Identifying thresholds in fish community dynamics and composition in response to altered hydroperiods in everglades marshes

Background/Question/Methods

The possibility of thresholds and other complex, non-linear dynamical properties of ecological systems is prompting a paradigm shift in ecosystem management practices. However, not all ecosystems exhibit threshold-like behavior and the dynamics associated with directional change in environmental drivers may well be ecosystem specific. Therefore, application of threshold models in restoration and management must be driven by empirical evidence for, and documentation of uncertainty associate with, threshold dynamics.  We used a multi-pronged approach to determine the presence of non-linear dynamics in Everglades fish community structure and process (biomass production).  First, we modeled time-series dynamics of small fish (<8 cm SL) biomass in freshwater marshes at 17 sites over periods ranging from 16 to 34 years. Then, we applied metric-based (changes in variance) and model-based (time-variation in autoregressive coefficients) indicators to detect possible non-linear changes in system state and stability. Second, we used distance-decay relationships to model long-term trends and turnover rates in composition. Third, we use Threshold Indicator Taxon Analysis (TITAN) to quantify species-level thresholds, their uncertainty, and community transitions in response to drought disturbance.

Results/Conclusions

After accounting for hydrological and seasonal variation in time-series dynamics, many sites exhibited emergent and cumulative effects of disturbance on biomass. Long-term declines in biomass correlated roughly with regional hydroperiods (7% to 11% declines). In addition, temporal changes in variance at 22% to 39% of study sites indicated potential non-linear dynamics.  Significant directional compositional changes were observed in 50% to 67% of regional study sites, with regional mean turnover rates of -0.021 to -0.014 yr^-1, respectively.  In short-hydroperiod regions, composition shifted to smaller-bodied species that maintain lower biomass, while in a longer hydroperiod region composition shifted to larger-bodied fishes. TITAN analysis confirms that most fish increased standing stock in response to reduced drought frequency (lengthened hydroperiods) and increased days since drying events, with many responding synchronously at approximately 250 days post-drought. The evidence for thresholds varied among species and localities, often accompanied by substantial uncertainty.  These results indicate that environmental change over the study period was not great enough to yield ecosystem state change, but was great enough to decrease fish standing stocks by ecologically significant amounts.