Relative Degrees of Equilibrium among North American Stream Fish Assemblages: Integrating Data from Multiple Sources to Quantify Assemblage Structure at Local, Regional, and Transcontinental Scales

EPA Grant Number: F5F11212
Title: Relative Degrees of Equilibrium among North American Stream Fish Assemblages: Integrating Data from Multiple Sources to Quantify Assemblage Structure at Local, Regional, and Transcontinental Scales
Investigators: McGarvey, Daniel J.
Institution: University of Alabama - Tuscaloosa
EPA Project Officer: Lee, Sonja
Project Period: August 1, 2005 through July 1, 2006
Project Amount: $111,000
RFA: STAR Graduate Fellowships (2005) RFA Text |  Recipients Lists
Research Category: Academic Fellowships


To achieve the ultimate objective of conservation biology – the preservation of biological diversity – ecologists and natural resource managers must redirect their focus from the autecological requirements of individual species to the collective needs of entire assemblages and communities. Such a holistic approach will, in turn, require a greater appreciation of community equilibrium. Equilibrium communities are those in which sympatric species coexist by interspecific partitioning of limiting niche resources, such as food and habitat. Alternatively, non-equilibrium communities tend to be structured by large-scale, historical events, such as migration and geological activity, as well as local, environmental factors. Community equilibrium is central to conservation biology, because it is the key to understanding how native communities were assembled, and how those communities will respond to contemporary management strategies. The objective of this research is to characterize the equilibrium status of North American fish assemblages, by analyzing assemblage data from Pacific Northwest, Southwest, and Southeast Rivers.


In order to determine whether each of the three regional fish assemblages have achieved equilibrium, there will be an integration of extensive fish distribution and abundance records from five independent agency and university sources, including the Environmental Protection Agency’s Environmental Monitoring and Assessment Program. Next, three critical tests of equilibrium will be performed:

  1. Competitive Co-occurrence Patterns – ecological theory predicts that competitively mediated, equilibrium assemblage structure will be reflected in nonrandom species co-occurrences; when species are interactively partitioning their environments, similar species should co-occur less often than expected under a random distribution, due to competitive exclusion. Using relative abundance data, this test will investigate whether fish assemblages in each of the three study regions express significantly non-random co-occurrences.
  2. Nested Subset Structure – nested distributions of species occur when differential migratory capabilities and/or resistances to local extirpation filter regional species pools into local subsets with few ubiquitous species, and many rare species. Because their formation is due primarily to migratory processes and selective extinction, nested subsets are indicative of non-equilibrium communities. Using historical and contemporary distribution data, this test will determine whether the fish assemblages in question are significantly nested, at local and regional scales.
  3. Community Saturation – equilibrium communities should be “saturated” with species, such that limiting similarity precludes further division of finite niche space, and local assemblages are highly resistant to nonnative invasions. To test the saturation hypothesis, plots of local vs. regional richness will be compared. When local richness increases as a linear function of regional richness, one may infer that local assemblages are not saturated, and that species are free to move among multiple localities within a region. Alternatively, if an asymptotic relationship is formed between local and regional richness, one may conclude that competitive exclusion is preventing additional species from joining local assemblages that have already been “filled.”


Effective conservation of North American fish diversity requires a rigorous understanding of community equilibrium. Equilibrium fish assemblages, which are structured by the distribution of critical resources (e.g., food and habitat), can be protected by ensuring the availability of those resources. Non-equilibrium assemblages, however, are structured by historical events (e.g., migration and tectonism), and may not respond to localized management efforts. This project will combine datasets from multiple government agencies and universities to determine whether fish assemblages have reached equilibrium in Pacific Northwest, Southwest, and Southeast rivers.

Expected Results:

Preliminary evidence suggests that the Northwest and Southwest assemblages are non-equilibrium communities, while the Southeast assemblages have achieved equilibrium; using the three critical tests of equilibrium, these anecdotal results will be substantiated or refuted. Such a holistic, large-scale approach will advance the conservation of not one, but all species of North American freshwater fishes. It will also serve as a model for future interagency and interdisciplinary collaboration.

Supplemental Keywords:

assemblage structure, biodiversity, biogeography, Columbia River basin, community equilibrium, community saturation, competition, conservation biology, EMAP, fish assemblage, interagency research, longitudinal zonation, Mobile River basin, Monte Carlo simulation, nested subset, neutral model, North American fishes, Pacific Northwest rivers, regional species pool, Rio Grande River basin, riverine fishes, Southeast rivers, Southwest rivers, species co-occurrence,, RFA, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Monitoring/Modeling, Habitat, Environmental Monitoring, habitat dynamics, habitat species co-occurrence, biodiversity, fish, habitat use, biodoversity, conservation biology, stream ecosystem

Progress and Final Reports:

  • Final