Trends and projections in invasive ecology: how susceptible to invasion are our marine ecosystems?

The spread of non-indigenous species (NIS) has been increasing in the last decades, with no signals of slowing down. As one of the main agents involved in the current biodiversity crisis, NIS lead to multi-faceted impacts on recipient communities and ecosystems. A fraction of NIS, known as invasive species, can alter ecosystem functioning and services, often associated with severe economic losses. However, before causing such impacts, all NIS need to arrive and establish in a new area and go through the same assembly processes that have shaped and structured the recipient communities. Hence, a better understanding of the underlying mechanisms behind NIS establishment is key to enhance our preventive actions directed to mitigate the spread and further effects of NIS. Addressing biological invasions using species traits (i.e. the fundamental properties directly related with the fitness of individuals) and a community perspective, can lead a new way to identify the potential characteristics of NIS that would make them prone to established and how they are associated with native species. In this thesis I explored several aspects of biological invasion in marine systems from a trait-based and community perspective.

First, I determined the degree of functional distinctiveness (i.e., niche overlap) between NIS and native species using benthic and coastal fish communities in the Baltic Sea. The Baltic Sea is characterized by an extended salinity gradient, which makes it a perfect case study to address the successful establishment of NIS within different communities that are in close proximity to each other. Moreover, I identified traits that might be beneficial for NIS and detect spatial patterns of distinctiveness in local communities. The results showed that NIS are not particularly dissimilar compared to native species in terms of their overall trait composition, but across the case study area display one or a few singular traits, such as unique burrowing abilities (benthos) or aggressive behaviour (fish). I also detected pronounced variation in the spatial patterns of distinctiveness, showing the capacity of NIS to exist within quite different native communities in the same region.

Secondly, the dominance of NIS is often overlooked, despite being a proxy of their invasion success. I investigated how abiotic and biotic factors together with the trait composition of NIS affected the degree of dominance (i.e. relative biomass) of two widespread NIS: the polychaete Marenzelleria and the round goby (Neogobius melanostomus). I observed that both environmental and biotic factors explain NIS dominance, with a slightly larger effect of the biotic variables. Interestingly, the dominance of both NIS was similarly affected by species richness and evenness, suggesting analogous assembly processes acting on both NIS. However, the two NIS showed different individual strategies based on their degree of niche overlap with natives, by preferring either a more specialized niche (Marenzelleria) or being better competitors (round goby).

Finally, I determined the global invasion risk posed by 121 potential non-indigenous fish species by incorporating the degree of niche overlap (i.e. functional distinctiveness) as an element of risk, together with the connectivity between regions and environmental suitability of NIS. Based on the assessment I identified the Caribbean Sea and Gulf of Mexico as the most sensitive regions to be invaded by non-indigenous fish, due to a natural environmental suitability and the availability of vacant niches for other tropical fish species, in particular. I also detected that the availability of niches may be critical in the future, especially in temperate areas currently unsuitable for warm-water NIS.

This thesis documents the widespread applicability of the trait-based approach and expands its current usage to address biological invasions in marine ecosystems. The results contribute to a better understanding of how species traits can influence the establishment and dominance of NIS, as well as pose a framework to assess and visualize the potential invasion risk of areas based on the trait similarity with natives. The conducted research opens up for multiple future research possibilities to expand our knowledge on the complex ecology of biological invasions, their associated impacts, and the detection of sensitive areas to invasion. This can help guide decision-making in conservation and marine spatial planning to mitigate the effects of NIS introductions.