Variation in the structure of food webs related to climate change and antropogenic disturbance

Climate change and anthropogenic pressure are deeply altering the global environments dramatically threating the persistence of biodiversity and ecosystems processes including the primary production, matter cycling and energy flux. However, the still scarce knowledge on the biodiversity organization in the food web structure, and not only on the species number, limits our power to predict the effects of environmental change and the possible response mechanism of biological community to environmental variations. This work aims to understand the mechanism underlying the community structure and its stability under different natural or anthropogenic pressures conditions. The food web structure and metrics, the main carbon pathways and the taxa that play a key role in the community organisation in aquatic ecosystems were identified and quantified. The extensive field sampling and stable isotope analysis of each individual collected in the study ecosystems allowed to reconstruct and compare highly detailed food webs and trophic niche at both spatio-temporal and hierarchical scale (from individuals to community). In addition, the new consideration of trophic specie based on restricted interval of two elements isotopic signals, proved to be an efficient approach to overcome the current limits of the classical food web reconstruction, that occur when the biodiversity and the possible trophic interactions between species within an ecosystem are particularly complex. Our results highlight that modifications in the availability and quality of basal resources alters the interactions among organisms at different hierarchal scale with cascade effects on the entire food web structure, complexity, and stability. In accordance with optimal foraging theory, the availability of food sources has imposed change of link density already starting from single individuals with important variation of recurring patterns in the food web structure. Specifically, the consumers diet breadth decreased with the per capita availability and quality of resources increased. This, in turn, led to a relatively simpler food web characterized by lower link density affects the stability of the community against biodiversity loss and biological invasions. Our results are of particular relevance providing the key information for understanding the mechanisms that structure the community and its possible response to the different disturbance conditions. The results support that the expected changes in the habitat physical conditions could be reflected in food web architecture and organisms with greater generalism and trophic plasticity may be able to persist after a disturb playing a key role in community stability. The structure and function of food webs could be an efficient measure to predict and mitigate the effect of induced both physical and biological change on natural communities, both in the short and medium-long term

Time- and depth-wise trophic niche shifts in Antarctic benthos

Climate change is expected to affect resource-consumer interactions underlying stability in polar food webs. Polar benthic organisms have adapted to the marked seasonality characterising their habitats by concentrating foraging and reproductive activity in summer months, when inputs from sympagic and pelagic producers increase. While this enables the persistence of biodiverse food webs, the mechanisms underlying changes in resource use and nutrient transfer are poorly understood. Thus, our understanding of how temporal and spatial variations in the supply of resources may affect food web structure and functioning is limited. By means of C and N isotopic analyses of two key Antarctic benthic consumers (Adamussium colbecki, Bivalvia, and Sterechinus neumayeri, Echinoidea) and Bayesian mixing models, we describe changes in trophic niche and nutrient transfer across trophic levels associated with the long- and short-term diet and body size of specimens sampled in midsummer in both shallow and deep waters. Samplings occurred soon after the sea-ice broke up at Tethys Bay, an area characterised by extreme seasonality in sea-ice coverage and productivity in the Ross Sea. In the long term, the trophic niche was broader and variation between specimens was greater, with intermediate-size specimens generally consuming a higher number of resources than small and large specimens. The coupling of energy channels in the food web was consequently more direct than in the short term. Sediment and benthic algae were more frequently consumed in the long term, before the sea-ice broke up, while consumers specialised on sympagic algae and plankton in the short term. Regardless of the time scale, sympagic algae were more frequently consumed in shallow waters, while plankton was more frequently consumed in deep waters. Our results suggest a strong temporal relationship between resource availability and the trophic niche of benthic consumers in Antarctica. Potential climate-driven changes in the timing and quality of nutrient inputs may have profound implications for the structure of polar food webs and the persistence of their constituent species, which have adapted their trophic niches to a highly predictable schedule of resource inputs

Effects of sea-ice persistence on the diet of Adélie penguin (Pygoscelis adeliae) chicks and the trophic differences between chicks and adults in the Ross Sea, Antarctica

In Antarctica, prey availability for the mesopredator Adélie penguin, Pygoscelis adeliae, depends on sea-ice dynamics. By affecting cycles of sea-ice formation and melt, climate change could thus affect penguin diet and recruitment. In the light of climate change, this raises concerns about the fate of this dominant endemic species, which plays a key role in the Antarctic food web. However, few quantitative studies measuring the effects of sea-ice persistence on the diet of penguin chicks have yet been conducted. The purpose of this study was to fill this gap by comparing penguin diets across four penguin colonies in the Ross Sea and evaluating latitudinal and interannual variation linked to different sea-ice persistence. Diet was evaluated by analysing the δ13C and δ15N values of penguin guano, and sea-ice persistence by means of satellite images. Isotopic values indicate that penguins consumed more krill in colonies with longer sea-ice persistence. In these colonies, the δ13C values of chicks were lower and closer to the pelagic chain than those of adults, suggesting that the latter apparently catch prey inshore for self-feeding and offshore for their chicks. The results indicate that sea-ice persistence is among the principal factors that influence the spatiotemporal variability of the penguins’ diet

The feeding behaviour of gall midge larvae and its implications for biocontrol of the giant reed. Insights from stable isotope analysis

The gall midge Lasioptera donacis, whose larval stage interferes with the reed’s leaf development, is a potential candidate agent for the biological control of Arundo donax. Reed infestation is always associated with the presence of a saprophytic fungus, Arthrinium arundinis, which is believed to provide food for the larvae. Larvae also interact with a parasitic nematode, Tripius gyraloura, which can be considered its natural enemy. To deepen our knowledge of the plant–fungus–insect trophic interactions and to understand the effects of the nematode on midge larval feeding behaviour, we applied stable isotope analysis, one of the most effective methods for investigating animal feeding preferences in various contexts. The results showed that on average the fungus accounted for 65% of the diet of the midge larvae, which however consumed the reed and the fungus in variable proportions depending on reed quality (expressed as the C:N ratio). No differences in feeding behaviour were observed between parasitised and non-parasitised midge larvae, indicating that nematodes have no effect in this regard. Due to its trophic habits, L. donacis could be an effective control agent of A. donax and these results need to be considered when implementing biological control measures

Changing isotopic food webs of two economically important fish in mediterranean coastal lakes with different trophic status

Transitional waters are highly productive ecosystems, providing essential goods and services to the biosphere and human population. Human influence in coastal areas exposes these ecosystems to continuous internal and external disturbance. Nitrogen-loads can affect the composition of the resident community and the trophic relationships between and within species, including fish. Based on carbon (δ13C) and nitrogen (δ15N) stable isotope analyses of individuals, we explored the feeding behaviour of two ecologically and economically important omnivorous fish, the eel Anguilla anguilla and the seabream Diplodus annularis, in three neighbouring lakes characterised by different trophic conditions. We found that A. anguilla showed greater generalism in the eutrophic lake due to the increased contribution of basal resources and invertebrates to its diet. By contrast, the diet of D. annularis, which was mainly based on invertebrate species, became more specialised, focusing especially on polychaetes. Our results suggest that changes in macroinvertebrate and fish community composition, coupled with anthropogenic pressure, affect the trophic strategies of high trophic level consumers such as A. anguilla and D. annularis. Detailed food web descriptions based on the feeding choices of isotopic trophospecies (here Isotopic Trophic Units, ITUs) enable identification of the prey taxa crucial for the persistence of omnivorous fish stocks, thus providing useful information for their management and habitat conservation

When climate change and overexploitation meet in volcanic lakes. The lesson from lake Bracciano, Rome’s strategic reservoir

Lakes worldwide have been strongly affected by several types of human-caused alteration, including changes in water level. This also affects deep lakes, including volcanic ones. Volcanic lakes in the Mediterranean area are of great importance for the local economy, but local human activities can threaten their rich biodiversity. As a European biodiversity hotspot and habitat of endemic species, the volcanic Lake Bracciano (Central Italy) is an ecosystem of primary conservation interest threatened by sharply falling water levels, particularly since 2017. It also plays a key role in human wellbeing by providing important ecosystem services including drinking water, fisheries and various recreational opportunities. Although the lake has historically been considered to enjoy good ecological status, various environmental problems, often amplified by water level changes, have arisen during the last two decades. Given this recent rapid evolution, the lake can be considered an example of a valuable ecosystem at risk as a result of increasing anthropogenic pressures. The aim of this review is to examine the changes that have affected the lake in the last 20 years, considerably reducing its capacity to provide ecosystem services, and to review existing and potential threats in order to better inform the management of such resources

Species richness and vulnerability to disturbance propagation in real food webs

A central issue in ecology is understanding how complex and biodiverse food webs persist in the face of disturbance, and which structural properties affect disturbance propagation among species. However, our comprehension of assemblage mechanisms and disturbance propagation in food webs is limited by the multitude of stressors affecting ecosystems, impairing ecosystem management. By analysing directional food web components connecting species along food chains, we show that increasing species richness and constant feeding linkage density promote the establishment of predictable food web structures, in which the proportion of species co-present in one or more food chains is lower than what would be expected by chance. This reduces the intrinsic vulnerability of real food webs to disturbance propagation in comparison to random webs, and suggests that biodiversity conservation efforts should also increase the potential of ecological communities to buffer top-down and bottom-up disturbance in ecosystems. The food web patterns observed here have not been noticed before, and could also be explored in non-natural networks

Trophic attractiveness for soil fauna of residues of Bt and near-isogenic maize. A C and N stable isotope-based study

Genetically modified (GM) crops are considered a good way to reduce insecticide use and the presence of certain agricultural pests, thus improving food and environmental safety. Nevertheless, effects of GM plant residues on the soil food web are still poorly understood. Zea mays L., enriched with a gene from Bacillus thuringiensis Berliner subsp. kurstaki (Bt) as a defence against the maize borer, is widely cultivated. In this study, we explored the in- vertebrate food webs associated with residues of Bt and non-Bt maize, respectively represented by DKC6575, with a Cry1Ab transgene (event MON810), and its near-isogenic Tietar variety, in a five-month field experiment. C and N stable isotopes and Bayesian mixing models were used to assess trophic niche metrics and track nutrient flows from maize residues and weeds occurring in maize crops to invertebrate detritivores and predators. While there were no initial differences in the structural components of maize residues between varieties, after five months of exposure in the field, the lignin content was higher and the organic matter content was lower in non-Bt than in Bt maize. Organic matter depletion over time was associated with a decrease in primary consumer abun- dance and an increase in their trophic niche width in both Bt and non-Bt maize, but it was faster in the former. The abundance of primary consumers and predators was higher in non-Bt than in Bt maize, but the distribution of organisms across trophic levels differed between varieties, with prey availability being lower in non-Bt than in Bt maize. This allowed Bt-associated predators to maintain a diet based primarily on the maize food chain through- out the experimental period. In contrast, non-Bt-associated predators were more dependent on weed-feeding prey by the end of the experiment, increasing the coupling between the maize and weed energy channels in the soil food web. Some taxon-specific effects were evident. Less vagile organisms such as Diptera had a specific diet mostly related to maize regardless of variety and time, while the diets of more vagile organisms like Coleoptera and Opiliones changed the most over time. Overall, our results suggest that the attractiveness of non-Bt maize residues for invertebrate consumers is higher than the Bt variety, although the higher recalcitrant content follow- ing decomposition reduces it over time. The attractiveness also affects the movement of invertebrate predators (potentially including pest control agents) and thus nutrient flows in tri-trophic detritus-based food webs in culti- vated and uncultivated plots

Horizontal and vertical food web structure drives trace element trophic transfer in Terra Nova Bay, Antarctica

Despite a vast amount of literature has focused on trace element (TE) contamination in Antarctica during the last decades, the assessment of the main pathways driving TE transfer to the biota is still an overlooked issue. This limits the ability to predict how variations in sea-ice dynamics and productivity due to climate change will affect TE allocation in the food web. Here, food web structure of Tethys Bay (Terra Nova Bay, Ross Sea, Antarctica) was first characterised by analysing carbon and nitrogen stable isotopes (δ13C, δ15N) in organic matter sources (sediment and planktonic, benthic and sympagic primary producers) and consumers (zooplankton, benthic invertebrates, fish and birds). Diet and trophic position were also characterised using Bayesian mixing models. Then, relationships between stable isotopes, diet and TEs (Cd, Cr, Co, Cu, Hg, Ni, Pb and V) were assessed in order to evaluate if and how horizontal (organic matter pathways) and vertical (trophic position) food web features influence TE transfer to the biota. Regressions between log[TE] and δ13C revealed that the sympagic pathway drives accumulation of V in primary consumers and Cd and Hg in secondary consumers, and that a coupled benthic/pelagic pathway drives Pb transfer to all consumers. Regressions between log[TE] and δ15N showed that only Hg biomagnifies across trophic levels, while all the others TEs showed a biodilution pattern, consistent with patterns observed in temperate food webs. Although the Cd behavior needs further investigations, the present findings provide new insights about the role of basal sources in the transfer of TEs in polar systems. This is especially important nowadays in light of the forecasted trophic changes potentially resulting from climate change-induced modification of sea-ice dynamics

Spatial and temporal diet variability of Adélie (Pygoscelis adeliae) and Emperor (Aptenodytes forsteri) Penguin. A multi tissue stable isotope analysis

The Ross Sea, Antarctica, supports large populations of Emperor Penguin (Aptenodytes forsteri) and Adélie Penguin (Pygoscelis adeliae), two key meso-predators that occupy high trophic levels. Despite these species are largely studied, little is known about their diet outside the breeding period. In the present study, we investigated the intra-annual diet of Adélie and Emperor Penguins belonging to five colonies in the Ross Sea through the stable isotope analysis of different tissues (feathers and shell membranes), synthetized in different seasons, and guano that indicates recent diet. Penguin samples and prey (krill and fish) were collected during the Antarctic spring–summer. δ13C and δ15N of tissues and guano indicate spatio-temporal variation in the penguin diet. The krill consumption by Adélie Penguins was lowest in winter except in the northernmost colony, where it was always very high. It peaked in spring and remained prevalent in summer. The greatest krill contribution to Emperor Penguin’s diet occurred in summer. The relative krill and fish consumption by both species changed in relation to the prey availability, which is influenced by seasonal sea ice dynamics, and according to the penguin life cycle phases. The results highlight a strong trophic plasticity in the Adélie Penguin, whose dietary variability has been already recognized, and in the Emperor Penguin, which had not previously reported. Our findings can help understand how these species might react to resource variation due to climate change or anthropogenic overexploitation. Furthermore, data provides useful basis for future comparisons in the Ross Sea MPA and for planning conservation actions