Prolonged prey suppression by carnivores — predator-removal experiments

Summary The hypothesis that carnivores can significantly suppress prey populations after they collapse during drought was tested by predator-removal experiments. Low populations of rabbits (Oryctolagus cuniculus) responded with significantly accelerated growth where foxes (Vulpes vulpes) and feral cats (Felis catus) were continually shot. Experiments in years of good pasture and poor were confirmatory. After only 14 months, the rabbits were well on their way to another eruption whereas untreated populations had remained low for 2.5 yrs until a second drought. These studies confirm the impact of carnivores found for low populations of cyclical prey but there was no measurable effect of predator-removal on the population declines in our studies. They were due to aridity and poor pastures. The concept of Environmentally Modulated Predation is presented. Only after the intervention of a widespread environmental event is such limiting predation possible. Drought is also the cause in arid Australia for dingoes (Canis familiaris dingo) preying seqenntially on rodents, rabbits and red kangaroos, while wildfire was the cause in temperate forests. Such environmental intervention may be more widespread than usually considered, triggering some apparent predator-prey cycles. The major factors limiting rabbits in inland Australia are: adequacy of green herbage during breeding, food scarcity during average summers, critical shortages of food and its low quality (including moisture content) during crashes in drought, followed by limiting predation. Contrasting life-histories are one cause for the ultimate escape of rabbit populations from limiting predation as rabbits can breed continuously but carnivores seasonally only. Patchy predation and alternate prey may also play a part.     

Strength of a Trophic Cascade Between an Apex Predator,Mammalian Herbivore and Grasses in a Desert Ecosystem Does Not Vary with Temporal Fluctuations in Primary Productivity

There has long been debate regarding the primacy of bottom-up and top-down effects as factors shaping ecosystems. The exploitation ecosystems hypothesis (EEH) predicts that predators indirectly benefit plants because their top-down effects limit herbivores’ consumption of plants, and that the strength of trophic cascade increases with increasing primary productivity. However, in arid environments, pulses of primary productivity produced by irregular rainfall events could decouple herbivore–plant and predator–prey dynamics if high conversion efficiency from seed biomass to consumers allows the rapid build-up of consumer populations. Here, we test predictions of the EEH in an arid environment. We measured activity/abundances of dingoes, red kangaroos and grasses, and diet of dingoes, in landscapes where dingoes were culled or not culled over 3 years. Dingo activity was correlated with rainfall, and their tracks were less frequent at culled sites. Kangaroo abundance was greater at sites where dingoes were culled and increased with rainfall in the previous 6 months. Grass cover was greater at sites where dingoes were not culled and increased with rainfall in the previous 3 months. During a period of average rainfall, dingoes primarily consumed rodents and increased their consumption of kangaroos during a period of drier conditions. Our results are consistent with the hypothesis that suppression of an apex predator triggers a trophic cascade, but are at odds with the EEH’s prediction that the magnitude of trophic cascades should increase with primary productivity. Our study demonstrates that temporal fluctuations in primary productivity can have effects on biomasses of plants and consumers which are in many ways analogous to those observed along spatial gradients of primary productivity.     

Does a top-predator provide an endangered rodent with refuge from an invasive mesopredator?

In arid environments, ecological refuges are often conceptualised as places where animal species can persist through drought owing to the localised persistence of moisture and nutrients. The mesopredator release hypothesis (MRH) predicts that reduced abundance of top-order predators results in an increase in the abundance of smaller predators (mesopredators) and consequently has detrimental impacts on the prey of the smaller predators. Thus according to the MRH, the existence of larger predators may provide prey with refuge from predation. In this study, we investigated how the abundance of an endangered rodent Notomys fuscus is affected by Australia's largest predator, the dingo Canis lupus dingo , introduced mesopredators, introduced herbivores, kangaroos and rainfall. Our surveys showed that N. fuscus was more abundant where dingoes occurred. Generalised linear modelling showed that N. fuscus abundance was associated positively with dingo activity and long-term annual rainfall and negatively with red fox Vulpes vulpes activity. Our results were consistent with the hypothesis that areas with higher rainfall and dingoes provide N. fuscus with refuge from drought and predation by invasive red foxes, respectively. Top-order predators, such as dingoes, could have an important functional role in broad-scale biodiversity conservation programmes by reducing the impacts of mesopredators.     

Foraging and diet of the red fox Vulpes vulpes in relation to variable food resources in Biatowieza National Park, Poland

Feeding ecology of red fox Vulpes vulpes was studied by scat analysis and snow-tracking m primeval temperate forest and adjacent meadows during four years (1985/86-1988/89) Winters varied from mild to unusually severe Main food resources for foxes were rodents of open meadows and river valleys (root vole Microtus oeconomus ). forest rodents (bank vole Clethrionomys glareolus and yellow-necked mouse Apodemus flavicollis ), hare Lepus europaetis and carcasses of wild boar Sus scrofa and red deer Cervus elaphus either killed by wolves and lynx or that had died from inanition Composition of fox diet m four cold seasons (autumn-winter) was compared to the abundance of main food resources Prolonged, sharp decline of Microtus was followed by only a twofold decrease of its share in fox diet Foxes continued to prey on declining Microlus The changes in the proportions of forest rodents and hare in fox diet clearly followed the fluctuations in numbers of these two prey Carcasses were alternative, buffer food to foxes and were taken considerably when Microlus and other prey were in low numbers or poorly accessible The depth of snow was the most important factor restricting foxes access to rodents Snow-tracking revealed that foxes dwelling in the forest widely used adjacent open areas In open meadows foxes mainly hunted for rodents, while in the forest the most significant foraging activity was scavenging Seasonal analysis of fox diet revealed that consumption of Microlus by foxes was stable throughout the year (37-47% of biomass consumed) Bank vole significantly contributed to fox diet in autumn, and hare in summer only Scavenging was most pronounced in winter and spring when carcasses made up 30% of biomass taken     

The relationship between intra–guild diet overlap and breeding in owls in Israel

Even though intra-guild predators frequently prey on the same species, it is unclear whether diet overlap between two predators is a source of interspecific competition or whether predators simply use the same abundant prey resource. We measured the extent to which the diets of barn owls (Tyto alba) and long-eared owls (Asio otus) in Israel overlap and examined whether yearly differences in diet overlap correlate with barn owl breeding success. Pianka’s index of niche overlap was positively related to barn owl population size but not to its breeding success. The number of breeding barn owls was higher when long-eared owls consumed more rodents, suggesting that diet overlap most likely increased when rodents became more abundant. Therefore, in Israel, when these two owl species prey more often on rodents, their diets are more similar and interspecific competition is reduced. Unlike sympatric populations in Europe, in years when rodents are less abundant in Israel long-eared owls switch to hunting alternative prey (e.g., birds), perhaps to avoid competition with barn owls.     

The feeding ecology of the dingo II. Dietary and numerical relationships with fluctuating prey populations in south-eastern Australia

The dietary and predator-prey relationships of Canis familiaris dingo were studied for 9 yr at a coastal site and for 1.5 yr at a montane site in south-eastern Australia. The percentage occurrences of items eaten were obtained from faeces, and the abundances of prey by counting water-birds, trapping small mammals, and tracking large and medium-sized mammals on specially prepared soil plots. Dingoes were also estimated by tracking. The diet was broad but predominantly mammalian (23 species). Dietary frequencies were grouped around three weight modes, 0.1, 1.25 and 16 kg, corresponding with bush rat (Rattus fuscipes), ringtail possum (Pseudocheirus peregrinus) and swamp wallaby (Wallabia bicolor). The amplitudes of those frequencies approximately doubled from mode to mode, viz. 4, 13.5 and 28.3%, respectively. Medium-sized mammals have been recognized as the staple prey because of their dependability. Their density estimates fluctuated least among prey-groups, and track records of dingoes in the mountains were significantly related to them. Other categories of prey were supplementary (large mammals), opportune (small mammals) and scavenged. Although there were general tendencies for dietary frequencies to follow prey abundances, significant functional and numerical responses were obtained only for water-birds (coot and swan). Their super-abundance in the mid-years of the coastal study and their highly clumped distribution were the likely causes. Predation was disproportionately severe on mammalian prey-classes after an extensive wildfire at the coastal site. Such predation may have suppressed populations of wallabies and kangaroo for 2 yr until the water-birds became super-abundant. The prevalence of wombats in the mountains may have induced heavy predation upon other less numerous large prey. Concepts of profitability in feeding appear to apply to the dingo more than those of optimization of time or energy. The decline in dingoes was correlated most with long-term declines in water-birds and medium-sized mammals. At the time, abundance estimates of wallabies and kangaroos were increasing and those species increasingly predominated in the diet. The ability to hunt co-operatively was apparently ineffectual in preventing decline in dingo numbers. It is suggested that pack size is related inversely to the level of temporal instability in the environment. Frequent wildfires may prevent staple (mediumsized) and supplementary (large) prey from being abundant simultaneously, a combination thought necessary for large pack size.     

Lizards reduce food consumption by spiders: mechanisms and consequences

Summary To determine the effect of lizards on webspider populations, we conducted a long-term field experiment in the Bahamas. Numbers of spider individuals were about 3 times higher in lizard-removal enclosures than in control enclosures with natural densities of lizards. Dietary analyses showed that lizards ate spiders and that lizard and spider diets overlapped substantially. Lizards reduced biomass of prey consumed by spiders; details indicated that they reduced biomass of large (> 4 mm) prey consumed by spiders more than biomass of small (4 mm) prey. Similarly, lizards reduced biomass of large aerial arthropods caught in sticky traps but not biomass of small aerial arthropods. We found no evidence that the lizard effect on prey consumption by spiders was caused by a spatial shift from areas with high aerial arthropod abundance to areas with low aerial arthropod abundance. Lizards reduced adult female cephalothorax width and fecundity of spiders. In a separate experiment, food-supplemented spiders were more fecund than control spiders. This study indicates that the interaction between lizards and spiders includes both predation and competition for food.     

Interspecific and Geographic Variation in the Diets of Sympatric Carnivores: Dingoes/Wild Dogs and Red Foxes in South-Eastern Australia

Dingoes/wild dogs (Canis dingo/familiaris) and red foxes (Vulpes vulpes) are widespread carnivores in southern Australia and are controlled to reduce predation on domestic livestock and native fauna. We used the occurrence of food items in 5875 dingo/wild dog scats and 11,569 fox scats to evaluate interspecific and geographic differences in the diets of these species within nine regions of Victoria, south-eastern Australia. The nine regions encompass a wide variety of ecosystems. Diet overlap between dingoes/wild dogs and foxes varied among regions, from low to near complete overlap. The diet of foxes was broader than dingoes/wild dogs in all but three regions, with the former usually containing more insects, reptiles and plant material. By contrast, dingoes/wild dogs more regularly consumed larger mammals, supporting the hypothesis that niche partitioning occurs on the basis of mammalian prey size. The key mammalian food items for dingoes/wild dogs across all regions were black wallaby (Wallabia bicolor), brushtail possum species (Trichosurus spp.), common wombat (Vombatus ursinus), sambar deer (Rusa unicolor), cattle (Bos taurus) and European rabbit (Oryctolagus cuniculus). The key mammalian food items for foxes across all regions were European rabbit, sheep (Ovis aries) and house mouse (Mus musculus). Foxes consumed 6.1 times the number of individuals of threatened Critical Weight Range native mammal species than did dingoes/wild dogs. The occurrence of intraguild predation was asymmetrical; dingoes/wild dogs consumed greater biomass of the smaller fox. The substantial geographic variation in diet indicates that dingoes/wild dogs and foxes alter their diet in accordance with changing food availability. We provide checklists of taxa recorded in the diets of dingoes/wild dogs and foxes as a resource for managers and researchers wishing to understand the potential impacts of policy and management decisions on dingoes/wild dogs, foxes and the food resources they interact with.     

Resolving the value of the dingo in ecological restoration

There is global interest in restoring populations of apex predators, both to conserve them and to harness their ecological services. In Australia, reintroduction of dingoes (Canis dingo) has been proposed to help restore degraded rangelands. This proposal is based on theories and the results of studies suggesting that dingoes can suppress populations of prey (especially medium‐ and large‐sized herbivores) and invasive predators such as red foxes (Vulpes vulpes) and feral cats (Felis catus) that prey on threatened native species. However, the idea of dingo reintroduction has met opposition, especially from scientists who query the dingo's positive effects for some species or in some environments. Here, we ask ‘what is a feasible experimental design for assessing the role of dingoes in ecological restoration?’ We outline and propose a dingo reintroduction experiment—one that draws upon the existing dingo‐proof fence—and identify an area suitable for this (Sturt National Park, western New South Wales). Although challenging, this initiative would test whether dingoes can help restore Australia's rangeland biodiversity, and potentially provide proof‐of‐concept for apex predator reintroductions globally.     

Annual and seasonal changes in diets of martens: evidence from stable isotope analysis

Theory predicts that generalist predators will switch to alternative prey when preferred foods are not readily available. Studies on the feeding ecology of the American marten (Martes americana) throughout North America suggest that this mustelid is a generalist predator feeding largely on voles (Microtus sp.; Clethrionomys sp.). We investigated seasonal and annual changes in diets of martens in response to the changing abundance of small rodents (Peromyscus keeni, and Microtus longicaudus) on Chichagof Island, Southeast Alaska, using stable isotope analysis. We hypothesized that martens would feed primarily on small rodents during years with high abundance of these prey species, whereas during years of low abundance of prey, martens would switch to feed primarily on the seasonally available carcasses of salmon. We also hypothesized that home-range location on the landscape (i.e., access to salmon streams) would determine the type of food consumed by martens, and martens feeding on preferred prey would exhibit better body condition than those feeding on other foods. We live-captured 75 martens repeatedly, from mid-February to mid-December 1992–1994. We also obtained marten carcasses from trappers during late autumn 1991 and 1992, from which we randomly sub-sampled 165 individuals. Using stable isotope ratios and a multiple-source mixing model, we inferred that salmon carcasses composed a large portion of the diet of martens in autumn during years of low abundance of rodents (1991 and 1992). When small rodents were available in high numbers (1993 and 1994), they composed the bulk of the diet of martens in autumn, despite salmon carcasses being equally available in all years. Selection for small rodents occurred only in seasons in which abundance of small rodents was low. Logistic regression revealed that individuals with access to salmon streams were more likely to incorporate salmon carcasses in their diet during years of low abundance of small rodents. Using stable isotope analysis on repeated samples from the same individuals, we explored some of the factors underlying feeding habits of individuals under variable ecological conditions. We were unable to demonstrate that body weights of live-captured male and female martens differed significantly between individuals feeding on marine-derived or terrestrial diets. Therefore, martens, as true generalist predators, switched to alternative prey when their principal food was not readily available on a seasonal or annual basis. Although salmon carcasses were not a preferred food for martens, they provided a suitable alternative to maintain body condition during years when small rodents were not readily available. Received: 1 May 1996 / Accepted: 24 February 1997     

Trophic status and feeding habits of the European Polecat Mustela putorius L. 1758

The trophic status of the European Polecat Mustela putorius was studied through a review of the diets of 18 different populations in Europe. Rodents represented the principal prey in 10 sites (55.5%) and were a prey of secondary importance in seven sites (38.8%). Anurans prevailed in three sites (17%) but constituted the second food category in eight others (44%). The ascendant hierarchical classification showed a relatively unvarying diet in Europe. The frequency of birds and invertebrates were important in the diversification of the diet, indicated by the value of the food niche index, whereas rabbits were more significant in southern regions. The constant incidence of predation on woodland rodents and amphibians gave a particular status to the Polecat and showed it to be a generalist feeder well adapted to the mid-European region.     

Assessing predation risk to threatened fauna from their prevalence in predator scats: dingoes and rodents in arid Australia

The prevalence of threatened species in predator scats has often been used to gauge the risks that predators pose to threatened species, with the infrequent occurrence of a given species often considered indicative of negligible predation risks. In this study, data from 4087 dingo (Canis lupus dingo and hybrids) scats were assessed alongside additional information on predator and prey distribution, dingo control effort and predation rates to evaluate whether or not the observed frequency of threatened species in dingo scats warrants more detailed investigation of dingo predation risks to them. Three small rodents (dusky hopping-mice Notomys fuscus; fawn hopping-mice Notomys cervinus; plains mice Pseudomys australis) were the only threatened species detected in <8% of dingo scats from any given site, suggesting that dingoes might not threaten them. However, consideration of dingo control effort revealed that plains mice distribution has largely retracted to the area where dingoes have been most heavily subjected to lethal control. Assessing the hypothetical predation rates of dingoes on dusky hopping-mice revealed that dingo predation alone has the potential to depopulate local hopping-mice populations within a few months. It was concluded that the occurrence of a given prey species in predator scats may be indicative of what the predator ate under the prevailing conditions, but in isolation, such data can have a poor ability to inform predation risk assessments. Some populations of threatened fauna assumed to derive a benefit from the presence of dingoes may instead be susceptible to dingo-induced declines under certain conditions.     

Diet of the Omani Owl,Strix butleri,near Nakhal,Oman

The diet of the Omani Owl, Strix butleri, was studied for the first time based on re- cently collected pellets. A total of 22 individual prey items represented three mam- mals (all rodents), at least two undetermined gecko species and one scorpion. By fre- quency, Acomys cahirinus constituted 31.8% and lizards 27.3%, while Gerbillus nanus and G. dasyurus were the least consumed prey items (4.5% each).     

Not all predators are equal: a continent‐scale analysis of the effects of predator control on Australian mammals

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How important is milk for near-weaned red kangaroos (<Emphasis Type="Italic">Macropus rufus</Emphasis>) fed different forages?

Red kangaroos (Macropus rufus) are large (>20 kg) herbivorous marsupials common to the arid and semi-arid regions of inland Australia, where drought is frequent. Young-at-foot (YAF) red kangaroos are the age/size class usually most affected by drought. Kangaroos at this YAF stage are making the transition from a milk-based diet to one of herbivory and an inability to adequately digest high-fibre feeds may contribute to their high mortalities during drought. We examined the role of milk in the nutrition of YAF red kangaroos fed forages of different fibre content and evaluated it as an extra energy and/or nitrogen source. Milk intake had little impact on the digestion of herbage by YAF red kangaroos fed low-fibre chopped lucerne (alfalfa) hay. Organic matter (OM) intake was 210+/-20 g day(-1) and 228+/-22 g day(-1), respectively, by YAF fed lucerne and lucerne with milk. Apparent digestibility of lucerne OM was ca. 55%, regardless of milk intake. Fed lucerne, with and without milk, YAF sustained growth rates of ca. 45 g day(-1). Conversely, even with a milk supplement, YAF red kangaroos ingested only 90+/-11 g day(-1) of high-fibre chopped oaten hay, of which they digested only ca. 36%. Despite milk intake, YAF fed chopped oaten hay lost between 0 and 75 g body mass day(-1) and were in negative nitrogen balance (-0.40+/-0.11 g N day(-1)). On all diets nitrogen loss was primarily as endogenous nitrogen (urinary and faecal) rather than as dietary nitrogen. Endogenous nitrogen losses were elevated in YAF fed chopped oaten hay, primarily as non-dietary faecal nitrogen. Overall, when high-quality feed was available, YAF were not markedly dependent on milk. However, YAF fed poor-quality chopped oaten hay would require up to 540 ml day(-1) of late-stage kangaroo milk to attain intakes of energy and nitrogen, and hence growth rates, comparable with those YAF fed lucerne.     

Inheritance of Glucose-6-phosphate Dehydrogenase Variation in Kangaroos

THE production of glucose-6-phosphate dehydrogenase (EC 11149, G6PD) in human¹, horse and donkey² and brown and blue hare³ cells is governed by genes carried by the X chromosome. Two electrophoretic forms of G6PD have been found in wallaroos and euros (Macropus robustus Gould) and one in red kangaroos (Macropus rufus (Desm.)); members of the marsupial family Macropodidae (kangaroos). This analysis used electrophoresis of red blood cell haemolysates on cellulose acetate⁴. No polymorphic populations were found⁵ but electrophoretic phenotypes of euros (Macropus robustus erubescens Sclater) were characterized by a single slow moving band (G6PD-S) while those of wallaroos (Macropus r. robustus Gould) had a single fast moving band (G6PD-F). Red kangaroo populations were uniformly G6PD-S.     

Food habits of the Indian fox (Vulpes bengalensis) in Kutch, Gujarat, India

We characterized the diet of the Indian fox (Vulpes bengalensis) during the breeding season in a semi-desert region of Western India. Diet was estimated using scat analysis. We used Index of Relative Importance (IRI) to determine the contribution of prey items in the diet of the Indian fox.Indian foxes were observed to feed on a wide variety of prey items. Arthropods were the most frequently occurring prey in their diet. IRI scores were highest for the group Coleoptera and Orthoptera followed by rodents, termites, Ziziphus fruits and spiny tailed lizards (Uromastyx hardwickii). IRI scores for rodents were higher for pups, differing significantly from proportions present in adult diet, thus indicating that they are crucial food items for the young ones. Prey proportions in the fox diet differed between the two habitats in the study area (grassland and scrubland).Our data suggest that the Indian fox is essentially an omnivore showing similar diet (in terms of high incidence of arthropods) to foxes inhabiting arid and semi-arid regions. The opportunistic and generalist strategy has probably helped the species to survive in varied habitats across the Indian subcontinent.     

Changes in desert avifauna associated with the functional extinction of a terrestrial top predator

We investigated how long‐term suppression of populations of a top predator, the dingo Canis dingo, affected composition of sympatric avifauna in Australian deserts, by surveying bird assemblages across ~80 000 km² of arid dune‐fields on either side of the Dingo Barrier Fence (DBF; a 5614 km‐long fence separating ecosystems in which dingoes are abundant from ecosystems in which dingoes are functionally extinct). Using fourth‐corner modelling, incorporating species’ traits, we identified apparent declines of sedentary birds that nest in low vegetation and small birds that feed primarily on grass seed, and increases in scavenging birds associated with the functional extinction of dingoes. Occupancy differed between sites inside and outside the DBF in 13 bird species. We hypothesise that these differences in bird assemblages across the DBF result, in part, from increases in kangaroos Macropus spp. and red foxes Vulpes vulpes in arid landscapes where dingoes have been removed. Our study provides evidence that the functional extinction of a large terrestrial predator has had pervasive ecosystem effects, including shifts in composition of avian assemblages.     

Joint use of habitat by red kangaroos and shorthorn cattle in arid central Australia

The distribution of cattle and red kangaroos among the major communities of a 170 km² paddock in central Australia was determined from 108 and 82 air surveys respectively over a four and a half year period. Fifty-nine surveys of each species were used in this report. Changes in use of the communities by the two species were analysed in relation to forage conditions using linear regression techniques. The two species showed trends in time of use of the open and wooded communities. Kangaroos used the mulga-perennial community (groved Acacia woodland with a shrub and perennial grass understorey) during good forage conditions and moved to the drought refuge open communities when forage conditions deteriorated. Cattle, on the other hand, used the open communities during good forage conditions and tended to move to the mulga communities and the hills when drought began. Mulga-annual (ungroved Acacia woodland with short grass and forb understorey) was the only major community which showed no clear linear relationship between kangaroo use and forage conditions. This might be a buffer area from which kangaroos come and go as other areas become more attractive as feeding areas. Cattle, however, show some preference for mulga annual during medium forage conditions. There appears to be little spatial interaction by the two species except during drought when kangaroos concentrate on the open communities and some cattle continue to feed in these communities. Community selection seems to be determined mainly by forage conditions, as there is no evidence that one species attracts or repels the other in spatial terms. In the conditions observed, the two species successfully coexisted with some control of numbers of cattle by man.     

The control of vertebrate pests by vertebrate predators

Carnivores can control mammalian pests for long periods, but only after pest numbers have been reduced by other means. In Australia, the cause is prolonged dry weather. The consequent low populations of rabbits can then be regulated by European foxes, feral cats and dingoes. Kangaroos, and probably feral goats and pigs, succumb to dingoes at the same time, as substitute prey for rabbits. In the general case, such regulatory predation may be triggered climatically, by disease or by human intervention. When predators are themselves pests to be controlled, integrated pest management may be required to avoid unwanted resurgences of other pests.