Are fundamental niches larger than the realized? Testing a 50-year-old prediction by Hutchinson

For more than 50 years ecological niches have been defined as combinations of multidimensional environmental conditions permitting a species to survive and reproduce. A fundamental niche ( N F ) is defined as the set of conditions within which a species can live in the absence of competitors, and a realized niche ( N R ) is a N F hypothetically reduced by competitive interactions (and some other limiting factors). This definition implies that N F is “larger” than N R , something that has been nearly universally accepted by ecologists. However, there have been few attempts at empirical tests. Here, we present a novel quantitative test using one-dimensional estimates of N F for 105 species of reptiles and amphibians, and estimates of N R obtained from ~1.4 x 10 4 field observations. To specify our test, we operationalize the original classification of niche types. Our results predominantly support the hypothesis that N F ‘is larger’ than N R and we highlight the theoretical and practical importance of quantifying niches.