Functional and numerical responses of shrews to competition vary with mouse density

Competitive release during fire succession influences ecological turnover in a small mammal community

Ecologists have long debated the relative importance of biotic interactions versus species-specific habitat preferences in shaping patterns of ecological dominance. In western North America, cycles of fire disturbance are marked by transitions between North American deermice (Peromyscus maniculatus), which predominate after wildfires, and southern red-backed voles (Myodes gapperi), which gradually replace deermice 3-4 years postfire and maintain dominance as forests mature. While this shift has been frequently documented, the processes that mediate this turnover are debated. One possibility is competitive release, which predicts a reduction in vole competition may contribute to niche expansion and population growth in deermice. Alternatively, turnover in both species may be shaped by differences in their preferred habitat and resource base, as predicted by optimum foraging theory. We evaluate these hypotheses using stable isotopes and spatial mark-recapture of deermouse and vole populations sampled prior to and following a fire as part of a longitudinal study in the Greater Yellowstone Ecosystem. Fire disturbance was associated with a 94% decrease in vole abundance but a 102% increase in deermice. Even after accounting for microhabitat, vole and deermouse populations were negatively correlated spatially and temporally (R = -0.45), and competitor abundance was more important prefire than postfire. When vole abundance was high (prefire), vole dietary niche space was seven times broader than that of deermice. Postfire, deermouse dietary niche nearly tripled and was enriched in ¹³ C (i.e., more C₄ plants), while voles occupied a slightly reduced dietary niche (79% of prefire breadth). Our results suggest deermice are experiencing ecological release due to a reduction in vole competition but vole shifts are largely driven by habitat preferences.

Isotopic Niche of Syntopic Granivores in Commercial Orchards and Meadows

Simple Summary

Granivorous murids, namely striped field (Apodemus agrarius), yellow-necked (Apodemus flavicollis), and harvest (Micromys minutus) mice, occur in a variety of habitats and live syntopically in agricultural areas. Agroecosystems may be quite complex isotopically with δ¹⁵N values being influenced by many internal and external fluxes. Using isotopic (δ¹⁵N and δ¹³C) compositions from hair samples, we analysed isotopic niches of granivores in apple and plum orchards, raspberry and currant plantations, and nearby meadows in Lithuania. As the main hypothesis, we expected differences in the isotopic niches of these species (being a proxy for their diet), minimising interspecific competition. Striped field and yellow-necked mice were trapped in every habitat. Therefore, syntopic co-occurrence of granivores depended on the presence of harvest mice in the apple orchards, raspberry plantations, and meadows that served as control habitats. All species were fully separated according to δ¹⁵N values, presuming different amounts of food of animal origin in their diet. The separation of species according to δ¹³C was not expressed in all habitats. The core dietary niches of these species were fully separated in the apple orchards and raspberry plantations. Intraspecific differences of the isotopic niche were not present in any of the three species: that is, resources were equally used by males and females, adults, subadults, and juveniles.

Abstract

In agricultural habitats, diets and trophic positions of syntopic granivorous small mammals are not known sufficiently. Agroecosystems may be quite complex isotopically and the most complex situation concerns the nitrogen-15 isotope as δ¹⁵N values are influenced by many internal and external fluxes. We analysed the isotopic niches of striped field (Apodemus agrarius), yellow-necked (Apodemus flavicollis), and harvest (Micromys minutus) mice living sympatrically and syntopically in apple and plum orchards, raspberry and currant plantations, and nearby meadows that were used as control habitats. Carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios from hair samples were used as a proxy for their diet. As the main hypothesis, we expected differences in the isotopic niches of these three species, minimising interspecific competition. All species were fully separated according to δ¹⁵N values, presuming different amounts of food of animal origin in their diet. The separation of species according to δ¹³C was not expressed in all habitats. The core dietary niches of these species were fully separated in the apple orchards and raspberry plantations. Intraspecific differences of the isotopic niche were not present in any of the three species: that is, resources were equally used by males and females, adults, subadults, and juveniles.

Using a blind test to assess the discriminant power of morphological traits to distinguish between similar shrew species

The Canary Islands are home to a large variety of endemic fauna. The Canary shrew (Crocidura canariensis) has a distribution restricted to Fuerteventura, Lanzarote and the islets of Lobos and Montaña Clara. One of the main threats to the insular fauna is the arrival of exotic species. The greater white-toothed shrew (Crocidura russula) is an easily transportable animal and a potential competitor for C. canariensis. Therefore, C. russula should be taken into account in the management protocols for invasive species. One of the most easily applicable techniques for detecting shrews is the analysis of pellets. This study aims to assess which morphological characters are diagnostic and easy to use when identifying both species of shrews. For this purpose, a blind specific assignment has been made using seven previously described characters and another three added in the present study. The results show that the observer’s experience did not improve the correct identification rate and that only three of the evaluated characters have a high discriminant capacity. Finally, it was found that the combined use of the maximum number of characters and the identification by two independent observers reduces the probability of making a mistake in the determination to minimum values.

Investigating effects of soil chemicals on density of small mammal bioindicators using spatial capture-recapture models

Monitoring the ecological impacts of environmental pollution and the effectiveness of remediation efforts requires identifying relationships between contaminants and the disruption of biological processes in populations, communities, or ecosystems. Wildlife are useful bioindicators, but traditional comparative experimental approaches rely on a staunch and typically unverifiable assumption that, in the absence of contaminants, reference and contaminated sites would support the same densities of bioindicators, thereby inferring direct causation from indirect data. We demonstrate the utility of spatial capture-recapture (SCR) models for overcoming these issues, testing if community density of common small mammal bioindicators was directly influenced by soil chemical concentrations. By modeling density as an inhomogeneous Poisson point process, we found evidence for an inverse spatial relationship between Peromyscus density and soil mercury concentrations, but not other chemicals, such as polychlorinated biphenyls, at a site formerly occupied by a nuclear reactor. Although the coefficient point estimate supported Peromyscus density being lower where mercury concentrations were higher (β = –0.44), the 95% confidence interval overlapped zero, suggesting no effect was also compatible with our data. Estimated density from the most parsimonious model (2.88 mice/ha; 95% CI = 1.63–5.08), which did not support a density-chemical relationship, was within the range of reported densities for Peromyscus that did not inhabit contaminated sites elsewhere. Environmental pollution remains a global threat to biodiversity and ecosystem and human health, and our study provides an illustrative example of the utility of SCR models for investigating the effects that chemicals may have on wildlife bioindicator populations and communities.

Using stable isotopes of plasma, red blood cells, feces, and feathers to assess mature-forest bird diet during the post-fledging period

Post-fledging use of early successional habitat by mature-forest birds is well-documented, but the important trophic factors driving this association remain poorly understood. We used stable isotope analysis of plasma, red blood cells, feces, and feathers to determine foraging preferences of three mature-forest bird species captured in 8-year-old clearcuts during the post-fledging period. We did not identify a significant source contribution in any consumer tissue combination using MixSIAR posterior distributions, but the position of consumer tissues in bivariate isotope mixing space suggested that all three mature-forest bird species acted as generalist insectivores during the post-fledging period. Furthermore, estimates of the proportional contribution of fruit to the overall diet were <0.13 for all Scarlet Tanager (Piranga olivacea (J.F. Gmelin, 1789)) tissue types, despite observational evidence of frugivory. We observed significant differentiation in core-niche-space estimates between the two obligate insectivores (Worm-eating Warbler (Helmitheros vermivorum (J.F. Gmelin, 1789)) and Ovenbird (Seiurus aurocapilla (Linnaeus, 1766))), suggesting that clearcuts provided adequate habitat to support the different foraging strategies and invertebrate preferences of both species. By using tissues with short turnover rates to address post-fledging trophic associations of mature-forest birds, we provide a comparison of isotopic values of multiple consumer tissue types in this context.

It's a trap: Optimizing detection of rare small mammals

Improving detection probabilities for rare species is critical when assessing presence or habitat associations. Our goal was to create a new small mammal trapping protocol that improved detection of rare species, such as the olive-backed pocket mouse (Perognathus fasciatus). We used three trap and bait types and trapped an area 4.4 times larger than the standard grid. We also assessed the effect of captures of non-target species on detection probability of pocket mice. Regardless of species, trap success was higher for Havaharts. We found that bait and trap type selection varied significantly by species, with pocket mice showing strongest selection for Havahart traps baited with bird seed. Increasing grid size, while maintaining a similar trapping effort, resulted in higher detection probability, although our analyses showed that effective grids can be about three-quarters of the size we use to achieve similar results. We were also able to demonstrate that by deploying a combination of different traps and baits it is possible to overcome the potential effect of non-target species (e.g., deer mice, Peromyscus maniculatus) on the detection probability of pocket mice. Our results show that simple changes to standard small-mammal trapping methods can dramatically increase the detectability of rare and elusive small mammals. Increasing detection probability of rare components of a community can improve the results and understanding of future studies.