Anthropogenic changes to the Holocene nitrogen cycle in Ireland

Nitrogen palaeo-isoscapes: Changing spatial gradients of faunal δ15N in late Pleistocene and early Holocene Europe

Nitrogen isotope ratio analysis (δ15N) of animal tissue is widely used in archaeology and palaeoecology to investigate diet and ecological niche. Data interpretations require an understanding of nitrogen isotope compositions at the base of the food web (baseline δ15N). Significant variation in animal δ15N has been recognised at various spatiotemporal scales and related to changes both in baseline δ15N, linked to environmental and climatic influence on the terrestrial nitrogen cycle, and animal ecology. Isoscapes (models of isotope spatial variation) have proved a useful tool for investigating spatial variability in biogeochemical cycles in present-day marine and terrestrial ecosystems, but so far, their application to palaeo-data has been more limited. Here, we present time-sliced nitrogen isoscapes for late Pleistocene and early Holocene Europe (c. 50,000 to 10,000 years BP) using herbivore collagen δ15N data. This period covers the Last Glacial-Interglacial Transition, during which significant variation in the terrestrial nitrogen cycle occurred. We use generalized linear mixed modelling approaches for interpolation and test models which both include and exclude climate covariate data. Our results show clear changes in spatial gradients of δ15N through time. Prediction of the lowest faunal δ15N values in northern latitudes after, rather than during, the Last Glacial Maximum is consistent with the Late Glacial Nitrogen Excursion (LGNE). We find that including climatic covariate data does not significantly improve model performance. These findings have implications for investigating the drivers of the LGNE, which has been linked to increased landscape moisture and permafrost thaw, and for understanding changing isotopic baselines, which are fundamental for studies investigating diets, niche partitioning, and migration of higher trophic level animals.

Interpreting past trophic ecology of a threatened alpine parrot, kea Nestor notabilis, from museum specimens

When ecosystems are under severe pressure or environments change, trophic position and intraspecific niche width may decrease or narrow, signalling that conservation action is required. In New Zealand, alpine and subalpine ecosystems have been extensively modified through farming since 19th-century European settlement, with consequences for indigenous species such as the kea Nestor notabilis. We investigated feather stable isotope values in the kea and predicted a lower trophic position in modern kea populations, to reflect reduced lowland habitat and a mixed diet with more plant material. We predicted that size and sex would influence trophic values in this sexually dimorphic species, with larger birds more likely to have a high protein diet. We examined potential dietary changes in 68 museum collected kea from 1880s to 2000s, first recording accession details including provenance and sex and measuring culmen length. We used bulk carbon and nitrogen stable isotope analyses (BSIAs) of feathers and a further feather subset using compound-specific stable isotope analyses of amino acids (CSIA-AA) to obtain isotopic values and estimate trophic position. BSIA showed δ¹⁵ N values in kea feathers declined through time and could indicate that early century kea were highly omnivorous, with δ¹⁵ N values on average higher than in modern kea. Variance in δ¹⁵ N values was greater after 1950, driven by a few individuals. Few differences between males and females were evident, although females in the south region had lower δ¹⁵ N values. There was a tendency for large male birds to have higher trophic values, perhaps reflecting dominant male bird behaviour noted in historical records. Nonetheless, CSIA-AA performed on a subset of the data suggested that variation in BSIA is likely due to baseline changes rather than relative trophic position which may be more homogenous than these data indicate. Although there was more variability in modern kea, we suggest caution in interpretation. Stable isotope data, particularly CSIA-AA, from museum specimens can reveal potential change in ecological networks as well as sexually dimorphic feeding patterns within species. The data can reveal temporal and regional variation in species trophic position and changes in ecosystem integrity to inform conservation decision-making.

Arthropod-Microbiota Integration: Its Importance for Ecosystem Conservation

Recent reports indicate that the health of our planet is getting worse and that genuine transformative changes are pressing. So far, efforts to ameliorate Earth's ecosystem crises have been insufficient, as these often depart from current knowledge of the underlying ecological processes. Nowadays, biodiversity loss and the alterations in biogeochemical cycles are reaching thresholds that put the survival of our species at risk. Biological interactions are fundamental for achieving biological conservation and restoration of ecological processes, especially those that contribute to nutrient cycles. Microorganism are recognized as key players in ecological interactions and nutrient cycling, both free-living and in symbiotic associations with multicellular organisms. This latter assemblage work as a functional ecological unit called "holobiont." Here, we review the emergent ecosystem properties derived from holobionts, with special emphasis on detritivorous terrestrial arthropods and their symbiotic microorganisms. We revisit their relevance in the cycling of recalcitrant organic compounds (e.g., lignin and cellulose). Finally, based on the interconnection between biodiversity and nutrient cycling, we propose that a multicellular organism and its associates constitute an Ecosystem Holobiont (EH). This EH is the functional unit characterized by carrying out key ecosystem processes. We emphasize that in order to meet the challenge to restore the health of our planet it is critical to reduce anthropic pressures that may threaten not only individual entities (known as "bionts") but also the stability of the associations that give rise to EH and their ecological functions.

Seaweed-eating sheep show that δ34 S evidence for marine diets can be fully masked by sea spray effects

RATIONALE

Stable sulfur isotope compositions (δ³⁴ S values) are a useful marker of terrestrial (lower δ³⁴ S) versus marine (higher δ³⁴ S) diets. In coastal areas, ³⁴ S-enriched sea spray can obscure these marine/terrestrial differences. We sought to establish whether δ³⁴ S values of sea spray-affected terrestrial fauna can be distinguished from those of marine-feeding terrestrial fauna.

METHODS

We measured bone and dentine collagen δ³⁴ S values, as well as stable carbon (δ¹³ C) and nitrogen (δ¹⁵ N) isotope compositions via continuous flow elemental analysis/isotope ratio mass spectrometry of 21 sheep (Ovis aries) raised on an island (North Ronaldsay, UK) of <7 km² that had widely divergent access to marine (seaweed) and heavily sea spray-affected terrestrial (grass) food sources. We also analyzed the bone collagen of marine and terrestrial fauna from this island.

RESULTS

Sheep bone collagen showed well-defined trends with highly significant correlations between δ¹³ C and δ¹⁵ N values indicative of feeding along a continuum of fully terrestrial to fully marine diets, consistent with other modern baseline data from marine and terrestrial animals in the same area. In contrast, δ³⁴ S values were generally elevated for all sheep and were not significantly correlated with either δ¹³ C or δ¹⁵ N values.

CONCLUSIONS

Our findings demonstrate that δ³⁴ S values are poorly suited for differentiating marine and terrestrial diets in terrestrial animals in areas with pronounced sea spray effects. Care must be taken to characterize the isotopic compositions of potential food items before δ³⁴ S values are used as a marker for reliance on marine protein in modern and ancient contexts.

Integrating fish scale and bone isotopic compositions for 'deep time' retrospective studies

Isotopic studies of archived fish scales have tremendous potential to develop long-term retrospectives that provide important insights into how humans have altered aquatic ecosystems. However, fish specimens in museum archives and other repositories typically date to time periods when the impacts of industrial societies may have already caused profound environmental changes. Archaeological fish bones offer an opportunity to bridge this key temporal gap by providing samples spanning from the recent past to as far back as the Pleistocene. Collagen is the primary protein component of both fish scale and bone, but the comparability of isotopic compositions from these tissues has not been established experimentally. To lay the framework for integrating isotopic datasets from these tissues, we compare δ¹³C and δ¹⁵N of bone and scale collagen, as well as other tissues, from fish with life-time controlled diets. Results show that while there is no difference in δ¹³C between scale and bone collagen, there may be a very slight but meaningful inter-tissue offset in δ¹⁵N (<0.3‰). We discuss potential sources of δ¹⁵N variation in scale and bone collagen measurements. Because there is no difference in scale and bone δ¹³C, and the observed offset in δ¹⁵N is very small (less than analytical uncertainty in many studies), our findings demonstrate that collagen isotopic compositions from these tissues should be directly comparable when integrating datasets from modern and ancient samples to build more powerful, millennium-scale isotopic times series. In linking isotopic compositions of collagen from modern, historical (scales), and archaeological (bones) fish, our findings open the way for more nuanced contextualization of how ecosystems functioned prior to large-scale exploitation and how they have responded to mounting anthropogenic pressures in the intervening centuries.

Feasting and Mobility in Iron Age Ireland: Multi-isotope analysis reveals the vast catchment of Navan Fort, Ulster

Navan Fort is an iconic prehistoric Irish ceremonial centre and the legendary capital of Ulster. The fort has produced an exceptional pig-dominated faunal assemblage that also contained a barbary macaque skull. Dating from the 4^th to 1^st century BC, it is likely to be a ceremonial feasting centre that may have drawn people and their animals from across Ulster and beyond. This study uses a multi-isotope (⁸⁷Sr/⁸⁶Sr, δ³⁴S, δ¹³C, δ¹⁵N) approach to identify non-local animals and reconstruct site catchment. New biosphere mapping means that isotope data can be more confidently interpreted and the combination of strontium and sulphur analysis has the potential to estimate origins. In the absence of human remains, fauna provide the best proxy for human movement. Results for the 35 analysed animals are wide-ranging, especially in terms of strontium (0.707–0.715), which has the largest range for an Irish site. Sulphur values are more restricted (13.1‰−17.1‰) but are high in the context of British and Irish data. Results provide clear evidence for animals (and thus people) coming from across Ulster and beyond, demonstrating the site’s wide catchment. Navan Fort was clearly a major ceremonial centre with far-reaching influence and hosted feasts that drew people and animals from afar.

Urban rats have less variable, higher protein diets

Over the past 1000 years, rats (Rattus spp.) have become one of the most successful and prolific pests in human society. Despite their cosmopolitan distribution across six continents and ubiquity throughout the world's cities, rat urban ecology remains poorly understood. We investigate the role of human foods in brown rat (Rattus norvegicus) diets in urban and rural areas over a 100 year period (ca AD 1790–1890) in Toronto, Canada using stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope analyses of archaeological remains. We found that rat diets from urban sites were of higher quality and were more homogeneous and stable over time. By contrast, in rural areas, they show a wide range of dietary niche specializations that directly overlap, and probably competed, with native omnivorous and herbivorous species. These results demonstrate a link between rodent diets and human population density, providing, to our knowledge, the first long-term dietary perspective on the relative value of different types of human settlements as rodent habitat. This study highlights the potential of using the historical and archaeological record to provide a retrospective on the urban ecology of commensal and synanthropic animals that could be useful for improving animal management and conservation strategies in urban areas.