PS 27-111
Habitat structure and ecosystem engineers: Critical components of sustainable habitat restoration for burrowing owls
Historically the California Ground Squirrel (Otospermophilus beecheyi) created burrows for Western Burrowing Owls (Athene cunicularia hypugaea) across large grassland areas, but squirrel populations have been impacted by pest eradication efforts and shifts in grassland structure due to exotic annual grass invasion. Many squirrels are now concentrated in disturbed edges of grasslands, placing owls associated with them in potentially maladaptive breeding sites. Our objective was to implement short-term restoration treatments to re-establish key ecological processes on protected reserve lands. We established 6 pairs of experimental plots, 0.8 hectare per plot, across 3 sites in San Diego County. Vegetation mowing and soil decompaction treatments were carried out annually to reduce grass density and thatch depth. We conducted squirrel translocation to accelerate squirrel settlement and activity in target sites, using soft release methods. The squirrel response was measured through burrow surveys, squirrel mark and recapture surveys, and radiotracking. The vegetation response was monitored through transect surveys, and the interaction of translocation and vegetation treatments was evaluated with repeated measures analysis. This is a collaborative multi-year research program, with agency partners and research conducted jointly by the San Diego Zoo Institute for Conservation Research and San Diego State University.
Results/Conclusions
After two consecutive years of vegetation treatment, density of springtime grass regrowth was reduced in mowing treatment subplots relative to control subplots. Squirrel overwinter survival was low but sufficient to establish small breeding populations. Squirrel burrows were largely concentrated in the plots that received squirrel translocation (96±1% burrows). Within plots receiving translocation, most burrows were observed in the mowed subplots (82±3%) compared to unmowed controls. Repeated measures analysis using disturbed apron area around burrow entrances as a proxy for squirrel activity provides additional evidence for a significant interaction of squirrel translocation and mowing (p<0.01). The combination of translocation and vegetation treatments supported significantly more activity than either treatment alone. The highest activity levels were observed after two years of treatments, but with 538 burrow entrances remaining through the third year, noteworthy and persistent engineering effects were achieved through squirrel activity. The results show that both vegetation management and squirrel establishment are needed to support self-sustaining burrow complexes in sites managed for burrowing owl populations.