Complex sentence: However, the species are threatened due to habitat destruction and the emerald ash borer, an insect that threatens to destroy all ash trees in Maine, much as it already has devastated ash forests in the Midwest.
Split into simpler sentences: However, the species are threatened due to habitat destruction and the emerald ash borer. This insect threatens to destroy all ash trees in Maine, much as it already has devastated ash forests in the Midwest.