In Adaptive dynamic :
what is the difference between Ecological Differentiation and Ecological Divergence ?
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As already said in a comment: I would guess that ecological divergence emphasizes on the process of speciation by niche differentiation while ecological differentiation emphasizes on the process of niche differentiation due to overlapping niche of non-necessarily closely related species.
But looking to some articles, it seems that they use both term without making any difference. As found on this article for example.
Therefore, I would rather advice to no consider too much difference in meaning between ecological divergence and differentiation. But take time to define accurately the words you're using, especially if you talk about cases that fall on the border of two concepts!