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Tree declines from invasive insect pests generate a variety of short- and long-term effects on forests along with impacts on ecosystems and wildlife. This chapter discusses the hemlock woolly adelgid's arrival at the hemlock forests in North America and describes the efforts to document the rate of adelgid infestation, hemlock decline, and environmental and vegetation changes.

The hemlock, a long-lived evergreen tree, suffered the heaviest decline from the landscape destruction following the arrival of Europeans in North America. Since the days of the Civil War, the hemlock has reasserted its way into the forests and returned to its prominence in the northern landscape and the eastern forests. This chapter studies the hemlock's past, present, and future, beginning with its prominence following centuries of abuse and exploitation. By examining the hemlock, important issues about forest conservation can be addressed. The chapter also describes the impact of the spread of hemlock woolly adelgid, an insect that found its way to the forests of North America and which is threatening the existence of the hemlock trees. It compares the advancing decline of the hemlock to that of chestnut in the early twentieth century and of the hemlock 5,000 years ago and then following European settlement.

This chapter discusses the use of simulation models to address ecological questions. Ecological modelling refers to computer simulations of ecological processses represented and linked together using mathematical and statistical functions. These models enable scientists to generate hypotheses about the ways ecosystems work. The chapter focuses on a forest model that has been used to anticipate the effects of adelgid-induced deaths of hemlocks in eastern North America.

The eastern hemlock is identified as a foundation species—a species that creates its own ecosystem and is intimately linked to the majority of other species in the system. This chapter discusses the characteristics of the eastern hemlock as a foundation species. Studies conducted have shown that the loss of hemlock in hemlock-dominated systems, whether from adelgid or logging, results in both short-term and long-term effects to associated plant and animals species along with shifts in ecological processes.