Eastern pondmussel (Ligumia nasuta) COSEWIC assessment and status report: chapter 8

Limiting factors and threats

The introduction and spread of non-native dreissenid mussels throughout the Great Lakes and connecting channels has led to dramatic declines of native freshwater mussels in colonized areas (see Habitat trends). Over 90% of historical records for the Eastern Pondmussel – the most for any species of unionid in Canada – are from areas now infested with zebra mussels and, thus, uninhabitable. Dreissenid mussels continue to threaten and limit the distribution of this species in the delta area of Lake St. Clair where most specimens collected between 2003 and 2005 were found in one small sheltered bay. Results of an unpublished study on the impacts of zebra mussels on five species of native mussels in Lake St. Clair showed that Ligumia nasuta had the lowest rate of survival (30%) and carried the heaviest load of attached zebra mussels relative to their size (Hunter pers. comm. 2004). Prior to the zebra mussel invasion, a general decline in water quality coupled with periods of low oxygen levels is believed to have been responsible for the decline in unionid densities from 10/m² in 1961 to 4/m² in 1982 in the western basin of Lake Erie (Nalepa et al. 1991). It is unlikely that zebra mussels could be introduced into the Lyn Creek drainage because the only standing waterbodies in the system are two small, wetland-surrounded ponds (Lambs Pond south of New Dublin and Lees Pond north of Lillies) with no boat access (BMNHC 2006).

Impacts of climate change on remaining populations of Ligumia nasuta and other unionids in the Great Lakes are likely to be severe. The potential impact of climate variability and change on the Great Lakes ecosystem is a topic of considerable research effort at present. Although a clear warming trend is indicated, the various climate models do not always agree on the magnitude and direction of other components, such as precipitation, and their effects on lake levels. Likely responses of the Great Lakes to climate variability and change are discussed in a recent Environment Canada report on threats to water availability in Canada (Environment Canada 2004). According to one model, net basin supply (precipitation plus runoff minus evaporation) to the lower lakes shows large decreases, with Lake St. Clair showing a dramatic decrease. Other simulations show decreases or even slight increases, but there is general agreement that climate warming will cause lake levels to drop. Impacts of lower lake levels on remnant unionid communities clinging to survival in the shallow (1.5 m or less) “flats” area of the St. Clair delta are likely to be significant. If the flats dry up, these communities would either be lost entirely or the mussels would move out of the flats and into deeper water where they would encounter high densities of zebra mussels and suffer considerable mortality.

Natural controls on the size and distribution of mussel populations include the distribution and abundance of their host fishes, predation, parasitism and disease. Unionids can not complete their life cycle without access to the appropriate glochidial host. If host fish populations disappear or decline in abundance to levels below that which can sustain a mussel population, recruitment will no longer occur and the mussel species may become functionally extinct (Bogan 1993).  As noted earlier (Life cycle and reproduction), the host fish(es) for Ligumia nasuta are currently unknown. Laboratory testing and field confirmation is required to identify the functional host(s) in Ontario waters. Follow-up studies on the health of host fish populations would then be needed to determine if access to glochidial hosts is a limiting factor for this mussel in Ontario. Freshwater mussels are known to be food sources for a variety of mammals and fishes (Fuller 1974). Predation by muskrats in particular may be a limiting factor for the Eastern Pondmussel in the delta area of Lake St Clair, because wetland areas with abundant emergent vegetation are the preferred habitat for muskrats (NatureServe 2005). Tyrrell and Hornbach (1998) and others have shown that muskrats are both size- and species-selective in their foraging, and can therefore significantly affect both the size structure and species composition of mussel communities. There have been several studies of muskrat predation on freshwater mussels (Neves and Odom 1989; Watters 1993-1994; Tyrell and Hornbach 1998), but these studies were not conducted in areas likely to support populations of the Eastern Pondmussel. Thus, the potential impact of muskrat predation on L. nasuta in Ontario needs further study. There are currently no data available on the impacts of parasitism or disease on Canadian populations of L. nasuta.

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