When the condition of an aspect of the environment has decreased to a point where its long-term viability is in question. It usually describes the result of several interacting factors (e.g. for species, reducing numbers, decreasing quality or extent of habitat, increasing pressures).  

Where ‘decline’ is applied to elements of environments (e.g. condition of vegetation as habitat) it means that changes have been sufficient to potentially affect the viability of species relying on those elements.