The ups and downs of ecosystem engineering by burrownesting seabirds on Triangle Island, British Columbia

Abstract

Some plant communities develop as a result of chemical and physical perturbations by burrowing seabirds and are affected when those activities are reduced. Declines in burrow density and population size of Cassin’s Auklets Ptychoramphus aleuticus at their largest breeding colony on Triangle Island, British Columbia, from 1989 to 2009, have been associated with decreases in tufted hairgrass Deschampsia cespitosa and increases in salmonberry Rubus spectabilis cover. These habitat changes may inhibit population recovery because Cassin’s Auklets prefer tufted hairgrass and tend to avoid tall salmonberry for nesting. Surveys conducted in 2014 suggested a reversal in the population trend and provided an opportunity to compare vegetation changes during periods of increasing and decreasing burrow density. We hypothesized that decreased burrowing by seabirds allows salmonberry to invade and outcompete grass as the dominant cover and that increased burrowing reverses those trends. We related changes in vegetation composition to trends in burrow numbers within permanent plots established to monitor Cassin’s and Rhinoceros Cerorhinca monocerata auklets on Triangle Island over a 25-year period. Trends in salmonberry cover and burrow numbers were negatively related. Decreases in salmonberry cover were directly correlated to increases in burrow numbers within plots, but, as found previously, increases in salmonberry cover were not directly correlated with decreasing numbers of burrows. There was no corresponding increase in grass cover with increasing burrow numbers. Vegetation damage and reduction due to seabird activities is probably more immediately apparent than establishment, spread or recovery of vegetation following seabird impacts. Our results confirm that biopedturbation by burrow-nesting seabirds negatively affects and reduces percent cover of woody shrubs, but only when burrow densities are increasing or high. At lower or declining burrow densities, seabird activities are inadequate to halt what may be the natural succession to mainly salmonberry cover on this non-forested island.