The Dispersal and Ecology of Zebra Mussels ( Dreissena polymorpha ).

1. By Mark Hager Competitiveeffects on native mussels Effects on Macroinvertabrates Native Fresh water mussels are currently at great risk of extinction, and pending zebra mussel dispersal will likely accelerate their demise. Due to human and natural causes 72% of Margaritiferidae and Unionidae families are at risk. Zebra mussels attach to native mussels and present a great deal of stress on native mussel’s survival. (Schneider et. al. 1998) A Case Study Macroinvertabrates have a different relationship with Zebra Mussels than most organisms. Increased Zebra Mussels in an area actually increases the abundance of macroinvertabrates, by creating habitat for them. The mussels change the floor of the water body and allow for more macroinvertabrate colonization. This is due to increased habitat from a larger surface area as well as complexity of structures. Studies have also found that benthic organic matter resulting from the mussels feces also has a positive effect on the macroinvertabrate community. (Horvath et. al. 1999) The Dispersal and Ecology of Zebra Mussels (Dreissena polymorpha). Four Ways That Zebra Mussels impair native Mussels • attachment interferes with movement • attachment interferes with feeding mechanisms • attachment slows growth • attachment changes physiology (Schneider et. al. 1998) Fig 1. Close up of Zebra Mussel Methods How are they dispersing and what effects will they have? This study took place in Christiana Creek in South West Michigan. Rocks with both live and dead mussels glued to them were placed in the stream. There were also 3 different densities of mussels deposited, low, medium and high. .(Horvath et. al. 1999) The Zebra Mussel is a nonnative invasive species that has since the 1980’s become an ecological disturbance as well as a large nuisance to people living in and around the Great Lakes Region. The mussel has extremely powerful rates of filtering which can reduce nutrient and energy levels for other organisms. Zebra Mussels have become a nuisance to humans as well by fouling man made structures that are underwater such as municipal water systems, and power plant turbines. The total repair and maintenance of such structures in Wisconsin during 1994 was estimated at 4 million dollars.( fig 2.) Although these clams have potential for much ecological and structural damage, they also have potential to be very helpful. As an invasive species they have taken over an underexploited niche and began to disperse. Due to thorough documentation from the beginning of their invasion they offer a unique opportunity to study the dispersal and of invasive species along with their ecological impacts. Such information may prove helpful in predicting dispersal and impacts future cases of invasive aquatic species and possibly all invasive species. (Buchman et.al. 1999)(Johnson et. al. 1996) Results • Areas of greater Zebra mussels densities had significantly higher macroinvertabrate Populations. (p<0.001). (Fig 9.) • There was no significant difference between live and dead mussel samples (P=.231) • Simpson's species diversity index showed no significant difference between different density of Zebra mussels in the sample=.185)(fig 10.) • There was a significant difference in family richness between different density of Zebra mussels in the sample<0.001)(fig 10.)(Horvath et. al.1999) Fig 6. Zebra mussels attached to a clam Zebra Mussels compete with many other organism besides native mussel species. They have a great capacity for filter feeding, which most aquatic species in the region do not. This along with their ability to disperse and grow very rapidly allows them to become dominant in many environments. Their rapid filter feeding ultimately limits The amount of energy and nutrients in the system, and negatively affects other species. (Schneider et. al. 1998)(Johnson et. al. 1996) Fig 3. Shopping Cart Covered in Zebra Mussels Fig(2). Pipe clogged by zebra mussels The Spread of the Zebra Mussel Zebra mussels are transported between water bodies by both natural and anthropocentric sources. Natural dispersal is largely due To the planktonic larval stage of the juvenile mussels. The planktonic stage which last from 5-10 days can allow juveniles in large numbers to leave their native water body and relocate via streams and outlets into other water bodies. The mussels also attach to submerged biota that can travel between lakes and streams. (Padilla et. al.1996) The anthropocentric dispersal is of more interest in the case of the zebra mussel. This is because it can be examined and managed to help prevent such spread. Byssal threads unique to the zebra mussel allow it to attach to recreational boaters vessels and associated equipment such as fishing gear and trailers. (Schneider et. al. 1998) • 7 Most Common mechanisms of transport from boating • (Most common in bold) • Attach to the hull or motor of boat • Attach to anchor or snag • Attach to plants entangles on trailer • Larvae in bilge water • Larvae in engine cooling water • Larvae in bait buckets • Larvae in Live wells (Johnson et. al. 1996) Fig 7. Crayfish overcome by Zebra Mussels Fig 9. The mean densities of macroinvertabrates collected for the control and low, medium and high Zebra Mussel densities .(Horvath et. al 1999) Discussion As presented in the middle column, recreational boating and zebra mussel spread are highly correlative. This suggest that boating is a major cause of overland transportation of the mussels into inland lakes and rivers. With this knowledge it Is important to focus management efforts on recreational boating, in particular those areas that have been documented as more frequent long distance recreational boat destinations. Specific concern should also be lent to boats exiting the great lakes Because those lakes are known to have the most abundant populations of zebra mussels. Methods and results • Randomized survey mailed out to 505,880 registered boaters in Wisconsin and adjacent states • Sent 12 times over two week periods • Survey asked where the boaters had used their boat, for how many days, and if they used great lakes as well as inland lakes • Surveys were used to estimate from where and the distance each respondent trailered their boat. (Buchman et.al. 1999) Fig 10. Macroinvertabrate diversity and richness on rock samples. (Horvath et. al. 1999) The increased Macroinvertabrate densities shown above is most likely due to an increase in habitat complexity and shading as a result of higher Zebra mussel densities. The small difference between dead and live Mussels suggests that physical habitat rather than the mussels biotic functions are responsible for the increase. Finally species richness increase may be a result of more complex habitat shapes and sizes. (Horvath et. al. 1999) Understanding these changes due to Zebra Mussel colonization are pivotal to creating a full picture of what will happen to communities and systems as Zebra Mussels disperse further across the northeast. Fig 8. Zebra mussels underwater Fig 4. Number of boaters per distance traveled 1989-1990. Arrows point to the percentage of boaters who traveled more then current interval (Buchman et. al. 1999) Fig 5. Map of recorded boater movements over extreme long distances ( >261 km) Numbers indicate the number of boaters that traveled path. (Buchman et. al. 1999) 1999. Buchman A.J. Lucy, Padilla K Dianna. Estimating the probability of long distance overland dispersal of invading aquatic species. Ecological Applications 9:254-265. 1999. Horvath G. Thomas, Martin M Kristine, Lamberti A Gary. Effect of Zebra Mussels, Dreissena polymorpha on Macroinvertabrates in a Lake-Outlet steam. Am. Midl. Nat 142:340-347. 1996. Johnson E. Ladd, Carlton T James. Post-establishment spread in large scale invasions: Dispersal mechanisms of the Zebra Mussel. Ecology 77: 1686-1690 1996. Padilla M. Dianna, Chotkowski M.A., Buchan J.A. Lucy. Predicting the spread of zebra mussels (Dressena polymorpha) to inland waters using boater movement patterns. Global Ecology and Biogeography Letters 5: 353-359. 1998. Schneider W. Daniel, Ellis D Christopher, Cummings S. Kevin. A Transportation Model Assessment of the risk to native mussel communities from Zebra mussel Spread. Conservation Biology 12 :788-800. When survey results were extrapolated over all registered boaters it was estimated that 75,573 boaters traveled long distances (>106km) and 10,524 traveled extreme long distances (>261 km). Previous studies have shown that about 8% of boats from this region carry Zebra Mussels. (Johnson et. al. 1996). Combining these two figures presents that 5895 long distance and 1289 extreme long distance boat trip carrying zebra mussels occur per year within the study site. Additionally the six inland lakes known to have zebra mussels in them during 1998 were found to be in the top 6.3% for usage of all 1147 commonly used water bodies. (Buchman et. al. 1999)