Native-exotic diversity relationships for Eastern Mediterranean fishes reveal a weak pattern of interactions

A critical question in invasion ecology is how the native species community and the exotic species affect one another. It is commonly postulated that exotics negatively impact the native community's biodiversity, and that a rich native community can inhibit the establishment of exotics. This hypothesis can be examined using the native-exotic richness relationship (NERR), where negative impacts would be reflected by negative correlations. We examined this hypothesis in the Mediterranean Sea, which is subject to an influx of Red Sea species, making it an invasion hotspot. We tested the potentially important, yet neglected, temporal dynamics of the NERR associated with increases in the richness of exotics over time. We used data from fish bottom trawl surveys conducted in the Eastern Mediterranean over 2 time periods, spanning 2 decades, characterized by a considerable increase in numbers of exotic species. While exotic richness in the first time period was uncorrelated to that of natives, a negative NERR was observed in the second time period. However, this pattern disappeared once we controlled for depth. Results did not change when we used a diversity index instead of richness alone or changed the scale of the analyses. Together, these results suggest that for fish communities over soft substrates, native species diversity does not strongly affect exotic species diversity, which hints at weak interactions between natives and exotics.