This is the 11486th most frequent Russian word.
долины
Valleys.
Here, 'долин' is the plural genitive case, indicating possession or relation of ecosystems to valleys.
Группа ученых изучает экосистемы долин.
A group of scientists is studying the ecosystems of the valleys.
Here, 'долины' is used to refer to physical valleys, geographical features in a natural landscape.
Эти цветущие долины прекрасно подходят для отдыха.
These blooming valleys are perfect for relaxation.
In this sentence, 'долинах' is in the plural prepositional case, referring to valleys depicted in artistic scenes.