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.

Fig 1. Location of the three studied Atlantic forest areas in São Paulo, Brazil.

(A) Carlos Botelho State Park (in dark green) and the upper part of the park, São Miguel Arcanjo field-station evidenced by a black dot, (B) Santa Virginia nucleus (in dark green) and their both field-station (Itamambuca e Vargem Grande) indicated by black dots. In light green we have the current distribution of the Serra do Mar massif in the Brazilian Atlantic Forest.

Fig 2. Isotope plot depicting means isotope values (δ¹⁵N and δ¹³C) of rodent (gray dots) and marsupial (blue dots) species from Atlantic rainforests, with respective standard isotope ellipses, for each studied site and all sites polled together.

Isotope values were corrected downward 2.7‰ (for δ¹⁵N) and 2.4‰ (for δ¹³C) to account for trophic enrichment. Isotope values of the potential food resources are represented in boxplots. Species code: 1 = Didelphis aurita; 2 = Gracilinanus microtarsus; 3 = Juliomys pictipes; 4 = Marmosops incanus; 5 = Metachirus nudicaudatus; 6 = Monodelphis Americana; 7 = Monodelphis iheringi; 8 = Monodelphis scalops; 9 = Monodelphis sp.; 10 = Philander frenatus; 11 = Akodon cursor; 12 = Akodon montensis; 13 = Blarinomys breviceps; 14 = Brucepattersonius soricinus; 15 = Delomys dorsalis; 16 = Delomys sublineatus; 17 = Euryoryzomys russatus; 18 = Necromys lasiurus; 19 = Oligoryzomys nigripes; 20 = Sooretamys angouya; 21 = Thaptomys nigrita; 22 = Trinomys iheringi.

Fig 3. Standard isotope ellipses (SEA_C) from different groups of locomotor habit in communities of small mammals in the Atlantic forest.