Species richness, latitude, and scale‐sensitivity

The latitudinal gradient of species richness is well documented for a variety of taxa in both terrestrial and aquatic environs. Moreover, a number of recent attempts to assess the effects of scale on the relationship have concluded that the latitudinal pattern is scale‐invariant. Nonetheless, the power of those approaches is predicated on precise knowledge of the forms of the latitudinal gradient, the area relationship, and their interaction. We used a model developed by J. Pastor, A. Downing, and H. E. Erickson for assessing the effects of scale on the productivity–diversity gradient to avoid such complications. More specifically, for 253 sets of nested quadrats (1000–25 000 km²) located throughout the New World, we parameterized the power function and determined whether those parameters varied in a systematic fashion with latitude. Significant latitude‐induced monotonic variation in the rate of species accumulation with area (z parameter) documented scale‐sensitivity for both bats and marsupials, with z decreasing toward tropical latitudes. Variation in the intercept parameter (C) reflected the latitudinal gradient in richness after adjustment for the latitude‐specific effects of area. Both bats and marsupials exhibited strong gradients of richness, with modal values in the tropics. Mechanisms affecting species richness, or the size of ranges and the juxtapositioning of their boundaries, may initiate scale‐sensitivity in many systems. A number of mechanistic or phenomenological models (environmental, geometric constraint, and Rapoport‐rescue hypotheses) thought to produce the latitudinal gradient also enhance the likelihood of scale‐sensitivity. Consequently, investigations of other macroecological gradients of richness (e.g., elevation, depth, and productivity) that should be affected by such factors are probably also scale‐sensitive.