Terrestrial Prey Research Articles

Taking the trophic bypass: aquatic-terrestrial linkage reduces methylmercury in a terrestrial food web.

Ecosystems can be linked by the movement of matter and nutrients across habitat boundaries via aquatic insect emergence. Aquatic organisms tend to have higher concentrations of certain toxic contaminants such as methylmercury (MeHg) compared to their terrestrial counterparts. If aquatic organisms come to land, terrestrial organisms that consume them are expected to have elevated MeHg concentrations. But emergent aquatic insects could have other impacts as well, such as altering consumer trophic position or increasing ecosystem productivity as a result of nutrient inputs from insect carcasses. We measure MeHg in terrestrial arthropods at two lakes in northeastern Iceland and use carbon and nitrogen stable isotopes to quantify aquatic reliance and trophic position. Across all terrestrial focal arthropod taxa (Lycosidae, Linyphiidae, Acari, Opiliones), aquatic reliance had significant direct and indirect (via changes in trophic position) effects on terrestrial consumer MeHg. However, contrary to our expectations, terrestrial consumers that consumed aquatic prey had lower MeHg concentrations than consumers that ate mostly terrestrial prey. We hypothesize that this is due to the lower trophic position of consumers feeding directly on midges relative to those that fed mostly on terrestrial prey and that had, on average, higher trophic positions. Thus, direct consumption of aquatic inputs results in a trophic bypass that creates a shorter terrestrial food web and reduced biomagnification of MeHg across the food web. Our finding that MeHg was lower at terrestrial sites with aquatic inputs runs counter to the conventional wisdom that aquatic systems are a source of MeHg contamination to surrounding terrestrial ecosystems.

Levels and temporal trends of persistent organic pollutants (POPs) in arctic foxes (Vulpes lagopus) from Svalbard in relation to dietary habits and food availability

Temporal trends of persistent organic pollutants (POPs) in arctic foxes (Vulpes lagopus) from Svalbard, Norway, were investigated in relation to feeding habits and seasonal food availability. Arctic foxes from Svalbard forage in both marine and terrestrial ecosystems and the availability of their food items are impacted by climatic variability. Concentrations of polychlorinated biphenyls (PCBs), organochlorinated pesticides (OCPs) and brominated flame retardants (polybrominated diphenyl ethers [PBDEs] and hexabromocyclododecane [HBCDD]) were analyzed in the liver of 141 arctic foxes collected between 1997 and 2013. Stable carbon isotope values (δ13C) were used as a proxy for feeding on marine versus terrestrial prey. The annual number of recovered reindeer carcasses and sea ice cover were used as proxies for climate influenced food availability (reindeers, seals). Linear models revealed that concentrations of PCBs, chlordanes, p,p′-DDE, mirex and PBDEs decreased 4–11% per year, while no trends were observed for hexachlorobenzene (HCB) or β-hexachlorocyclohexane (β-HCH). Positive relationships between POP concentrations and δ13C indicate that concentrations of all compounds increase with increasing marine dietary input. Increasing reindeer mortality was related to lower HCB concentrations in the foxes based on the linear models. This suggests that concentrations of HCB in arctic foxes may be influenced by high mortality levels of Svalbard reindeer. Further, β-HCH concentrations showed a positive association with sea ice cover. These results in addition to the strong effect of δ13C on all POP concentrations suggest that climate-related changes in arctic fox diet are likely to influence contaminant concentrations in arctic foxes from Svalbard.

The relationship between individual habitat use and diet in brown trout

SummarySalmonids exhibit considerable variation within and between individuals in growth, diet, foraging strategy and habitat use, but little is known about how these characteristics covary. Previous work has shown that habitat use strongly influences growth rates in brown trout (Salmo trutta).We examined individual variation in diet of PIT‐taggedSalmo truttain three stream enclosures in relation to individual habitat use, size, sex and growth. Each enclosure consisted of a fine substratum pool and a coarse substratum riffle. By placing antennae between these habitats, we continuously monitored habitat use under field conditions. Fish were recaptured four times over the 2‐month study period, and diet, which was examined through stomach flushing, was related to habitat use over the previous 48 h. Individual fish growth was also measured.Based on habitat use, trout were classified as either movers or stayers, with stayers being of two types, those using only pools and those using only riffles and movers using and swimming between pools and riffles. The stayers in pools took more terrestrial prey than the stayers in riffles, whereas the latter fed more on aquatic invertebrates such as the crustaceanGammarus pulex,the plecopteranLeuctraand cased caddis larvae. Movers had diets intermediate between the stayers in pools and the stayers in riffles.Results of canonical correspondence analysis showed that variation in diet amongst individual fish over the study period was significantly influenced by enclosure, growth and % time spent in the pool during the day. Graphical models of diet analysis showed that population and mean individual prey diversity tended to differ amongst enclosures and suggested that stayers in pools consumed a greater prey diversity, whereas stayers in riffles consumed more prey. Discriminant analysis of diets revealed significant discrimination by habitat and sex on two of the four sampling occasions, based on daytime habitat use, but only on one date based on night‐time habitat use. Stoneflies and terrestrial prey contributed most to the separation.

Effects of riparian canopy cover on salmonid diet and prey selectivity in low nutrient streams.

This study focuses on stream sections within a relatively low nutrient catchment in south-east Ireland in an attempt to characterize the probable effects of riparian canopy on salmonid diet and prey selectivity within two size classes of nursery stream. Sampling found that brown trout Salmo trutta diet changed significantly in response to riparian canopy regardless of stream size. The observation that S. trutta within unshaded stream sites did not feed on drifting terrestrial prey items to the same extent as those within shaded streams was not due to a lack of availability of this food source. There was no evidence to suggest that S. trutta selectively choose particular prey items.

Comparative analysis of the Yellow-legged Gull’s (Larus michahellis(Naumann, 1840)) trophic ecology in two colonies of the Central Coast of Algeria

The Yellow-legged Gull’s diet (Larus michahellis) was studied by analyzing 118 pellets of adults collected in two colonies of the Algerian Central coast. This study has resulted in the identification of 89 food items. The dietary diversity (73 food items) of the Yellow-legged Gull on the Aguelli Island was found to be richer than that on the Tigzirt Island where only 44 food items were identified. It is surprising that the proportion of inorganic remains in pellets of Yellow-legged Gull adults on the Aguelli Island (33.24%) is the largest, followed by the food category represented by terrestrial prey (20.74%), and vegetation remains ranking last (20.45%). Meat remains (31.95%) are strongly represented in the composition of the Yellow-legged Gull’s pellets in the colony of the Tigzirt Island. As far as marine vertebrates are concerned, their share in the diet accounts for about 21.3%, while that of plant remains for 19.5% and ranks third. With regard to the foraging habitat, it appears that Yellow-legged G...

Intricate predatory decisions by a mosquito-specialist spider from Malaysia

Paracyrba wanlessi is a southeast Asian jumping spider (Salticidae) that lives in the hollow internodes of fallen bamboo and preys on the larvae, pupae and adults of mosquitoes. In contrast to Evarcha culicivora, an East African salticid that is also known for actively targeting mosquitoes as preferred prey, there was no evidence of P. wanlessi choosing mosquitoes on the basis of species, sex or diet. However, our findings show that P. wanlessi chooses mosquitoes significantly more often than a variety of other prey types, regardless of whether the prey are in or away from water, and regardless of whether the mosquitoes are adults or juveniles. Moreover, a preference for mosquito larvae, pupae and adults is expressed regardless of whether test spiders are maintained on a diet of terrestrial or aquatic prey and regardless of whether the diet includes or excludes mosquitoes. Congruence of an environmental factor (in water versus away from water) with prey type (aquatic versus terrestrial mosquitoes) appeared to be important and yet, even when the prey were in the incongruent environment, P. wanlessi continued to choose mosquitoes more often than other prey.

Mammalian mesopredators on islands directly impact both terrestrial and marine communities.

Medium-sized mammalian predators (i.e. mesopredators) on islands are known to have devastating effects on the abundance and diversity of terrestrial vertebrates. Mesopredators are often highly omnivorous, and on islands, may have access not only to terrestrial prey, but to marine prey as well, though impacts of mammalian mesopredators on marine communities have rarely been considered. Large apex predators are likely to be extirpated or absent on islands, implying a lack of top-down control of mesopredators that, in combination with high food availability from terrestrial and marine sources, likely exacerbates their impacts on island prey. We exploited a natural experiment--the presence or absence of raccoons (Procyon lotor) on islands in the Gulf Islands, British Columbia, Canada--to investigate the impacts that this key mesopredator has on both terrestrial and marine prey in an island system from which all native apex predators have been extirpated. Long-term monitoring of song sparrow (Melospiza melodia) nests showed raccoons to be the predominant nest predator in the Gulf Islands. To identify their community-level impacts, we surveyed the distribution of raccoons across 44 Gulf Islands, and then compared terrestrial and marine prey abundances on six raccoon-present and six raccoon-absent islands. Our results demonstrate significant negative effects of raccoons on terrestrial, intertidal, and shallow subtidal prey abundance, and point to additional community-level effects through indirect interactions. Our findings show that mammalian mesopredators not only affect terrestrial prey, but that, on islands, their direct impacts extend to the surrounding marine community.

Prey perception of predation risk: volatile chemical cues mediate non-consumptive effects of a predator on a herbivorous insect.

Predators can affect prey in two ways-by reducing their density (consumptive effects) or by changing their behavior, physiology or other phenotypic traits (non-consumptive effects). Understanding the cues and sensory modalities prey use to detect predators is critical for predicting the strength of non-consumptive effects and the outcome of predator-prey encounters. While predator-associated cues have been well studied in aquatic systems, less is known about how terrestrial prey, particularly insect larvae, detect their predators. We evaluated how Colorado potato beetle, Leptinotarsa decemlineata, larvae perceive predation risk by isolating cues from its stink bug predator, the spined soldier bug, Podisus maculiventris. When exposed to male "risk" predators that were surgically manipulated so they could hunt but not kill, beetles reduced feeding 29% compared to controls. Exposure to risk females caused an intermediate response. Beetles ate 24% less on leaves pre-exposed to predators compared to leaves never exposed to predators, indicating that tactile and visual cues are not required for the prey's response. Volatile odor cues from predators reduced beetle feeding by 10% overall, although male predators caused a stronger reduction than females. Finally, visual cues from the predator had a weak effect on beetle feeding. Because multiple cues appear to be involved in prey perception of risk, and because male and female predators have differential effects, beetle larvae likely experience tremendous variation in the information about risk from their local environment.

Trophic ecology of the Grey Wagtail Motacilla cinerea before and during the breeding season in the region of Bejaia (Algeria)

The diet composition of the Grey Wagtail Motacilla cinerea of the Babor Range is followed by analysis of faecal samples (90 faeces) before and during the breeding season 2010. The Grey Wagtail's diet varies depending on the stage of the breeding cycle at the southern edge of their breeding area in North Africa (Bejaia). The diet consists predominantly of aquatic preys (51.79%), with Coleoptera being the most frequent constituent (n=331, 45.5%). During the pre-laying period (February–March), the diet was variable (91 prey-taxa and H’=3,36 bits) and preys of medium size (5 to 8mm) were most common. During the incubation period (April–May), preys were mainly aquatic (60%) and larger (20 to 32mm). At the end of the breeding season (June–July), there was a greater occurrence of terrestrial preys (31 aquatic versus 30 terrestrial taxa).

Partial Dietary Segregation between Adult and Nestling BluethroatsLuscinia svecica

Several studies have suggested dietary segregation between nestling and adult birds resulting from both dietary requirements of offspring and distance to the foraging sites. We examined the diet in terms of composition and dimension, as the weight, habitat and taxonomy of prey in nestling and adult male and female Bluethroats Luscinia svecica spp. cyanecula at a recently colonized area in a mosaic of wetland (with sewage water) and terrestrial habitats in south-west Poland. On the basis of faecal samples collected between 2009–2012 from several broods and 94 adults, we found that nestlings had significantly lower diet diversity, consisting of heavier prey items. Comparing the proportion of seven major food types (order of insects/invertebrate class) we showed that the diet composition of adult and nestling Bluethroats differed significantly. The diet of nestlings contained significantly more soft-bodied prey items, namely Diptera and Lepidopteran larvae, and significantly fewer Coleoptera and Hemiptera. Furthermore, since adult showed marked decrease (contrary to young Bluethroats) of diet diversity and number of prey in the progress of the breeding season, hence our entire findings can imply that nestlings are fed in a selective manner receiving more profitable (soft-bodied and terrestrial) prey, and adults adjust their diet consuming less profitable (more chitinized and smaller) prey. This ultimately suggests the partial dietary segregation between nestling and adult birds, both in the term of size (biomass) and composition of prey. The use of a broad spectrum of food items and various prey types, and particularly the low dependence of nestlings on aquatic insects, suggests that Bluethroats have very plastic dietary requirements, which is most likely a factor facilitating the recent population recovery and adaptation to ecotonal zones between moist/wetland and terrestrial habitats with abundant moist soil, in newly settled areas of Europe.

Linking Wolf Diet to Changes in Marine and Terrestrial Prey Abundance

Since most wolf (Canis lupus) diet studies have been conducted in inland ecosystems, comparatively few data are available on diets of wolves in coastal systems. We investigated the diet of wolves in Glacier Bay, Alaska, from 12 May to 28 June in both 2010 and 2011. Although we identified 12 different prey species, including birds and small to medium-sized mammals, in wolf scats, moose (Alces alces) was the most frequent food item, observed in 80% of all scats. In contrast, a study conducted in 1993 in an area 37 km away found harbor seal (Phoca vitulina richardii) in 41% of wolf scats. Although we cannot account for differences in sampling design between the two studies, wolf diets may have changed between the two time periods.

Parvalbumin and calbindin expression in parallel thalamocortical pathways in a gleaning bat, Antrozous pallidus

The pallid bat (Antrozous pallidus) listens to prey-generated noise to localize and hunt terrestrial prey while reserving echolocation to avoid obstacles. The thalamocortical connections in the pallid bat are organized as parallel pathways that may serve echolocation and prey localization behaviors. Thalamic inputs to the cortical echolocation call- and noise-selective regions originate primarily in the suprageniculate nucleus (SG) and ventral division of medial geniculate body (MGBv), respectively. Here we examined the distribution of parvalbumin (PV) and calbindin (CB) expression in cortical regions and thalamic nuclei of these pathways. Electrophysiology was used to identify cortical regions selective for echolocation calls and noise. Immunohistochemistry was used to stain for PV and CB in the auditory cortex and MGB. A higher percentage (relative to Nissl-stained cells) of PV(+) cells compared with CB(+) cells was found in both echolocation call- and noise-selective regions. This was due to differences in cortical layers V-VI, but not layers I-IV. In the MGB, CB(+) cells were present across all divisions of the MGB, with a higher percentage in the MGBv than the SG. Perhaps the most surprising result was the virtual absence of PV staining in the MGBv. PV staining was present only in the SG. Even in the SG, the staining was mostly diffuse in the neuropil. These data support the notion that calcium binding proteins are differentially distributed in different processing streams. Our comparative data, however, do not support a general mammalian pattern of PV/CB staining that distinguishes lemniscal and nonlemniscal pathways.

A Comparative Analysis of the Diets ofVaranusalbigularisandVaranus niloticusin South Africa

We present data from gut content analyses of Varanus albigularis (savanna monitor) and V. niloticus (water monitor) in South Africa. Both species are generalist, opportunistic feeders. We did not detect any sex-based differences in the diet of V. albigularis, and there were relatively high levels of dietary overlap between the species, although there were significant differences for certain prey types. These differences match differences in habitat use between the species: the more aquatic V. niloticus consumed aquatic prey, such as amphibians and crabs, more often than did V. albigularis. Varanus albigularis included more terrestrial prey such as diplopods in its diet. Tortoises and millipedes were also prominent in the V. albigularis diet, constituting an important component of dietary intake. Few of our samples from V. albigularis had empty stomachs, suggesting that this species may occupy a lower trophic level than its Australian counterparts. Although there is a large degree of overlap in the diets of the two species, there is a trend for V. albigularis to eat more slow-moving prey. These differences result in the species foraging at different trophic levels.

A comparative analysis of the diets of Varanus albigularis and Varanus niloticus in South Africa

We present data from gut content analyses of Varanus albigularis (savanna monitor) and V. niloticus (water monitor) in South Africa. Both species are generalist, opportunistic feeders. We did not detect any sex-based differences in the diet of V. albigularis, and there were relatively high levels of dietary overlap between the species, although there were significant differences for certain prey types. These differences match differences in habitat use between the species: the more aquatic V. niloticus consumed aquatic prey, such as amphibians and crabs, more often than did V. albigularis. Varanus albigularis included more terrestrial prey such as diplopods in its diet. Tortoises and millipedes were also prominent in the V. albigularis diet, constituting an important component of dietary intake. Few of our samples from V. albigularis had empty stomachs, suggesting that this species may occupy a lower trophic level than its Australian counterparts. Although there is a large degree of overlap in the diets of the two species, there is a trend for V. albigularis to eat more slow-moving prey. These differences result in the species foraging at different trophic levels.

Ecological opportunities and intraspecific competition alter trophic niche specialization in an opportunistic stream predator

Many generalist populations are composed of specialized individuals that use a narrow part of the population's niche. Ecological theories predict that individual specialization and population trophic niche are determined by biotic interactions and resource diversity emerging from environmental variations (i.e. ecological opportunities). However, due to the paucity of empirical and experimental demonstrations, the genuine importance of each of these drivers in determining trophic niche attributes is not fully appreciated. The present study aimed at determining the population level and individual responses of brown trout (Salmo trutta) to variations in ecological opportunities (terrestrial prey inputs) and autochthonous prey communities among 10 stream reaches along a riparian condition gradient using individual longitudinal monitoring and stable isotope analyses. Our results suggested that trophic niche diversity varied along the environmental gradient, while individual trophic specialization was indirectly driven by ecological opportunities through strengthened intraspecific competition. Individual diet was repeatable over the study period, and the growth rate of juvenile brown trout increased with their specialization for aquatic predatory invertebrates. Our findings highlight the dual influences of intraspecific competition and ecological opportunities on individual trophic specialization and population trophic niche.

Food Web Linkages Demonstrate Importance of Terrestrial Prey for the Threatened California Red-Legged Frog

Restoration efforts are ongoing to protect the threatened California Red-Legged Frog (Rana draytonii) from further decline, but conserving species that have aquatic and terrestrial life stages can be challenging. For example, although it is clear that aquatic habitat must be protected for breeding, less is known about the importance of terrestrial habitat. Food web linkages remain largely unknown for this species yet would offer insight on the source of prey items and, thus, the importance of each type of habitat. We used three methods to analyze food web linkages for the California Red-Legged Frog: 1) stable isotopes collected from frogs and key species in their ecosystem; 2) stomach contents sampled from live frogs in the wild; and 3) stomach contents sampled from museum specimens. The stable isotope mixing model found 99.7% of R. draytonii diet came from terrestrial prey. Wet and dry season stomach content samples flushed from captured frogs had 90% terrestrial prey, and museum specimens contained 82% terrestrial prey. These data suggest that conservation efforts should protect riparian and upland habitats adjacent to aquatic habitats in addition to aquatic habitats.

Carbon and nitrogen ratios of aquatic and terrestrial prey for freshwater fishes

Ecological subsidies are materials and energy that cross a boundary between two ecosystems. An example is terrestrial leaf litter that falls into a stream to ultimately become the basis of the stream food web. Terrestrial arthropods that fall into streams are often an important food resource for fishes, but variation in the nutritional quality of aquatic versus terrestrial items has not been fully assessed. Carbon and nitrogen ratios in invertebrates roughly reflect relative amounts of structural chitin and since chitin is mostly indigestible to fishes, it can be an indicator of food quality, where a greater amount of chitin (or greater C:N) indicates lower quality. To better understand potential differences in the quality of terrestrial versus aquatic arthropods, we sampled terrestrial and aquatic arthropods during winter, spring, and summer, and measured their molar C:N values. We tested arthropod C:N values for origin (terrestrial vs. aquatic), taxonomy (order level), and time of year (winter, spring, and summer). We did not detect significant differences in any of these comparisons. The average molar C:N (± 1 SD) for aquatic arthropods (n = 81 samples) was 5.0 ± 0.6 and ranged from 4.7 to 5.6 and that for terrestrial arthropods (n = 42 samples) was 5.1 ± 0.7 and ranged from 4.4 to 5.6. Molar C:N values were not different among aquatic and terrestrial arthropods (df = 74.3, t = −0.995, p = 0.418) and did not differ across arthropod taxa (F7,50 = 1.9, p = 0.087). Any trends in molar C:N variability in arthropods, whether they be aquatic or terrestrial, are probably due to relative amounts of structural chitin. Since chitin is largely undigested in fish diets, molar C:N might be important in considering the true benefits of terrestrial subsidies on fish life histories.

Competitive interactions and resource partitioning between northern spotted owls and barred owls in western Oregon

Competitive interactions and resource partitioning between northern spotted owls and barred owls in western Oregon

Prey Vulnerability Limits Top-Down Control and Alters Reciprocal Feedbacks in a Subsidized Model Food Web

Resource subsidies increase the productivity of recipient food webs and can affect ecosystem dynamics. Subsidies of prey often support elevated predator biomass which may intensify top-down control and reduce the flow of reciprocal subsidies into adjacent ecosystems. However, top-down control in subsidized food webs may be limited if primary consumers posses morphological or behavioral traits that limit vulnerability to predation. In forested streams, terrestrial prey support high predator biomass creating the potential for strong top-down control, however armored primary consumers often dominate the invertebrate assemblage. Using empirically based simulation models, we tested the response of stream food webs to variations in subsidy magnitude, prey vulnerability, and the presence of two top predators. While terrestrial prey inputs increased predator biomass (+12%), the presence of armored primary consumers inhibited top-down control, and diverted most aquatic energy (∼75%) into the riparian forest through aquatic insect emergence. Food webs without armored invertebrates experienced strong trophic cascades, resulting in higher algal (∼50%) and detrital (∼1600%) biomass, and reduced insect emergence (−90%). These results suggest prey vulnerability can mediate food web responses to subsidies, and that top-down control can be arrested even when predator-invulnerable consumers are uncommon (20%) regardless of the level of subsidy.

Population genetic structure of gray wolves (Canis lupus) in a marine archipelago suggests island-mainland differentiation consistent with dietary niche.

BackgroundEmerging evidence suggests that ecological heterogeneity across space can influence the genetic structure of populations, including that of long-distance dispersers such as large carnivores. On the central coast of British Columbia, Canada, wolf (Canis lupus L., 1758) dietary niche and parasite prevalence data indicate strong ecological divergence between marine-oriented wolves inhabiting islands and individuals on the coastal mainland that interact primarily with terrestrial prey. Local holders of traditional ecological knowledge, who distinguish between mainland and island wolf forms, also informed our hypothesis that genetic differentiation might occur between wolves from these adjacent environments.ResultsWe used microsatellite genetic markers to examine data obtained from wolf faecal samples. Our results from 116 individuals suggest the presence of a genetic cline between mainland and island wolves. This pattern occurs despite field observations that individuals easily traverse the 30 km wide study area and swim up to 13 km among landmasses in the region.ConclusionsNatal habitat-biased dispersal (i.e., the preference for dispersal into familiar ecological environments) might contribute to genetic differentiation. Accordingly, this working hypothesis presents an exciting avenue for future research where marine resources or other components of ecological heterogeneity are present.