Why is Henry David Thoreau significant to environmental science?

depends on how you'll define science.

No doubt his work had a significant impact on the environmental movement, starting from the taxidermists and bird watchers in the 19th century. These 19th century taxidermists came from Europe to NorthAmerica to not only preserve but also to catalogue new animal and plant species. Can't think of the names now, maybe if you asked a college/university prof in the subject they'd be happy to answer in very long phone calls XD.

But yeah, environmental science as an independent discipline didn't exist until the 70s at least, and didn't flourish until the 90s. It's a very recent phenomenon, Thoreau would have had nothing to do with it. His work are studied in philosophical/theoretical classes, I guess as a moral compass of a sort. I'd like to think that we're well past his mystical view of nature, but I'm no prof, and it's still good to know how everything came about. Can't judge if you don't know about what you're judging right? :D

Henry David Thoreau(1817-1862) a great personality was interested in natural history,environmental history and carried out pioneering work in this field when there was very little information about environment was known.The information's will be available in Wekipedia .

Thoreau was a preservationist who appreciated nature because of it's beauty and wrote about it. He doesn't really have anything to do with environmental science. One could argue that perhaps those who read his books became more aware of environmental issues/natural beauty and were therefore more motivated to protect it.

He wasn't...? He has no right to be on that list. He was significant to literature more than anything else.