Seth D. Newsome

A niche for isotopic ecology

Fifty years ago, GE Hutchinson defined the ecological niche as a hypervolume in n-dimensional space with environmental variables as axes. Ecologists have recently developed renewed interest in the concept, and technological advances now allow us to use stable isotope analyses to quantify these niche dimensions. Analogously, we define the isotopic niche as an area (in δ-space) with isotopic values (δ-values) as coordinates. To make isotopic measurements comparable to other niche formulations, we propose transforming δ-space to p-space, where axes represent relative proportions of isotopically distinct resources incorporated into an animals tissues. We illustrate the isotopic niche with two examples: the application of historic ecology to conservation biology and ontogenetic niche shifts. Sustaining renewed interest in the niche requires novel methods to measure the variables that define it. Stable isotope analyses are a natural, perhaps crucial, tool in contemporary studies of the ecological niche.

Combining sources in stable isotope mixing models: alternative methods

Stable isotope mixing models are often used to quantify source contributions to a mixture. Examples include pollution source identification; trophic web studies; analysis of water sources for soils, plants; or water bodies, and many others. A common problem is having too many sources to allow a unique solution. We discuss two alternative procedures for addressing this problem. One option is a priori to combine sources with similar signatures so the number of sources is small enough to provide a unique solution. Aggregation should be considered only when isotopic signatures of clustered sources are not significantly different, and sources are related so the combined source group has some functional significance. For example, in a food web analysis, lumping several species within a trophic guild allows more interpretable results than lumping disparate food sources, even if they have similar isotopic signatures. One result of combining mixing model sources is increased uncertainty of the combined end-member isotopic signatures and consequently the source contribution estimates; this effect can be quantified using the IsoError model (http://www.epa.gov/wed/pages/models/isotopes/isoerror1_04.htm). As an alternative to lumping sources before a mixing analysis, the IsoSource mixing model (http://www.epa.gov/wed/pages/models/isosource/isosource.htm) can be used to find all feasible solutions of source contributions consistent with isotopic mass balance. While ranges of feasible contributions for each individual source can often be quite broad, contributions from functionally related groups of sources can be summed a posteriori, producing a range of solutions for the aggregate source that may be considerably narrower. A paleohuman dietary analysis example illustrates this method, which involves a terrestrial meat food source, a combination of three terrestrial plant foods, and a combination of three marine foods. In this case, a posteriori aggregation of sources allowed strong conclusions about temporal shifts in marine versus terrestrial diets that would not have otherwise been discerned.

Agricultural origins and the isotopic identity of domestication in northern China

Stable isotope biochemistry (δ13C and δ15N) and radiocarbon dating of ancient human and animal bone document 2 distinct phases of plant and animal domestication at the Dadiwan site in northwest China. The first was brief and nonintensive: at various times between 7900 and 7200 calendar years before present (calBP) people harvested and stored enough broomcorn millet (Panicum miliaceum) to provision themselves and their hunting dogs (Canis sp.) throughout the year. The second, much more intensive phase was in place by 5900 calBP: during this time both broomcorn and foxtail (Setaria viridis spp. italica) millets were cultivated and made significant contributions to the diets of people, dogs, and pigs (Sus sp.). The systems represented in both phases developed elsewhere: the earlier, low-intensity domestic relationship emerged with hunter–gatherers in the arid north, while the more intensive, later one evolved further east and arrived at Dadiwan with the Yangshao Neolithic. The stable isotope methodology used here is probably the best means of detecting the symbiotic human–plant–animal linkages that develop during the very earliest phases of domestication and is thus applicable to the areas where these connections first emerged and are critical to explaining how and why agriculture began in East Asia.

Using Isoscapes to Trace the Movements and Foraging Behavior of Top Predators in Oceanic Ecosystems

The stable isotope composition of animal tissues can provide intrinsic tags to study the foraging and migratory ecology of predators in the open ocean. Chapter 13 (this volume) demonstrated that by comparing the isotope values of an animal and its local prey or environment, the animal’s movements can be estimated, given that isotopic variation exists between habitats. The utility of using geographical variations in stable isotopes values, or isoscapes to study the movements of marine predators has been limited because of our lack of knowledge on the spatial variation of the carbon, nitrogen, and oxygen isotope values in the open ocean.

Tools for quantifying isotopic niche space and dietary variation at the individual and population level.

Abstract Ecologists are increasingly using stable isotope analysis to inform questions about variation in resource and habitat use from the individual to community level. In this study we investigate data sets from 2 California sea otter (Enhydra lutris nereis) populations to illustrate the advantages and potential pitfalls of applying various statistical and quantitative approaches to isotopic data. We have subdivided these tools, or metrics, into 3 categories: IsoSpace metrics, stable isotope mixing models, and DietSpace metrics. IsoSpace metrics are used to quantify the spatial attributes of isotopic data that are typically presented in bivariate (e.g., &dgr;13C versus &dgr;15N) 2-dimensional space. We review IsoSpace metrics currently in use and present a technique by which uncertainty can be included to calculate the convex hull area of consumers or prey, or both. We then apply a Bayesian-based mixing model to quantify the proportion of potential dietary sources to the diet of each sea otter population and compare this to observational foraging data. Finally, we assess individual dietary specialization by comparing a previously published technique, variance components analysis, to 2 novel DietSpace metrics that are based on mixing model output. As the use of stable isotope analysis in ecology continues to grow, the field will need a set of quantitative tools for assessing isotopic variance at the individual to community level. Along with recent advances in Bayesian-based mixing models, we hope that the IsoSpace and DietSpace metrics described here will provide another set of interpretive tools for ecologists.

The shifting baseline of northern fur seal ecology in the northeast Pacific Ocean

Historical data provide a baseline against which to judge the significance of recent ecological shifts and guide conservation strategies, especially for species decimated by pre-20th century harvesting. Northern fur seals (NFS; Callorhinus ursinus) are a common pinniped species in archaeological sites from southern California to the Aleutian Islands, yet today they breed almost exclusively on offshore islands at high latitudes. Harvest profiles from archaeological sites contain many unweaned pups, confirming the presence of temperate-latitude breeding colonies in California, the Pacific Northwest, and the eastern Aleutian Islands. Isotopic results suggest that prehistoric NFS fed offshore across their entire range, that California populations were distinct from populations to the north, and that populations breeding at temperate latitudes in the past used a different reproductive strategy than modern populations. The extinction of temperate-latitude breeding populations was asynchronous geographically. In southern California, the Pacific Northwest, and the eastern Aleutians, NFS remained abundant in the archaeological record up to the historical period ≈200 years B.P.; thus their regional collapse is plausibly attributed to historical hunting or some other anthropogenic ecosystem disturbance. In contrast, NFS populations in central and northern California collapsed at ≈800 years B.P., long before European contact. The relative roles of human hunting versus climatic factors in explaining this ecological shift are unclear, as more paleoclimate information is needed from the coastal zone.

Variation in δ13C and δ15N diet–vibrissae trophic discrimination factors in a wild population of California sea otters

The ability to quantify dietary inputs using stable isotope data depends on accurate estimates of isotopic differences between a consumer (c) and its diet (d), commonly referred to as trophic discrimination factors (TDFs) and denoted by Δc-d. At present, TDFs are available for only a few mammals and are usually derived in captive settings. The magnitude of TDFs and the degree to which they vary in wild populations is unknown. We determined δ13C and δ15N TDFs for vibrissae (i.e., whiskers), a tissue that is rapidly becoming an informative isotopic substrate for ecologists, of a wild population of sea otters for which individual diet has been quantified through extensive observational study. This is one of the very few studies that report TDFs for free-living wild animals feeding on natural diets. Trophic discrimination factors of 2.2‰ ± 0.7‰ for δ13C and 3.5‰ ± 0.6‰ for δ15N (mean ± SD) were similar to those reported for captive carnivores, and variation in individual δ13C TDFs was negatively but significa...

Polybrominated Diphenyl Ether (PBDE) Levels in Peregrine Falcon (Falco peregrinus) Eggs from California Correlate with Diet and Human Population Density

Peregrine falcons are now considered a conservation success story due in part to the phasing out of harmful contaminants that adversely affected reproduction. Recent studies have shown that peregrine eggs collected from California cities, however, have high levels of the higher-brominated polybrominated diphenyl ethers (SigmaPBDE(183-209)), a class of industrial flame retardants, in comparison to published data for other wildlife. Sources of these high PBDE levels and unusual PBDE profiles are unknown. Here we analyzed the stable carbon (delta(13)C), hydrogen (deltaD), and nitrogen (delta(15)N) isotope composition of peregrine eggs collected from urban and nonurban habitats. We found that delta(13)C values were significantly higher in urban versus nonurban eggs, suggesting that urban peregrines indirectly receive anthropogenic subsidies via their consumption of prey reliant on corn-based anthropogenic foods. delta(15)N and deltaD values were significantly lower in urban versus nonurban eggs, reflecting differences in dietary diversity and food/water sources available to peregrines in each habitat. These patterns suggest a link between an anthropogenic diet and high levels of SigmaPBDE(183-209) in California peregrines, and identify anthropogenic food as a potentially important PBDE exposure pathway for urban wildlife. If diet is an important PBDE exposure pathway for peregrines, continued high body burdens of SigmaPBDE(183-209) may be a potential risk to ongoing peregrine conservation efforts in California.

Stable isotopes evaluate exploitation of anthropogenic foods by the endangered San Joaquin kit fox (Vulpes macrotis mutica)

Abstract The unprecedented rate of urbanization over the past several decades is a major concern for conservation globally and has given rise to the multidisciplinary field of urban ecology. This field explores the direct and indirect effects of human activities on food-web dynamics, community structure, and animal behavior in highly modified urban ecosystems. Urban ecosystems are typically characterized by reduced species diversity but increased abundance of a few species able to exploit anthropogenic food sources. For many urban mammalian and avian species direct resource subsidization is difficult to assess using traditional means such as scat analysis. Here we show how stable isotope analysis can be used to assess the exploitation of anthropogenic foods in an endangered carnivore, the San Joaquin kit fox (Vulpes macrotis mutica) inhabiting the southern San Joaquin Valley in California. Examination of carbon (δ13C) and nitrogen (δ15N) isotope data shows that kit foxes living in urban Bakersfield, California, extensively exploit anthropogenic foods, which sharply contrasts with dietary data derived from scat analysis. Urban kit foxes had significantly higher δ13C and lower δ15N values than foxes from adjacent nonurban areas and had similar isotope values as Bakersfield human residents, which suggests a shared food source. In contrast, examination of isotopic data for nonurban kit foxes shows that they largely consume the most abundant natural prey species found in their scats. Stable isotope analysis offers a rapid and cost-effective means of evaluating the degree to which urban wildlife populations exploit anthropogenic foods in areas where native C4 vegetation is relatively uncommon or absent, important in assessing the direct impacts of human activities on food-web dynamics in urban ecosystems. We anticipate that the isotopic gradients used here will be useful in assessing the exploitation of anthropogenic foods in other urban wildlife populations.

Historical Ecology and Biogeography of North Pacific Pinnipeds: Isotopes and Ancient DNA from Three Archaeological Assemblages

ABSTRACT Zooarchaeology has the potential to make significant contributions to knowledge of pinniped biogeography of import to both archaeologists and environmental scientists. We analyzed northern fur seal remains found in three archaeological sites located along the outer coast of the Northeast Pacific Ocean: Cape Addington Rockshelter in southeast Alaska, Ts’ishaa on the west coast of Vancouver Island, and the Netarts Sandspit site on the Oregon Coast. These three sites occur along an 850 km stretch of coastline between 45° to 55° N. and 123° to 134° W., far southeast of the primary breeding area for northern fur seals today, located on the Pribilof Islands at 57° N. 170° W. We use ancient DNA (aDNA) and carbon (δ13C) and nitrogen (δ15N) isotopes to investigate whether northern fur seal remains from these archaeological sites originated with migratory Pribilof Islands populations. For sites located in Oregon and points north, the isotope values are not distinct from those of the Pribilof fur seals. Although aDNA was recovered from three pinniped species (northern fur seal, Steller sea lion, and Guadalupe fur seal), the paucity of published genetic data from modern northern fur seals prevents us from distinguishing the archaeological specimens from modern Pribilof seals.