Stable isotopes reveal limited Eltonian niche conservatism across carnivore populations

Abstract

Niche conservatism—the retention of ecological traits across space and time—is an emerging topic of interest because it can predict responses to global change. The conservation of Grinnellian niche characteristics, like species‐habitat associations, has received widespread attention, but the conservation of Eltonian traits such as consumer–resource interactions remains poorly understood. The inability to quantify Eltonian niches through space and time has historically limited the assessment of Eltonian niche conservatism and the dynamics of foraging across populations. Consequently, the relative influence of endogenous factors like phylogeny versus exogenous features like environmental context has rarely been addressed. We tested Eltonian niche conservatism using a paired design to compare foraging among four populations of American martens Martes americana and Pacific martens Martes caurina, morphologically and ecologically similar sister taxa that are allopatrically distributed throughout western North America. We developed a three‐stage isotopic framework and then quantified dietary niche overlap between the sister species and paired island‐mainland sites to assess the relative influence of endogenous (i.e., species) versus exogenous (i.e., environment) factors on Eltonian niches. First, we calculated pairwise dietary overlap in scaled δ‐space using standard ellipses. We then estimated proportional diets (“p‐space”) for individuals using isotopic mixing models and developed a novel utilization distribution overlap approach to quantify proportional dietary overlap. Lastly, we estimated population‐level proportional diets and quantified the differential use of functional prey groups across sites. We detected no pairwise overlap of dietary niches in δ‐space, and distributions of individual diets in p‐space revealed little overlap in core diets across populations. All pairwise comparisons of individuals revealed significant differences in diet, and population‐level comparisons detected contrasting use of functional prey groups. We developed a multi‐faceted isotopic framework to quantify Eltonian niches and found limited evidence of Eltonian niche conservatism across carnivore populations. Our findings are consistent with the growing recognition of dietary plasticity in consumers and suggest that consumer–resource dynamics are largely driven by exogenous environmental factors like land cover and community composition. These results illustrate the context‐dependent nature of foraging and indicate consumer functionality can be dynamic. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13266/suppinfo is available for this article.

Biotic factors influencing the unexpected distribution of a Humboldt marten (Martes caurina humboldtensis) population in a young coastal forest

“…Yet, nearly impenetrable shrub layer may also reduce the ability of birds to detect predators such as martens that are actively foraging underneath the shrubs and limiting vertical escape routes [52]. In addition, ericaceous shrubs provide fruit that are readily consumed by martens, a result (also observed using stable isotope assessment of diet [53]), potentially allowing them to opportunistically prey on vertebrates while foraging for fruit. Diet of North American marten populations appear to vary [49,53], suggesting generalizing across populations may not be valid even within this subspecies.…”

“…Here, we were able to demonstrate a broad variety of passerines consumed. Importantly, shorebirds including the Federally Threatened snowy plover ( Charadrius nivosus ) were not identified as consumed, tentatively suggesting martens were not a threat to this species which is associated with open beaches (see also [53]).…”

Trophic niche size and overlap decreases with increasing ecosystem productivity

“…Part of the solution to these issues with stable isotope space is to use mixing models, which determine the percent contributions of sources to consumer which are considered mixtures of those sources (Moore and Semmens 2008, Parnell 2008, Phillips et al 2014, Stock et al 2018. Converting isotope data into contributions of source channels to consumers allows for direct comparison of isotope data to more traditional ecological information, (Newsome et al 2007, Hopkins III and Kurle 2016, Manlick et al 2019). Newsome's DietSpace incorporates source information to create metrics that assess dietary specialization and similarity of individuals within populations, demonstraing how source contribution information can break free of the limitations of isotope space itself and generate meaningful insight into the dynamics of the trophic niche.…”

“…The incorporation of source contribution information into isotope niche metrics can account for variability introduced in this way (Newsome et al 2007(Newsome et al , 2012. Isotope mixing models function as a tool for both the incorporation of source variation into the determination of the percent contributions of sources to consumer mixes (Moore and Semmens 2008, Parnell 2008, Phillips et al 2014, Stock et al 2018 and to set the framework for the quantification of a trophic niche in terms of the sources availible to consumers in an environment (Flaherty and Ben-David 2010, Cummings et al 2012, Newsome et al 2012, Hopkins III and Kurle 2016, Manlick et al 2019. Newsome et al (2012) developed 'DietSpace' metrics that linked information about the spatial arrangment of isotope values in bivariate space with Bayesian mixing model results to quantify diet specialization.…”

Status of Pacific Martens (Martes caurina) on the Olympic Peninsula, Washington

“…Despite the influence of snow on habitat partitioning of fishers and martens in some regions (e.g., Manlick et al 2017, both species occur in low-elevation coastal forests in California and Oregon that are characterized by dense shrubs year round (Klug 1997, Slauson et al 2007, Thompson 2008, Moriarty et al 2016. Complex communities can co-exist with ample food resources, and marten diets are broad in coastal environments (Slauson and Zielinski 2017, Eriksson et al 2019, Manlick et al 2019. Thus, available evidence indicates that both species once occupied low-elevation coastal forests on the Peninsula.…”