This is the 11042nd most frequent Russian word.
гибелью
"Гибелью" means "by death" or "by ruin/destruction" in English.
Here, "гибелью" relates to a state or event of death or destruction, often sudden or tragic.
Он спас себя от гибели, проявив храбрость.
He saved himself from death by showing courage.
"Гибелью" in this context refers to the irreversible destruction or demise of the ecosystem.
Экосистема находилась под угрозой гибели из-за загрязнения.
The ecosystem was under the threat of destruction due to pollution.
"Гибелью" is used here to signify the historical event of a civilization's decline or disappearance.