Rather than viewing the city as an artificial landscape completely under human control and the rat as a foreign invader of our territory, we need to see the rat for what it is - a wild animal that shares the urban ecosystem with us. Although cities are not often thought of as ecosystems, adoption of an ecosystem-based perspective may provide new insights and opportunities for dealing with rats and rat-related issues.

Urban ecosystems are often conceptualized as complex adaptive systems (CAS) (Desouza and Flanery, 2013; Meerow et ah, 2016). A CAS is a set of component parts that interact to form a system that has emergent properties unique from the components in isolation, and that has the capacity to change over time (see Chapter 13 on complexity and One Health). Each component may, itself, be complex (Levin, 1998; Meerow et ah, 2016). Indeed, the urban ecosystem has been described as CAS composed of intertwined socio-economic, infrastructural, material and energy, and governmental networks (Meerow et ah, 2016). It would be beneficial to supplement this characterization with an additional level that includes biological networks (Figure 21.1) inclusive of all life forms existing in a city - human, wildlife, microbiota, etc.

Once we recognize that we are managing a system, it becomes clear that leadership and strategic planning are critical (Ernston et ah, 2010). The very concept of a system is that the whole is more than the sum of its parts, which is antithetical to our current reductionist approach to rats. It also follows that if we are going to manage a system, then we must understand it. This, in turn, provides justification for substantive, long-term investments in collecting data on rat infestations, the specific conditions that support them, and the issues that they may cause (Parsons et ah, 2018).

The CAS approach can be used to provide solutions that better address those issues. Rat control interventions are often undertaken with the goal of reducing rat-associated public health risks - particularly the risk of pathogen transmission from rats to people. These interventions usually focus on trapping or poisoning based on the perspective that, regardless of the nature of the problem, reducing

A conceptual model of an urban ecosystem based on the framework proposed by Meerow et al., 2016

FIGURE 21.1 A conceptual model of an urban ecosystem based on the framework proposed by Meerow et al., 2016.

the number of rats will somehow make it better. However, it has been shown that, for certain rat-related diseases, rats are more likely to carry zoonotic pathogens after the intervention compared to before (Lee et ah, 2018). This surprising finding is predictable once rats are considered in the broader context of their own ecology, the ecology of the pathogens they carry, and their interactions with the urban environment. Specifically, urban rats have evolved to form tight family units with limited home ranges owing to a resource-rich environment and barriers to movement (e.g. rats avoid crossing roads) (Davis, 1953). For pathogens that are transmitted among rats through specific social interactions within the colony, removal of key family members through trapping and poisoning can result in social strife and increased opportunities for disease transmission as rats fight to restore social order (Lee et ah, 2018).

Revisiting the rat control programme described earlier with a CAS approach would require us to understand how rats interact with each other, how they interact with the microbiota they carry, and how both interact with urban infrastructure and resources. Using this perspective, rather than killing rats, one might seek to alter the environment in a manner that decreases the number of rats it can support (e.g. through altering waste management, repairing aging infrastructure), so there is a gradual decline in total rat population while maintaining social order (Colvin and Jackson, 1999).

The rats, however, are not the only complex beings in the urban ecosystem. Just as the impact of humans on rats can be multifaceted and unpredictable, so too can the impact of rats on humans. Rats are usually considered to be a health risk to people only if they are found to be a source of infectious disease. However, it is increasingly being recognized that chronic exposure to rats can have significant negative mental health impacts, particularly among impoverished, inner city residents (Wundram and Ruback, 1986; Lam et al., 2018; Byers et ah, 2019). It has been shown that rats can elicit fear, anger, stress, worry, and exhaustion, as well as sleep disturbances and avoidance of activities (e.g., urban gardening) that could lead to rat contact (Byers et ah, 2019). A CAS lens further enriches this understanding by identifying the fact that rat infestations are just one component of a constellation of negative environmental factors afflicting impoverished city dwellers; rats, in other words, are inextricably intertwined with a variety of social justice issues (Wundram and Ruback, 1986; Lam et ah, 2018; Byers et ah, 2019). For example, being forced to live with rats can be seen as a symptom or symbol of an individual’s lack of control over their lives and living conditions, which is compounded by the perceived neglect of those who may have the power to improve these conditions (e.g. municipal governments and slum landlords) (Byers et ah, 2019). A more sophisticated understanding of the potential consequences of rat-human contact not only brings to light a significant and widespread public health risk that has hitherto been completely ignored, but it also highlights the importance of understanding the interface between rats and people.

Further to the concept of interfaces, a CAS lens would also find fault with our current approach to rats, which is largely focused on “things” - the rat, the human, the environment, etc. By contrast, a key aspect of systems thinking is the understanding that the relationships between the components of a system often contribute more to the function of the system than the components themselves (Levin, 1998; Meerow et al., 2016). Thus, a CAS lens would have us direct the majority of our time and energy to identifying, understanding, and ultimately modifying these relationships or interfaces. For example, a safe, healthy, and controlled physical home environment is a multidimensional component of an individual’s mental and physical health (Cornwell, 2014; Pruss-Ustun et ah, 2017). Rats have a propensity for invading this environment and can be extremely difficult to get rid of. Thus, the home environment is the place where the relationship between rats and people is least tenable (Wundram and Ruback, 1986; Himsworth et ah, 2013, Lam et ah, 2018). In our current rat management paradigm, private residences are often the areas most ignored by municipal powers; however, a CAS lens would show us that they are an area of particular vulnerability and should be prioritized for municipal intervention and assistance. Similarly, since factors associated with urban poverty promote rat infestations and rat-human conflict (Clinton, 1969; Wundram and Ruback, 1986; Himsworth et ah, 2013), impoverished inner city areas should also receive special attention.

Finally, the CAS approach puts some completely novel tools in our toolbox for dealing with urban rats. Instead of focusing on responding to problems as they occur, the CAS lens introduces the concept of resilience (Desouza and Flanery, 2013; Ernston et ah, 2010; Meerow et ah, 2016). Resilience, or the ability of a system and the members within in it, to function in the face of disturbances (Desouza and Flanery, 2013; Meerow et ah, 2016), is a helpful concept, because it shifts the perspective from one that is reactionary and myopic (i.e. focused on a single issue) to one in which we can predict, prepare for, or even prevent a multitude of challenges. One example of resilience-based rat control would be a comprehensive waste management programme that prevents rats from accessing garbage (a common food source), thereby reducing the capacity of the urban environment to sustain rat infestations (Colvin and Jackson, 1999). Another would be the development and enforcement of more stringent bylaws that enshrine the right of low-income tenants to live in a healthy and rat-free environment. This latter strategy is particularly pertinent as a disordered living environment is a central contributor to a number of physical and psychological ailments (Cornwell, 2014; Pruss-Ustun, 2017), so this one intervention could increase resilience to diverse array of insults. All-in-all, then, a focus on resilience introduces many new opportunities and approaches to managing the urban rat problem, very few of which have anything to do with killing rats or even with the rats themselves.

It is of note that the concept of resilience has many parallels with the public health concept of harm reduction (see Chapter 6 for details on harm reduction and Chapter 19 for an example of its use in practice). Harm reduction is mainly used to deal with the impact of drug use, and it refers to policies, programmes, and practices that aim to minimize negative health, social, and legal impacts. Similar to interventions rooted in resilience theory, those based on harm reduction principles may have a broad range of predicted and unforeseen benefits. For example, in Vancouver, Canada, supervised injection sites are an effective harm reduction tool that produce a range of health and community benefits, including reductions in overdose deaths, public drug use, and syringe sharing, as well as increased uptake of detoxification services and other community and medical resources (Wood et al., 2006; Marshall et ah, 2011). Given that a major risk factor for rat exposure among impoverished inner city residents is engaging in outdoor injection drug use (McVea et ah, 2018), these injection sites may also help to reduce rat exposure and associated harms.


One of the challenges of a CAS approach is that it is so complex that it may be difficult for municipal leaders and decision-makers to operationalize. There are, however, several different perspectives that could be employed to overcome this obstacle.

For example, an understanding of the urban ecosystem and its interconnected networks can be used as a scaffolding on which to develop more comprehensive, efficient, and effective rat monitoring and mitigation programmes (Figure 21.1). Governance networks must be understood in order to identify stakeholder concerns and objectives (i.e. why do stakeholders care about rats and what do they want done about them), as well as jurisdictions, resources, and tools for intervention (i.e. the area or scenario being governed, availability of funding, expertise, and other supports; and methods to deploy, enforce, monitor, and maintain the intervention). Socio-cultural and socio-economic networks will need to be understood in order to establish priorities (i.e. who is most at risk and/or most likely to benefit from an intervention) and to develop programmes with the greatest chance for success (i.e. what are potential barriers and opportunities for acceptance of and compliance with the intervention). Urban infrastructure and form networks must be understood to identify factors that sustain infestations and to ensure that rodent management is considered during urban planning and management of the built environment and infrastructure.

Ultimately, though, the benefit of a systems approach lies in its ability to change our perspective on rats and rat-related issues. Therefore, it may be more helpful to shift the conversation away from developing a single set of policies or procedures that best embody the CAS approach, to using CAS as a way of thinking, or a lens through which a problem can be viewed (Meerow et al., 2016). This will allow us to reap the benefits of this thinking regardless of the scale, scope, or nature of the programme in question.


Seeing through the lens of a CAS requires a paradigm shift in our way of viewing our world and ourselves in it. It requires that we focus on the forest rather than the trees; that we become aware of relationships rather than entities. Our goal ought to be that we work to minimize friction that is potentially imbedded in the infinite interaction zones within a CAS. Such a viewpoint shifts the emphasis away from winners and losers, or oppressors versus the oppressed, to one of creating resilience, so that members of an ecosystem may peacefully exist alongside one another.

Ultimately, adopting this lens may help humans acquire the kind of humility that is necessary to truly understand that we cannot control the world - even one as seemingly under our control as a city. However, the “adaptive” component of the CAS approach teaches us that through innovation, leadership, thoughtful action, and strategic planning, we can “trigger a transition of the system to a more preferable regime” (Ernston et al., 2010). We are all, and always will be, members of a CAS, and as such, human well-being depends on our stewardship of that system. With regard to rats, it will likely never be the case that rats are welcome into our homes; however, it is possible for us to live with them in a mutually healthy arrangement.


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