Regulation of Tree Squirrel Populations with Immunocontraception: a Fox Squirrel Example

Tree squirrels (Sciurus spp.) are highly successful immigrants to urban and suburban areas in North America and Europe, causing both economic and ecological damage. Control of such invasive populations is challenging but of increasing importance to local managers. We studied an invasive population of fox squirrels (S. niger) and applied an immunocontraceptive vaccine in an experimental study of demographic control. Here we integrate our data into a simple stage-structured population model to simulate population reduction under 2 treatment levels (60 and 80% of females, respectively) for vaccines lasting from 1 to 5 years. Contraception can be an effective means of reducing fox squirrel populations if the vaccine is effective for ≥2 years or if ≥71% of the females are treated. Over a 15-year period, fewer individuals require treatment at a treatment rate of 0.8 versus 0.6 due to a declining population size. This study illustrates how a simple population model may guide local resource managers in the design of control strategies for invasive species.