Competitive Replacement of Invasive Congeners May Relax Impact on Native Species: Interactions among Zebra, Quagga, and Native Unionid Mussels

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

Numbers and wet weight of all attached dreissenids (>2 mm in size) and separately by species (<i>Dreissena polymorpha</i> and <i>D. rostriformis bugensis</i>) per infested host unionid (mean ± standard error, median, lower and upper quartiles, sample size) in studied lakes in 2011–2012.

<p>Numbers and wet weight of all attached dreissenids (>2 mm in size) and separately by species (<i>Dreissena polymorpha</i> and <i>D. rostriformis bugensis</i>) per infested host unionid (mean ± standard error, median, lower and upper quartiles, sample size) in studied lakes in 2011–2012.</p

Map of sampling locations surveyed in 2011–2012.

<p>Each nearshore location (dots) was sampled at several (from 1 to 5) standard collection sites (0.5 ha surveyed for two person hours of search time). The inset gives sampling locations in inland Michigan lakes (Burt, Douglas, and Paradise).</p

Infestation of unionids by <i>Dreissena</i> spp. in the waterbodies surveyed in 2011–2012.

<p>Percent unionids infested by dreissenid mussels, the ratio of total wet weight of attached <i>Dreissena</i> spp. (>2 mm in size) to host unionid wet weight (mean ± standard error, median, lower and upper quartiles, sample size in parentheses), percent unionids with past infestation (unionids with or without <i>Dreissena</i> and with byssal threads) and percent <i>D. polymorpha</i> of total lake-wide dreissenid abundance are given for each waterbody studied (the lower Great Lakes, Lake St. Clair, the Detroit River, and inland lakes in north Michigan (Burt, Douglas, and Paradise lakes)).</p>a<p>Time since the first recorded finding. No <i>D. r. bugensis</i> was reported from lakes Burt, Douglas and Paradise.</p>b<p>D. Zanatta, unpublished data.</p><p>Infestation of unionids by <i>Dreissena</i> spp. in the waterbodies surveyed in 2011–2012.</p

Infestation parameters of unionids by species collected from waterbodies dominated by zebra or quagga mussels.

<p>Proportion of unionids infested by dreissenids by unionid species, proportion of unionids with evidence of past infestation (regardless of <i>Dreissena</i> presence), <i>Dreissena</i> spp./host unionid wet weight ratios, and the proportion of uninfested unionids of those with past infestation in waterbodies dominated by <i>D. polymorpha</i> (Lake St. Clair, the Detroit River, Paradise, Douglas and Burt lakes in Michigan, magenta bars) and in lakes Erie and Ontario dominated by <i>D. r. bugensis</i> (purple bars).</p