The Push and Pull of Niche in Rodents

This blog post is provided by Pei-Jen L. Shaner and tells the #StoryBehindThePaper for the paper “Niche overlap in rodents increases with competition but not ecological opportunity: A role of inter-individual difference“, which was recently published in the Journal of Animal Ecology. She explores how different rodent species in Taiwan have different dietary niches to allow coexistence between individuals and species.

Animals living in the same area share a common resource pool. The way they share may determine how species co-exist. The resource use of an animal population is often referred to as their ‘ecological niche’, which has a quantifiable size determined by how individuals use resources relative to one another (individuals’ relative positions within the population niche). For example, a population that consumes four types of foods has a niche size two times larger than another population that consumes two types of foods. A population’s niche size expands as more different kinds of resources become available (increased resource diversity).

Furthermore , a population’s niche size varies with competition. Intra-specific competition (competition among individuals of the same species) tends to ‘pull’ individuals apart in niche position, which expands the population’s niche size. This is because individuals of the same species have very similar resource use efficiency, hence the best way to avoid intra-specific competition is for individuals to use different subsets of the resources. In contrast, inter-specific competition (competition from other species) tends to ‘push’ individuals tighter in niche position, which reduces the population’s niche size. This is because each species tends to be better at utilizing a different subset of the resources hence excluding other species from using it.

How individuals adjust their niche positions under intra- and inter-specific competition determines the size and overlap of population niches in a biological community, and consequently, the number of species that can co-exist in the community. In this study we used stable isotope data (a proxy for ecological niche) of 635 individual rodents from four species across nine sites in the montane region of Taiwan (Figure 1) to explore the effects of competition and resource diversity on individual niche position, and the size and overlap of population niche.

Figure 1. Photos of the study sites and rodent species in the montane region of Taiwan. (a) subalpine environment at a site with altitude of 2990 m, (b) mixed deciduous forests at a site with altitude of 1843 m, (c) typical understory habitats for the rodents, (d) rodent trapping by one of the co-authors Ling-hua Ke, (e) Taiwan field mouse, (f) Formosan white-bellied rat, (g) Taiwan vole, (h) Père David’s vole. (Photo Copyright: Pei-Jen L. Shaner (b,e,g,h); Ling-hua Ke (a,c,d,f))

The island of Taiwan is mountainous with altitudes ranging from sea level to nearly 4000 m. In the montane region at the higher altitude (>1500 m), the vegetation is dominated by mixed deciduous forests (broadleaf trees and conifers), which gradually transitions into subalpine forests (conifers) at very high altitude (>3000 m). The ground-dwelling rodent communities in this montane region are dominated by four species, Taiwan field mouse (Apodemus semotus), Formosan white-bellied rat (Niviventer culturatus), Taiwan vole (Alexandromys kikuchii), and Père David’s vole (Eothenomys melanogaster) (Figure 1). All four species are diet generalists. However, Taiwan field mouse and Formosan white-bellied rat are known omnivores feeding on various plants and animals, whereas the two vole species are mainly herbivorous. The Taiwan voles in particular have a narrow diet favoring Yushan cane, which is in contrast to the broad diet of Père David’s voles comprising several dozen plant species.

One interesting and novel aspect of this study is that niche variation is quantified with geometric patterns of individuals’ niche positions. For example, population niche size is quantified with an ellipse area that is enclosed by individuals’ niche positions, and inter-individual niche difference was quantified with ‘uniqueness’, which is the average closeness of individuals within the population niche (Figure 2). With such measures, we were able to show that population niche size is constrained by resource diversity but not competition, and given the population niche size, increased competition tends to increased inter-individual niche difference as well as inter-specific niche overlap.

Figure 2. Illustration of geometric measures of population niche size and inter-individual difference in niche. Using one of the sites as an example, the center panel shows the niche positions of the individuals belonging to the four rodent species (Taiwan field mouse: orange; Formosan white-bellied rat: blue; Taiwan vole: green; Père David’s vole: magenta), with the ellipses representing their population niche size (the solid and dashed lines indicate ellipse areas enclosed by 40% and 95% of the individuals within the population). Rodent niche positions are shown alongside the basal resource (plants) values for reference. The outer panels are the ‘uniqueness’ of individuals within the population niche (counter-clock: Taiwan field mouse, Formosan white-bellied rat, Père David’s vole, Taiwan vole). For all panels, the small symbols denote individuals’ niche positions, and the two axes represent stable carbon (horizontal) and nitrogen (vertical) isotope values.     

These findings suggest that given a population niche size set by resource diversity, competition creates a push-and-pull dynamic in which individual rodents tend to maintain their uniqueness in resource use from other individuals of the same species at the cost of overlapping in resource use with other species. Therefore, these rodents may achieve co-existence through their ability to balance intra- and inter-specific competition by adjusting niche at individual and population niche.

Author biography

Dr. Shaner is a wildlife ecologist and faculty member at Department of Life Science, National Taiwan Normal University. Her research centers on foraging behavior, niche variation and ecological niche modeling. 

Read the paper

Shaner, P-J. L., & Ke, L-h. (2022). Niche overlap in rodents increases with competition but not ecological opportunity: A role of inter-individual difference. Journal of Animal Ecology, 00, 1– 14.

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