CSA’s Dr. Mark Fonseca Publishes Paper on Extreme Events and Resilience Thresholds
CSA Ocean Sciences Inc.’s (CSA) Chief Sciences Officer Dr. Mark Fonseca recently authored a new peer-reviewed paper, Separating catastrophe from the norm: detecting extremes and resilience thresholds, which was part of the Proceedings of the 7th Tampa Bay Area Scientific and Information Symposium (Session 10: Episodic and catastrophic events) held earlier this year.
Detection of catastrophic events in ecology has a long history but is often overlooked in definitions of resilience thresholds. Eclipsed by headline-grabbing events such as hurricanes, consequential small-scale extreme events often occur but can be missed when measured by means and medians. Extreme events have five main components that must be considered to understand their effects on resources: severity, geographic extent, duration, frequency, and sequence. In this paper, an analysis of historic wind data along the North Carolina coast was performed and compared with seagrass landscape changes over time to determine what aspects of extreme events may influence the structure of seagrass landscapes.
Dr. Mark Fonseca, Chief Sciences Officer, CSA Ocean Sciences. (Photo credit: CSA Ocean Sciences)
The paper also explored what types of events should be classified as a “disaster” or a “catastrophe.” These terms are often used interchangeably but a disaster is properly defined as being comparatively localized, imposing a selective pressure that leaves some representative members of a population to recover. By contrast, a catastrophe was defined as being widespread and so severe that no representatives are left to repopulate at a landscape scale, leading to a long-term (e.g., time measured in years or decades) shift in space occupation. In the case of seagrasses, this equates to a shift to an unvegetated but persistent, stable state of the seafloor.
In the analysis, spanning decades of hourly wind data along the North Carolina coast, one extreme event stood out, the March 1993 “Storm of the Century.” This brief but intense event affected thousands of hectares of North Carolina seagrass landscapes with effects ranging from complete elimination of open water, wave-exposed beds to no effect at all on seagrass beds in sheltered environments. Nonetheless, the March 1993 event was seen to be a disaster but not a catastrophe, as affected beds recovered in approximately 4–5 years.
Detection of extreme, “clock-re-setting” events that occur over seagrass landscapes can provide critical insights for mapping and delineation efforts and perceptions of ecosystem resilience. For example, if such events had occurred just prior to a survey, the distribution and abundance of seagrass habitat may well be in a state of recovery, including still being at a stage of complete absence. This would lead to a misclassification of the seafloor as being habitat incapable of supporting seagrass. Recovery from an unvegetated state back to a seagrass landscape may be especially challenging as other factors come into play, including availability of colonizing propagules, physical isolation of a site from propagule sources, and biological disturbance, which may act to hold the landscape in a stable, un-vegetated state for time perhaps measured in years. Thus, discrimination of what may or may not be potential habitat should consider time since the last “clock-re-setting” extreme event to identify resilient habitat recovery. Doing so illustrates the need to re-examine continuous data and identify environmental thresholds in the context of extremes to separate catastrophes from the norm and clarify resilience thresholds.