Prey Shortage Research Articles

Influence of food shortage during the summer on body composition and reproductive hormones in the red fox, Vulpes vulpes

Some mammalian carnivores compensate for prey shortage during the summer by increasing their consumption of wild berries. We tested whether such prey shortage affected all body components (e.g., fat, protein, water) equally, and whether it could affect reproduction. Two groups of wild red foxes (Vulpes vulpes) were kept in captivity: the first group was fed a control diet of dog food, and the second group was fed a diet of dog food and fresh apples for 8 weeks during midsummer. The experimental diet contained 95% of the energy content of the control diet but only 55% of the protein and 46% of the fat content. Foxes fed the experimental diet simultaneously lost body fat and protein on the apple diet, but body reserves were quickly restored upon return to the control diet. We found no evidence that protein and energy deficiency during summer impaired reproduction the following spring.

Cannibals and parasites: conflicting regulators of bimodality in high latitude Arctic char, Salvelinus alpinus

Unexploited populations of Arctic char (Salvelinus alpinus) sampled in autonomous lake ecosystems in northern Svalbard demonstrate extraordinary catch curves with age and size frequency distributions characterized by discrete bimodality. Analyses of size‐age relationship, summer diet and food‐related intestinal parasite intensities of modal char groups revealed a pattern of discrete ontogenetic niche shifts. Life‐history changes at age 10–15 and size 200–300 mm/50–300 g involved shifting from an initial mode of small‐sized, slow‐growing and sexually mature individuals feeding on micro‐crustaceans and aquatic insects (Chironomidae, Trichoptera), to a terminal mode of large‐sized and fast‐growing cannibals. Cannibalism, however, was found to result in accumulation of cestodan parasites, of which Diphyllobothrium ditremum increases age‐related mortality rates and may be lethal at 1500–2000 plerocercoids. Genetically allopatric populations with cannibalism demonstrated a female‐biased sex ratio, primarily in the initial mode, suggesting sexual asynchrony in their ontogeny. By contrast, a small population of large‐sized, non‐cannibalistic Arctic char feeding exclusively on the large amphipod Gammaracanthus lacustris, demonstrated unimodal size and age frequency distribution, faster growth, an excess of males and lower parasite burden. Seasonal prey shortage and slow juvenile growth in association with fitness components favoring large body size is a suggested mechanism for inducing cannibalism. Although not the basic cause of bimodality as such, it is concluded that ontogenetic niche shift by cannibalism reinforces discrete age modal divergence resulting in the numerical preponderance of large‐sized individuals in these marginal char populations. Cannibalism is thus considered an important strategy for survival of landlocked Arctic char in the High Arctic. As a conflicting cost to the more efficient use of available energy by larger individuals, the accumulation of cestodan parasites in cannibals, however, will reduce the survival rate of old individuals and accelerate their termination within this modal group.

Predation by Australian magpies (Gymnorhina tibicen) on pasture invertebrates: Are non‐territorial birds less successful?

AbstractDiet composition and prey intake rates of Australian magpies in a New Zealand population were investigated to determine whether flocking, non‐territorial birds were less successful predators of pasture invertebrates than territorial birds. Time‐activity budgets showed that flock magpies foraged throughout the day during February and March, indicative of a prey shortage at that time. Prey abundance was therefore measured in the autumn months but there was no evidence of a large quantitative difference in the food supply of flock and territorial magpies. Significantly fewer invertebrates were collected from pitfall traps in the flock foraging area, but these were counterbalanced by significantly more scarab beetle larvae and more flying insects, obtained by sweep‐netting. When compared with territorial magpies, flock birds exhibited similar peck rates, prey intake rates, and earthworm intake rates. While there were qualitative differences in the diets of flock and territorial magpies, determined by faecal analysis, five of seven prey items analysed were captured in the same relative proportions by the flock and territorial birds. Earthworms exceeded their relative availability in the diets of flock birds, and more scarab larvae were found in the diets of territorial magpies. Non‐territorial magpies were therefore no less successful than territorial birds during the autumn, and it is suggested that the function of flocking behaviour may eventually be discovered through consideration of why non‐territorial birds never occur as solitary individuals, rather than emphasizing comparisons with territorial magpies.

Dietary shifts, niche relationships and reproductive output of coexisting Kestrels and Long-eared Owls.

Food samples of breeding Kestrels (Falco tinnunculus) and Long-eared Owls (Asio otus) were collected in the peak and low phase of their preferred prey (Microtus voles) in western Finland. Diets of pairs that bred as neighbours (≤1 km) with interspecifics were compared with those of non-neighbours. In both species, neighbouring pairs fed less on Microtus voles and more on alternative prey than did non-neighbours. Competition theory predicts that diet overlap should be lower during prey shortage and that diet similarity should be especially reduced in neighbouring pairs. Observations were consistent with expectations: diet similarity was lower in the low vole years and neighbouring pairs showed less diet overlap that non-neighbours. Differences in habitat composition and prey availability at the sample sites should not confuse the results. In addition to the high diet similarity, hunting habitats and nest sites of the species overlapped almost completely; they only showed clear temporal segregation in hunting. Probably because of food competition, the neighbouring pairs of both species produced significantly fewer young than the non-neighbours. These results contrast with the view that the diet composition and dietary shift of rodent-feeding predatory birds can be interpreted in terms of simple opportunistic foraging. In the breeding season, interspecific competition for food seems to be an important factor that affects the niches of these species, especially in northern areas, where the seasonal low phase of voles in spring and the number of alternative prey are lower than in more southern areas.

Dietary shifts of sympatric buteos during a prey decline.

Diets of nesting Red-tailed Hawks (Buteo jamaicensis) and Ferruginous Hawks (Buteo regalis) were sampled before and after a decline in the hawks' principal prey species. Diets of pairs that shared their foraging ranges with interspecifics were contrasted with those of pairs whose home ranges did not overlap with interspecifics. Current theory predicts that diets should diverge during prey shortages and that overlap should be especially reduced in ranges shared with interspecifics. Species composition of the two hawks' diets diverged during the prey shortage, but the divergence was most pronounced in hawks that did not share foraging ranges with interspecifics. In contrast to predictions, the two species converged on similar-sized prey during the prey shortage. Available data on differences in habitat composition and prey availability at the sample sites did not explain the deviations from the predicted response. Although our findings on diet shifts differ from those of most other studies, the implications are consistent with observations of others who have studied assemblages of mammal-feeding raptors. We conclude that diet composition of the hawks we studied was not directly affected by interspecific competition and that shifts in diet overlap during prey shortages do not necessarily imply that interspecific competition is occurring.

Growth patterns and food habits of Baetis rhodani, Capnia pygmaea and Diura nanseni in a West Norwegian river

As a part of weir‐pond‐ecosystem project, the growth patterns and food habits of Baetis rhodani (Ephemeroptera), Capnia pygmaea (Plecoptera), and Diura nanseni (Plecoptera) were investigated. B. rhodani was bivoltine. The winter generation consisted of three cohorts. C. pygmaea and D. nanseni accomplished their nymphal growth in one year. The predominate food consumed by B. rhodani and C. pygmaea was detritus. Variations in the diet of these detritivores are discussed in relation to microdistribution and possible competition. The most common prey items of the carnivorous D. nanseni were chironomids and nymphs of B. rhodani and C. pygmaea. Prey shortage is suggested to be the reason for small D. nanseni eating detritus.