During the previous three decades, much has been discussed about the ecological consequences of urbanization. Without doubt, our world is becoming increasingly urbanized – and not just in the western world, but increasingly in Latin America and Asia as well. Some scientists emphasize the increasing rates of deforestation caused by greater urbanization, while others highlight accelerating climate change and rising sea levels that could wreak havoc on low-lying areas. However, a novel study conducted by Professor James R. Miller of the University of Illinois at Urbana-Champaign and graduate researchers Jason D. Fischer, Sarah H. Cleeton, and Timothy P. Lyons, explores the effect of urbanization on vertebrate communities. Their research, reported in a September 2012 BioScience article, discusses the ecological effects of urbanization, particularly in Europe and North America.
We naturally tend to assume that the growth of urban centers necessarily leads to the decline of all animals; however, the actual effect of urbanization on biodiversity is surprisingly complex. The largest carnivorous predators, such as black bears and mountain lions, cannot sustain themselves amidst human development. Their absence allows smaller predators – termed mesopredators by Miller et al – to proliferate dramatically in urban areas. Some mesopredators, such as coyotes, foxes, and hawks, are abundant in urban habitats but prefer to minimize contact with humans. Others, including introduced species such as domestic cats and raccoons, increase proportionally with human density. Based on the increased predator proliferation in urban areas, we would naturally assume that prey numbers are diminished. However, the research conducted by Miller et al demonstrates that paradoxically, prey populations actually have greater survival rates in urban areas. In effect, the predator pressure on prey populations is reduced in urban areas, despite an increase in predator populations.
Figure 1: Diagram by Miller et al; This graphic represents the paradoxical relationship between increased predator numbers and decreased predation pressure in urbanized areas.
Professor Miller and his research group conducted a large-scale study of predator-prey relationships to investigate this apparent paradox. Their results suggested five interrelated reasons that could explain this counterintuitive relationship. The first reason, described as “predator subsidy consumption” addresses the large quantities of food we provide to wildlife, intentionally or not. This leads to a shift in predator diets away from vertebrate prey, and thus lowers the predation pressure despite high predator populations. Another reason – one much more straightforward – cited by Miller et al focuses on the hyper-abundance of prey populations in urban habitats. The higher prey survival rates indicate that the prey populations must be increasing faster than the predator populations. A third reason identified by the study addresses predators becoming increasingly specialized on particular prey species, which reduces the predation pressure on other prey populations. Oftentimes, the specialized prey is non-native, and as a result, the native species experience greater survival rates in urban areas. The final two reasons used to explain the predation paradox involve changes in predator and prey compositions, respectively. The predator species in cities are different from those in surrounding rural areas; Miller et al hypothesize that the species in urban areas exert less pressure on prey populations. The prey composition also changes in urban landscapes; most of the species that remain in urban habitats are generally well-adapted to urban predators. On the other hand, those that failed to adapt to urban development probably became locally extinct due to habitat destruction, and not due to predation.
These five reasons explaining the predation paradox are hypotheses, not confirmed rules. Miller et al discuss several limitations to their study and possible ways to address them. The foremost problem is that urbanization is not a linear process from rural to urban, but has several different pathways and different configurations of urban landscapes. In addition, the investigations have not been tested on multiple predator and prey species simultaneously, and such studies are absolutely necessary to identify a causal relationship between urbanization and predation pressure. The researchers believe that each rationale must be independently verified by testing their effects simultaneously on predator and prey, in order to determine whether the model adequately explains the paradox. Miller et al also address the advanced in new DNA technology, termed “DNA barcoding”, that allows predator-prey relationships to be monitored over a longer duration. Ultimately, while their research highlights five hypotheses that may explain the paradox between increasing predator populations and decreased predation pressure, more comprehensive studies are needed to fully validate each hypothesis.
By: Prateek Bhattacharya
Fischer, J. D., Cleeton, S. H., Lyons, T. P., Miller, J. R. (2012), Urbanization and the Predation Paradox: The Role of Trophic Dynamics in Structuring Vertebrate Communities, BioScience, 62: 809-818.