Because it actually meant that their singing birds do not simply represent the border territory under their control as scientists previously thought but somehow mysteriously linked very closely with the growth of plants and spread their seeds In other words the singing of birds to promote healthy growth and development of plants

Please fix the punctuation.
Because it actually meant that their singing birds do not simply represent the border territory under their control, as scientists previously thought, but somehow mysteriously linked very closely with the growth of plants and spread their seeds. In other words, the singing of birds to promote healthy growth and development of plants.