In my last post on the sword fern die-off phenomenon, we reported that the problem was getting worse. Another year has gone by and my students have continued to monitor the die-off, giving us 2 years of data. Here (link takes you to slides put together by my undergraduate students Tristan O’Mara and Kramer Canup), based on our most recent survey data collected in October 2016, we report the same thing again. All of our study plots had a net loss of ferns, with 30% increase in outright dead ferns across all the plots from 2015 to 2016. The affected area of the park has doubled in size from 5.7 acres in 2015 to 11 acres in 2016. Qualitative observations in the spring of 2017 suggest that the die-off continues to spread throughout the park. We continue to monitor the ferns and we hope to have a new quantitative update in the fall of 2017. Additionally, we have located other sites in Seattle and the Puget Sound Region that seem to be exhibiting signs of the same die-off.
In a related study, my student, Justin Beach, has set up exclosures to test the effects of herbivory by native mountain beavers (Aplodontia rufa) on sword ferns in Seward Park. Seward Park is one of the last places within the city of Seattle to host populations of this unique burrowing rodent endemic to the Pacific Northwest. While there does already seem to be some noticeable effect of beaver herbivory on ferns in the control plots, we remain dubious that mountain beavers are the root cause of die-off, namely because die-off also seems to be occurring at other sites where mountain beavers are absent (not to mention that die-off is not occurring in many other places where mountain beavers are present–the ferns and beavers have probably co-evolved and successfully co-existed over millions of years). The rationale for studying mountain beavers in Seward Park is 1) there has been little work done on the effects of mountain beavers on ecosystem structure anywhere, and 2) predators of mountain beavers have been absent in Seward Park for some time, so it is possible that mountain beaver densities in the park are artificially high, leading to more extreme negative effects on their primary food source (sword fern). If the die-off continues at its current rate, however, it raises the more alarming prospect that one of the last remaining populations of native mountain beavers within the Seattle city limits will become endangered due to loss of ferns, one of their primary food sources, especially in winter.