Collapse of trophic‐niche structure in belowground communities under anthropogenic disturbance

Water-Level Fluctuation Control of the Trophic Structure of a Yangtze River Oxbow

Seasonal water-level fluctuations can profoundly impact nutrient dynamics in aquatic ecosystems, influencing trophic structures and overall ecosystem functions. The Tian-e-Zhou Oxbow of the Yangtze River is China's first ex situ reserve and the world's first successful case of ex situ conservation for cetaceans. In order to better protect the Yangtze finless porpoise, the effects of water-level fluctuations on the trophic structure in this oxbow cannot be ignored. Therefore, we employed stable isotope analysis to investigate the changes in the trophic position, trophic niche, and contribution of basal food sources to fish during the wet and dry seasons of 2021-2022. The research results indicate that based on stable isotope analysis of the trophic levels of different dietary fish species, fish trophic levels during the wet season were generally higher than those during the dry season, but the difference was not significant (p > 0.05). Fish communities in the Tian-e-Zhou Oxbow exhibited broader trophic niche space and lower trophic redundancy during the wet season (p < 0.05), indicating a more complex and stable food web structure. In both the wet and dry seasons, fish in the oxbow primarily relied on endogenous carbon sources, but there were significant differences in the way they were utilized between the two seasons (p < 0.05). In light of the changes in the trophic structure of the fish during the wet and dry seasons, and to ensure the stable development of the Yangtze finless porpoise population, we recommend strengthening the connectivity between the Tian-e-Zhou Oxbow and the Yangtze River.

Carbon and nitrogen isotopic similarity between the endangered Darwin's fox (Lycalopex fulvipes) and sympatric free-ranging dogs in Chiloé Island, Chile

Darwin's fox is an opportunistic omnivorous predator native to Chile classified as endangered by the IUCN Red List. Habitat use by Darwin's foxes can be negatively affected by the presence of free-ranging dogs that range freely across native and non-native habitats and can be a source of fox mortality. The objective of this study was to analyze the isotopic similarity of Darwin's fox and sympatric free-ranging dogs in Chiloé Island to determine the impact of anthropogenic environmental alterations on wild predators. We use hair samples to characterise and compare their δ¹³C and δ¹⁵N values and to evaluate isotopic similarity and isotope niches overlap. A generalised linear model was used to associate the isotope value with landscape variables (forest cover and vegetation type) and distance to the nearest house. We found no significant differences in δ¹³C or δ¹⁵N values between foxes and dogs, and a marginally significant isotope niche overlap (59.4 %). None of the selected variables at landscape and site scale were related to isotope values. Although our study is not a probe of direct contact between foxes and free-ranging dogs, the high isotopic similarity highlights the risk of pathogen spillover from free-ranging dogs to Darwin's foxes.

Impacts on food web properties of island invertebrate communities vary between different human land uses

Human land use is of growing concern for island ecosystems. Besides direct impacts on biodiversity, land uses can alter the functioning and structure of ecosystems. Central to this are impacts on food webs. The release of additional nutrients from human origin, habitat homogenization, or environmental filtering due to human land use can change the diet of individual consumer species (i.e., their trophic niches) and the distribution and overlap of trophic niches within a food web. However, it remains largely unclear whether the effects on food web properties vary between the different and predominant human land uses present on islands. Here, we investigated the impact of two dominant human land uses on small oceanic islands (i.e., urban and tourism development) and tested if and how different land uses on islands affect food web structure. To disentangle human land uses, we investigated islands, which were either privately owned by a tourist facility (i.e., exclusively tourism land use) or experienced urban development from the local population (i.e., urban land use), or remained uninhabited, serving as reference sites free of direct land use. Using stable isotope analysis, we show that isotope signature, trophic (isotopic) niches, and overall food web properties of the investigated island invertebrate communities were significantly changed under both land use regimes. While trophic diversity was reduced and trophic niche widths increased under tourism land use, the investigated food webs showed reduced trophic diversity at the food web base and a more uneven trophic niche distribution under urban land use. In summary, these findings show that different human land uses can have contrasting impacts on oceanic island food webs. As oceanic islands experience rapidly growing human land conversion, our results indicate that they may also face increasing yet unpredictable long-term changes in food web dynamics.

Trophic niche but not abundance of Collembola and Oribatida changes with drought and farming system

Higher frequencies of summer droughts are predicted to change soil conditions in the future affecting soil fauna communities and their biotic interactions. In agroecosystems drought effects on soil biota may be modulated by different management practices that alter the availability of different food resources. Recent studies on the effect of drought on soil microarthropods focused on measures of abundance and diversity. We here additionally investigated shifts in trophic niches of Collembola and Oribatida as indicated by stable isotope analysis (¹³C and ¹⁵N). We simulated short-term summer drought by excluding 65% of the ambient precipitation in conventionally and organically managed winter wheat fields on the DOK trial in Switzerland. Stable isotope values suggest that plant litter and root exudates were the most important resources for Collembola (Isotoma caerulea, Isotomurus maculatus and Orchesella villosa) and older plant material and microorganisms for Oribatida (Scheloribates laevigatus and Tectocepheus sarekensis). Drought treatment and farming systems did not affect abundances of the studied species. However, isotope values of some species increased in organically managed fields indicating a higher proportion of microorganisms in their diet. Trophic niche size, a measure of both isotope values combined, decreased with drought and under organic farming in some species presumably due to favored use of plants as basal resource instead of algae and microorganisms. Overall, our results suggest that the flexible usage of resources may buffer effects of drought and management practices on the abundance of microarthropods in agricultural systems.

Variation in Community-Level Trophic Niches of Soil Microarthropods With Conversion of Tropical Rainforest Into Plantation Systems as Indicated by Stable Isotopes (15N, 13C)

Land-use change is threatening biodiversity worldwide and is predicted to increase in the next decades, especially in tropical regions. Most studies focused on the response of single or few species to land-use change, only few investigated the response of entire communities. In particular the response of belowground communities to changes in land use received little attention. Oribatid mites (Oribatida, Acari) are among the most abundant soil animals, involved in decomposition processes and nutrient cycling. Oribatid mite species span a wide range of trophic niches and are known to sensitively respond to changes in land use. Here, we investigated shifts in the community-level trophic niche of oribatid mites with the conversion of rainforest into rubber and oil palm plantations. Due to a wider range of resources in more natural ecosystems, we expected the community-level trophic niche to shrink with conversion of rainforest into plantations. As the conversion of rainforest into plantations is associated with reduced availability of litter resources, we expected the average trophic level (indicated by the 15N/14N ratio) to be higher and basal resources (indicated by the 13C/12C ratio) to shift toward living plant material in rubber and oil palm plantations. Our analysis showed that community-level trophic niches in rainforest and rubber agroforest (“jungle rubber”) were separated from those in monoculture plantation systems, indicating a trophic niche shift with land-use intensification. As hypothesized, oribatid mites shifted their diet toward predation and/or scavenging and toward the plant-based energy channel with transformation of rainforest into plantations. Exceptionally low minimum 13C/12C ratios in rubber plantations suggest that certain oribatid mite species in this land-use system use resources not available in the other studied ecosystems. We detected high isotopic uniqueness in oil palm plantations suggesting a low trophic redundancy and thus high vulnerability of trophic functioning in this system in comparison to rainforest. Overall, the results suggest that the conversion of rainforest into plantations is associated with pronounced shifts in community-level trophic niches of mesofauna detritivores with potential major consequences for the functioning of the decomposer system.

Trophic niche differentiation and utilisation of food resources in Collembola is altered by rainforest conversion to plantation systems

Intensively managed monoculture plantations are increasingly replacing natural forests across the tropics resulting in changes in ecological niches of species and communities, and in ecosystem functioning. Collembola are among the most abundant arthropods inhabiting the belowground system sensitively responding to changes in vegetation and soil conditions. However, most studies on the response of Collembola to land-use change were conducted in temperate ecosystems and focused on shifts in community composition or morphological traits, while parameters more closely linked to ecosystem functioning, such as trophic niches, received little attention. Here, we used stable isotope analysis (¹³C and ¹⁵N) to investigate changes in the trophic structure and use of food resources by Collembola in Jambi province (Sumatra, Indonesia), a region that experienced strong deforestation in the last decades. Isotopic values of Collembola from 32 sites representing four land-use systems were analyzed (rainforest, rubber agroforest, rubber (Hevea brasiliansis) and oil palm (Elaeis guineensis) monoculture plantations). Across Collembola species Δ¹³C values were highest in rainforest suggesting more pronounced processing of litter resources by microorganisms and consumption of these microorganisms by Collembola in this system. Lower Δ¹³C values, but high Δ¹³C variation in Collembola in oil palm plantations indicated that Collembola shifted towards herbivory and used more variable resources in this system. Small range in Δ¹⁵N values in Collembola species in monoculture plantations in comparison to rainforest indicated that conversion of rainforest into plantations is associated with simplification in the trophic structure of Collembola communities. This was further confirmed by generally lower isotopic niche differentiation among species in plantations. Across the studied ecosystems, atmobiotic species (Symphypleona and Paronellidae) occupied the lowest, whereas euedaphic Collembola species occupied the highest trophic position, resembling patterns in temperate forests. Some species of Paronellidae in rainforest and jungle rubber had Δ¹⁵N values below those of leaf litter suggesting algivory (Salina sp.1, Callyntrura sp.1 and Lepidonella sp.1), while a dominant species, Pseudosinella sp.1, had the highest Δ¹⁵N values in most of the land-use systems suggesting that this species at least in part lives as predator or scavenger. Overall, the results suggest that rainforest conversion into plantation systems is associated with marked shifts in the structure of trophic niches in soil and litter Collembola with potential consequences for ecosystem functioning and food-web stability.

Trophic level and basal resource use of soil animals are hardly affected by local plant associations in abandoned arable land

Plants provide resources and shape the habitat of soil organisms thereby affecting the composition and functioning of soil communities. Effects of plants on soil communities are largely taxon-dependent, but how different functional groups of herbaceous plants affect trophic niches of individual animal species in soil needs further investigation. Here, we studied the use of basal resources and trophic levels of dominating soil meso- and macrofauna using stable isotope ratios of carbon and nitrogen in arable fallow systems 3 and 14-16 years after abandonment. Animals were sampled from the rhizosphere of three plant species of different functional groups: a legume (Medicaco sativa), a nonlegume herb (Taraxacum officinale), and a grass (Bromus sterilis). We found virtually no consistent effects of plant identity on stable isotope composition of soil animals and on thirteen isotopic metrics that reflect general food-web structure. However, in old fallows, the carbon isotope composition of some predatory macrofauna taxa had shifted closer to that of co-occurring plants, which was particularly evident for Lasius, an aphid-associated ant genus. Trophic levels and trophic-chain lengths in food webs were similar across plant species and fallow ages. Overall, the results suggest that variations in local plant diversity of grassland communities may little affect the basal resources and the trophic level of prey consumed by individual species of meso- and macrofauna belowground. By contrast, successional changes in grassland communities are associated with shifts in the trophic niches of certain species, reflecting establishment of trophic interactions with time, which shapes the functioning and stability of soil food webs.