General linear model with a Gaussian error assessing the relationships between gray wolf Canis lupus consumption of large- to kilogram and medium-sized to kilogram ungulates and anthropogenic foods livestock and garbage.
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Silhouettes are adapted images courtesy of the Integration and Application Network, University of Maryland. Under more natural circumstances, gray wolves are known to switch to alternate prey when their primary wild ungulate prey declines. Such prey includes beavers, lagomorphs, microtine rodents, birds, fish, and, on occasion, other carnivores Newsome et al.
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However, a case study in Iran highlights that gray wolves may still consume high quantities of anthropogenic foods even when wild prey are abundant Hosseini-Zavarei et al. This tendency reflects the opportunistic nature of wolves and their underlying attraction to foods that can be readily obtained and eaten with low energetic cost or risk of harm. Given that wolves consistently and sometimes heavily rely on anthropogenic foods, it is surprising that relatively little attention has been paid to this aspect of their foraging ecology or to its potential consequences.
Although the exact reasons for gray wolves using anthropogenic foods are not clear, there is emerging recognition that this practice can dramatically alter their ecology and behavior Newsome et al.
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It is likely that the availability of anthropogenic foods could alter the ecological relationships of wolves, both intraspecifically e. Thus, where anthropogenic foods are abundant, the ecological effects of wolves will likely differ from those in systems with low or no human presence or effects figure 2. A hypothetical comparison of gray wolf Canis lupus ecological effects in wilderness areas left and human-modified systems right where there are abundant anthropogenic foods.
For example, if gray wolves focus on eating anthropogenic foods, their main prey e. In support of this hypothesis, a recent review revealed that at least 36 species of terrestrial predator more than 1 kilogram in body size use anthropogenic foods in 34 countries worldwide Newsome et al. In the presence of these resources, there are documented effects on predator abundance, dietary preferences, life-history parameters, and movements, as well as negative effects on co-occurring species that become more susceptible to predation Newsome et al.
To further elaborate on this emerging issue, we highlight several case studies below demonstrating that anthropogenic food availability can modify intra- and interspecific interactions involving predators, and we discuss how these changes could heighten the potential for conflict with humans. These case studies also illustrate the diverse ways that predators adapt to human-modified environments, including being driven to eat anthropogenic foods.
Independent studies on Australian dingoes Canis dingo ; Crowther et al. In the case of dingoes, access to food scraps at a waste facility resulted in decreased home-range areas and movements, larger group sizes, and altered dietary preferences to the extent that they filled a similar dietary niche to domestic dogs Newsome et al. Moreover, the population of subsidized dingoes was a genetically distinct cluster, possibly because of founder effects Newsome et al. Similar responses have been documented for red foxes in Israel: In the presence of anthropogenic foods, foxes had smaller home ranges and much higher mean survival rates than in the absence of these foods Bino et al.
Genetic drift and genetic differentiation have also been found between rural and urban red fox populations in Zurich Switzerland , because of the urban population being founded by a small number of individuals Wandeler et al. Bears also frequently take advantage of anthropogenic foods, and access to these foods influences bear reproductive success, dietary preferences, and several life-history traits Newsome et al.
Interestingly, though, higher age-specific mortality of black bears has been documented in urban areas, where they forage on garbage, in comparison with wildland populations because of elevated human-caused mortalities Beckmann and Lackey Therefore, an increase in anthropogenic food supply may not always result in gains in predator fitness, especially if there is a high probability of conflict with humans akin to an ecological trap. In the Greater Gir Landscape of Western India, the last remaining population of Asiatic lions Panthera leo persica has increased over the last five decades in response to a series of conservation measures Banerjee et al.
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Although ostensibly a conservation success story, this development has been accompanied by an increase in livestock depredation in areas surrounding the Gir Protected Area GPA , with depredation hotspots falling along the movement paths of dispersing lions Meena et al. By implication, the continued recovery of the Gir lion population is likely to spur increased dispersal as individuals seek space outside the crowded GPA, and this could exacerbate the potential for livestock depredation Banerjee et al.
At the same time, intensive use of livestock as prey and frequent contact with humans has apparently led to a level of habituation that enables tourists to make close observations of Asiatic lions without the protection of a vehicle Packer This kind of close contact centered on the use of anthropogenic foods can certainly drive ecotourism. However, it is also reminiscent of the process by which wolves putatively embarked on the path to domestication and, if pervasive, could at least modify the ecological relationships of lions in the Greater Gir ecosystem.
Furthermore, such contact has the potential to promote negative interactions between lions and humans, which might ultimately jeopardize lion conservation. Human impacts on ecosystems continue to expand through the conversion of land to agriculture and for suburban and urban development.
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As this encroachment continues, it is well acknowledged that more species will use anthropogenic foods where available Oro et al. However, less well known is that climate change could exacerbate this phenomenon and even force species to use anthropogenic foods.
For example, in some parts of their range, polar bears Ursus maritimus are spending more time on land as an apparent response to the loss of sea ice that has accompanied climate warming Stirling and Derocher This habitat shift has been linked to modified interactions both among polar bears e. It has also been associated with elevated polar bear—human conflict.
For example, over a three-decade period at Churchill, Manitoba, earlier ice-breakup dates in western Hudson Bay were accompanied by an increase in the number of problem bears Towns et al. Accordingly, and especially given that continued Arctic warming is forecasted, climate-driven conflict between polar bears and humans is likely to increase. Much of this conflict will stem from close bear—human contact and the potential for habituation near areas where anthropogenic subsidies are reliably available. Since the British colonization of the Americas in approximately , the distribution and population size of the gray wolf have been greatly reduced, largely because of direct persecution but also because of habitat loss Ripple et al.
Conversely, coyotes Canis latrans —typically subordinate mesopredators where wolves are extant—are now expanding their distribution and abundance and adapting to and successfully exploiting human-modified ecosystems Newsome and Ripple The reduced access to mates of small and isolated wolf populations likely facilitated mating between wolves and coyotes, particularly in northeastern North America, where there are now distinct hybrid zones vonHoldt et al.
In these hybrid zones, some coyote populations became more wolf-like because of the infusion of genes affecting body size and skeletal proportions vonHoldt et al. Ongoing hybridization is likely where wolves persist at low numbers, but access to anthropogenic resources in human-modified environments could increase the probability of non-aggressive contact between the two species. This scenario is plausible because coyotes survive and thrive in urban and fragmented landscapes Fedriani et al.
The extent to which wolves will actually occupy urban areas is also unclear, because this ultimately depends on human tolerance Treves and Bonacic Studies on mountain lions Puma concolor demonstrate that large carnivores can persist in fragmented urban landscapes, although selection or use of urban areas appears to be highest for females in comparison with males, and mountain lions appear to adopt behaviors that reduce encounters and potential conflicts with humans Benson et al.
How do we predict the outcomes for gray wolves when they become heavily reliant on anthropogenic foods? Several viable hypotheses can be derived from the case studies highlighted above. First, the extensive use of anthropogenic foods by wolves in human-dominated landscapes is likely to result in changes to group sizes, diets, home ranges, and sociality, as has been demonstrated in dingoes, red foxes, and bears. These changes are often accompanied by disrupted ecological relationships e. Accordingly, their documentation would raise questions about a how broadly insights into the role played by wolves gleaned from protected areas such as Yellowstone can be applied in areas that have been grossly modified by humans and b the extent to which wolf recolonization can facilitate ecosystem restoration.
Second, although gray wolves move freely in anthropogenic habitats, the survival rates and causes of mortality of wolves in these systems will likely differ from those in less disturbed settings.
Rates of population increase could maximize with abundant food resources, but in combination with likely increased human—wolf conflicts and therefore greater human persecution of wolf populations, observed rates of increase could become negative, as has occurred in a case study on bears Beckmann and Lackey In support of the latter possibility, a recent study of three wolf populations in the northwest United States showed that although anthropogenic mortality was partly compensatory, it became increasingly pronounced and additive in areas where wolves had the highest exposure to humans and livestock Murray et al.
Therefore, the gradient from wildlands to human-dominated landscapes is likely to be characterized by a shift from predominantly non-human to anthropogenic mortality of wolves. This switch may lead to lower wolf densities, but it should not preclude the emergence of a commensal canid. Third, the continued use of anthropogenic foods by gray wolves could result in evolutionary divergence, as in the case of dingoes Newsome et al.
The commensal process by which wolves appear to have first embarked on the pathway to domestication was likely followed by a period of purposeful and accidental selective breeding for simple beneficial traits, such as tameness Trut et al. More recently, humans have bred dogs for aesthetic values, all of which in turn resulted in the diversity of breeds that exists today.
There is growing agreement, however, that the process of wolf domestication had advanced considerably prior to the onset of directed breeding Larson et al. Therefore, there is no need to invoke artificial selection as a basis for hypothesizing that human-associated wolves are likely to diverge from those subsisting on more natural foods. Rather, as in the Late Pleistocene, when founder groups of less fearful wolves moved toward nomadic encampments to scavenge kills Driscoll et al. Accordingly, if extant wolves continue to increase their reliance on anthropogenic foods, we should expect to observe evidence of dietary niche differentiation and, over time, the development of genetic structure that could signal incipient speciation.
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Niche segregation driven by social or ecological forcing may commonly herald incipient speciation, even at local scales and within populations in which gene flow appears to be unimpeded e. Fourth, natural hybridization can lead to speciation or incorporation of new, often suboptimal genes into wild populations Mallet Anthropogenic habitat modifications and resource subsidies are likely to increase the opportunity for hybridization. However, for many plants how selection exerted by humans interacts with selective forces that have acted on plants throughout their history, and that continue to act upon them in domesticated environments, has not been investigated McKey et al.
Studies of chili Capsicum spp. Capsaicinoids, the compounds that give chili its pungency, protect the fruits against fungal attack. Such selective pressures have given us pungent chilies. Interestingly, humans use capsaicinoids and other plant-defensive compounds found in spices for the same function that they perform in wild Capsicum.
Spices, including chili, have strong antimicrobial properties. The quantities and diversity of spices used in traditional cuisines, and their antimicrobial effects, show latitudinal gradients: spice use is greatest in tropical climates, where cooked food is more rapidly contaminated by bacteria Sherman and Billing Comparing recipes of the same country, more spice is used in meat recipes than in vegetable recipes, because unlike plants, animals lose their defenses immunity at death, so that meat dishes are more rapidly contaminated Sherman and Hash These patterns are repeated all over the world, showing convergent cultural evolution in response to similar selective pressures.
This pattern has never been explored for traditional cuisines of Amazonia. Functional analyses of Amazonian diets could be very instructive.