Formation and Loss of Self-Organized Pattern in the Florida Everglades
Speaker: James B. Heffernan, Florida International University
Series: CEE Departmental Seminars
Location: Bowen Hall Auditorium
Date/Time: Thursday, November 18, 2010, 4:30 p.m. - 5:30 p.m.
Regular, self-organized spatial patterning have been observed in a diverse, large, and growing list of terrestrial, wetland, and aquatic ecosystems. The occurrence of such patterns reflects the operation of spatially-dependent feedbacks whose strength and directionality dictate both the geometry of patterning and its stability and resilience. When these feedbacks are sufficiently strong, pattern formation can exhibit global bi-stability, meaning that the same set of boundary conditions permits the persistence of either patterned or homogenous configurations. As a result, establishment or loss of patterning may occur via catastrophic transitions whose reversal is confounded by hysteretic responses to restoration. The development of tools that can identify such transitions before they occur is thus both a theoretical and applied imperative.
The peatlands of the Florida Everglades were historically dominated by an elongate mosaic of high elevation ridges supporting productive emergent communities and lower elevation sloughs inhabited by floating and submerged plants. Hydrologic changes in the modern, engineered Everglades have been accompanied by visible degradation of vegetation patterning, and have prompted efforts to understand the mechanisms that created this landscape, determine hydrologic thresholds that allow its persistence, and develop indicators of pending pattern loss. Achieving these objectives requires recognition of the ridge-slough landscape as a coupled plant-soil system that interacts reciprocally with hydrologic regime.
In this seminar, I will present recent and ongoing research that demonstrates the importance and utility of this perspective by combining geostatistical analysis of soil surface elevations and a simple model of ecohydrologic feedbacks in the ridge-slough landscape. Geostatistical analysis of soil microtopography demonstrates that 1) in intact landscapes, soil elevation exhibits distinct bi-modality, anisotropy (directionality), and spatial autocorrelation; 2) degradation of soil elevation bimodality precedes loss of vegetation pattern; and 3) increased soil elevation variance may serve as a leading indicator of microtopographic pattern loss. Second, a simple analytical model demonstrates that landscape discharge competence 1) produces anisotropic feedbacks and therefore is a potential mechanism of anisotropic pattern formation and 2) can lead to global bi-stability of patterned and unstructured landscapes. Discharge competence is thus a plausible mechanism for Everglades pattern formation even in the absence of sediment redistribution.