Chapter 3: Isotopic analysis of small mammal faecal samples
3.4 Discussion
104 impact seed dispersal, potentially reducing recruitment of native vegetation species relying on small mammals for seed dispersal. .
Most of the species of small mammals displayed δ13C values between -30 and -24‰, indicating that plants with C3 metabolism are important basal sources in the diet of these species. These findings suggest that none of the species sampled appear to be eating C4 grass, which was not abundant across the sampled regions. These grasses have been shown to be a declining component in South Coast Renosterveld, although previous studies have suggested that this region is unlikely to have been dominated by C4 grasses historically (Curtis & Bond, 2013). Within the agricultural matrix, C4, crops are not common in the region, with wheat and barley (C3) being prevalent. This supports the finding of a limited C4contribution in the diets of these small mammals. There appears to be a strong C3 component in the diets of all species sampled, although results on the lower end of the δ13C axis indicate the possibility of a small contribution of C4 to the diets of some species. All of these results suggest large variability amongst the plants, plant parts (seeds versus leaves) and of diet of the small mammals captured in these fragments. These small mammals appear to occupy a broad isotopic niche space with species distributed across different trophic levels and relying on diverse basal carbon sources (C3 and C4 plants). Galetti et al. (2016) suggest habitat modification, such as in habitat fragmentation, may simplify the vertical structure of ecosystems and altering the diversity of basal resources, and thereby negatively affecting small mammal communities, resulting in shifts in feeding habits. Isotopic separation of species is not evident from these results, with huge variation within and between species, making it difficult to establish diets from this isotopic analysis of faecal samples. Whether there are foraging peculiarities specific to this study area, or sampling was flawed, is unclear.
Van den Heuvel and Midgley (2014) suggest that any attempt to explore trophic position of small mammals, by placing species on the spectrum from obligate herbivore (lowest δ15N), obligate granivore (intermediate δ15N) to obligate insectivore (high δ15N) on the basis of δ15N will require detailed site-specific information, such as on available plant material, with seeds having higher isotope values than leaves. They further suggest that the isotope method is more useful for species at extreme ends of trophic feeding levels, than for the majority of species which have intermediate isotope levels. The majority of species captured in this study occupy intermediate isotope levels, being mainly generalist species with plastic feeding
105 habits, making inferences on trophic level difficult. Furthermore, Sponheimer et al. (2003a) found that one of the primary losses of dietary nitrogen in large mammalian herbivores was through 15N-enriched faeces. This has the potential to mislead the interpretation of dietary nitrogen through isotopic analysis of faeces, and may be the case with small mammals.
Through controlled experimentation, they also found that mammalian herbivores with the same diet can have δ15N values that differ by as much as 3.6‰, suggesting that interspecific physiological differences can result in greater shifts in δ15N than trophic level increases. In addition, δ15N values for plants, a source of dietary nitrogen for herbivorous mammals, can vary considerably due to physiological and abiotic factors (Sponheimer et al., 2003a). Hwang, Millar & Longstaffe (2007) found that for small mammals, faecal δ15N was enriched by ~2.5‰
than diet. All of these factors lead to the conclusion that δ15N ratios can be somewhat variable and misleading. The results of this study do not indicate δ15N trophic separation, but rather suggest that δ15N from small mammal faecal samples provides a weak signal for trophic niche separation.
From Chapter 2 it is clear that Rhabdomys pumilio is abundant in the region where sampling took place, being found in even the smallest fragments of renosterveld. This result agrees with the literature in suggesting that R.pumilio is a ubiquitous species in southern Africa (Kingdon & Happold, 2014). This prompted further qualitative investigation into the carbon and nitrogen isotopic signatures of R.pumilio from the different fragments sampled.
Differences in the range of δ13C between individuals captured at small, medium, and large fragments were evident. The range of δ13C for individuals captured at small fragments is the greatest, with this range decreasing with increasing fragment size. It is assumed that habitat quality is poorest in smaller fragments where the impacts of fragmentation, such as increased edge effects and isolation, are greatest, having generally negative impacts on indigenous species (Saunders, Hobbs & Margules, 1991; Fahrig, 2003). From these results it seems that individuals in these smaller fragments are less discriminating in their diet, with δ13C ranging from -28.06‰ to -22.38‰, due possibly to abiotic forcing and a lower availability of their preferred food resource and subsequent incorporation of lower‐quality dietary items that are usually ignored by individuals in the larger fragment. This result would be in line with the optimal foraging theory, which predicts that an overall reduction in per capita food availability leads to an increase in the number of items incorporated into the diet of organisms (Pyke,
106 1984), as has been observed in small mammal communities elsewhere (see Muñoz‐Lazo et al., 2019). This could also suggest that these individuals are feeding on food resources from adjacent land more than individuals in larger fragments. Carbon isotopes from samples taken in large fragments had the lowest variability, ranging from -28.06‰ to -26.09‰, suggesting R.pumilio found in large fragments are not as restricted in terms of diet than those in smaller fragments, being more discerning in their diet, and consuming a preferred subset of the food resources available in the fragment. As such, it appears that the results of the faecal δ13C analysis indicate larger dietary breadth in individuals captured in smaller fragments than those captured in large fragments. This shift in dietary breadth may have important implications for conservation and restoration of renosterveld, as the ecological role of small mammals may be altered through altered feeding habits as a result of habitat fragmentation and disturbance, either diminishing positive contributions to restoration, such as seed dispersal, or by promoting negative contributions such as increased insect consumption.
Micaelamys namaquensis and Elephantulus edwardii have high overlaps in dietary and microhabitat requirements (Lancaster & Pillay, 2010; Abu Baker & Brown, 2012). These species were, however captured within the same trapping grid in this study, indicating that these species are not in direct competition, despite their overlap in habitat requirements.
Lancaster and Pillay (2010) found that there was an observed lack of aggression, with direct competition between these species appearing weak, suggesting that mutual avoidance is potentially providing a mechanism for minimising interspecific interactions, and promoting coexistence, although their study examined Elephantulus myurus, closely related to Elephantulus edwardii . Mean isotopic values for M.namaquensis and E.edwardii do not show considerable differences in δ13C (-27.80‰ and -25.64‰ respectively), and in δ15N (5.79‰ and 5.65‰ respectively), suggesting that there are not strong dietary differences between these species, and that interspecific competition does not appear to be the major factor causing patterns of trophic differentiation in this community, as has been found in similar habitats (Codron et al., 2015).
This analysis
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