hornleaf riverweed
... that the cause of the decline in hornleaf riverweed, a foundation species in swift-flowing rivers in North America, is unclear?
... that the cause of the decline in <b id="mwAns"><a href="./Podostemum_ceratophyllum" id="mwAnw" rel="mw:WikiLink" title="Podostemum ceratophyllum">hornleaf riverweed</a></b>, a <a href="./Foundation_species" id="mwAn0" rel="mw:WikiLink" title="Foundation species">foundation species</a> in swift-flowing rivers in North America, is unclear?