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Cryptic Tolerant Species and Their Potential Effect on Index of Biotic Integrity (IBI) Scores, with the Presentation of an Alternative Metric of Stream Health




Hilburn, Bryson

Type of Degree

Master's Thesis


School of Fisheries, Aquaculture, and Aquatic Sciences

Restriction Status


Restriction Type

Auburn University Users

Date Available



Metrics such as the Index of Biotic Integrity (IBI) are often used by management agencies to estimate the abstract property of stream health. These metrics are usually predicated on the belief that certain fish species are tolerant to environmental perturbation while others are sensitive. Species are usually designated as either tolerant or sensitive in these analyses based on inherent ecological or taxonomic characteristics. However, previous literature has shown that certain species from ecological or taxonomic “sensitive” groups experience increased abundance in degraded streams. We term such species “cryptic tolerants”. Using a stream fish assemblage dataset of 433 unique sample locations across the state of Alabama and the National Landcover Dataset (NLCD) (Dewitz 2021), our objectives were to 1) identify the most common cryptic tolerant species, 2) investigate how cryptic tolerant species might inflate metrics of stream health, and 3) compare an alternative metric of stream health in which species are statistically defined rather than defined using the traditional trait-based approach. We identified cryptic tolerants using Nonmetric Multidimensional Scaling in each ecoregion. A series of regressions revealed that the proportion of cryptic tolerant species decreased in response to the proportion of forested land in catchments while the proportion of true sensitives increased in all ecoregions except for the Cumberland Plateau. A metric that simply used the percentage of statistically defined, non-tolerant species generally had lower p-values and higher r2 values than IBI scores when both were regressed against percentage of forest in catchment. However, both metrics had low degrees of correlation with expected disturbance, indicating a univariate metric may be inadequate to characterize stream health. Our results highlight a potential issue with applying the IBI to diverse southeastern systems, which may be alleviated by designating species sensitivity based on empirical response to disturbance rather than taxonomic or ecological characteristics.