Carolyn M. Kurle

Stable isotope assessment of temporal and geographic differences in feeding ecology of northern fur seals (Callorhinus ursinus) and their prey

We investigated the feeding ecology and foraging location of migrating and nursing northern fur seal (Callorhinus ursinus) adult females and migrating juvenile males from the Pribilof Islands, Alaska, using carbon (δ13C) and nitrogen (δ15N) isotope analysis of fur seal skin and whole potential prey. Post-parturient and lactating females had mean δ15N values significantly (0.8‰) higher than pregnant, migratory females, and δ13C values that were not significantly different. Two opportunistically collected, migrating, nulliparous females had mean δ13C values 1.1‰ lower than migrating, pregnant females, and δ15N values that were not different. Pregnant, migratory females had mean δ13C values significantly (~1.5‰) higher than migratory juvenile males, and mean δ15N values significantly (~0.6–1.6‰) higher than migratory juvenile males. The exception was one group of juvenile males from St. Paul Island with mean δ15N values that were not significantly different from migrating females. The mean δ15N values of pregnant females indicate they were feeding at a higher trophic level than juvenile males during migration. The higher mean δ13C values for pregnant females suggest they were feeding coastally during the spring migration, while juvenile males and nulliparous females were feeding offshore. The higher δ15N values for post-parturient, lactating females over migrating, pregnant females point to either a trophic shift in diet over time, or a more likely 15N-enrichment due to negative nitrogen balance caused by the nutritional stress of lactation and the feeding/fasting regime experienced by females. Similar mean δ13C values for migrating and breeding-season females indicate that both groups were feeding in coastal, on-shelf domains during their respective time periods. Similar mean δ15N values for nulliparous and pregnant females indicate they were feeding at similar trophic levels despite indications of feeding in separate ecosystems during migration. Using a δ15N shift of 2–3‰ per trophic level, we made general inferences about the trophic levels at which northern fur seals were feeding. The interpretation of our δ15N data indicates that migrating pregnant females, lactating females and the majority of migrating juvenile males consumed prey with mean δ15N values between 14.2‰ and 15.2‰, 15.1‰ and 16.1‰, and 13.6‰ and 14.6‰, respectively. Probable fur seal prey was analyzed as well. Walleye pollock showed progressive 15N and 13C-enrichments with age. Mean δ15N and δ13C values of 3- to 4-year-old fish were ~6.0‰ and 1.1‰ higher, respectively, than values for 0-age pollock. Atka mackerel also showed isotopic enrichment with age. The δ15N and δ13C values of large fish were 0.8‰ and 0.3‰ higher, respectively, than values for smaller fish.

Introduced rats indirectly change marine rocky intertidal communities from algae- to invertebrate-dominated

It is widely recognized that trophic interactions structure ecological communities, but their effects are usually only demonstrated on a small scale. As a result, landscape-level documentations of trophic cascades that alter entire communities are scarce. Islands invaded by animals provide natural experiment opportunities both to measure general trophic effects across large spatial scales and to determine the trophic roles of invasive species within native ecosystems. Studies addressing the trophic interactions of invasive species most often focus on their direct effects. To investigate both the presence of a landscape-level trophic cascade and the direct and indirect effects of an invasive species, we examined the impacts of Norway rats (Rattus norvegicus) introduced to the Aleutian Islands on marine bird densities and marine rocky intertidal community structures through surveys conducted on invaded and rat-free islands throughout the entire 1,900-km archipelago. Densities of birds that forage in the intertidal were higher on islands without rats. Marine intertidal invertebrates were more abundant on islands with rats, whereas fleshy algal cover was reduced. Our results demonstrate that invasive rats directly reduce bird densities through predation and significantly affect invertebrate and marine algal abundance in the rocky intertidal indirectly via a cross-community trophic cascade, unexpectedly changing the intertidal community structure from an algae- to an invertebrate-dominated system.

The effects of sex, tissue type, and dietary components on stable isotope discrimination factors (Δ13C and Δ15N) in mammalian omnivores

We tested the effects of sex, tissue, and diet on stable isotope discrimination factors (Δ13C and Δ15N) for six tissues from rats fed four diets with varied C and N sources, but comparable protein quality and quantity. The Δ13C and Δ15N values ranged from 1.7–4.1 ‰ and 0.4–4.3 ‰, respectively. Females had higher Δ15N values than males because males grew larger, whereas Δ13C values did not differ between sexes. Differences in Δ13C values among tissue types increased with increasing variability in dietary carbon sources. The Δ15N values increased with increasing dietary δ15N values for all tissues except liver and serum, which have fast stable isotope turnover times, and differences in Δ15N values among tissue types decreased with increasing dietary animal protein. Our results demonstrate that variability in dietary sources can affect Δ13C values, protein source affects Δ15N values even when protein quality and quantity are controlled, and the isotope turnover rate of a tissue can influence the degree to which diet affects Δ15N values.

High-Arctic butterflies become smaller with rising temperatures

The response of body size to increasing temperature constitutes a universal response to climate change that could strongly affect terrestrial ectotherms, but the magnitude and direction of such responses remain unknown in most species. The metabolic cost of increased temperature could reduce body size but long growing seasons could also increase body size as was recently shown in an Arctic spider species. Here, we present the longest known time series on body size variation in two High-Arctic butterfly species: Boloria chariclea and Colias hecla. We measured wing length of nearly 4500 individuals collected annually between 1996 and 2013 from Zackenberg, Greenland and found that wing length significantly decreased at a similar rate in both species in response to warmer summers. Body size is strongly related to dispersal capacity and fecundity and our results suggest that these Arctic species could face severe challenges in response to ongoing rapid climate change.

Variation in the stable carbon and nitrogen isotope discrimination factors from diet to fur in four felid species held on different diets

Abstract Stable carbon (13C/12C; δ13C) and nitrogen (15N/14N; δ15N) isotope ratios are best used to assess wild animal diets when the isotopic differences between consumers and diets are known. These differences are called discrimination factors (expressed with Δ notation). We report the 1st Δ13C and Δ15N values between diet and fur from captive individuals held on controlled diets for 7 months and representing 4 felid species: African lions (Panthera leo), bobcats (Lynx rufus), Canada lynx (Lynx canadensis), and mountain lions (Puma concolor). All animals were fed a mix of diet items (beef, beef rib, a commercial carnivore diet, chicken, mice, rats, turkey, and turkey wings) that was consistent throughout their molting period. Weekly diet composition was determined by the percentage of mass of each diet item and overall δ13C and δ15N values were calculated for each animals diet. The mean Δ13C and Δ15N values (± SD) between felid fur and their non–lipid-extracted diets were 1.1‰ ± 0.2‰ and 3.5‰ ± 0.0‰, respectively (African lion, n = 1 animal sampled at 2 intervals); 5.5‰ ± 0.5‰ and 4.1‰ ± 0.1‰, respectively (bobcats, n = 3); 2.4‰ and 3.3‰, respectively (Canada lynx, n = 1); and 4.7‰ ± 0.6‰ and 4.5‰ ± 0.2‰, respectively (mountain lions, n = 2). Variations in Δ13C and Δ15N values among species were likely due to dietary differences and we recommend the use of the Δ13C (5.5 ± 0.5) and Δ15N (4.1 ± 0.1) values obtained from the bobcats for future determinations of wild felid foraging ecology as they were held on diets composed of 100% whole animals and animal parts, which best reflects diets of wild felids.

Effects of demineralization on the stable isotope analysis of bone samples

RATIONALE The sampling of sequential, annually formed bone growth layers for stable carbon (δ(13)C values) and nitrogen (δ(15)N values) isotope analysis (SIA) can provide a time series of foraging ecology data. To date, no standard protocol exists for the pre-SIA treatment of cortical samples taken from fresh, modern, bones. METHODS Based on the SIA of historical bone, it is assumed that fresh bone samples must be pre-treated with acid prior to SIA. Using an elemental analyzer coupled to an isotope ratio mass spectrometer to measure stable carbon and nitrogen ratios, we tested the need to acidify cortical bone powder with 0.25 M HCl prior to SIA to isolate bone collagen for the determination of δ(13)C and δ(15)N values. We also examined the need for lipid extraction to remove potential biases related to δ(13)C analysis, based on a C:N ratio threshold of 3.5. RESULTS It was found that acidification of micromilled cortical bone samples from marine turtles does not affect their δ(15)N values, and the small effect acidification has on δ(13)C values can be mathematically corrected for, thus eliminating the need for pre-SIA acidification of cortical bone. The lipid content of the cortical bone samples was low, as measured by their C:N ratios, indicating that lipid extracting cortical bone samples from modern marine turtles is unnecessary. CONCLUSIONS We present a standard protocol for testing fresh, modern cortical bone samples prior to SIA, facilitating direct comparison of future studies. Based on the results obtained from marine turtle bones, pre-acidification and lipid removal of cortical bone are not recommended. This is especially useful as there is frequently not enough bone material removed via micromilling of sequential growth layers to accommodate both acid treatment and SIA.

Measuring the realized niches of animals using stable isotopes: from rats to bears

© 2015 The Authors. Stable isotope analysis is a powerful method for estimating the impacts animals have on their environment (resource use), revealing their ecological niches. We demonstrated the use of a stable isotope mixing model for measuring the ecological niches of consumers. In particular, we used the model IsotopeR to estimate the resource use of two species with complex, omnivorous diets: invasive Norway rats from the Aleutian Islands, AK, and American black bears from Yosemite National Park, CA. Marginal posterior distributions for major food sources (for populations, groups and individuals) described the resource axes that partly define the realized niches of these omnivores. We used measures of these resource axes to inform resource management in the Aleutians and Yosemite. Results from our analyses confirm that coastal rats did not rely on marine birds on rat-infested islands in the Aleutians. Instead, rats foraged primarily on terrestrial plants and preferred amphipods when they were available. We also use stable isotopes to confirm that plants and acorns are the largest contributors to black bear nutrition in Yosemite and learned that female bears foraged for acorns and pine nuts more heavily than males. Although it is unclear if Norway rats can maintain viable populations in the Aleutians without access to marine-derived animal protein, results from our analyses suggest their dependence on such nutrients. In addition, sex-specific differences in foraging for high-fat acorns and pine nuts in Yosemite suggest black bear populations in the Sierra Nevada may be limited by the productivity and health of hard mast species. As demonstrated here, stable isotope analysis has wide applicability for investigating the resource use and ecological niches of animals. We anticipate and encourage its rapid development in this fundamental field of ecology.

Methods for sampling sequential annual bone growth layers for stable isotope analysis

Methods in Ecology and Evolution 2016 doi: 10.1111/2041-210X.12522 Methods for sampling sequential annual bone growth layers for stable isotope analysis Calandra N. Turner Tomaszewicz 1,2 *, Jeffrey A. Seminoff 2 , Larisa Avens 3 and Carolyn M. Kurle 1 Division of Biological Sciences, Ecology, Behavior and Evolution Section, University of California, San Diego, La Jolla, CA 92093-0116, USA; 2 Southwest Fisheries Science Center, NOAA, National Marine Fisheries Service, La Jolla, CA 92037, USA; and 3 Southeast Fisheries Science Center, NOAA, National Marine Fisheries Service, Beaufort, NC 28516, USA Summary 1. Stable carbon (d 13 C) and nitrogen (d 15 N) isotope analysis (SIA) has proven useful in addressing fundamental questions in ecology such as reconstructing trophic interactions, habitat connections and climate regime shifts. The temporal scales over which SIA can be used to address ecological problems vary depending on the protein turnover times of the analysed tissue. Hard, inert tissues, such as teeth, bones and mollusc shells, grow in regular intervals (i.e. daily or annually), and sequential sampling of these growth layers provides a time series of isotopic patterns. As a result, SIA on these tissues is useful for elucidating behaviour and ecology of animals over time, especially those with cryptic life-history stages, such as marine turtles that retain growth layers in their humerus bones. To date, there exists no standard protocol for the sequential sampling of cortical bone samples taken from fresh, modern samples for SIA. 2. We tested two different methods, micromilling untreated bone cross sections and biopsy coring bone cross sec- tions processed for skeletochronology, for sequentially sampling individual growth layers from marine turtle humerus bones. 3. We present a standard protocol for sequential bone growth layer sampling for SIA, facilitating direct compar- ison of future studies. We recommend using the micromilling sampling technique on untreated bone cross sec- tions, as it facilitated higher precision sampling of growth layers that were not affected by chemical processing, and minimized sample handling, thereby reducing chances for contamination. 4. This is the first study to present a standardized method to sequentially sample annual bone growth layers for stable isotope analysis and facilitates direct comparison among future studies. Key-words: bone, collagen, marine turtles, sequential sampling, skeletochronology, stable isotope analysis Introduction Stable carbon (d 13 C) and nitrogen (d 15 N) isotope analysis (SIA) of organic matter is a powerful tool used in ecological studies to elucidate diet, trophic level, habitat use and migra- tion of a wide variety of taxa in both marine (e.g.Vander Zan- den & Rasmussen 2001; Michener & Lajtha 2007; Graham et al. 2010; Newsome, Clementz & Koch 2010) and terrestrial (e.g. Koch, Fogel & Tuross 1994; Hobson, Barnett-Johnson & Cerling 2010) systems. Examination of both d 13 C and d 15 N values from animal tissues allows for reconstruction of animal movement patterns due to spatial variation in these values that reflect differential carbon and nitrogen processing at the base of terrestrial and marine food webs (DeNiro & Epstein 1978; Rau et al. 1983; Clementz & Koch 2001; McMahon, Hamady & Thorrold 2013). Different tissues incorporate and retain stable isotopes from the diet at varying rates, allowing researchers to investigate for- aging ecology over multiple time-scales by sampling-specific *Correspondence author. E-mail: cturnert@ucsd.edu tissues (Hobson 1999; Dalerum & Angerbj€ orn 2005; Reich, Bjorndal & Martinez del Rio 2008; Kurle 2009). Many hard tissues, such as bone, teeth, otoliths, corals and bivalve shells, do not have regular cellular turnover; instead, subsequent lay- ers formed during growth are retained. These inert layers pre- serve their original chemical composition, thereby reflecting the stable isotope values of the environment and the prey con- sumed during the formation of a particular growth layer (e.g. Elorriaga-Verplancken et al. 2013). This creates a time series of data reflecting an animal’s diet and location when layers are formed at regular time intervals (e.g. days for otolith rings, or years for bone, tooth, coral and tree rings). Sequential SIA of growth layers has been conducted on tis- sues such as otolith and teeth (e.g. Schwarcz et al. 1998; Hob- son 1999; Newsome et al. 2006; Elorriaga-Verplancken et al. 2013) with promising results for reconstructing habitat use pat- terns for migratory megafauna. For some marine turtle spe- cies, humerus bone tissue is deposited in annual layers (e.g. Snover et al. 2011) and, recently, sequential SIA of marine tur- tle bone growth layers identified by skeletochronology has been successful, generating a time series reflecting the diet and

Diet-tissue stable isotope (Δ13C and Δ15N) discrimination factors for multiple tissues from terrestrial reptiles

RATIONALE Stable isotope analysis is a powerful tool for reconstructing trophic interactions to better understand drivers of community ecology. Taxon-specific stable isotope discrimination factors contribute to the best use of this tool. We determined the first Δ(13)C and Δ(15)N values for Rock Iguanas (Cyclura spp.) to better understand isotopic fractionation and estimate wild reptile foraging ecology. METHODS The Δ(13)C and Δ(15)N values between diet and skin, blood, and scat were determined from juvenile and adult iguanas held for 1 year on a known diet. We measured relationships between iguana discrimination factors and size/age and quantified effects of lipid extraction and acid treatment on stable isotope values from iguana tissues. Isotopic and elemental compositions were determined by Dumas combustion using an elemental analyzer coupled to an isotope ratio mass spectrometer using standards of known composition. RESULTS The Δ(13)C and Δ(15)N values ranged from -2.5 to +6.5‰ and +2.2 to +7.5‰, respectively, with some differences among tissues and between juveniles and adults. The Δ(13)C values from blood and skin differed among species, but not the Δ(15)N values. The Δ(13)C values from blood and skin and Δ(15)N values from blood were positively correlated with size/age. The Δ(13)C values from scat were negatively correlated with size (not age). Treatment with HCl (scat) and lipid extraction (skin) did not affect the isotope values. CONCLUSIONS These results should aid in the understanding of processes driving stable carbon and nitrogen isotope discrimination factors in reptiles. We provide estimates of Δ(13)C and Δ(15)N values and linear relationships between iguana size/age and discrimination factors for the best interpretation of wild reptile foraging ecology.

Applications of stable isotope analysis in mammalian ecology.

In this editorial, we provide a brief introduction and summarize the 10 research articles included in this Special Issue on Applications of stable isotope analysis in mammalian ecology. The first three articles report correction and discrimination factors that can be used to more accurately estimate the diets of extinct and extant mammals using stable isotope analysis. The remaining seven applied research articles use stable isotope analysis to address a variety of wildlife conservation and management questions from the oceans to the mountains.