Two methods for estimating climatic response functions by measuring realized climatic niche spaces are compared. The first, direct gradient analysis, utilizes measures of abundance of the species at sites spanning the climatic range of the species' occurrence. The second, geographic distribution analysis, quantifies the proportion of the species' total areal occurrence that is found within each climatic zone. A geographic information system is used to digitize species distribution maps and climatic isoline maps, and to compute areas of coincidence. The methods are compared, using independent data sets, on eight species of Californian coastal sage scrub. The position of species maxima along climatic gradients coincided for 75% of the 24 cases in which climatic response from the two methods was compared. This result suggests that the quicker and less expensive geographic distribution analysis method may have value in generating first approximations to climatic response functions. The geographic distribution analysis method is also used to characterize the realized climatic niche space for an additional eight species of Californian hard chaparral. Most species' response surfaces from data throughout the species' range were approximately bell shaped along climatic axes, providing some confirmation of theoretical expectation. The smoothness of such curve shapes will, however, be somewhat sensitive to the scale of sample resolution.
