DOI: 10.5061/dryad.wstqjq2ph
title: Niche overlap in rodents increases with competition but not ecological opportunity: A role of inter-individual difference

description: Niche variation at population level mediates niche packing (i.e., patterns
 of species’ spread within the niche space) and species coexistence at
 community level. Competition and ecological opportunity (resource
 diversity) are two of the main mechanisms underlying niche variation.
 Dense niche packing could occur through increased niche partitioning or
 increased niche overlap. In this study we used stable carbon and nitrogen
 isotope data of 635 individual rodents from 4 species across 9 sites in
 the montane region of a subtropical island to test the effects of
 competition and ecological opportunity on population isotope niche size,
 inter-individual niche difference within population, and inter-specific
 niche overlap within community. We used the Bayesian Standard Ellipse Area
 (SEAB, the ellipse area enclosed by carbon and nitrogen isotope values of
 organisms on a bi-plot) to estimate population niche size and
 inter-specific niche overlap. Inter-individual niche difference within
 population was quantified as isotopic divergence and isotopic uniqueness.
 We used rodent abundance (the number of unique individuals captured) to
 measure competition and plant isotope niche size (plant SEAB) to measure
 ecological opportunity. The rodents experienced competition as evidenced
 by a negative relationship between population change rate and conspecific
 abundance. Rodent population niche size increased with ecological
 opportunity but not competition. The inter-individual niche difference
 (isotopic uniqueness) increased with competition (inter-specific
 competition only) but not ecological opportunity. At community level,
 inter-specific niche overlap (herbivore—omnivore pair only) increased with
 competition (the combined abundance of the pair) but not ecological
 opportunity. This study demonstrated that isotope niche variation of the
 rodents could be hierarchically influenced by ecological opportunity and
 competition, with the former setting the limit of population niche size
 across communities and the latter shaping inter-individual niche
 difference and inter-specific niche overlap within communities. Under
 strong intra-specific competition and limited ecological opportunity for
 niche expansion, individuals may choose to increase their isotopic
 uniqueness from conspecifics at the cost of overlapping with
 heterospecifics of different trophic roles within the community niche
 space as overall competition increases. Denser niche packing of these
 rodent communities might be achieved through increased niche overlap.
description: This dataset contain stable carbon and nitorgen isotope data of
 rodents and plants collrected from a total of 9 sites spanning 1800-3000 m in
 altitude in Taiwan. All sites are mainly composed of forests, with varying
 degrees of shrublands, grasslands and/or farmlands mixed in. The data
 file included two worksheets. The first worksheet contained stable isotope
 values of hair samples from 4 rodent species (<em>Alexandromys
 kikuchii, Apodemus semotus, Eothenomys melanogaster, Niviventer
 culturatus</em>; same species could be represented at more than one
 site), and the second contained stable isotope values of foliar samples
 from 92 species (the number of plant species sampled ranged from 10 to 25 across
 the 9 sites; same species could be represented at more than one
 site). We collected a
 small amount of hair from the lower back  each adult rodent. We used only one hair sample from each unique
 individual. There was no distinct seasonal molting in these
 rodents. Adult hair therefore is likely replaced as needed, and may
 reflect diet incorporated over a few months. We obtained plant foliar isotopic values from
 literature (site WL; see related works) and samples of the common plant
 species collected at each
 site opportunistically during rodent trapping (one foliar sample per plant
 species).  Rodent hair samples were lipid-extracted in 2:1
 chloroform:methanol solution for 24 hours, rinsed with distilled water,
 and oven-dried at 55°C for 24-48 hours. Approximately 1±0.2 mg of the hair
 tissue were loaded into tin capsules for isotope analysis. The plant
 foliar samples were rinsed with distilled water, oven-dried at 55°C for
 48-72 hours, and grounded into find powder. Approximately 3 mg of plant
 foliar samples were loaded into tin capsules for isotope analysis. Stable
 carbon and nitrogen isotope analysis was performed at UC Davis Stable
 Isotope Facility (ThermoFinnigan Delta Plus, Bremen, Germany). 
description: The mean isotope values of the plants at each site should be
 subtracted from rodent hair values if cross-site comparision of rodent
 isotope niche is to be compared.