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Hemlock Headwater Ecosystems are known for their cool temperature environment with a running stream with brook trout and of course their Hemlock trees. In the northeastern side of the United States, this seems to be changing. Due to the invasive species of the Hemlock Woolly Adelgid, the once thriving Hemlock trees are now losing their needles and in the process, affecting the whole ecosystem around them. The Hemlock Woolly Adelgid affects the Hemlock trees by feeding on the base of their needles and can kill a mature Hemlock tree within three to five years (Walker, Copenheaver, and Zink-Sharp 2014. p. 595). The Hemlock Woolly Adelgid is native to eastern Asia and were brought to the United States through human assisted easy movement; when we move, organisms travel with us. Because the Hemlock Woolly Adelgid feeds on the base of the needles, this invasive species causes a decrease in canopy density and ULCR ratio. .
The aftermath of the Hemlock Woolly Adelgid doesn't stop there. After feeding on the Hemlock trees, the Hemlock Woolly Adelgid, because of its destruction to the trees, in turn affects the rest of the Hemlock Headwater Ecosystem. Since the Hemlock Woolly Adelgid feeds on the base of the Hemlock tree needles, this creates problems for other biological factors in the ecosystem. With the needles falling, it creates aerial litter that will then fall into the stream while increasing the temperature of the stream because of the openings in the canopy density. .
With the stream temperature rising, this causes the water oxygen levels to plummet because of the decrease in dissolved oxygen levels. The rising temperatures will then, in turn, affect the fish and macro invertebrates in the stream. Rising temperature will also affect conductivity in the stream which will affect the fish in the long run. Hemlock Headwater Ecosystem of Blacklick in Central Pennsylvania showed a decrease in canopy density along with ULCR but it was concluded that the macro invertebrate diversity was still quite high.
