The relationship of environmental (ED) or habitat (HD) diversity of a landscape with its species richness (S) is of much interest. Based on underlying speciesarea (SA) curves, we show that the standard linear relationship lacks theoretical support. The SED relationship is the product of numerous SA curves per habitat and number of distinct habitats in the landscape. We recognize three basic SED patterns: convex, unimodal, concave, based on three fundamental SA curves: power, logarithmic and sigmoid. The preponderance of positive linear or absence of SED/HD relationships reported so far can be attributed to six causes. These include: only testing for linear relationships; limited data sets that exclude small, unique or isolated habitats; regressions are against non-causal variables; and use of biased data that have not been ground-truthed. Hump-backed SED curves should apply widely in regions with species-rich biota and need to resurrected, provided data collected are sufficiently comprehensive and accurate.