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Niche overlap in rodents increases with competition but not ecological opportunity: A role of inter-individual difference

Citation

Shaner, Pei-Jen; Ke, Ling-hua (2022), Niche overlap in rodents increases with competition but not ecological opportunity: A role of inter-individual difference, Dryad, Dataset, https://doi.org/10.5061/dryad.wstqjq2ph

Abstract

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.

Methods

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 (Alexandromys kikuchii, Apodemus semotus, Eothenomys melanogaster, Niviventer culturatus; 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). 

Usage Notes

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.

Funding

Taiwan’s Ministry of Science and Technology, Award: MOST 104-2621-B-003-004-MY3