“Latitudinal Diversity Gradient — “latitudinal” meaning how far north or south of the equator, “diversity” meaning the number of species, and “gradient” meaning the transition between high and low. We’ll just call this mouthful the LDG for short. “
What this article is saying that the Latitudinal Diversity Gradient does not have a constant slope.
There are changes in North America where the curve flattens and creates a zone, specifically the North America Latitudinal Diversity Zone.