Evaluating resilience: The implications of invasive species and natural water-level fluctuation on Great Lakes coastal wetlands

Coastal wetlands of the Laurentian Great Lakes must be resilient to maintain diverse vegetation. When resilience is reduced by anthropogenic stress, coastal wetlands become vulnerable to invasive species, even where water levels vary naturally. I clarified three practical methods for evaluating resilience. Aerial photograph interpretation allowed me to quantify the historical pattern of vegetation change and identify recent deviations from that pattern. I created the Species Dominance Index (SDI) to identify dominant species and characterize their behavior. Lastly, I evaluated wetland seed banks by comparing seedlings that emerged from soil to a conceptual model of resilience. All three approaches detected symptoms of lost resilience associated with the invasion of Typha x glauca (cattails) under naturally fluctuating water levels in the coastal wetlands of Green Bay, Lake Michigan. Air photo interpretation uncovered deviation from the historical pattern during the most recent low water period, in which wet meadow habitat was lost to Typha x glauca. The SDI found invasive Typha to be dominant in many Green Bay wetlands and many instances of "monotype" dominance behavior, which is associated with low species richness. Evaluation of the seed bank showed a variation in the resilience conveyed by the seed bank in five Green Bay coastal wetlands, but all five wetland seed banks contained a preponderance of the invasive Lythrum salicaria (purple loosestrife), which threatens their potential for future regeneration. My research clearly shows that Typha x glauca has achieved widespread dominance and that resilience can be lost even when water levels vary widely. That reduction in resilience can, however, be detected before degradation is complete, giving managers an opportunity to restore resilience.