<ref>D. Ludwig, D. D. Jones, C. S. (1978). HollingQualitative Analysis of Insect Outbreak Systems: The Spruce Budworm and Forest. </ref> | <ref>D. Ludwig, D. D. Jones, C. S. (1978). HollingQualitative Analysis of Insect Outbreak Systems: The Spruce Budworm and Forest. </ref> |