Patterns in thermal niche equilibria of global seaweed distributions - results from biogeographic studies with implications for invasion biology and forecasting of climate change effects

The thesis covers the topics of bioinvasions and climate change in the context of global seaweed distributions under the application of niche identification tools such as ecological niche identification and species distribution models. In the first chapter, I identify the fundamental and realized niches of the invasive red alga Capreolia implexa in Chile, and use these to identify its suitable habitat range and project changes in suitability under global warming. I conclude that C. implexa’s suitable range in South America is much larger than currently occupied, which might imply a huge potential for further spread. Under global warming, the suitable range will dramatically be reduced. In the second chapter, I assess the invasive potential of non-polar seaweeds to invade Antarctica, and conclude that such species might pose an invasion risk. This challenges the climate matching hypothesis which assumes that rather polar species pose a risk. As such, the risk of invasion for Antarctica is probably much larger than previously assumed. In the third chapter, I compare fundamental and realized niches of 126 seaweed species and assess how well distributions can be predicted based on thermal tolerance limits. I report that especially cold-tolerances are poor predictors of distributional patterns, while warm-tolerances are better predictors. I conclude that many seaweed species have cold-tolerances which are not discernable from their distributions. Further, approximately 60% of global seaweed species will face major challenges under global warming, as their distributional limits closely match their warm-tolerance limits, or lie even beyond.
In conclusion, I point out challenges in thermal niche identification of seaweeds and illustrate and discuss consequences for forecasting of bioinvasions and climate change effects. Further, I highlight that global seaweed distributions will face major threat under warming conditions.