Abstract
Ecological (isotopic) niche refers to a surface in a two-dimensional space, where the axes correspond to environmental variables that reflect values of stable isotopes incorporated in an animal's tissues. Carbon and nitrogen stable isotope ratios (δ13C-δ15N) notably provide precious information about trophic ecology, resource and habitat use, and population dynamics. Various metrics allow for isotopic niche size and overlap assessment. In this paper, we advocate α-minimum convex polygons (MCP) - that have long been used for home range estimation – as a relevant tool for isotopic niche size, overlap, and characteristics. The method allows for outlier rejection while being suited to data that are not Gaussian in the bivariate isotopic (δ13C-δ15N) space. The proposed indicators are compared to other existing approaches and are shown to be complementary. Notably an indicator of divergence within the niche is introduced, and allows for comparisons at low (n > 6) and different sample sizes. The R code is made publicly available and will enable ecologists to perform isotopic niche comparison, contraction and expansion assessment, and overlap, based on various methods.
Highlights
Stable isotope analysis provides deep insight into a great variety of trophic and ecological processes
Carbon and nitrogen stable isotope ratios in the δ-space have been successfully used to understand the trophic dynamics in marine systems, to trace the pathway of organic matter of different origins through aquatic food webs (Fry and Scherr, 1984; Kaehler et al, 2000; Pinnegar and Polunin, 2000; Briand et al 2016), to measure the impact of invasive species (O’Farrell et al, 2014), the impact of pred ators on preys (Gallagher et al, 2017), or to support the resource breadth hypothesis (Rader et al, 2017), to cite a few applications
It may be traced back to Mohr (1947), and ecologists routinely use the 95% minimum convex polygon (MCP) instead, to rule out occasional sallies, see e.g, Van Beest et al (2010). Inspiring from these works, we advocate the use of MCPs for the study of isotopic niches, as a basis for complementary and relevant indicators we introduce
Summary
Stable isotope analysis provides deep insight into a great variety of trophic and ecological processes. In Jackson et al (2011), the authors proposed Gaussian ellipses as an alternative to convex hulls, and introduced the indicator SEA which is the area of the theoretic confidence ellipse containing 40 % of a bivar iate Gaussian having the same covariance matrix as the data. Both in dicators were empirically compared (Syvaranta et al, 2013). They are simple to use even for researchers who are not familiar with R, and allow for reproduction of the indicators and figures of the paper
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