**5. Causes for the decline in freshwater mussel abundance and diversity**

There are many causes for the decline in freshwater mussel biodiversity (Strayer et al., 2004; Downing et al., 2010). Dudgeon et al. (2006) describe five major contributors to the loss of freshwater biodiversity in general: over-exploitation, pollution, flow modification, exotic species invasion, and habitat degradation. These five factors are also driving the decline in freshwater mussel biodiversity and, along with the threat of global climate change, can create smaller and more isolated populations susceptible to genetic bottlenecks and burdened with extinction debts.

#### **5.1 Commercial harvesting**

144 Biodiversity Loss in a Changing Planet

As suspension feeders, Unionids can remove large amounts of phytoplankton, bacteria, and inorganic nutrients from the water column, enhancing water clarity and quality (Strayer et al., 1999). When present in large numbers, they can filter an amount of water equal to or greater than that of daily stream discharge. In a study conducted in the River Spree in Germany, Welker and Walz (1998) found that freshwater mussels created a zone of "biological oligitrophication" by decreasing phytoplankton and phosphorus in the water column. Unionids can also play other important roles in nutrient cycling, such as removing pelagic nutrient resources and depositing them into nearby sediments as faeces or psuedofaeces (Roditi et al., 1997; Spooner and Vaughn, 2006). Mussels also influence nutrient cycling by serving as nutrient sinks in growing populations, or as nutrient sources

The presence of live mussels can increase in sediment organic matter, which has been shown to positively influence abundance and diversity of other benthic invertebrates and phytoplankton (Spooner and Vaughn, 2006). Benthic invertebrate diversity can also be increased by the presence of mussel shells (Allen and Vaughn, 2011). Other benthic organisms use the shell as habitat and flow refuges, and in large numbers, the presence of mussel shells can increase landscape-level species diversity and abundance (Gutierrez et al., 2003). The influence of Unionids on benthic communities is so great that Aldridge et al., (2006) found that the abundance of freshwater mussels successfully predicted invertebrate abundance and richness in seven lowland rivers in the UK. Mussels also act as environmental engineers, bioturbating the sediment as they move both vertically and horizontally (Allen and Vaughn, 2011). This activity can increase the depth of oxygen penetration in the sediment, homogenize sediment particle size (McCall et al., 1995), and affect the flux rates of solutes between the sediment and water column (Matisoff et al., 1985).

Freshwater mussels are declining in both species richness and abundance, which can reduce their influence on ecosystem functioning and have multiple negative impacts on the ecosystem as a whole. If unioniddiversity declines but total abundance remains the same, these ecological functions should continue being performed if all mussel species perform these functions at equivalent rates (i.e. are functionally redundant). However, both common and rare species are in decline (Vaughn, 1997; Vaughn & Taylor, 1999), and it has been shown that some mussel species are more effective in carrying out the ecosystem functions described above (McCall et al., 1995; Vaughn et al., 2007). It is likely, therefore, that the ecological functions performed by freshwater mussels will continue to decline along with mussel populations, which can significantly impact the overall ecological functioning of

**5. Causes for the decline in freshwater mussel abundance and diversity** 

There are many causes for the decline in freshwater mussel biodiversity (Strayer et al., 2004; Downing et al., 2010). Dudgeon et al. (2006) describe five major contributors to the loss of freshwater biodiversity in general: over-exploitation, pollution, flow modification, exotic species invasion, and habitat degradation. These five factors are also driving the decline in

**4.1 Removing suspended particles** 

**4.2 Benthic influences** 

in declining ones (Vaughn and Hakencamp, 2001).

**4.3 Ecological impacts of declining Unionid populations** 

freshwater systems (Vaughn, 2010).

Humans have gathered freshwater mussels for meat, pearls, and mother-of-pearl shells for thousands of years, although commercial harvesting on a large scale did not begin in North America until the early 19th century (Strayer et al., 2004). During this period, commercial musselers harvested untold numbers of unionids for their pearls, which were sold in domestic and international markets. Local populations of mussels were decimated following exhaustive harvesting, after which time the musselers moved on to other, previously untapped, streams (Anthony and Downing, 2001). Overharvest made marketable pearls rarer, and the pearl fishery declined near the end of the century. Around the same time, however, new manufacturing processes allowed for the production of clothing buttons from North American mussel shells, and another round of unregulated exploitation occurred that devastated many populations that had been missed by the pearl frenzy in the previous decades (Neves, 1999). As plastic buttons began to replace those made from mussel shells in the 1930s and 40s, the rising market of the Japanese cultured pearl industry sparked a new demand for mussel shells. It was found that beads of freshwater mussel shells, when placed inside saltwater pearl oysters, made superior nuclei for the formation of cultured pearls (Anthony and Downing, 2001). This most recent boom has lasted until the mid 1990's, when a combination of declining mussel stocks, increased regulation, foreign competition, and disease outbreaks in Japanese pearl oysters has significantly reduced freshwater mussel harvest in North America (Neves, 1999).

#### **5.2 Pollution**

Because mussels are such long-lived organisms, chronic exposure to pollutants can cause direct mortality or reduced fitness. This pollution can come from many different sources, such as municipal wastewater effluent, industrial waste, and agricultural and mining runoff (Bogan, 1993), and because unionids live in the sediment, the legacy effects of accumulated toxins can have long-term effects on populations (Strayer et al., 2004). Freshwater mussels can suffer direct mortality from acute or long-term exposure to high levels of organic and inorganic pollutants, and experience sublethal effects on growth, enzyme production, abnormal shell growth, reduced metabolism, and reduced fitness in general (Keller et al., 2007). Because of their complex life cycles, there are several critical life stages where unionids can be exposed to these pollutants, and each stage can have different sensitivities to them (Cope et al., 2008).

In addition to chemical toxicants, excessive sediment can also be a pollutant. Poor agricultural and forestry practices, benthic disturbance by dredging operations, runoff from construction sites, road building, urbanization, loss of riparian vegetation, erosion of stream banks, and changes in hydrologic patterns all contribute to unnaturally high amounts of fine particle sedimentation that affects mussels directly by clogging gills and feeding siphons, and indirectly by blocking light necessary for algal production (Brim Box and Mossa, 1999) and reducing visibility needed for fish hosts to find the lures of breeding female mussels (Haag et al., 1995). Siltation can also create a hardpan layer in the substrate, making it unsuitable for burrowing in (Gordon et al., 1992).

Biodiversity Loss in Freshwater Mussels: Importance, Threats, and Solutions 147

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

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

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

disturbance, especially in their larval stages (Brim Box and Mossa, 1999).

**5.5 Habitat destruction and alteration** 

the system (Brierley and Fryirs, 2005).

**5.6 Climate change** 
