Identifying the elusive dwarf wedgemussel habitat through modeling and field approaches

Abstract

Habitat identification is an important step in the conservation of at-risk species, but difficult due to the small, fragmented populations of rare species. In particular, fine-scale habitat features that constrain species occurrence may not be captured by landscape models. Thus, I used both modeling and field-based approaches to identify habitat characteristics for the endangered dwarf wedgemussel, Alasmidonta heterodon. Community analyses and modeling were combined to identify characteristics of suitable habitat for A. heterodon in the Maryland Coastal Plain. Community analyses suggested that landscape, rather than biological, surrogates would be preferable for predictive habitat modeling. Subsequent MaxEnt modeling associated A. heterodon habitat in the Maryland Coastal Plain with the following variables: depth to the water table, pasture/hay land cover, woody wetlands, low intensity development, Tertiary-aged sediments, and minimum elevation. The results from this model directed field work to evaluate thermal, geochemical, and physical characteristics of A. heterodon reaches throughout the species' range. Paired air-water temperature sensors placed in A. heterodon reaches suggested a potential thermal threshold of 29°C. Southern sites had higher maximum water temperatures but exhibited less diurnal variation and lower rates of temperature change than northern sites; characteristics that suggest intermediate to deep groundwater sources. Physical and chemical characteristics were measured along the length of Flat Brook, a stream with A. heterodon in the Delaware River basin. Data indicated that the mussel occurred in reaches that were stable during bankfull and lower discharges and water chemistry data indicated saturation with respect to aragonite during summer base flow. Field studies suggest several potential essential habitats for A. heterodon: (1) habitats with stable streambeds at bankfull and lower discharges; (2) stream waters in equilibrium with aragonite precipitation during baseflow conditions; and (3) habitats with maximum temperatures < 29°C and stable thermal regimes. The different thermal regimes and contributing groundwater sources between northern and southern populations suggest that geographic region be a consideration in species' reintroductions. Until essential habitats are identified and management plans instituted, all populations should be similarly protected and the loss of individual populations prevented.