2009 BES Annual Meeting Presentation and Poster Abstracts
Considerations for analyzing ecological responses to anthropogenic disturbance gradients: a case for Threshold Indicator Taxa Analysis (TITAN)
Co-Authors: Matthew Baker and Ryan King
Abstract: Existing methods for identifying ecological community thresholds are designed for univariate indicators or multivariate dimension-reduction of community structure. Neither is sensitive to responses of individual taxa with low occurrence frequencies or highly variable abundances, properties of the vast majority of taxa in community datasets. We introduce Threshold Indicator Taxa ANalysis (TITAN) to identify and assess synchronous changes along an environmental gradient as evidence for community thresholds. TITAN integrates occurrence, abundance, and directionality of taxa responses and tracks cumulative responses of declining and increasing community subsets. Resampling procedures measure both indicator reliability and purity, and assess uncertainty surrounding the existence of community change points. Using two simulated datasets, TITAN correctly identified taxon and community thresholds in more than 99% of all 500 iterations. Other change-detection techniques did not distinguish directional taxa responses, and resulted in greater uncertainty or biased estimates that failed to capture underlying thresholds in >40% of iterations. Retrospective analysis of macroinvertebrate responses to an urbanization gradient illustrated disparate thresholds for declining (0.81-3.3%) and increasing (6.8-26.6%) taxa, whereas the published threshold estimate of 10-30% urban land cover missed the declining-taxa threshold because it could not distinguish their synchronous decline from the gradual increase in ubiquitous taxa. By deconstructing communities to assess synchrony of taxon-specific change points, TITAN provides a sensitive and precise alternative to existing methods for assessing community thresholds. TITAN has tremendous potential to inform conservation of rare or threatened species, classify response to specific environmental stressors, identify reference conditions, and to develop regulatory criteria.