A tale of two niches: methods, concepts, and evolution

Abstract

Being snapshots in time, the ranges of species may fall short of representing all of the geographic or environmental-space that these taxa are able to occupy. This has important implications for niche studies, yet most comparative studies overlook the transient nature of species' distributions and assume that they are at equilibrium. We review the methods most widely used for niche comparisons today and suggest a modified framework to describe and compare niches based on snapshot range data of species. First, we introduce a new environmental-space-based Niche Equivalencestatistic to test niche similarity between two species, which explicitly incorporates the spatial distribution of environments and their availability into statistical tests. We also introduce a new Background statistic to measure the ability of this Niche Equivalence statistic to detect differences based on the available environmental-space. These metrics enable fair comparisons between different geographies when the ranges of species are out of equilibrium. Based on distinct parameterizations of the new Equivalence and Background statistics, we then propose a Niche Divergence test and a Niche Overlap test, which allow assessment of whether differences between species emerge from true niche divergences. These methods are implemented in a new R package, 'humboldt' and applied to simulated species with pre-defined niches. The new methods improve accuracy of niche similarity and associated tests-consistently outperforming other tests. We show that the quantification of niche similarity should be performed only in environmental-space, which is less sensitive than geographic space to the spatial abundance of key environmental variables. Further, our methods characterize the relationships between non-analogous and analogous climates in the species' distributions, something not available previously. These improvements allow assessment of whether the different environmental-spaces occupied by two taxa emerge from true niche evolution, as opposed to differences in life history and biological interactors, or differences in the variety and configuration of environments accessible to them.

Multi-scale habitat modelling and predicting change in the distribution of tiger and leopard using random forest algorithm

“…A non-significant equivalency statistic and a significant background statistic support the underlying null hypothesis that species environmental niches are identical. A statistically significant equivalency statistic, regardless of the significance of background statistics, results in the rejection of the null hypothesis of niche equivalency 44 . If both the equivalency statistic and background statistic are statistically non-significant, it implies that observed niche similarity is a result of space limitations and that there is a low power for the equivalence statistic to detect the meaningful and significant differences among the species niches 44 .…”

“…We used ENMs of tiger and leopard (Fig. 2) to perform Schoener's niche equivalency (identity) test (D) and Warren's niche background test (I) 37 using the R package 'Humboldt' 44 . Niche equivalency is a one-tailed statistical test used to test out the null hypothesis that two species have identical environmental niches.…”

“…The niche equivalency test compares the observed niche similarity between the ENMs of two species and a niche background test assesses the power to detect the differences between the ENMs of two species. The values of niche similarity (D) range from 0 indicating complete dissimilar niches to 1 indicating complete similar niches 37,44 . The statistics calculate how similar the occupied niches of two species are to each other based on original input occurrences by calculating the Schoener's D. The observed values of Schoener's D are then compared to the indices obtained by resampling and reshuffling the species occurrence locations.…”

“…Thus higher the proportion truncated, greater the risk that occupied niche poorly reflects the species fundamental niche. Brown and Carnaval 44 introduced the concept of the Potential Niche Truncation Index (PNTI) implemented in package 'Humboldt' which quantifies the amount of observed E-space truncated by the available E-space. It specifically measures the overlap between the 5% kernel density isopleths of species E-space and the 10% density isopleths of the available or accessible E-space in the environment.…”

“…We use niche overlap test and niche divergence test to recognize the differences in environmental niches that emerge from the true niche divergence instead of other subtle causes such as difference in life history strategies, or simply due to the space limitations within the habitats 44 . Niche overlap test or niche equivalency test estimates the degree of similarity between the occupied niches and niche divergence test or niche background test estimates the portion of the accessible environment space shared by two species 44 . The niche overlap test determines how equivalent (or dissimilar) the occupied niches of two species are given the common environmental space in which they occur and in turn the niche divergent test determines the significant differences in the environmental space occupied by two species.…”