**5.4 Non-native organisms**

Invasion of exotic species is a global phenomenon that threatens terrestrial and aquatic ecosystems alike. The zebra mussel (*Dreissena polymorpha*) and Asian clams (*Corbicula fluminea*) are the two non-native species of greatest concern in North America (although there is some debate over the impact of *C. fluminea*) (Strayer et al., 1999). *D. polymorpha* is highly invasive and fecund, and will attach to any solid substance including the shells of living Unionid mussels. They can occur in densities greater than 750,000 individuals/m2 , with veliger (their pelagic larvae) densities reaching 400/liter of water (Leach, 1993). Zebra mussels spread rapidly, and one group of researchers has noted a 4-8 year delay from time of introduction of *D. polymorpha* and extirpation of Unionid mussels in many ecosystems (Ricciardi et al., 2003). They compete for food and habitat with native mussels, although it is believed that epizoic colonization (infestation) of the surface of Unionid mussel shells is the most direct and ecologically destructive characteristic of *D. polymorpha* (Hunter and Bailey, 1992; Mackie, 1993). Infestation densities of zebra mussels have been found to exceed 10,000/Freshwater mussel (Nalepa et al., 1993).

Fig. 4. Invasive zebra mussels, *Dreissena polymorpha,* infesting a native fatmucket, *Lampsilis siliquoidea*. Photo courtesy of Chris Barnhart (http://unionid.missouristate.edu).

#### **5.5 Habitat destruction and alteration**

146 Biodiversity Loss in a Changing Planet

Restriction or alteration of flow patterns is another major cause of mussel biodiversity loss. The construction of dams restricts the timing, frequency, and magnitude of natural flow regimes, and affects mussels by altering the stability of the substrate, the type and amount of particulate organic matter (an important food source for mussels), the temperature of the water, and water quality (Poff et al., 2007). Studies have shown decreased mussel populations below large dams, with populations increasing with increased distance downstream from dams and with increasing flow stability (Strayer, 1993; Vaughn and Taylor, 1999). Altered flow regimes after dam construction have been implicated in the extinction of several mussel species, and have resulted in the local extirpation of many more (Layzer et al., 1993). Dams also impair recruitment of juveniles by restricting access to host fish and dispersal of glochidia (Watters, 1999). Urbanization of catchment basins can also alter flow regimes by increasing the amount of impervious cover and channelizing storm runoff, causing higher, faster, and more frequent erosive storm flow events (Walsh et al., 2005). Direct withdrawals of surface and ground water for human consumption can also reduce available habitat, increase water temperatures, and impair mussels' ability to feed,

Invasion of exotic species is a global phenomenon that threatens terrestrial and aquatic ecosystems alike. The zebra mussel (*Dreissena polymorpha*) and Asian clams (*Corbicula fluminea*) are the two non-native species of greatest concern in North America (although there is some debate over the impact of *C. fluminea*) (Strayer et al., 1999). *D. polymorpha* is highly invasive and fecund, and will attach to any solid substance including the shells of living Unionid mussels. They can occur in densities greater than 750,000 individuals/m2 , with veliger (their pelagic larvae) densities reaching 400/liter of water (Leach, 1993). Zebra mussels spread rapidly, and one group of researchers has noted a 4-8 year delay from time of introduction of *D. polymorpha* and extirpation of Unionid mussels in many ecosystems (Ricciardi et al., 2003). They compete for food and habitat with native mussels, although it is believed that epizoic colonization (infestation) of the surface of Unionid mussel shells is the most direct and ecologically destructive characteristic of *D. polymorpha* (Hunter and Bailey, 1992; Mackie, 1993). Infestation densities of zebra mussels have been found to exceed

Fig. 4. Invasive zebra mussels, *Dreissena polymorpha,* infesting a native fatmucket, *Lampsilis* 

*siliquoidea*. Photo courtesy of Chris Barnhart (http://unionid.missouristate.edu).

respire, and reproduce (Golladay et al., 2004; Hastie et al., 2003).

10,000/Freshwater mussel (Nalepa et al., 1993).

**5.3 Flow alteration** 

**5.4 Non-native organisms** 

Many researchers believe that habitat destruction and alteration are one of the greatest threats to freshwater ecosystems and mussel populations worldwide (Ricciardi and Rasmussen, 1999; Richter et al., 1997; Sala et al., 2000; Osterling et al. 2010). Habitat modification is a general term that encompasses many of the threats described earlier, such as sedimentation, flow alteration, substrate modification, and others, but also include activities such as gravel and sand mining, channelization for boat transportation, clearing of riparian vegetation, and bridge construction (Watters, 1999). Increasing amounts of sediments, either from land surface runoff or instream erosion, is one of the largest contributors to mussel habitat loss, as it makes existing habitat unsuitable for many mussel species (Brim Box and Mossa, 1999). Altered stream behavior caused by modified flows, poor riparian zone management, and runoff from impervious cover can also result in habitat loss through bed scouring, channel morphology changes, and altered sediment regimes in the system (Brierley and Fryirs, 2005).

Headcutting, channelization, and other modifications in river geomorphology are also major causes of habitat alteration in mussel species. Headcutting occurs when an alteration on the bottom of a stream causes a localized washout that progressively moves up the river channel, deepening and widening the channel and releasing large amounts of sediment into the water column. Not only does this process physically destroy mussel habitat, the release of sediment smothers previously suitable downstream habitat as well (Harfield, 1993). Many rivers and streams have been channelized to allow easier boat and barge traffic and for transport of felled logs downstream. Dredging stream channels deposits huge amounts of sediment on the stream bottom, smothering mussels already present and preventing recolonization of future generations. Dredging also drastically alters the natural flow regime and homogenizes habitat, the natural flow regime, and results in habitat homogenization (Watters, 1999). Instream gravel mining operations have been shown to modify the spacing and structure of pools and riffles, change species diversity and abundance of fishes and invertebrates, and alter ecosystem functioning in streams (Brown et al., 1998). These changes can strongly impact freshwater mussels, as most unionids have evolved to thrive in shallow riffle areas with stable, moderately coarse substrate, and are extremely intolerant to disturbance, especially in their larval stages (Brim Box and Mossa, 1999).

#### **5.6 Climate change**

There is now strong evidence that both global and regional climate change is occurring and will cause an increase in mean air temperature, more erratic precipitation patterns, and more severe floods and droughts. (Bates et al., 2008) These changing patterns pose serious threats to both terrestrial (Thomas et al., 2004) and freshwater (Sala et al., 2000) ecosystems. One group of researchers predicted that up to 75% of fish species could become extinct in rivers suffering from declining flows as a result of both climate change and human withdrawals (Xenopoulos et al., 2005). Most of the research done on the effects of climate change in freshwater systems has focused on fish and other vertebrates, with very little direct study of the effect on unionids. However, it is well known that temperature affects several aspects of mussel physiology and life history, including reproduction, growth, and recruitment of juveniles (Bauer, 1998; Kendall et al., 2010; Roberts and Barnhart, 1999). It is possible that some mussel species will be able to acclimate to a gradual increase in water temperature, but it is the predicted spikes in maximum temperature and prolonged duration of high temperatures that are likely to impact many mussel populations, especially

Biodiversity Loss in Freshwater Mussels: Importance, Threats, and Solutions 149

Because of the growing awareness of the importance of freshwater mussel diversity and freshwater ecosystems in general, there have been increasing efforts to restore and rehabilitate mussel populations and their habitats. Most strategies focus on reversing the root causes of the decline in unionid abundance and diversity listed in the preceding

Although the commercial harvest of freshwater mussels has greatly contributed to the historic decline of Unionids, it is not generally considered to be a major threat to them at present. There are several reasons for the reduction in commercial harvesting of freshwater mussels. The replacement of mussel shell with plastics in the 1940s and 50s in the button industry reduced demand for shells, and more recently the collapse of the Japanese oyster pearl fishery has reduced the demand for pearl nuclei in that industry (Neves, 1999). Enforced regulation on commercial harvesting, as well as low prices for mussel shell, have

Although water pollution has significantly declined in many industrial countries thanks to national-level legislation such as the Clean Water Act in the United States and the Water Resources Act in the UK, it is still a major threat to freshwater ecosystems and unionid mussels in most parts of the world. Acute toxicity studies in freshwater mussels have been performed on only a small number of known organic and inorganic contaminants present in the surface water of North America, and sublethal toxicity studies are even more rare (Keller et al., 2007). More studies are needed on a broader array of substances to provide regulators with better information for setting acceptable pollution standards in surface waters where

Non-point source nutrient and sediment pollution from agriculture, timber extraction, and urban runoff is regularly cited as one of the most serious threats to freshwater ecosystems (Richter, 1997). Best management practices that control runoff into surface water have been shown in numerous studies to improve the physical and chemical quality of streams (Caruso, 2000; D'Arcy & Frost, 2001; Lowrance et al., 1997). One of the most effective ways controlling sediment and nutrient inputs into streams is an intact, functional riparian zone. Well-vegetated riparian zones slow and reduce surface run-off into streams, capture large amounts of sediment in the runoff, store excess nutrients for uptake into riparian vegetation, and stabilize stream banks which further reduces instream sedimentation (Allan, 2004).

Reversing the trend of increasing human control of the flow of rivers and streams worldwide is not likely in the near future. As the human population grows over the foreseeable future, the global demand for domestic and irrigation water is projected to increase correspondingly (Robarts and Wetzel, 2000). Although the world's rivers have been fragmented and controlled by more than 1 million dams (Jackson et al., 2001), there are methods of operating these dams to minimize the negative effects they have on downstream ecosystems. In several case studies in the United States, water managers, conservation organizations, and scientists have attempted to regulate releases from dams to mimic the

**6. Solutions to the decline of freshwater mussels** 

**6.1 Reduction in commercial harvesting** 

freshwater mussels are found.

section, along with restoring and protecting existing mussel populations.

also provided a respite for mussel populations (Strayer et al., 2004).

**6.2 Best management practices to reduce pollution** 

**6.3 Restoring natural and adequate stream flows** 

in small streams where water temperature is more closely linked to air temperature (Hastie et al., 2003).

The change in precipitation patterns could also impact mussel populations through increased flooding and prolonged droughts. Although periodic, low-intensity flooding can have beneficial effects on mussel populations such as flushing fine sediments and pollutants out of substrates (Gordon et al., 1992), extreme storm events can dislodge mussels from the sediment and alter mussel bed habitat (Hastie et al., 2001). In a record multi-year drought in Georgia, Golladay et al. (2004) observed a greater than 50% loss in total mussel abundance in some reaches in the study area. As mussels are limited in their ability to move horizontally, they are unlikely to reach refuges in response to complete dewatering of their habitat. Even reduced flows can have negative effects on respiration, feeding, growth, and glochidial recruitment; and can increase predation by terrestrial consumers like raccoons (Golladay et al., 2004; Hastie et al., 2003).

The response by unionid mussels to climate change will vary depending on several factors. Geographic location will play an important role as climate change is expected to affect different parts of the world differently (Parry et al., 2007). Climate change, as with most types of ecological changes, will produce winners as well as losers (McKinney and Lockwood, 1999; Somero, 2010). Endemic species with restricted geographical ranges are expected to be especially hard hit (Malcolm et al., 2006), as are species that are already close to their upper thermal tolerance ranges (Spooner and Vaughn, 2008). The threat of climate change does not exist in isolation. It also interacts with other disturbances such as land use, direct human-caused flow alterations, and biotic exchange of non-native species; and the severity of these other threats along with geographic location will influence the effects caused by a changing climate (Sala et al., 2000).

#### **5.7 The extinction debt**

As serious as the current conservation status of many freshwater mussels are, there most likely exists a substantial extinction debt in many mussel populations (Haag, 2010). Freshwater mussels naturally exist in spatially "patchy" populations separated by areas occupied by no or only a few individuals. These patches remained connected, however, by glochidia transported by host fishes travelling throughout the matrix of mussel beds and unoccupied areas (Strayer, 2008a). Thus, population declines caused by stochastic events such as major floods or droughts could be restored through recruitment from neighboring populations. Many of the threats unionids are facing today, though, such as the building of dams, decline or extinction of host species, increased difficulty of host fish finding female mussels' "lures" or conglutinates due to decreased visibility, and lack of suitable habitat for juvenile mussels, limit reproductive success and gene flow between patches.

As pelagic spawners that release sperm into the water column, it has also been shown that reproductive success declines dramatically with decreasing mussel density, with almost no fertilization occurring at densities below 10 individuals/m2 (Downing et al., 1993). This lack of reproductive connectivity creates a genetic bottleneck in the remaining populations. These life history characteristics, along with the well-documented decline in mussel diversity and abundance, point to significant future losses in even seemingly stable mussel populations unless action is taken to reduce the perturbations causing the initial decline and increase connectivity between populations (Haag, 2010).
