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 (NF) is defined as the set of conditions within which a species can live in the absence of competitors, and a realized niche (NR) is a NF hypothetically reduced by competitive interactions (and some other limiting factors). This definition implies that NF is "larger" than NR, 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 NF for 105 species of reptiles and amphibians, and estimates of NR obtained from ~1.4 x 104 field observations. To specify our test, we operationalize the original classification of niche types. Our results predominantly support the hypothesis that NF 'is larger' than NR and we highlight the theoretical and practical importance of quantifying niches.