Long term Trends in Introduced Eurasian Magpie (Pica pica) Abundance: Agricultural Land Expansion Effects on Jeju Island

Abstract

Island agro-ecosystems are highly vulnerable to ecological disturbances caused by introduced vertebrates. The Eurasian magpie (Pica pica), intentionally introduced to Jeju Island in 1989, has since established and expanded across urban and agricultural landscapes. Long-term monitoring from 2006 to 2021 recorded an increase in the island’s magpie population from 9,500 to 35,094 individuals, showing a steady upward trend with only a brief period of stabilization. Annual line transect and point count surveys were analyzed together with agricultural census data on cultivated land area at the municipal scale. In Jeju city, magpie population size showed a positive but statistically non-significant relationship with cultivated land area (R² = 0.18, P = 0.105), whereas in Seogwipo city, a significant positive association was observed (R² = 0.31, P = 0.025). Residual analysis revealed that deviations from model predictions differed between the two municipalities, with higher-than-expected values occurring primarily in horticultural farmland areas. These findings demonstrate regional variation in population response to cultivated land expansion and provide quantitative evidence of long-term population growth of magpies on Jeju Island