An alternative hypothesis for invasional meltdown in the Great Lakes: General facilitation by Dreissena

Background/Question/Methods

Biological invasions occur naturally, but the rate has been rapidly increasing due to human intervention, leading to the collapse of biogeographical barriers and a loss of regional biotic distinctiveness.  One proposed reason for this increased rate is that “some invaders alter habitat conditions in favor of other invaders, thereby creating a positive feedback system that accelerates the accumulation of nonindigenous species and their synergistic impacts.”  This idea has been coined an “invasional meltdown” and was suggested to be occurring in the Great Lakes. The Great Lakes have been cited as an example of invasional meltdown in over 100 articles; however, we suggest an alternative explanation.  We hypothesized that strong interactors, such as non-native dreissenid mussels modify the environment, resulting in population level changes in both native and non-native species. The two dreissenid species in the Great Lakes are the zebra and quagga mussels (Dreissena polymorpha and D. bugensis).  Literature reviews were conducted on native and non-native species of benthic invertebrates, fish, macrophytes, and phytoplankton that have known interactions with dreissenids. and we determined if the interactions were positive, negative, or of no effect.  
Results/Conclusions

There is little evidence that dreissenids are strong facilitators of fish or phytoplankton.  Dreissenids strongly facilitate macrophytes and benthic invertebrates; however there was no difference between native and non-native species. Additionally, we examined the timing of the introduction of facilitating species and the species that were being facilitated. No relationship was found between the time of introduction of facilitating and facilitated species.  This analysis further suggests that true ‘invasional meltdown’ interactions are possible in only about ~50% of the cases analyzed.  Although dreissenids do facilitate the population growth and expansion of non-native species, we have shown that dreissenids proportionally facilitate as many native species as non-native species.  We suggest that dreissenids cause strong interactions and have been able to change the benthos in a way that facilitates many organisms, spanning multiple trophic levels, of both non-native and native species.