Competitive replacement of invasive congeners may relax impact on native species: interactions among zebra, quagga, and native unionid mussels
Positive interactions among invaders that may enhance their probability of survival and thus cause a greater impact on the recipient community received much attention in invasion biology. However, the opposite phenomenon when two invaders impact each other negatively and thus reduce invasion success and potentially the overall effect on the native ecosystem, has been largely overlooked. Determining whether the effects of multiple invaders will be superadditive, or subadditive, is critical for developing global management priorities to protect native species in advance of future invasions. Over the past century, the decline of freshwater bivalves of the family Unionidae has been greatly accelerated by the invasion of Dreissena spp. Currently quagga mussels (Dreissena rostriformis bugensis) are outcompeting the zebra mussels (D. polymorpha) in most of the Great Lakes they invade. The purpose of this study was to evaluate the current infestation rates of unionids by zebra (Dreissena polymorpha) and quagga (D. rostriformis bugensis) mussels in the lower Great Lakes region 25 years after they nearly extirpated native unionids. In 2011-2012, we collected infestation data for over 4000 unionids from 26 species at 198 nearshore sites in lakes Erie, Ontario, and St. Clair, the Detroit River, and inland Michigan lakes and compared those results to studies from the early 1990s.
We found that the frequency of unionid infestation by Dreissena has recently declined, and the number of dreissenids attached to unionids in the lower Great Lakes has fallen almost ten-fold since the early 1990s. We also found that the rate of infestation depends on the dominant Dreissena species in the lake: zebra mussels infested unionids much more often and in greater numbers. Consequently, the proportion of infested unionids, as well as the number and weight of attached dreissenids were lower in waterbodies dominated by quagga mussels. The decrease in unionid infestation that accompanied the competitive replacement of zebra with quagga mussels may result in reduced overall impact on native unionids. This is the first large-scale systematic study that revealed how minor differences between two taxonomically and functionally related invaders may have large consequences for native communities they invade.