Terrestrial Prey Research Articles

An Audacious Maneuver: First Record of Leopardus guigna in the Marine Environment

The Güiña (Leopardus guigna), the smallest Neotropical feline, inhabits central and southern Chile and western Argentina. This communication reports the first documented instance of a güiña swimming in a marine environment, observed in the Refugio Channel, which separates Refugio Island from the mainland in Northern Patagonia, Chile. In April 2023, a local resident recorded video footage of a güiña swimming near the eastern shore of the channel, emerging from the water, shaking off, and climbing a tree to groom itself. This observation suggests that the güiña might use the seacoast when searching for food, particularly during periods of low terrestrial prey availability during the winter. The ability of the güiña to adapt to such environments underscores the species’ ecological flexibility, previously undocumented in this context, and highlights the need for integrating marine resources into the species’ conservation strategies. The video’s quality is limited due to the simplicity of the recording device, but it provides crucial visual evidence of this behavior.

Terrestrial generalist predators have larger isotopic niches but rely less on aquatic prey at a natural compared to a post-mining lakeshore

The flooded pits from former opencast mining (post-mining lakes) are young aquatic ecosystems often with low primary productivity due to the high acidity. The limited availability of nutrients and simplified aquatic food webs in these artificial lakes may affect the matter fluxes towards food webs in adjacent terrestrial habitats. To understand the impact of emerging prey from a post-mining lake on terrestrial food webs and associated arthropod communities, we compared the emergence and activity of arthropods as well as the isotopic niches of generalist predators and their potential prey between lakeshores of a post-mining and a natural lake. We used aquatic and terrestrial emergence tents to sample potential prey that emerged locally (Collembola and Diptera) and pitfall traps to sample terrestrial arthropod predators (Carabidae and Araneae) from lakeshores. Community and stable isotope (δ13C and δ15N) analyses were performed to compare diversity and isotopic niches of generalist arthropod predators between the natural and post-mining lakeshores. The number of emerging aquatic and terrestrial prey did not differ significantly between lakes, but the δ15N values of aquatic emerging dipterans differed between the two lakes. Predator community diversity did not differ significantly between lakeshores, but generalist predator species from the natural lakeshore generally had larger isotopic niche sizes than the same species at the post-mining lakeshore indicating a more diverse diet. Additional estimates of diet composition suggest that predators at the natural lakeshore utilized aquatic prey to a lesser degree than at the post-mining lake, but still had a more diverse diet.

Influence of the dietary contribution of terrestrial insects to the condition factor of bleak Alburnus alburnus in a highly polluted lowland river.

Bleak Alburnus alburnus is a highly abundant but understudied fish species, and we know little about the trophic ecology of populations inhabiting rivers in central Europe. From an ecosystem perspective, this fish species is interesting as it is known to feed on surface insects, thereby linking the terrestrial with the aquatic habitat. In a previous study, we demonstrated that this flux is intensified, and dietary contribution of terrestrial insects is higher in fish inhabiting sections of the Spree River, Germany, that are polluted from iron oxides occurring from former lignite mining activities, and thus are characterized by lower abundances of aquatic insects. As terrestrial insects can be considered as food of lower quality (measured as long-chained polyunsaturated fatty acids, n-3 LC-PUFAs) compared to aquatic prey, it is reasonable to assume that the higher contribution of terrestrial insects is related to a lower body condition in fish. In this study, we explore the trophic ecology of riverine A. alburnus and their fitness consequences of feeding on terrestrial insects. We therefore modeled a terrestrial index from stable isotopes of hydrogen (δ2H) measured in the A. alburnus muscle tissue and compared individuals caught in locations upstream of a dam that were greatly influenced by iron oxides, with individuals caught in sections located downstream of a dam where passive remediation technologies are applied. The terrestrial index was significantly higher in A. alburnus caught in locations at high-iron concentrations, characterized by low abundances of aquatic prey, compared to A. alburnus caught in unpolluted habitats at low-iron concentrations. In contradiction to our hypothesis, the terrestrial index had no significant effect on the body condition of A. alburnus (measured as Fulton's condition factor K) in the sections downstream of the dam (i.e., at low-iron concentrations) and a significant positive, albeit weak, effect in sections upstream of the dam (i.e., at high-iron concentrations). However, the condition factor was generally lower in the high-iron section, potentially related to more direct effects of the iron oxide. We conclude that in A. alburnus, terrestrial insects can be considered as the less-favored food, unless the fish occur in environments where the aquatic food is of limited availability. Further research is needed to evaluate the direct and indirect effects, including the internal n-3 LC-PUFA synthesis as an adaption toward low-quality terrestrial prey on the fitness consequences of A. alburnus.

Declining freshwater habitats in Greenland may shift diets of the ubiquitous wolf spider Pardosa glacialis

Freshwater habitats in Arctic landscapes provide essential food resources to predaceous terrestrial fauna. However, climate change threatens the continuation of aquatic resources as increased temperatures prompt widespread drying. We investigated how declines in surface waters in the Arctic may have implications for the diets of the wolf spider Pardosa glacialis (Araneae: Lycosidae), an abundant top predator in western Greenland known to consume both aquatic and terrestrial prey. We estimated the abundance of ground-dwelling prey taxa near and far from shallow ponds to compare resource availability for P. glacialis. We also measured the stable isotopes ratios of carbon and nitrogen of potential prey taxa and used linear mixing models to estimate the relative proportions of aquatic and terrestrial prey in the diets of P. glacialis collected near ponds versus drier, upland locations. Overall, near ponds there was a 3.7 times greater abundance of ground-dwelling organisms compared to locations far from ponds. Near ponds, depleted δ13C values of P. glacialis compared to the upland locations were explained by the consumption of aquatic insects, which were relatively depleted in 13C compared to terrestrial ground-dwelling and canopy taxa. Our linear mixing model indicated that the mean relative contribution of aquatic taxa to the diets of P. glacialis at pond locations was ~ 23% versus ~ 0% at upland locations, though these results were accompanied by significant uncertainty. Our results emphasize the importance of Arctic ponds in providing habitat to potential prey of terrestrial predators. We highlight how climate change, and the continued disappearance of aquatic habitats, could disrupt critical linkages between aquatic insects and terrestrial predators.

Analysis of ontogenetic changes in head shape and diet in a catfish with moderately enlarged jaw adductors (Clariallabes melas)

Jaw adductor hypertrophy, or the presence of enlarged jaw-closing muscles, has arisen several times independently in African clariid catfish. Previous work has demonstrated that species characterized by enlarged jaw adductors may have unusual ways of foraging such as terrestrial foraging and prey capture, and often include a large proportion of hard and terrestrial prey in their diet. However, relatively little is known about species of the genus Clariallabes with an intermediate degree of jaw adductor hypertrophy. In the present study we present data on head shape and diet for a range of sizes of specimens of a poorly known species of Clariallabes, C. melas. Our data show that growth patterns in this species and the previously studied 
 C. longicauda are similar in some ways (e.g. positive allometry of the growth of the jaw muscles) but different in others (negative allometry in hyoid width). However, C. melas has a smaller head for its body size in all dimensions. Due to the large number of empty stomachs we encountered, dietary data remain preliminary, but suggest a varied diet including both hard and soft prey. Our data show that a large amount of variation in head shape may exist even among related species and that species with jaw adductor hypertrophy generally show positive allometry in the growth of the jaw adductors and associated structures. Whether jaw muscle hypertrophy is an adaptive trait in clariid catfish awaits further comparative analyses testing for the evolutionary association between jaw adductor hypertrophy and the inclusion of hard prey into the diet.

The expansion of the Atlantic–Mediterranean ghost crab Ocypode cursor (Linnaeus, 1758): Distribution, environmental niches and future perspectives

AbstractThe tufted ghost crab Ocypode cursor (Linnaeus, 1758) found in the Mediterranean Sea and the Atlantic Ocean is currently a great example that elucidates concerns within scientific and conservation communities. This species, native to the subtropical Atlantic Ocean and the warmest southeastern Mediterranean Sea (Egypt), has been extending its distribution in both regions since the 1980s, likely due to the warming sea temperatures. These small nocturn crabs are typically found inhabiting sandy beaches and dune environments. This species is an opportunistic predator eating terrestrial and marine prey, especially on sea turtle eggs. Despite its status as a threatened species by two European conventions, there is a lack of knowledge of its ecology and biology. Sometimes considered as an indicator of good ecological status of beaches where it lives, this ghost crab could nevertheless benefit from climate change to extend its distribution range. This review aims to create a baseline on the current knowledge and gaps in the published scientific literature on the ghost crab. Additionally, through an analysis of the existing literature, we also offer insights into the potential risk of beach erosion associated with the expansion of this species.

Allochthonous marsh subsidies enhances food web productivity in an estuary and its surrounding ecosystem mosaic.

Terrestrial organic matter is believed to play an important role in promoting resilient estuarine food webs, but the inherent interconnectivity of estuarine systems often obscures the origins and importance of these terrestrial inputs. To determine the relative contributions of terrestrial (allochthonous) and aquatic (autochthonous) organic matter to the estuarine food web, we analyzed carbon, nitrogen, and sulfur stable isotopes from multiple trophic levels, environmental strata, and habitats throughout the estuarine habitat mosaic. We used a Bayesian stable isotope mixing model (SIMM) to parse out relationships among primary producers, invertebrates, and a pelagic and demersal fish species (juvenile Chinook salmon and sculpin, respectively). The study was carried out in the Nisqually River Delta (NRD), Washington, USA, a recently-restored, macrotidal estuary with a diverse habitat mosaic. Plant groupings of macroalgae, eelgrass, and tidal marsh plants served as the primary base components of the NRD food web. About 90% of demersal sculpin diets were comprised of benthic and pelagic crustaceans that were fed by autochthonous organic matter contributions from aquatic vegetation. Juvenile salmon, on the other hand, derived their energy from a mix of terrestrial, pelagic, and benthic prey, including insects, dipterans, and crustaceans. Consequently, allochthonous terrestrial contributions of organic matter were much greater for salmon, ranging between 26 and 43%. These findings demonstrate how connectivity among estuarine habitat types and environmental strata facilitates organic matter subsidies. This suggests that management actions that improve or restore lateral habitat connectivity as well as terrestrial-aquatic linkages may enhance allochthonous subsidies, promoting increased prey resources and ecosystem benefits in estuaries.

Diet Diversity of the Fluviatile Masu Salmon, Oncorhynchus masou (Brevoort 1856) Revealed via Gastrointestinal Environmental DNA Metabarcoding and Morphological Identification of Contents.

Masu salmon, Oncorhynchus masou (Brevoort 1856), a commercially important fish species endemic to the North Pacific Ocean, attained national second-level protected animal status in China in 2021. Despite this recognition, knowledge about the trophic ecology of this fish remains limited. This study investigated the diet diversity of fluviatile Masu salmon in the Mijiang River, China, utilizing the gastrointestinal tract environmental DNA (GITeDNA) metabarcoding and morphological identification. The results revealed a diverse prey composition, ranging from terrestrial and aquatic invertebrates to small fishes. The fluviatile Masu salmon in general consumed noteworthily more aquatic prey than terrestrial prey. There were much more prey taxa and a higher diet diversity detected by GITeDNA metabarcoding than by morphological identification. GITeDNA metabarcoding showed that larger and older Masu salmon consumed significantly more terrestrial insects than aquatic prey species did, with 7366 verses 5012 sequences in the group of ≥20 cm, 9098 verses 4743 sequences in the group of ≥100 g and 11,540 verses 729 sequences in the group of age 3+. GITeDNA metabarcoding also showed size- and age-related diet diversity, indicating that the dietary niche breadth and trophic diversity of larger and older Masu salmon increased with food resources expanding to more terrestrial prey. Terrestrial invertebrates of riparian habitats play a vital role in the diet of fluviatile Masu salmon, especially larger individuals, highlighting their importance in connecting aquatic and terrestrial food webs. Conservation plans should prioritize the protection and restoration of riparian habitats. This study advocates the combined use of GITeDNA metabarcoding and morphological observation for a comprehensive understanding of fish diet diversity.

Sustained use of marine subsidies promotes niche expansion in a wild felid

The use of marine subsidies by terrestrial predators can facilitate substantial transfer of nutrients between marine and terrestrial ecosystems. Marine resource subsidies may have profound effects on predator ecology, influencing population and niche dynamics. Expanding niches of top consumers can impact ecosystem resilience and interspecific interactions, affecting predator-prey dynamics and competition. We investigate the occurrence, importance, and impact of marine resources on trophic ecology and niche dynamics in a highly generalist predator, the caracal (Caracal caracal), on the Cape Peninsula, South Africa. Caracals have flexible diets, feeding across a wide range of terrestrial and aquatic prey. We use carbon and nitrogen stable isotope analysis of fur samples (n = 75) to understand trophic position and niche shifts in coastal and inland foragers, as well as the implications of a diet rich in marine resources. We found significant differences in isotope signatures between these groups, with higher δ13C (P < 0.05) and δ15N values (P < 0.01) in coastal foragers. Isotope mixing models reveal that these elevated signatures were due to non-terrestrial food subsidies, where approximately a third of coastal foraging caracal diet comprised marine prey. The addition of marine prey species to diet increased both the trophic level and isotope niche size of coastal foraging caracals, with potential impacts on prey populations and competition. Our results suggest that marine prey are an important dietary resource for coastal foraging caracals, where seabirds, including two endangered species, are a major component of their diet. However, there are likely risks associated with these resource benefits, as routine consumption of seabirds is linked with higher pollutant burdens, particularly metals. Increased encounters between this terrestrial predator and seabirds may be a result of increased mainland colonies due to changes in habitat availability and the highly opportunistic and generalist foraging behaviour of a native predator.

Inter- and intrapopulation resource use variation of marine subsidized western fence lizards

Marine resource subsidies alter consumer dynamics of recipient populations in coastal systems. The response to these subsidies by generalist consumers is often not uniform, creating inter- and intrapopulation diet variation and niche diversification that may be intensified across heterogeneous landscapes. We sampled western fence lizards, Sceloporus occidentalis, from Puget Sound beaches and coastal and inland forest habitats, in addition to the lizards’ marine and terrestrial prey items to quantify marine and terrestrial resource use with stable isotope analysis and mixing models. Beach lizards had higher average δ13C and δ15N values compared to coastal and inland forest lizards, exhibiting a strong mixing line between marine and terrestrial prey items. Across five beach sites, lizard populations received 20–51% of their diet from marine resources, on average, with individual lizards ranging between 7 and 86% marine diet. The hillslope of the transition zone between marine and terrestrial environments at beach sites was positively associated with marine-based diets, as the steepest sloped beach sites had the highest percent marine diets. Within-beach variation in transition zone slope was positively correlated with the isotopic niche space of beach lizard populations. These results demonstrate that physiography of transitional landscapes can mediate resource flow between environments, and variable habitat topography promotes niche diversification within lizard populations. Marine resource subsidization of Puget Sound beach S. occidentalis populations may facilitate occupation of the northwesternmost edge of the species range. Shoreline restoration and driftwood beach habitat conservation are important to support the unique ecology of Puget Sound S. occidentalis.

Integrated approach for the isotope trophic position of black-tailed gull (Larus crassirostris) eggs over a decade: Combining stable isotopes of amino acids and fatty acids composition

Recently, compound-specific isotope analysis (CSIA) using the amino acid nitrogen stable isotope ratio (δ15NAAs) has been widely used for accurate estimation of trophic position (TP). In addition, a quantitative fatty acid signature analysis (QFASA) offers insights into diet sources. In this study, we used these techniques to estimate the TP for seabirds that rely on diverse food sources across multiple ecosystems. This allows for the proper combination of factors used in TP calculation which are different for each ecosystem. The approach involved the application of a multi-mixing trophic discrimination factor (TDF) and mixing β which is a Δδ15N between trophic and source amino acid of primary producer. Since the black-tailed gulls (BTGs) are income-breeding seabirds, which rely on energy sources obtained around their breeding sites, they and their eggs could be useful bioindicators for environmental monitoring. However, the ecological properties of BTGs such as habitats, diets, and TP are not well known due to their large migration range for wintering or breeding and their feeding habits on both aquatic and terrestrial prey. In this study, the eggs were used for estimating TP and for predicting TP of mother birds to overcome difficulties such as capturing birds and collecting non-invasive tissue samples. Eggs, sampled over a decade from three Korean islands, showed spatial differences in diet origin. Considering both the food chain and physiology of BTG, the TP of eggs was estimated to be 3.3–4.0. Notably, the TP was significantly higher at site H (3.8 ± 0.1) than at site B (3.5 ± 0.2), which indicated a higher contribution of marine diet as confirmed by QFASA. Using a reproductive shift of δ15NAAs, the TP of the mother birds was predicted to be 3.6–4.3, positioning them as the top predator in the food web. The advanced integration of multiple approaches provides valuable insights into bird ecology.

Species‐specific foraging behavior and diets of stream salmonids: An implication for negative impacts on native charr by nonnative trout in Japanese mountain streams

AbstractSalmonids have been introduced globally as a food source and recreational fishing target. In Japan, brown trout (Salmo trutta) and brook trout (Salvelinus fontinalis) were introduced in the 19th century and have since spread. In many headwater streams, native white‐spotted charr (Salvelinus leucomaenis) are thought to be experiencing negative impacts from these species. The current study examined foraging behavior, microhabitat use, and diet overlap of these three species in Kamikochi, Nagano Prefecture: one of Japan's premier mountain areas. In Kamikochi, many spring‐fed headwater streams are currently dominated by these invasive salmonids and white‐spotted charr have declined drastically over the last half century. Underwater video analysis revealed that while total foraging rates and foraging modes were similar between the three species, brook trout and white‐spotted charr foraged benthically more frequently than brown trout. Microhabitat water depth and flow velocity were similar between species, and fish size had a positive effect on water depth and flow velocity in all three species. Diet analysis indicated that brook trout and white‐spotted charr diets were nearly identical, comprised primary of aquatic invertebrates, while brown trout preyed on a mix of terrestrial and aquatic invertebrates, as well as amphibians and fish. These results indicate that in Kamikochi, the decline of white‐spotted charr is likely most influenced by direct competition with brook trout for prey resources. However, brown trout likely also predate on juvenile white‐spotted charr, while also possibly causing a foraging niche shift of white‐spotted charr, and have ecosystem‐level impacts due to predation on terrestrial prey.

Incremental evolution of modeling a prognosis for polar bears in a rapidly changing Arctic

Updating predictions of the response of high-profile, at-risk species to climate change and anthropogenic stressors is vital for informing effective conservation action. Here, we review two prior generations of Bayesian network probability models predicting changes in global polar bear (Ursus maritimus) population status, and provide a contemporary update based on recent research findings and sea-ice projections by newer climate models. We compare predictions of polar bear population response from all 3 models among four circumpolar Arctic ecoregions, using sea ice projections based on three IPCC greenhouse gas emissions scenarios (SSP2.6, 4.5, 8.5). Consistent with the previous two model generations, polar bears will continue to experience increasing probability of declining or greatly declining populations throughout the 21st century, varying by emission scenario. Populations within the Polar Basin Divergent Ice Ecoregion have the highest predicted probability of declines, but predictions were slightly less dire relative to the previous model generation. Most of the influence, denoted by model sensitivity analysis, is from expected degradation and loss of sea ice and reduced access to marine prey. The lack of terrestrial prey adequate to substitute for loss of access to marine prey, as well as human-caused bear mortality associated with hunting and defense of life and property encountered when polar bears are increasingly forced ashore also contributed to predicted declines. Although some tidewater glacial fjords and other localized onshore resources may provide local refugia, their benefit is transient. Our findings continue to inform priorities for inventory, monitoring, and research needs, and suggest that similar updates to models of other at-risk species can capitalize on the comparison framework we present here.

Small stream predators rely heavily on terrestrial matter energy input in the fall, regardless of riparian buffer size

Stream ecosystems are reliant on the reciprocal exchange of terrestrial and aquatic energy subsides to maintain a productive and stable food web. Land use around streams can have strong effects on the size and availability of resource subsidies for stream and riparian predators such as fish and spiders. A common forestry technique around streams is the establishment of forested buffers to protect aquatic and riparian ecosystems from upland disturbances. Buffer size may determine prey abundance, richness, and spatial extent of prey reach into both the aquatic and terrestrial systems. To test the effects of forested buffers subsidy direction, we explored the carbon and nitrogen stable isotope signatures of brown trout (Salmo trutta), Tetragnathidae and Lycosidae spiders, and their aquatic and terrestrial prey sources around twelve streams in southern Sweden. For both predator groups, buffer presence showed no effect on resource subsidy source. We found that both brown trout and spiders are significantly reliant on terrestrial sources of prey for their diets in the fall. To support the terrestrial subsidy into small streams it is vital to maintain ecologically functional riparian zones by conserving complex surrounding habitats that optimize habitat and both terrestrial and aquatic prey diversity.

DNA metabarcoding reveals rangewide variation in aquatic diet of a riparian avian insectivore, the Prothonotary Warbler

Abstract Riparian avian insectivores not only depend on terrestrial insect prey but also benefit from the inclusion of aquatic prey during critical life-history periods. Diets identified herein show that Prothonotary Warbler (Protonotaria citrea) nestlings were provisioned with aquatic prey throughout the breeding season across their range, but with variation in prey frequency of occurrence and taxonomy. Anthropogenic activity and climate change may impact the trophic link especially between aquatic and riparian habitats by altering the presence, abundance, and timing of prey availability. Thus, we used DNA metabarcoding of fecal samples to quantify the frequency of occurrence of nestling diet items at 9 sites across their breeding range that differed in expected aquatic prey consumption. We analyzed spatial and temporal differences in the occurrence and multivariate diet assemblages of each prey source. Lepidoptera was the predominant terrestrial prey occurring in diets across space and time, whereas emergent aquatic insects and freshwater mollusks in aquatic diet exhibited greater variation. The frequency of emergent aquatic prey occurrence in nestling diets ranged from 61% to 100% across sites and was greater for early-season nestlings. The seasonal decrease in aquatic prey consumption indicates a potential temporal shift in the nutritional landscape from aquatic to terrestrial prey sources and a possible nutritional phenological mismatch for early nestlings as climate change advances the timing of insect emergence. Our findings also suggest that Prothonotary Warblers respond to environmental variability by consuming alternative prey and argue for future research investigating the extent to which shifting diets have nutritional consequences for riparian nestlings.

The trophic ecology of an invasive predator in a novel ecosystem: Green sunfish Lepomis cyanellus in a restored urban stream

Rapid urbanization will increase the number of novel stream ecosystems in the Southeastern United States. The green sunfish (Lepomis cyanellus) is a globally widespread, invasive species that is particularly well-adapted to urban stream conditions. The trophic ecology of green sunfish is understudied, especially in the novel ecosystems where they appear to thrive. We assessed predation by green sunfish in the food web of a heavily engineered and restored urban stream in Raleigh, North Carolina. We sampled fish species composition, size structure, abundance, the benthic macroinvertebrate assemblage, and fish diets during two seasons. Green sunfish was the sole species inhabiting the study stream, with the exception of a single goldfish. The population size structure indicated potential overcrowding in our study ecosystem. Chironomidae was the most common taxa in both the benthic macroinvertebrate assemblage and in fish diets during the summer. We observed a seasonal shift in diets to lesser prey-specific abundance of Chironomidae and a greater overall abundance of terrestrial prey from summer to winter. Green sunfish can persist in small restored urban streams of the Southeast US where virtually no other fish occur, and they utilize benthic invertebrates and terrestrial prey as resources.

Bioenergetic responses of a stream food web to habitat restoration: interactions between Brown trout and invertebrate prey resources

Stream habitat restoration has the potential to rehabilitate degraded stream communities, but the bioenergetic impact of restoration on prey resources and utilization by salmonids has been understudied. We measured the responses of Brown trout (Salmo trutta) populations, aquatic and terrestrial prey resources, and prey resource utilization by trout, before (2008–2014) and after (2015–2020) large‐scale habitat restoration in the Upper Arkansas River, a previously listed US EPA superfund site located in the western United States. We observed significant increases in Brown trout populations following restoration, as well as complex alterations to invertebrate prey resources that generally resulted in increased benthic biomass but reductions in adult aquatic insects and terrestrial invertebrates. Population‐level metabolic demand by trout increased 25% after restoration, and prey utilization shifted with reduced consumption of adult aquatic insects and greater consumption of terrestrial invertebrates, relative to their availability. We observed reduced biomass of prey resources consumed by trout with increasing trout population size, and while the total number of invertebrates consumed increased after restoration, actual invertebrate biomass consumed decreased. Our results suggest that restoration was effective in increasing targeted Brown trout populations, but prey resources were altered by physical changes in habitat and increased resource utilization by trout. Habitat restoration projects that are primarily designed to increase trout populations may benefit by focusing on improving prey resources and utilization to support the bioenergetic needs of the restored fishery.

Visual spatial location influences selection of instinctive behaviours in mouse.

Visual stimuli can elicit instinctive approach and avoidance behaviours. In mouse, vision is known to be important for both avoidance of an overhead threat and approach toward a potential terrestrial prey. The stimuli used to characterize these behaviours, however, vary in both spatial location (overhead or near the ground plane) and visual feature (rapidly expanding disc or slowly moving disc). We therefore asked how mice responded to the same visual features presented in each location. We found that a looming black disc induced escape behaviour when presented overhead or to the side of the animal, but the escapes produced by side-looms were less vigorous and often preceded by freezing behaviour. Similarly, small moving discs induced freezing behaviour when presented overhead or to the side of the animal, but side sweeps also elicited approach behaviours, such that mice explored the area of the arena near where the stimulus had been presented. Our observations therefore show that mice combine cues to the location and features of visual stimuli when selecting among potential behaviours.

Complex Life Histories Alter Patterns of Mercury Exposure and Accumulation in a Pond-Breeding Amphibian.

Quantifying how contaminants change across life cycles of species that undergo metamorphosis is critical to assessing organismal risk, particularly for consumers. Pond-breeding amphibians can dominate aquatic animal biomass as larvae and are terrestrial prey as juveniles and adults. Thus, amphibians can be vectors of mercury exposure in both aquatic and terrestrial food webs. However, it is still unclear how mercury concentrations are affected by exogenous (e.g., habitat or diet) vs endogenous factors (e.g., catabolism during hibernation) as amphibians undergo large diet shifts and periods of fasting during ontogeny. We measured total mercury (THg), methylmercury (MeHg), and isotopic compositions (δ 13C, δ15N) in boreal chorus frogs (Pseudacris maculata) across five life stages in two Colorado (USA) metapopulations. We found large differences in concentrations and percent MeHg (of THg) among life stages. Frog MeHg concentrations peaked during metamorphosis and hibernation coinciding with the most energetically demanding life cycle stages. Indeed, life history transitions involving periods of fasting coupled with high metabolic demands led to large increases in mercury concentrations. The endogenous processes of metamorphosis and hibernation resulted in MeHg bioamplification, thus decoupling it from the light isotopic proxies of diet and trophic position. These step changes are not often considered in conventional expectations of how MeHg concentrations within organisms are assessed.

Impact across ecosystem boundaries – Does Bti application change quality and composition of the diet of riparian spiders?

Emerging aquatic insects link aquatic and adjacent terrestrial food webs by subsidizing terrestrial predators with high-quality prey. One of the main constituents of aquatic subsidy, the non-biting midges (Chironomidae), showed altered emergence dynamics in response to the mosquito control agent Bacillus thuringiensis var. israelensis (Bti). As riparian spiders depend on aquatic subsidy, they may be affected by such changes in prey availability. Thus, we conducted a field study in twelve floodplain pond mesocosms (FPMs), six were treated with Bti (2.88 × 109 ITU/ha, VectoBac WDG) three times, to investigate if the Bti-induced shift in chironomid emergence dynamics is reflected in their nutritional value and in the diet of riparian spiders. We measured the content of proteins, lipids, glycogen, and carbohydrates in emerged Chironomidae, and determined the stable isotope ratios of female Tetragnatha extensa, a web-building spider living in the riparian vegetation of the FPMs. We analysed the proportion of aquatic prey in spiders' diet, niche size, and trophic position. While the content of nutrients and thus the prey quality was not significantly altered by Bti, effects on the spiders' diet were observed. The trophic position of T. extensa from Bti-treated FPMs was lower compared to the control while the aquatic proportion was only minimally reduced. We assume that spiders fed more on terrestrial prey but also on other aquatic organisms such as Baetidae, whose emergence was unaffected by Bti. In contrast to the partly predaceous Chironomidae, consumption of aquatic and terrestrial primary consumers potentially explains the observed lower trophic position of spiders from Bti-treated FPMs. As prey organisms vary in their quality the suggested dietary shift could transfer previously observed effects of Bti to riparian spiders conceivably affecting their populations. Our results further support that anthropogenic stressors in aquatic ecosystems may translate to terrestrial predators through aquatic subsidy.