Trophic Niche Differentiation in Rodents and Marsupials Revealed by Stable Isotopes

Abstract

Tropical rainforests support the greatest diversity of small mammals in the world, yet we have little understanding about the mechanisms that promote the coexistence of species. Diet partitioning can favor coexistence by lessening competition, and interspecific differences in body size and habitat use are usually proposed to be associated with trophic divergence. However, the use of classic dietary methods (e.g. stomach contents) is challenging in small mammals, particularly in community-level studies, thus we used stable isotopes (δ13C and δ15N) to infer about trophic niche. We investigated i) how trophic niche is partitioned among rodent and marsupial species in three Atlantic forest sites and ii) if interspecific body size and locomotor habit inequalities can constitute mechanisms underlying the isotopic niche partitioning. We found that rodents occupied a broad isotopic niche space with species distributed in different trophic levels and relying on diverse basal carbon sources (C3 and C4 plants). Surprisingly, on the other hand, marsupials showed a narrow isotopic niche, both in δ13C and δ15N dimensions, which is partially overlapped with rodents, contradicting their description as omnivores and generalists proposed classic dietary studies. Although body mass differences did not explained the divergence in isotopic values among species, groups of species with different locomotor habit presented clear differences in the position of the isotopic niche space, indicating that the use of different forest strata can favor trophic niche partitioning in small mammals communities. We suggest that anthropogenic impacts, such as habitat modification (logging, harvesting), can simplify the vertical structure of ecosystems and collapse the diversity of basal resources, which might affect negatively small mammals communities in Atlantic forests.

Defaunation and fragmentation erode small mammal diversity dimensions in tropical forests

“…Adaptation of terrestrial, fossorial, aquatic and arboreal habitat indicate where a species spends its time foraging. These adaptations can provide niche partitioning, thus avoiding competition between species along the forest (Prevedello et al 2010, Püttker et al 2013, Galetti et al 2016). Period of activity (diurnal, crepuscular and nocturnal) has been associated with the sharing of resources and is considered one of the most important components of the niche (Schoener 1974, Graipel et al 2003.…”

“…Food resource type is closely related with growth and reproduction and directly associated to the average fitness of organisms (Kozłowski 1992, Lindstrom 1999. The partitioning of food resources between co-occurring species can favor coexistence via lessening the niche overlap and the intensity of interspecific competition (Brown and Lieberman 1973, Palmer et al 2003, Gibbes and Barrett 2011, Galetti et al 2016). Most of these traits have been shown to be robust indicators for where a species falls along a few key dimensions of mammalian functional variation (Flynn et al 2009, Carvalho et al 2010, Safi et al 2011 and we provide the linear correlation values between traits (Supplementary material Appendix 2 Fig.…”

Resource partitioning confirmed by isotopic signatures allows small mammals to share seasonally flooded meadows

“…The arrival of new species and resulting changes in food webs (Baltensperger et al, 2015) may be buffered by compensation from complementary species. Rodent species are characterized by different diets, and thus, the isotopic niche of this complex taxonomic group is broad (Galetti, Rodarte, Neves, Moreira, & Costa-Pereira, 2016). The diets of voles and mice partially overlap, but fundamentally differ from other small mammals such as marsupials and shrews (Baltensperger et al, 2015;Butet & Delettre, 2011;Galetti et al, 2016;Symes et al, 2013).…”

rKIN: Kernel‐based method for estimating isotopic niche size and overlap

“…This complexity can limit our ability to interpret stable isotope results including the various niche metrics. For example, differences in resource availability and the isotopic signatures of basal sources among communities influence the positions of consumers in bivariate space Layman et al, 2007 In most isotopic studies, the foraging behaviour of individuals is unknown and inferred from the isotopic data themselves (Botta et al, 2017;Galetti et al, 2016;Gallagher et al, 2017;Sepulveda et al, 2012;Staudinger et al, 2014). Even when corrected for resource or basal (end-member) signatures (via mixing models; Newsome et al, 2007;Newsome et al, 2012;Post, 2002), niche estimates are difficult to compare among study systems.…”

“…The package also produces estimates of sampling error. Indeed, most current studies of dietary specialization, niche partitioning and competition among individuals, populations and species rely on MCP and SEA techniques to delineate isotopic niche estimates (Botta et al, ; Galetti, Rodarte, Neves, Moreira, & Costa‐Pereira, ; Gallagher, Shiffman, Byrnes, Hammerschlag‐Peyer, & Hammerschlag, ; Staudinger, McAlarney, McLellan, & Pabst, ; but see Sepulveda, Lowe, & Marra, ).…”