Complex sentence: The marsh was included in bird breeding habitat studies in 1986--1987 and again in 2005, revealing that diversity of bird species is rapidly decreasing as the red-winged blackbird becomes dominant.
Split into simpler sentences: The marsh was included in bird habitat studies in 1986--1987 and again in 2005. The results revealed that the diversity of breeding bird species is steadily decreasing as the red-winged blackbird becomes dominant.