Abstract
Determining when and where the ecological impacts of invasive species will be most detrimental and 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. 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 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. 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.
Highlights
Positive interactions among invaders that may enhance their probability of survival and cause a greater impact on the recipient community [1] received much attention in invasion biology
The purpose of the present study was to access the unionid infestation by D. polymorpha and D. r. bugensis in the nearshore waters of the lower Great Lakes region 25 years after the dreissenid invasion, addressing the following hypotheses: (1) dreissenid species differ in their intensity of unionid infestation, and (2) unionid species, depending on taxonomy or life history, differ in their resistance to dreissenid infestation
We found that occurrence and intensity of unionid infestation by Dreissena spp. have declined since initial invasion, and the number of dreissenids attached to unionids in the lower Great Lakes has decreased to almost one tenth of the numbers found in the early 1990s
Summary
Positive interactions among invaders that may enhance their probability of survival and cause a greater impact on the recipient community [1] received much attention in invasion biology. The opposite phenomenon when two invaders impact each other negatively (‘‘invasional interference’’) and reduce invasion success and potentially the overall effect on the native ecosystem, has been overlooked [2]. Determining whether the effects of multiple invaders will be superadditive, additive, or subadditive, has critical implications for the Holy Grail of invasion biology – predicting impacts of invasive species [3], and for prioritization of management efforts [4]. Negative effects of Dreissena polymorpha on unionids were first documented in Europe in the 1930s [10],[11]. Heavy infestations of Dreissena may reduce glycogen reserves in host unionids [20],[24],[25] and cause depletion of their energy stores and total biomass [26]. Direct attachment was proven to be a very important component of the total effects of zebra mussel populations on unionids [28],[29]
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