Tag: Evolutionary Ecology of Urban Form

“Complex causal structures of neighbourhood change” is published!

One key way that evolutionary processes occur is via feedback loops. A classic way to model such feedback loops is in functional terms. Arthur Stinchcombe articulated the elemental structure of functional explanations in his 1968 book, Constructing Social Theories. In our recently published article, “Complex causal structures of neighbourhood change,” we try to revive this model and demonstrate its value for studying the evolution of cities.

The above figures shows Stinchcombe’s model on the left, and our reformulation of the model for neighbourhood evolution. It codified the causal structure of a complete functional explanation in terms of four core elements:

  1. The consequence that tends to be maintained, which also functions indirectly as a cause of the behaviour or social arrangement to be explained. This is H, the “Homeostatic” variable. Though H may tend to be stable empirically, its stability is maintained against pressures to change it, such as in the case of body temperature.
  2. The social arrangement or behaviour that impacts H, the explanandum. This is S, the “Structure.” In a functional model, Structures tend to maintain Homeostasis. For example, sweat glands tend to maintain body temperature.
  3. Tensions that tend to upset Homeostasis, unless Structures maintain it. This is T, the “tension” variable. If physical activity or air temperature did not alter body temperature, there would likely be no structure to counteract the tensions they create.
  4. Processes that reinforce or select for the S’s (structures) that maintain H (homeostasis). When H is threatened or pressured, these forces increase the activity of S when T (tensions) are higher and decrease when H is maintained. For example, sweat glands generate more sweat (S) when body temperature (H) is not maintained at normal levels due to a certain phenomenon (T). Since this structure helps to maintain H in equilibrium, it will tend to be selected or reinforced.

Stinchcombe’s diagram may be intuitively mapped onto familiar neighbourhood dynamics. For example, we may treat as Homeostatic (H) variables neighbourhood character, style, or scene (such as distinctive shops, restaurants, venues, or groups), Tension (T) variables as pressures to change that character (from, for example, new groups with divergent tastes), and Structure (S) variables as activities that maintain that character (such as Business Improvement Association sponsored festivals, political advocacy, or increased participation in venues and activities distinctive to that scene).

Based on this simple representation, we formulate an initial set of propositions regarding the presence and strength of 1) a functional relationship and 2) a homeostatic response, which can be seen in the paper in more detail.

The key value of such models from the point of considering urban evolution is that treat both persistence and change as a dynamic process. Urban forms of life are retained when there exist structures that preserve them when new challenges. If such structures respond effectively to tensions, there is a tendency for them to be selected and reinforced over time, generating both a pattern of structural retention and possible evolutionary histories of such structures. This idea is scarred further in Part III of “Towards a Model of Urban Evolution,” in our discussion of “retention hypotheses.”

Using data drawn from Yelp.com, we find considerable evidence that the sort of functional process envisaged in the model is a common feature of urban evolution. And in the process we develop novel methods for using data from Yelp and similar sources for such analyses.

We see great potential for using these models and methods for characterizing neighbourhoods in new ways. In contrast to the typical approach, which does so primarily by their demographics or built form, our proposed functionalist approach would identify neighbourhoods with more or less latent potential to resist tensions. In this way, neighbourhoods that look otherwise similar could be shown to have very different probabilities of maintaining their identity over time, thereby allowing planners and policymakers to take these latent functional capacities into account.

While incorporating novel data sources and methods would, to some extent, be challenging, doing so would be in line with parallel proposals. Indeed, local jurisdictions routinely use big data in multiple ways: traffic demand management (using GPS and sensor data), land use (using remotely sensed data), public health (COVID sewage testing), commercial health (using payments data), and more. Our methods could be used in a similar way to monitor tendencies toward neighbourhood change.

From the point of view of social science research more generally, perhaps the biggest result of our study is the possibility of reviving interest in functional explanation. While functional explanation has been characterized as “what any science does,” it has largely fallen out of favour in social science. We review common criticisms, and show that they do not apply to a properly specific functional model of the sort we propose.

At the same time, we find considerable evidence that functionalist motifs are commonplace in neighborhood change research. Researchers typically appeal to functionalist motifs when they discuss for example the capacity of local groups to push back against tensions or challenges as a key mechanism producing continuity or change.  However, we found no examples in the neighbourhood change literature where an author who utilized a functionalist motif articulated the motif in an explanatory model that would render it testable. Instead, much neighbourhood change research remains largely descriptive, mapping types and directions of change across a range of variables.

We hope one result of our study is to illustrate a path for remedying this situation, which in turn would help to more formally incorporate evolutionary thinking into urban research.

new paper published! A Markov model of urban evolution: Neighbourhood change as a complex process

Generative models of urban form

Continuing themes discussed in an earlier post, this paper develops a Markov model of urban evolution, using Toronto as a case study. The paper is available here (free and open access!).

It builds out the implications of some common experiences. We’ve all probably noticed that some parts of cities change fast and others look pretty much the same today like they did 20 years ago. And where things are changing, it isn’t random: a dense retail district isn’t going to change any time soon into a suburban bedroom community, or vice versa.

These are simple obervations, but following through their implications leads to some interesting insights.

1. The variability of structure. If we think of “structure” as the degree to which current conditions tend to reproduce themselves, this means that structure varies. Some places have deeply entrenched structures that are very hard to alter; others are volatile.

2. The temporality of space and the spatiality of time. Time doesn’t flow at the same pace from one place to another; part of what makes a place what it is involves how fast it is changing. Where it is coming from and where it is going is part of what it is here and now.

3. The contextuality of change. Changes of one type or another operate within a broader order that limits their likelihood of occurring elsewhere. Within certain boundaries, changes of one type to another may be very common, whereas on the other side of this horizon they can be quite rare. This means that even if change is common within these boundaries, the overall pattern of an urban system can persist.

4. Non-linear thresholds. Just as the butterfly flapping its wings far away can lead to big changes elsewhere in a complex system, small quantitative changes at key points in a complex urban order can produce large and indirect changes elsewhere.

Here are some key paragraphs that speak to these observations:

While the city exhibits a high degree of continuity, its degree of structure varies. Rather than assume there is an equally powerful structure at work throughout, an important question concerns how much of a given urban environment is structured at all, and to what degree. We find that much of Toronto is very deeply structured, so that it is highly likely to reproduce itself. In fact, some of the most “creative” parts of the city in terms of who is there and what they are doing—areas in which young, highly educated arts and technology workers predominant—are the most stable. By contrast, other parts of the city exhibit creativity where the urban fabric itself is in a state of transition in which neighbourhood forms themselves rise and fall more rapidly. There are, therefore, at least two types of urban creativity at work here. One appears to thrive within a stable urban context that supports a specific set of groups and activities; in the other, the urban form itself is in a more fluid state of experiment and transformation….

The non-counterfactual results (0 change, the darkest green dot) in Fig 6 show how the city would evolve if it continued forward according to its current trajectory. The dominant trend would be increased socioeconomic polarization: for example, elite suburban neighbourhoods would increase their share of the overall population of neighbourhoods by around 7.5 percentage points (from 8 to 15.5%), and “young urban professional” and “established creative” areas by around 5 percentage points (from 11 and 7 percent, respectively). All together, these three upper status areas would grow from around 26 to 35% of the city as a whole. At the same time, middle income diverse suburban neighbourhoods would decline (by about 5 percentage points), along with most of the city’s ethnic working and service class communities, as well as its “mixed creative” neighbourhoods. If unchecked, current trends point toward a solidification of the “divided city” [70]….

Following ideas from complexity theories of cities, we explore the extent to which small initial changes, when repeatedly iterated, can lead to relatively large and sometimes unexpected changes, both direct and indirect. Specifically, we examine different scenarios representing changes λ starting from 1% (λ = 0.01) with 1% increments up to 25% (upper-bound for a valid Markov chain in the interventions considered), to the probability that three neighborhood types—UP = {“black predominant”,“mixed suburban”, “mixed creative”}—would appear in three entrenched neighborhood types—DOWN = {“elite suburban”, “established creative”, “young urban professionals This imagined intervention represents a strategic planning decision to promote interchange among parts of the city that rarely interact and to induce change in some of the city’s most entrenched upper status areas (where reproduction rates are near or above .9). Indeed, transitions between these neighbourhood types are exceedingly rare: all are below 3% and most are near 0. Comparing these scenarios allows us to investigate threshold effects….

Three key points stand out in examining the counterfactual scenarios in Fig 6. First, in line with complexity theories, small initial changes can have big effects. In both scenarios, the growth of “young urban professional,” “elite suburban,” and “established creative” areas is substantially reduced. By contrast, the decline in the city’s occupationally and ethnically diverse areas is reduced or stabilized in “mixed creative,” “chinese predominant,” “portuguese predominant,” and “south asian predominant” areas. In some cases, such as predominantly black neighbourhoods, the trend reverses to net growth. Second, we see some signs of non-linear thresholds, again in line with complexity theories of cities. The incremental change from a .01 change in transition probabilities to a .02 change in transition probabilities generates relatively sharp downstream effects, most strikingly in the case of “young urban professional,” “elite suburban,” and “black predominant” neighbourhood types. However, the effects are non-linear and diminish at higher levels. For example, there is very little difference in the effect of a change from 12% vs. 13%. This non-linearity makes sense in the context of these specific scenarios: we are altering transitions that in the non-counterfactual scenario are very rare. Therefore, lower values (e.g. 1% or 2%) represent the initial introduction of a process that rarely occurred previously. As values increase, the process is in place, and additions do not change the situation as much beyond a certain threshold. The bunching in Fig 6 at higher values shows us approximately where this threshold is for the scenarios in this experiment. And third, we see evidence of indirect effects characteristic of complex systems. While we did not make any change to the transition probabilities for “south asian predominant” or “tower” neighbourhoods, their relative footprint in the city grew compared to the non-counterfactual scenario.

All in all, these results show that in a complex dynamic interacting system, small quantitative changes at critical points can potentially make a substantial qualitative difference. Connecting disconnected and divided upper status areas with lower status areas reduces the isolation of these parts of the city, and helps others to retain their foothold. This, in turn, reveals another sign of a complex system: changes in one part reverberate in others.

Public/Private Thresholds


Population formetics and the circulation of urban forms

This research project looks deeper into the evolution of threshold spaces in the built environment to help us understand the forces that contribute to their adaptation and reproduction in cities. Fueled by the current resurgence in both cultural value and range of activities afforded by them, these spaces reflect shifts in how the built environment enhances or diminishes levels of human interaction. When thinking of public/private thresholds, the word “porch” might come to mind first. Indeed, where privacy is thought of as the domestic, literature on these liminal spaces is dominated by the typology of the porch. Still, people are describing time spent not only on their porches, but on their verandas, galleries, and balconies. Perry (1985) argues that the porch extends the sphere of control from the house into the public arena, while at the same time bounding the public space. More than the mere boundary between public and private, this project’s goal is to uncover the spatial and social differences in the diverse typologies through their evolution. As a work-in-progress, this post outlines some preliminary findings from the project and opens the door to feedback, advice, or any questions sparked by the content.

In Porches of North America (2012) by Thomas Durant Visser, an important resource for this research, the author defines the porch in “the broad contemporary meaning of an identifiable building feature that is open on at least one side or serves as a covered entry and is large enough to shelter at least one person” (p. vii). However, this broad description has also been used for verandas, galleries and piazzas. There isn’t a clear consensus among existing literature on what the exact differences are, as the terms often bleed into each other. These terms have historically been used interchangeably depending on geographic location or social status, rather than describing a morphological difference. For example, “gallery” was most often used in gulf states and French settled regions of Canada (Visser, 2012; Kahn & Meagher, 1990). Further, some scholars claim that the terms gallery, veranda, and piazza were only used to signal a higher social status to the porch, rather than pointing to a spatial difference (Kahn & Meagher, 1990).

Domestic and Non-Domestic Threshold Typologies
Domestic Additions and Special Use Typologies

As a starting point in detangling the evolution of these terms and typologies, Appendix A classifies these threshold spaces by distilling them into a short description based on their historical use or origin and schematic plan drawing, largely based on Davida Rochlin’s 1976 thesis on the American porch. This series focuses solely on front-facing thresholds, excluding those that faced inner courtyards (such as loggias) or away from the public street, such as back decks and terraces. Rather than organizing by time, the study separates domestic from other non-domestic typologies, as research shows that these liminal spaces had their origins outside of the home. For instance, the Hourd, a medieval scaffolding device used for battle, is considered a potential precursor to the cantilevered balcony. The domestic typologies can also be further separated into spaces created through addition, such as enclosed porches or cloth awnings. Notably absent in this first series is the word “porch” by itself, due to the difficulty in distilling into one defined drawing or definition as previously described above.

Appendix B is a timeline diagram that studies when the previously noted typologies emerged, when they rose in popularity, and potential lineages between them. It is notable how their prevalence was mostly concentrated between the early 19th century to early 20th century. Where there are American and non-American typologies directly following each other, there is a suggested lineage, such as the French galleries and the American gallery. In addition, it is evident in this diagram how little is known of the potential lineage from indigenous and afro-Caribbean typologies. While most pattern books refer solely to European precedents (Downing, 1852) some southern American scholars claim that the front porch was imported through European settlers of Caribbean islands, due to climatic similarities (Perry, 1985; Donlon, 1996).

Typology and Styles Timeline

Along the top of the diagram are important events that marked a change in use or prominence; here we see the effect of the introduction of pattern books resulting in general diffusion of the form, but also that of war and the introduction of new technologies such as the automobile and air conditioning, resulting in an increase in privacy around the 1940’s (Visser, 2012; Wilson-Doenges, 2001). Enclosed and screened porches increased in popularity; layers added on to make them more of a secluded transition space rather than outdoor living spaces. However, even the “transition” quality of them is questionable. Perry (1985) makes a claim that glassed-in enclosures erase the quasi-public nature of the threshold, rather extending the private sphere of the house. By 1990, car garages were a widespread feature of most new houses being built, taking over a large part of the front facade where porches once were. In this way, the garage became the main access to public space, effectively disrupting the threshold at the porch where these worlds overlap. People retreated to the back deck (or more recently, the internet and social media) for social gatherings, preferring a life separated from the noisy and dirty street.

Appendix B also started classifying popular housing styles that were recognized by their porch as a defining feature. In North America, we tend to associate Victorian houses with large threshold spaces, for example. Here we can see a “call out” of four style groups, expanded on in Appendix C, which starts to match popular styles with typologies or words used in association with that style (whether through drawings or written word). The styles were defined by these typologies, but not vice versa. For example, style group 2 encompasses a wide range of terms, despite its short time period. In Appendix C, we see this group is associated with the Queen Anne style. Considering the sense of security and comfort associated with threshold spaces during this period (Visser, 2012),  the variety of typologies employed in Queen Anne homes reinforces a linkage between the accumulation of terms and their affordance of opportunities for social engagement.

Mapping connections between popular housing styles and typologies

Though most of existing literature claims an approximately 100 year time period when porches were most popular, we are seeing a comeback in the past few months due to the COVID-19 pandemic lockdown. The term ”porch sitting” was popular in both that time period and today, as it is now considered one of the safest ways to connect with the public. One of the findings from Appendix B is that an increase in popularity corresponds with the increased use of these spaces as community network building, rather than for climate control. On a larger scale, the affordances fluctuate between more social activities and storage/climate control, as seen at the bottom of the diagram. Appendix D looks deeper into this accumulation of affordances throughout time. Further research is needed to describe the discarding of affordances, as the diagram shows a time when these activities were popular but not whether they persisted. It is interesting to note the evolution of these spaces as mediators between sacred spaces to mediators between public/private spaces – has privacy become sacred?

Accumulation of Affordances over Time

The two last studies, Appendix E and F, look into house plans found in popular pattern books and kit homes, mostly during the time they were most popular up to the 1970’s when the back deck really took over and words like “concrete slab” started replacing the front porch. Click here and here to look closer at the plan analysis. The typologies are highlighted in these plans and color coded for an approximate comparison of size and location. As a general trend in the mid-19th century, verandas were larger and more rectangular in shape, at least 12’ deep and 14’ long. Porches were also included in the plan, but these tended to be smaller areas right where the door was, making them more square in shape and generally not wider than 8’. These findings are consistent with some descriptions found in literature where the authors attempt to clarify subtle differences in typologies (Visser, 2012). In the early 1900’s, porches became larger, taking the place of verandas. This corresponds to the rise of the “leisure class” (Kahn & Meagher, 1990) and the advent of electrical lighting, allowing verandas, or porches, to become deeper. By the end of WWII, these spaces diminished in size, if included in house plans at all. An important finding from this study is the gradual consolidation around the word “porch”, as represented by pink in the diagram. This is best observed in Appendix F, showing a “figure ground” drawing series of the plans analyzed.

The size of thresholds have a linear relationship with the accumulation of affordances. Smaller porch designs from factors described above resulted in less activities taking place on them, thereby reducing their importance in people’s conception of the home, contributing to its continuing decline. This is consistent with Wilson-Doenges’ (2001) research of factors that increase or decrease front porch use in a post 1970’s neighborhood in the United States. This study found that other than pull factors that lead to lifestyles no longer supporting front porch use, small “cartoon” porches where activities are limited is a push factor that reduces porch use.

Popular Plans from Pattern Books and Kit Homes Analysis

It is important to note the limitations and biases in the studies performed for this project, notably in the timeline and plan study (Appendix B and E). For instance, the plan study does not consider regional preferences for the terms, as these plans might have been published in parts of the continent where they speak differently. Further, it is mostly concentrated on those published in the United States, marking a bias away from Canada. Frequency of terms or typologies found in these plans are not reflected, as porches were included less often after 1920. However, rather than it being an exhaustive survey of the frequency of a certain typology or exact timeline of its existence, these studies suggest both a general shift in the size and use of them, and a general resurgence of it with respect to the gaining/loss of a certain affordance. Moreover, it suggests a direction of research we could take with machine learning.

Moving forward in this project, in addition to refining and continuing the studies performed, there are a few questions and possible avenues to explore:

  • What exactly caused the word “porch” to absorb the wide range of terms and typologies previously used for front facing thresholds?
  • Further research into balconies as an important threshold typology. What does it mean when the boundary is not physically accessible to the street, while still visually accessible? This is especially important today as it has seen an increase in affordances attributed to these spaces (food basket delivery in Italy, concerts, exercise).
  • Does the linear relationship between affordances and size still hold true today? Is the dimension of threshold spaces more a matter of “social distancing”?
  • In the study of the plans, one observation was that the “word” porch was also used over the years to describe the side and back outside features. Why did that stop being used, and why did the “back porch” change to “back deck”? Is it simply a manner of taking off the roof?
  • Further research into the evolution of a specific typology through a derailed study of how they were represented in plans over the years.
  • Further research into using Google street map view. A preliminary study was done on this, finding it hard to observe a difference over the years shown other than updating the style of the porch. It seems most of the major changes to front facing thresholds occurred before 2007, when GSM was not yet available in Toronto.
  • “Push” and “pull” factors (such as Wilson-Doenges 2001 work) from porch use during the Covid-19 pandemic.
  • Looking in more detail at a specific lineage as suggested in Appendix B. What are the formal linkages between the typologies, and how did that affect the affordances available through time?
  • Translate main findings from this report into UGP’s formal evolutionary model

Exploring LA’s Restaurant Niches

LA’s Mexican and Chinese Restaurant Niches

A niche, as Popielarz and Neal say in their 2007 article is a set of environmental conditions under which a species thrives. As part of the Urban Genome Project, we are searching for the characteristics of ‘niches’ that allow for urban forms to reproduce, and how these characteristics may have changed over time. Here, we demonstrate our approach by applying the niche concept to Los Angeles’ restaurant scene. Stay tuned for more detailed studies. 

These maps show the neighborhoods with and without Mexcian and Chinese Restaurants. Red-shaded zip codes have restaurants of a certain type, dark ones do not. Given the abundance of these restaurants forms in LA as a whole, it’s reasonable to conclude that most black-shaded areas do not have the conditions required to sustain a certain restaurant species. 

Data for the maps comes from recent Yellow Pages directories, generously shared with us by the “friendly cities lab”. It may not be fully representative of all restaurants but there’s no reason to believe that it is biased.

According to this data:

  • Nearly half of all zip codes with restaurants have both Chinese and Mexican restaurants, 19% just have Mexican restaurants and  17% just have Chinese.
  • There is only a loose correlation betweeen the number of Chinese and Mexican restaurants in a zip code(r=.16)
  • 56% of ‘Chinese-Only’ niches are in LA City but only 32% of Mexican Only ones are.
  • Long Beach, East Los Angeles, and Santa Clarita stand out as cities with more Chinese restaurant deserts; Glendale stands out as a city with more Chinese Restaurant Deserts.  

You can explore the trends with the maps pasted below, or at the app here.

Mexican Restaurant Niches

Map by Fabio Dias

Chinese Restaurant Niches

Map by Fabio Dias