Changes in Stream Fish Community Structure

How much?

The United States has upwards of 1000 species of native stream fishes that occur in combinations of interacting species called “communities.” Stream fish communities are dynamic: the community in a given body of water may vary over time relative to presence or absence of given species, relative abundances of species, demographic structure of the constituent species and more. Historically, the underlying assumption about community dynamics was that communities were in equilibrium such that mature communities tended to have a predictable composition with respect to species present and their relative abundances. Later, community structure was thought to be driven by responses to disturbance and, as such, the expectation for community dynamics shifted from equilibrium to non-equilibrium. But some authors proposed models suggesting that, although communities do not remain in a strict equilibrium, they tend to display “loose equilibrium” in which community composition varies around some central condition.  In our book, Stream Fish Community Dynamics: A Critical Synthesis, we have used our own research (or that of our graduate students) over a 40-year period, to describe community

dynamics of streams in the central United States and to examine underlying mechanisms that may explain community variation over time. Using our own data gave us a unique perspective based on our personal knowledge of and an intimate acquaintance with all of the datasets and experiments that form the basis of this book.

Why?

As we approached the writing of a synthesis about stream fish community dynamics, we considered many different approaches. We finally decided on a book that integrated a broad, field collection–based view of the temporal dynamics of natural fish communities (in many kinds of stream systems) with all of the experiments we had done (in systems from bench-top to field enclosure/exclosures

to large artificial streams) to try to understand underlying mechanisms for the composition and dynamics of the native fish communities in our study regions. Accordingly, we went back in time to review all of our experimental work on subjects such as tolerances of fish for harsh environmental conditions, responses of fish to flowing water stress, resistance of fish to catastrophic disturbances like flood and drought, effects of mesopredators on minnows, the ways the fish themselves affected stream ecosystems, or similar topics. As a result of integrating for this book all our observations of fish in the field, the results of a wide variety of experiments, and our empirical data on variation in fish communities over short (months) to long (decades) periods of time, we came to what we think is a better understanding of our fishes and their communities than we had previously achieved over 40+ years of more discrete individual research projects.

What does it mean? 

Our analyses suggest that dynamics of the stream fish communities that we studied most often match expectations of loose equilibrium, even when community structure is temporarily changed by disturbance events such as floods and droughts. In fact, we recommend that community ecologists adopt this as the “expected” dynamic against which to evaluate significant community change. But this model requires data over long periods and we hope researchers and funding agencies will value long-term studies and make the effort to continue those long enough to truly understand the vagaries of community structure. Finally, we hope that our attempts at uncovering mechanisms that drive community dynamics will inspire others to continue studies of basic biology of fishes, intra- and interspecific interactions, and community effects on ecosystem properties.

 

William J. Matthews and Edie Marsh-Matthews are Professors Emeritus at the University of Oklahoma. Their latest book, Stream Fish Community Dynamics: A Critical Synthesis, is available now.