> At first, biologists investigating the cause of the freshwater mussel die-off suspected human activity — contamination or pollution. Rivers and streams in the coal seam-striped southern Appalachian Mountains have a long history of environmental degradation. But pollution tends to be indiscriminate, and not every species is being affected.
If American English isn’t your first language, please understand the emphasis in my phrasing of “one has to think...”
"Has to think" is not a common construction, if you're trying to communicate something across the lines of "can't help but think".