically for natives in the warm treatment, whereas performance among non-natives would be affected less or not at all. Our results, however, do not support a significant interaction between range andtemperature . Instead, they reflect a strong main effect of range on both RGR and thallus brittleness. While the RGR was over 5-fold higher for non-natives, which would suggest better performance, non-natives were also 3-fold more likely to develop the thallus brittleness symptom (Figure 2C, D). Therefore, non-native hosts do not simply perform better, but may be more susceptible to disease at the same time. Increased growth rates have been typically linked with invasions (Van Kleunen et al., 2010) and it has also been found that the selection for such invasiveness may come at the cost of other traits, such as for instance, traits linked to stress resistance or disease (Burns, Halpern, & Winn, 2007; Lambers & Poorter, 2004). However, Sotka et al. (Sotka et al., 2018) found in a common garden study that in response to heat (40 °C), native G. vermiculophylla were more likely to bleach compared to non-natives. In our experiment the occurrence of bleaching was overall low and there was no detectable difference between natives and non-natives. Possibly, bleaching is more likely to occur at higher temperatures than the treatment applied here, but the thallus brittleness we observed (andSotka et al.; 2018, did not) may be a symptom that is more specific to non-native G. vermiculophylla . We note that Sotka et al. (2018) used apices in their experiments and may therefore have not been able to observe thallus brittleness.