From: ASSESSMENT OF SPECIES DIVERSITY IN THE MIXEDWOOD PLAINS ECOZONE
FRESHWATER MOLLUSCA
(Gastropoda and Bivalvia)


David W. Barr
Department of Invertebrate Zoology
Royal Ontario Museum



INTRODUCTION

The Mollusca are a significant component of freshwater ecosystems (McMahon 1991 provides a recent summary), and the Mixedwood Plains Ecozone is home to some of the highest diversity exhibited by this group in Canada. The four regions of the zone support all or part of the range of some 79 distinguishable taxa of bivalves and some 52 distinguishable taxa of gastropods. The term taxon is used with respect to these entities because, while most are recognized at the species level, a number in either class have been given subspecies names on the basis of distinguishable shell characteristics, although we have no modern systematic studies to establish the status of these entitites. In order, however, to avoid losing potentially useful data on both systematics and biogeography, all currently recognized taxa will be treated here in an equivalent fashion, leaving to future work judgements on their precise biological status.

The study zone for this analysis is composed of four ecoregions (see Fig. C-1, crayfish chapter). The substantial geographic extent of the zone results in different parts of the four component ecoregions being subjected to climatic regimes that vary substantially in their significance for the ecology of freshwater organisms. For instance the number of lake ice-free days varies from over 297 in the southwestern tip of region 135 to as low as 215 in the northwestern most limit of region 134 (Barr, unpublished data).

Fig.C-1

The Mixedwood Plains Ecozone occupies all or part of a drainage area which may be characterized as one large watershed, the Great Lakes- St. Lawrence Basin, or as 5 principal regional watersheds, the Lake Huron Basin, the Lake Erie Basin, the Lake Ontario Basin, the Ottawa River Basin and the St. Lawrence River Basin (see Fig. C-1, crayfish chapter). The geography of watersheds in the region has direct significance for the biogeography of freshwater organisms (for example, Mandrak and Crossman 1992).

North America is the world centre of evolutionary radiation in freshwater bivalves, principally those of the family Unionidae (Pennak 1989). The centre of the most intense diversity for this family is the upper Mississippi and Ohio drainage basin which is contiguous with the western-most regions of our study area. This results in total unionid diversity levels, for instance, of 70 in Ohio, 66 in Michigan and 80 in Kentucky. Clearly the maximum diversity levels in the Mixedwood Plains Ecozone are peripheral to this centre, although they are high for Canada and have the additional interest that they represent populations peripheral to the ranges of a number of the species involved. Peripheral populations not only commonly represent unique genetic variation for the species but also represent that area where limiting environmental constraints on range come into effect.

Available data on the freshwater mollusc fauna for the study area may be grouped broadly into two classes: that based on museum specimen records and that based on a wide variety of field studies resulting in published data but not on the deposition of voucher specimens in readily accessible collections. Data from museum collections have been summarized principally by LaRocque and Oughton (1937) and by Clarke (1973, 1981). Similar studies in contiguous regions have been published by Jokinen (1992) and Clarke and Berg (1959) for New York, Burch and Patterson (1976) for Michigan, and Parmalee (1967) for Illinois.

Data on freshwater molluscs occurring in the Mixedwood Plains Ecozone that arise from field studies have been much more widely reported. Some of these studies have concentrated on one or more elements of the molluscan fauna: for example Mackie and Topping (1990), Di Maio and Corkum (1995). Others record data on molluscs as a component of broader faunal analyses: for example Barton (1986, 1988), Bailey et al. (1995), Day et al. (1995), Reynoldson and Day (1994), Johnson and McNeil (1986) and Crowder et al. (1986).

A particularly large segment of the recent literature deals prinicipally with aspects of the biology of the Zebra Mussel (Dreissena polymorpha ), a recently introduced bivalve with a substantial impact on the environment and significant economic implications: for example Wisendon and Bailey (1995), Hebert et al. (1989), Griffiths (1993), Dermott et al. (1993), Leach (1993) and Stewart and Haynes (1994).

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