North America's endemic flock of ~300 species has been identified by The Nature Conservancy as the most imperiled group of plants or animals on the continent; dozens of species are already extinct and >100 species are in danger of extinction. Nevertheless, hypotheses about the natural and anthropogenic factors that regulate unionoid populations often are vague and poorly tested, and we believe we are a long way from understanding what regulates unionoid populations. Such incomplete understanding obviously hampers efforts to manage populations of rare unionoids.
We have been working over the past several years to test and extend hypotheses about what determines the distribution and abundance of unionoid populations. Initially focusing on habitat, we showed that physical variables traditionally thought to regulate distribution (e.g., sediment grain size, current speed) have almost no explanatory power (Strayer and Ralley, JNABS 12: 247-258), whereas sediment stability during floods appears satisfactorily to explain local patchiness of unionoid communities. We also have presented evidence to suggest that inadequate food, low interstitial dissolved oxygen, and crayfish predation may limit unionoid populations (Strayer and Smith. Freshwat. Biol. 36: 771-779; Sparks and Strayer, JNABS 17: 129-134; Klocker and Strayer, Northeastern Naturalist 11:167-178).
A serious problem that will ultimately face unionoid ecologists (and ecologists in general) is understanding how multiple controlling factors work together to determine the patterns of distribution and abundance that we see in nature. Ecologists often focus on one limiting factor at a time, but real populations are confronted simultaneously by multiple factors, which often interact. Thus, habitat, food, fish hosts, dispersal, and predators probably act jointly to regulate pearly mussel populations. Even if we knew how each of these factors in isolation acted on unionoid populations, integrating them together into a single explanatory framework would not be a trivial problem. Over the next five years, we hope to begin to explore how these multiple controlling factors act on unionoid populations.
Finally, methods to assess the size, extent, and temporal trends of unionoid populations often are informal and unsatisfactory. We have spent some of our time developing and testing better (faster, cheaper, more precise) methods for assessing mussel populations (Strayer et al., JNABS 15: 308-317; Strayer et al. In: K.S. Cummings, A.C. Buchanan, C.A. Mayer, and T.J. Naimo (eds). Conservation and management of freshwater mussels II. Initiatives for the future, pp. 163-169; Strayer, Conserv. Biol.13: 1034-1038). Dave Smith and David Strayer just published a guide for assessing mussel populations (American Fisheries Society Monograph 8:103 pp.)
For images of these fascinating animals, see Chris Bernhart's Unio Gallery.