**1. Introduction**

Today, in an increasingly urbanized world, scientists, conservationists, and politicians agree that understanding the patterns that explain the biodiversity of cities and conserving this biodiversity along with its ecological functions is a priority within urban planning and nature conservation [1–4]. Typically, the growth of cities has been linked to biodiversity loss [5–7]. However, recent studies have demonstrated that cities are exploited by more species than previously thought (sometimes reaching higher population densities than in their original habitats [2,8]), and are even capable of hosting endangered species [9–12]. Factors related to cities that influence species composition and their exploitation of this novel ecosystem include urban structure and development [6,13,14], different types of pollution found in cities (e.g., noise [15], artificial light [16], chemical contamination [17]), and the existence of refuge and food sources [18]. For vertebrates such as birds, urbanization acts as a filter, but a combination of traits, including phenotypic and behavioral flexibility, dispersal strategies, and niche flexibility, allow a certain number of species to exploit cities [19–22]. The same can be attributed to the other most common vertebrates in cities, mammals, as certain reproduction-related traits and their behavioral flexibility favor the successful use of cities for a limited number of taxa [23,24]. Other factors influencing urban species composition include diet-related aspects, as species that thrive in cities usually

**Citation:** Luna, Á.; Romero-Vidal, P.; Arrondo, E. Predation and Scavenging in the City: A Review of Spatio-Temporal Trends in Research. *Diversity* **2021**, *13*, 46. https://doi. org/10.3390/d13020046

Academic Editor: Luc Legal Received: 22 October 2020 Accepted: 28 December 2020 Published: 25 January 2021

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can feed on a wide variety of foods, which sometimes are directly or indirectly related to humans [19,25]. Meat is an abundant and heterogeneously distributed food resource present in urban and peri-urban areas [22,26–28]. It appears in the form of both domestic and wild vertebrates, and also as carrion (i.e., roadkill and anthropogenic refuse [29,30]). This mostly occurs in cities with inefficient waste treatment, but also in landfills and ports located in typical peri-urban areas [26–28]. The diet of several urban carnivores has been assessed, and as such we have gained an appreciation both for the items they consume and the prevalence of the different sources of food. Thus, different prey form part of the urban diet of raptors, mammals, and other carnivores [31,32]. Moreover, a growing amount of research reports the progressive inclusion of human refuse in the diet of urban animals in the form of organic and inorganic garbage [33–35]. In general, urban carnivores range from obligate carnivores, such as the barn owls (*Tyto alba*) [31], to generalist omnivores, such as the red fox (*Vulpes vulpes*) [36] and corvids [37], which eat meat, vegetables, fruits, and berries, with different preponderance of each food source depending on the species. Therefore, urban carnivorous species can consume meat (i.e., vertebrate biomass) in two different ways: by predation (actively hunting their prey) [32,38,39] or by scavenging (eating the remains of already dead animals or garbage) [27,40]. Between these two extremes, several species act as facultative scavengers, consuming both live prey and carrion or human refuse, such as the coyotes (*Canis latrans*) [41] and gulls [42].

The feeding behavior observed by carnivores in cities contributes to the structure of natural urban communities, for example, by limiting the population size of their prey [43]. In some cases, they can provide benefits to humans, especially when they prey upon certain synanthropic species (e.g., feral pigeons [44] or rodents [31]). An example is found in Indian cities, where the consumption of stray dogs by leopards is expected to reduce disease transmission [45]. However, the constant availability of refuse can also alter the diet preferences of carnivores in anthropized areas, affecting the trophic dynamics and population densities of urban carnivores and their potential prey [29,30,46–48]. Regarding scavengers, they play additional and concrete roles in cities, by contributing to accelerating the process of carrion decomposition [49,50]. Moreover, the consumption of subsided food and the high abundance of prey in cities can have a profound effect at the individual level, influencing the growth rate, body condition, and survival of carnivores. In addition, there are also obvious negative consequences, for example, when feeding on resources with low nutritional value or indigestible waste [30,51]. Furthermore, carnivores in cities are also confronted with new threats, common to the urban environment, such as an increased probability of collision with buildings or vehicles [27,52,53]. Attending to this, cities may become ecological traps for certain species [54–56]. However, although mostly unstudied, urban exploitation by carnivores may play a potential role in their conservation, as observed for some scavengers feeding mostly on human subsided food [57,58].

In recent years, different reviews have analyzed the knowledge generated within urban ecology, and have proposed how this discipline can advance along multiple research directions [59,60]. Focusing on urban biodiversity studies, McPhearson et al. [61], established the acquisition of a better understanding of how the ecological community structure, including invasive species, affects ecosystem dynamics, structure, and function in cities as a future priority. Moreover, Magle et al. [60] revealed how urban ecology research is biased towards birds and mammals, is generally focused on behavior and conservation issues, and is carried out to a lesser extent in less developed countries. Apparently, studies focused on the diet of carnivores, especially scavengers, seem to have been of little priority, despite the importance they may have for understanding urban wildlife communities, the ecosystem services they provide, and the potential conservation implications.

In this study, we aim to review the scientific literature on urban carnivores, distinguishing between the predatory and scavenging function of this ecological group, to offer an integrative first approach about the spatio-temporal trends observed in the scientific literature focused on this topic. Concerning the information contained in the articles, we also explored in which locations within the urban matrix the studies were conducted (the

interior of the city vs. the peri-urban areas), and, in the case of scavengers, we analyzed if the studies were carried out based on natural conditions or developing experiments with carrion provided by the researchers. Moreover, we explored the diversity of carnivores reported in the reviewed articles, together with the primary physical and behavioral traits that may contribute to explaining their presence in the reviewed studies. Considering that scavenger impact on ecology has been little studied until recent decades [62], we hypothesize that most of the published research concerning carnivores' diet and presence in cities has been devoted more to predators than to scavengers. In addition, following the suggestions from Magle et al. [60] about the current bias in urban ecology research on developed countries, we try to confirm that this trend also occurs in the case of carnivorous studies in cities. Additionally, we consider that the carnivores and scavenger richness obtained in our review would be higher, with many species recorded in studies from Africa, Latin America and Asia. To test this hypothesis, the greater species richness of tropical regions in which most of the developing countries are located [63,64] must be considered, as this logically leads to greater chances of more species potentially exploiting cities. Moreover, the poorer waste management of developing countries offers greater possibilities for feeding for scavengers in the form of all types of refuse, contributing to the presence of different scavengers. Lastly, attending to the characteristics of the vertebrates that most commonly exploit cities, we predict that the most represented predators and scavengers will be synanthropic species, mainly mammals and birds, with nocturnal habits that would permit these species to avoid potential conflict with humans.
