Alternative Prey Source Research Articles

Integrating technologies provides insight into the subsurface foraging behaviour of a humpback whale (Megaptera novaeangliae) feeding on walleye pollock (Gadus chalcogrammus) in Juan de Fuca Strait, Canada.

Subsurface foraging is an important proportion of the activity budget of rorqual whales, yet information on their behaviour underwater remains challenging to obtain. Rorquals are assumed to feed throughout the water column and to select prey as a function of depth, availability and density, but there remain limitations in the precise identification of targeted prey. Current data on rorqual foraging in western Canadian waters have thus been limited to observations of prey species amenable to surface feeding, such as euphausiids and Pacific herring (Clupea pallasii), with no information on deeper alternative prey sources. We measured the foraging behaviour of a humpback whale (Megaptera novaeangliae) in Juan de Fuca Strait, British Columbia, using three complimentary methods: whale-borne tag data, acoustic prey mapping, and fecal sub-sampling. Acoustically detected prey layers were near the seafloor and consistent with dense schools of walleye pollock (Gadus chalcogrammus) distributed above more diffuse aggregations of pollock. Analysis of a fecal sample from the tagged whale confirmed that it had been feeding on pollock. Integrating the dive profile with the prey data revealed that the whale's foraging effort followed the general pattern of areal prey density, wherein the whale had a higher lunge-feeding rate at the highest prey abundance and stopped feeding when prey became limited. Our findings of a humpback whale feeding on seasonally energy-dense fish like walleye pollock, which are potentially abundant in British Columbia, suggests that pollock may be an important prey source for this rapidly growing whale population. This result is informative when assessing regional fishing activities for semi-pelagic species as well as the whales' vulnerability to fishing gear entanglements and feeding disturbances during a narrow window of prey acquisition.

Examination of a humpback whale subsurface foraging on patchy prey in Juan de Fuca Strait

Contemporary data for humpback whale prey in British Columbia, Canada, are biased toward prey types that require surface feeding, with no information on the whales’ feeding behaviour at depth on alternative prey sources. Here, we present evidence that a humpback whale in Juan de Fuca Strait was subsurface feeding on juvenile pollock. We combined data from a multisensor, animal-borne tag with a vessel-mounted, Acoustic Zooplankton and Fish Profiler to describe the foraging effort of the whale relative to its prey. Analysis of a fecal sample from the tagged whale also revealed juvenile pollock. This work suggests a comprehensive sampling framework for the deeper foraging humpback whales in B.C. that are difficult to access for setting up quantitative data collection programs.

Temporal variation in the niche partitioning of Lake Michigan salmonines as it relates to alewife abundance and size structure

Stable isotope analyses offer a useful means for quantifying ecological niche dimensions, though few studies have examined isotopic response of an ecological community with respect to resource gradients such as fluctuations in prey availability. Stable carbon and nitrogen isotopes were measured for Lake Michigan salmonines and their prey collected from 2014 to 2016. Bayesian ellipse and mixing model analyses were used to quantify isotopic niche characteristics and diets, respectively, among species and years. During the 3-year study period, abundance and size structure of preferred alewife (Alosa pseudoharengus) prey changed substantially and offered an opportunity to explore predator isotopic niche response and diet shifts along a prey resource gradient. Results suggested increased reliance on alewives, especially small alewives, over the study period and were consistent with greater availability of this prey. However, differential use of alewife size classes and alternative prey sources by salmonine predators was apparent, which suggested possible resource partitioning. Characterization of ecological niche overlap using stable isotopes likely requires consideration of shared resource availability as well as specific prey and habitat preferences.

Environmental fluctuation and shifting predation pressure contribute to substantial variation in early marine survival of steelhead

SteelheadOncorhynchus mykisssmolts must migrate through distinct freshwater, estuarine, and coastal habitats, each with unique biological and physical characteristics, on their route from natal streams to the ocean to complete their life cycle. Fewer than 15% of steelhead smolts survived their migration from the Nisqually River through Puget Sound to the Pacific Ocean from 2006 to 2009 and in 2014. Rapid smolt migration coupled with evidence of smolt mortality at harbor sealPhoca vitulinahaulouts during 2014 indicated that predation by pinnipeds was an important source of mortality. When Puget Sound temperatures increased with the Northeast Pacific heat wave from late 2014 to 2016, steelhead smolt survival probabilities increased to 38% in 2016 and 2017, then decreased again as Puget Sound water temperatures cooled. Many ecological changes accompanied the increased marine temperatures, including the increased abundance of northern anchovyEngraulis mordaxin Puget Sound. The years of lowest smolt mortality coincided with abundant larval and post-larval anchovy during the previous year; and we provide data from telemetered harbor seals and steelhead indicating that the resulting high abundance of age-1+ anchovy provided an alternative prey source for predators of migrating steelhead smolts. Identification of ecological mechanisms that drive patterns in the survival of Endangered Species Act-threatened steelhead provides critical understanding of the systems within which management strategies must operate.

Predation risk is a function of alternative prey availability rather than predator abundance in a tropical savanna woodland ecosystem

Typically, factors influencing predation risk are viewed only from the perspective of predators or prey populations but few studies have examined predation risk in the context of a food web. We tested two competing hypotheses regarding predation: (1) predation risk is dependent on predator density; and (2) predation risk is dependent on the availability of alternative prey sources. We use an empirical, multi-level, tropical food web (birds–lizards–invertebrates) and a mensurative experiment (seasonal fluctuations in abundance and artificial lizards to estimate predation risk) to test these hypotheses. Birds were responsible for the majority of attacks on artificial lizards and were more abundant in the wet season. Artificial lizards were attacked more frequently in the dry than the wet season despite a greater abundance of birds in the wet season. Lizard and invertebrate (alternative prey) abundances showed opposing trends; lizards were more abundant in the dry while invertebrates were more abundant in the wet season. Predatory birds attacked fewer lizards when invertebrate prey abundance was highest, and switched to lizard prey when invertebrate abundance reduced, and lizard abundance was greatest. Our study suggests predation risk is not predator density-dependent, but rather dependent on the abundance of invertebrate prey, supporting the alternative prey hypothesis.

Marine subsidies change short-term foraging activity and habitat utilization of terrestrial lizards.

Resource pulses are brief periods of unusually high resource abundance. While population and community responses to resource pulses have been relatively well studied, how individual consumers respond to resource pulses has received less attention. Local consumers are often the first to respond to a resource pulse, and the form and timing of individual responses may influence how the effects of the pulse are transmitted throughout the community. Previous studies in Bahamian food webs have shown that detritivores associated with pulses of seaweed wrack provide an alternative prey source for lizards. When seaweed is abundant, lizards (Anolis sagrei) shift to consuming more marine‐derived prey and increase in density, which has important consequences for other components of the food web. We hypothesized that the diet shift requires individuals to alter their habitat use and foraging activity and that such responses may happen very rapidly. In this study, we used recorded video observations to investigate the immediate responses of lizards to an experimental seaweed pulse. We added seaweed to five treatment plots for comparison with five control plots. Immediately after seaweed addition, lizards decreased average perch height and increased movement rate, but these effects persisted for only 2 days. To explore the short‐term nature of the response, we used our field data to parametrize heuristic Markov chain models of perch height as a function of foraging state. These models suggest a “Synchronized‐satiation Hypothesis,” whereby lizards respond synchronously and feed quickly to satiation in the presence of a subsidy (causing an initial decrease in average perch height) and then return to the relative safety of higher perches. We suggest that the immediate responses of individual consumers to resource pulse events can provide insight into the mechanisms by which these consumers ultimately influence community‐level processes.

Seasonal foraging strategies of Alaskan gray wolves (Canislupus) in an ecosystem subsidized by Pacific salmon (Oncorhynchusspp.)

Despite frequent observations of gray wolves (Canis lupus L., 1758) using nonungulate prey, the seasonal and interannual variation in the use and relative importance of alternative prey sources to gray wolf diets have not been studied at the individual scale. We used stable isotope analysis (δ13C and δ15N) of guard hair and blood components (clot and serum) collected over 4 years to examine the occurrence, extent, and temporal variation of Pacific salmon (genus Oncorhynchus Suckley, 1861) as a food resource by both individual wolves and social groups in Lake Clark National Park and Preserve in southwestern Alaska, USA. Our results demonstrate substantial variability in the use of salmon over time. During summer, diets of five wolves consisted of at least 50% salmon, while the diets of 17 wolves consisted of primarily terrestrial prey. Over 3 years, one group of wolves consistently consumed salmon in summer and switched to terrestrial prey in winter. Prey choices were generally similar within social groups; however, the degree to which individuals consumed salmon was highly variable. The use of salmon as exhibited by wolves in Lake Clark is likely widespread where salmon are abundant and this finding should be taken into consideration in the conservation and management of wolves and their prey.

Landscape context influences nest survival in a Midwest grassland

Although the management and restoration of habitat is the key method to conserve species of interest, local habitat management often fails to elicit desired responses in populations. Landscape features beyond the local habitat scale affect the population dynamics of ring-necked pheasants (Phasianus colchicus), but the mechanism behind this response is unknown. One possibility is that nest survival, which is primarily reduced by nest predation, is regulating pheasant responses to the landscape. We investigated the extent to which land use affected nest survival by studying 202 artificial nests on 12 Conservation Reserve Program (CRP) fields in Nebraska, USA with varying surrounding land-use practices. After running a hierarchical analysis of competing models, we found that predicted nest survival increased as the amount of CRP, winter wheat, and pastureland surrounding a CRP field increased, whereas increasing fallow fields was correlated with decreased nest success. Our findings support theoretical and empirical evidence that nest predation rates are shaped by predator search efficacy. Changing the relative availability of nesting habitat that potentially holds alternative prey sources in our study affected nest survival rates, possibly by altering the search area of opportunistic nest predators. The similarities between the landscape relationships that predict nest survival and landscape predictors of pheasant abundance indicate that nest survival may potentially act as the mechanism shaping population dynamics within an ever changing farmland ecosystem. We recommend that managers consider the land use surrounding areas under consideration for habitat improvement to enhance conservation investments. © 2016 The Wildlife Society.

Changes in depth occupied by Great Lakes lake whitefish populations and the influence of survey design

Understanding fish habitat use is important in determining conditions that ultimately affect fish energetics, growth and reproduction. Great Lakes lake whitefish (Coregonus clupeaformis) have demonstrated dramatic changes in growth and life history traits since the appearance of dreissenid mussels in the Great Lakes, but the role of habitat occupancy in driving these changes is poorly understood. To better understand temporal changes in lake whitefish depth of capture (Dw), we compiled a database of fishery-independent surveys representing multiple populations across all five Laurentian Great Lakes. By demonstrating the importance of survey design in estimating Dw, we describe a novel method for detecting survey-based bias in Dw and removing potentially biased data. Using unbiased Dw estimates, we show clear differences in the pattern and timing of changes in lake whitefish Dw between our reference sites (Lake Superior) and those that have experienced significant benthic food web changes (lakes Michigan, Huron, Erie and Ontario). Lake whitefish Dw in Lake Superior tended to gradually shift to shallower waters, but changed rapidly in other locations coincident with dreissenid establishment and declines in Diporeia densities. Almost all lake whitefish populations that were exposed to dreissenids demonstrated deeper Dw following benthic food web change, though a subset of these populations subsequently shifted to more shallow depths. In some cases in lakes Huron and Ontario, shifts towards more shallow Dw are occurring well after documented Diporeia collapse, suggesting the role of other drivers such as habitat availability or reliance on alternative prey sources.

Effects of Prey Abundance on Breeding Season Diet of Northern Goshawks (Accipiter gentilis) Within an Unusual Prey Landscape

A critical element of diet analysis is species adaptability to alternative prey sources. The breeding-season diet of Northern Goshawks (Accipiter gentilis) includes both mammalian and avian species, varies geographically, and is often dependent upon tree squirrels of the genera Sciurus and Tamiasciurus. We studied alternative prey sources of Northern Goshawks in the South Hills of south-central Idaho, an area where tree squirrels are naturally absent and other prey frequently important in the diet of goshawks, such as smaller corvids, are uncommon. We quantified the diet of goshawks using nest cameras and surveyed abundance of prey using line transects. We found that goshawks consumed roughly 18.5% birds and 78.7% mammals by biomass, with diet dominated by the Belding's ground squirrel (Urocitellus beldingi, also known as Spermophilus beldingi; 74.8% of total biomass consumed); however, the percentages of mammals and birds in the diet varied between years. The diet was low in diversity, with high overlap among nests, indicating a strong local dependence on the dominant food item. Lastly, the proportion of mammalian prey in the diet was greater in larger broods than in smaller broods. This study provides new insight into the adaptability of the goshawk, particularly in areas with unique prey assemblages.Un elemento crítico del análisis de la dieta es la adaptabilidad de las especies a fuentes de presa alternativas. La dieta de la estación reproductiva de Accipiter gentilis varía geográficamente e incluye especies de mamíferos y aves, y depende a menudo de ardillas arborícolas de los géneros Sciurus y Tamiasciurus. Estudiamos fuentes de presa alternativas de A. gentilis en las South Hills del centro sur de Idaho, un área donde las ardillas arborícolas están naturalmente ausentes y donde son poco comunes otras presas frecuentemente importantes en la dieta de A. gentilis, como córvidos más pequeños. Cuantificamos la dieta de A. gentilis colocando cámaras en los nidos y medimos la abundancia de presas usando transectas lineales. Encontramos que los individuos de A. gentilis consumieron aproximadamente 18.5% de aves y 78.7% de mamíferos en términos de biomasa, con una dieta dominada por la ardilla Urocitellus beldingi, también conocida como Spermophilus beldingi (74.8% de la biomasa total consumida). Sin embargo, los porcentajes de mamíferos y aves en la dieta variaron entre años. La dieta fue poco diversa, con un elevado solapamiento entre nidos, indicando una fuerte dependencia local en la presa dominante. Por último, la proporción de presas de mamíferos en la dieta fue mayor en las nidadas más grandes que en las pequeñas. Este estudio brinda una nueva perspectiva sobre la adaptabilidad de A. gentilis, particularmente en áreas con un ensamble de presa único.

Western Burrowing Owls (Athene cunicularia hypugaea) Eavesdrop on Alarm Calls of Black‐Tailed Prairie Dogs (Cynomys ludovicianus)

AbstractAnimals sharing a common habitat can indirectly receive information about their environment by observing information exchanges between other animals, a process known as eavesdropping. Animals that use an auditory alarm calling system are an important indirect information source for eavesdropping individuals in their environments. We investigated whether Western burrowing owls (Athene cunicularia hypugaea) nesting on black‐tailed prairie dog (Cynomys ludovicianus) colonies responded to broadcasts of prairie dog alarm calls. Western burrowing owls are closely associated with black‐tailed prairie dogs in Colorado and neighboring states on the Great Plains of the United States. Prairie dog burrows in active colonies can serve as nesting sites for Western burrowing owls, and prairie dogs may act as an alternative prey source for predators, potentially decreasing the burrowing owls' risk of predation through the dilution effect. Burrowing owls nesting on prairie dog colonies may also eavesdrop on prairie dog alarm calls, enhancing their survival and nesting success on prairie dog colonies. We performed broadcast experiments with three different sounds: a prairie dog alarm call, a biological control (cattle mooing), and a non‐biological control (an airplane engine), and characterized burrowing owl responses as either alert or relaxed. For each sound stimulus, we recorded the time to first alert response to broadcast sounds (latency) and also how frequently the target burrowing owl exhibited an alert response within the first ten seconds of the broadcast (intensity). Burrowing owls reacted more quickly to the prairie dog alarm than to the biological control. They significantly increased the intensity of alert behaviors in response to broadcasts of the alarm, but did not show an increased reaction to either the biological or the non‐biological control. Our results suggest that burrowing owls nesting on prairie dog colonies eavesdrop on, and increase their alert behaviors in response to, prairie dog alarm calls.

Predators, alternative prey and climate influence annual breeding success of a long‐lived sea duck

1. Perturbations to ecosystems have the potential to directly and indirectly affect species interactions, with subsequent impacts on population dynamics and the vital rates that regulate them. 2. The few long-term studies of common eider breeding ecology indicate that reproductive success is low in most years, interrupted by occasional boom years. However, no study has explicitly examined the drivers of long-term variation in reproductive success. 3. Here, we use encounter history data collected across 41 years to examine the effects of arctic foxes (a terrestrial nest predator), local abundance and spatial distribution of lesser snow geese (an alternative prey source), and spring climate on common eider nest success. 4. Eider nest success declined over the course of the study, but was also highly variable across years. Our results supported the hypothesis that the long-term decline in eider nest success was caused by apparent competition with lesser snow geese, mediated by shared predators. This effect persisted even following a large-scale exodus of nesting geese from the eider colony. Nest success was also lowest in years of low arctic fox index, presumably driven by prey switching in years of low small mammal availability. However, increased snow goose abundance appeared to buffer this effect through prey swamping. The effect of spring climate depended on the stage of the breeding season; cold and wet and warm and dry conditions in early spring were correlated with decreased nest success, whereas warm and wet conditions in late spring increased eider nest success. 5. These results underscore the significance of both trophic interactions and climate in regulating highly variable vital rates, which likely have important consequences for population dynamics and the conservation of long-lived iteroparous species.

ROLE OF DISEASE PROPAGATION IN MIGRATORY BIRD POPULATION

Chatterjee considered a predator–prey model with avian migration in the migration prey population [S. Chatterjee, Alternative prey source coupled with prey recovery enhance stability between migratory prey and their predator in the presence of disease, Nonlinear Anal. Real World Appl. 11 (2010) 4415–4430]. In this paper, we modify and analyze the model by taking time dependent parameters and the general functional response into consideration. The conditions for the persistence of the system and the extinction of the disease are obtained. The global attractivity of the system is also studied. By numerical simulations, we find that the qualitative behavior of the system independent on the choice of the functional response. Moreover, it is observed that the infection rate, recruitment rate and the predation rate play a vital role in predicting the behavior of the dynamics.

Alternative prey source coupled with prey recovery enhance stability between migratory prey and their predator in the presence of disease

An eco-epidemiological model is considered where the prey population is migratory in nature. To incorporate the temporal pattern of the avian migration into the model, a time dependent recruitment rate was considered with a general functional response. In the numerical simulation we substitute the general functional response with Holling type-I and Holling type-II functional responses. It was observed that the qualitative behaviour of the system does not depend on the choice of the functional responses. The results showed that the system could be made disease free by either decreasing the contact rate or simultaneously increasing the predation and the recovery rate. Moreover, it was observed that the presence of an alternative food source for the predator population helps in the coexistence of all the species.

Effects of sample size on numerical estimates of diel prey consumption in a fish population

Numerical data based on stomach content analysis of the zooplanktivorous freshwater fish, Retropinna semoni, were used to examine the effect of fish sample size on mean counts of dominant prey items. Fifty adult R. semoni were collected from throughout the open‐water of Lake Benanee, Australia at each of five times over a diel period. Bootstrapping was used to generate confidence intervals around sample means, and markedly more accurate means were obtained from samples collected in the day than the night. High variation in night samples was the by‐product of a diurnal feeding regime. Traditional sample sizes of 10 to 15 stomachs resulted in reasonable confidence intervals of sample means derived from collections on the first day, corresponding to uniform feeding patterns in the population. However, increased sample sizes were required to describe more complex feeding behaviour on the second day, when a proportion of the population switched to an alternative prey source.

Juvenile White Shrimp Litopenaeus setiferus Predation on Macrobenthic and Zooplanktonic Prey

An important part of quantifying predator-prey dynamics is to understand the contribution of various prey types to the predator diet. Although the role of juvenile penaeid shrimp as benthic predators is well established, recent research has indicated that penaeids are also capable of utilizing the energy available in zooplanktonic form. Yet the relative role of juvenile penaeids as direct zooplankton predators, capturing and consuming zooplankton from the water column—as compared with their role as benthic predators of animals that live on, or have fallen to, the bottom remains unknown. Two separate sets of laboratory experiments were conducted to determine the density-dependent predation rates of juvenile Litopenaeus setiferus feeding directly on (1) zooplanktonic prey Artemia salina in the water column and (2) benthic prey Mercenaria mercenaria in sediment. The average proportional mortalities for M. mercenaria (43% to 97%) were much higher than for A. salina (-10% to 28%). At low prey densities, proportional prey mortality increased with increasing prey densities for both M. mercenaria and A. salina, indicating a type III functional response by L. setiferus. A maximum consumption rate of 476 M. mercenaria/L. setiferus/hr was estimated in the benthic feeding experiments, whereas predator satiation was not observed even at the highest A. salina densities. High proportional M. mercenaria mortalities across moderate densities (1,000-2,500 m -2 ) suggest the potential for L. setiferus regulation of Mercenaria populations. Negative proportional A. salina mortalities at low and moderate densities suggest that L. setiferus are not efficient zooplankton predators. The variable consumption rates displayed by L. setiferus feeding on different prey types are a demonstration of their dietary plasticity as omnivores. Although L. setiferus are more effective as benthic predators, their ability to capture and consume prey from the water column suggests that zooplankton may be a viable alternative prey source when benthos are unavailable or inaccessible.

The importance of the cost of swimming to the foraging behavior and ecology of larval cod (Gadus morhua) on Georges Bank

Abstract Energy expenditure by larval cod, Gadus morhua , during foraging was quantified based upon laboratory observations of search behavior and measurements of the cost of swimming. A large-volume (250-L) observation system employing stereo-paired video cameras was developed to record foraging behavior in three dimensions, and a respirometry system was developed to measure the cost of swimming of individual larvae. Application of the derived cost of swimming model to activity levels observed within the large observation tank showed that activity was a substantial and variable component of a larval cod's total bioenergetic budget. The estimated routine metabolic rate of the freely swimming larvae was 3.8–5 times greater than the measured basal rate depending upon their activity level. This is greater than the range of routine factorial scopes previously reported for larval fish in general. Future trophodynamic models developed for small marine fish larvae should consider these greater scaling factors when estimating the active metabolic rate of larvae foraging in the ocean. A trophodynamic model for larval cod on Georges Bank was developed incorporating observed foraging behavior, measured swimming costs, and the theoretical effect of turbulence on predator–prey contact rates. This model was used to estimate the prey density required to meet the estimated minimum daily metabolic demand. The estimated nauplius and copepodite concentrations required for the survival of small larvae were within the range of mean homogeneous springtime concentrations observed on Georges Bank. However, for the smallest post-yolk-sac larvae (5 mm), favorable low-turbulence foraging conditions, encountering patches of high prey density, or exploitation of alternative prey sources such as protozoans may be necessary.

Breeding Biology of Steller's Eiders (<i>Polysticta stelleri</i>) near Barrow, Alaska, 1991–99

The breeding biology of Steller's eiders (Polysticta stelleri) near Barrow, Alaska, was studied from 1991 to 1999. The number of nests found per year ranged from 0 to 78. Mean clutch size was 5.4 (SD = 1.6, n = 51), incubation period was 24 days, and Mayfield nest success ranged from 0 to 35%. Egg survival was 24% overall (n = 451). Most nests were found on the rims of low-centered polygons near ponds with emergent vegetation. Marked broods remained within 700 m of their nest sites, and duckling survival was low. Steller's eiders nested in five of the nine years studied, corresponding with years of high lemming density and nesting pomarine jaegers (Stercorarius pomarinus) and snowy owls (Bubo scandiacus). Steller's eiders may choose to nest only in years with abundant lemmings for two reasons: first, abundant lemmings provide an alternative prey source for foxes and other predators of eiders; second, jaegers and owls defending their own nests may inadvertently provide protection to eiders nesting nearby.

Temporal variation of predation on rodents and shrews by small African forest carnivores

AbstractAnalysis of 904 small carnivore scats collected over a two‐year period in a seasonal forest in the south‐western Central African Republic yielded the remains of 732 shrews, rodents (<1 kg), and bats comprising at least 34 species. Most scats belonged to the long‐nosed mongoose, Herpestes naso, the numerically dominant carnivore in the study area. Shrews were the principal small mammal prey with respect to number of individuals and frequency of occurrence. The mean monthly frequency of occurrence of shrews was 21.7% and of small rodents (<100 g) was 20.8%. Both shrews and small rodents occurred significantly more often in scats collected during dry season months, and live‐trapping of rodents over a 22‐month period revealed similar peaks in availability. In contrast to many mammalian carnivores, shrews did not appear to be used as an alternative prey source: not only did consumption levels of both prey types fluctuate in synchrony, but the availability of rodents apparently had no effect upon the likelihood of including shrews in the diet. Preferential use of small mammals by long‐nosed mongooses was tested for the 10 rodent species for which abundance indices were calculated. Mongooses preyed selectively on several rodent species (Praomys spp., Deomys ferrugineus, Stochomys longicaudatus, and Grammomys rutilans), while the most common rodent in the study area, Hylomyscus spp., was found in scats at lower frequencies than expected. In accordance with the habitat specificities of these ‘preferred’ rodent prey, it is suggested that mongooses and perhaps other small carnivores were foraging in micro‐habitats characterized by higher‐than‐average vegetation understorey thickness where the simultaneous rate of encounter with arthropods, their main prey, and shrews was apt to be high.

Evidence for food competition between mink (Mustela vison) and otter (Lutra lutra) on Scottish islands

Food competition between American mink and otters was measured by comparing the diets of sympatric mink and otter populations with those of allopatric populations. Niche breadth was narrower for otters than mink. Niche breadth was wider for both mink and otters on islands where they co‐existed in comparison to that of the allopatric populations. Niche overlap was lower in sympatric populations on islands with mammalian prey, however, niche overlap was not reduced on small islands without mammalian prey. Obtained data suggest that mink and otter compete for food resources and, when alternative prey sources are available, mink become more generalist predators to avoid competition with otters. However, when alternative prey sources are not available, both species become more generalist.