Wild vegetation in a vacant lot suggests there is very little social control over an area, and that may encourage criminal activity. It is suggestive of a place where people aren’t paying attention; where neighbours are not coming together or looking out for their environment or each other. The direct opposite is conveyed when there is a very well-maintained vegetated landscape.
Professor Jeremy Mennis
Research from Temple University in Philadelphia, Pennsylvania, suggests that, rather than contributing to higher rates of crime, vegetation and greenery in cities can actually suppress some types of criminal activity. While convention has traditionally dictated that high levels of vegetation in urban planning abet crime, this paper shows that the presence of grassy areas, trees and shrubs was associated particularly with a reduced incidence of robbery and aggravated assault in Philadelphia.
In an interview with ScienceOmega.com
, Jeremy Mennis, Associate Professor in the Department of Geography and Urban Studies at Temple, explained how the study questions received wisdom on vegetation in urban settings.
"Although there is not necessarily a hard and fast rule, for a very long time common wisdom has suggested that, in an area with a lot of vegetation, a situation is created where criminal activity can be concealed," he began. "It means there are places for criminals to hide and that potential victims can’t see assailants."
Promoting visibility has, for a long time, been considered key to reducing criminal activity. So, what prompted the professor to carry out this analysis and to effectively reassess of the situation?
"I have been interested for a while in the relationship between vegetation and socioeconomic status, especially in relation to issues like poverty, educational attainment, and the ability of people in higher socioeconomic classes to buy into more vegetated environments," he remarked.
However, Professor Mennis insisted that the real credit has to go to undergraduate student Mary Wolfe – first author on the paper which was reported in the journal Landscape and Urban Planning
– as she brought the relationship between crime rates and vegetation to his attention.
Along with Professor Mennis, Wolfe analysed data on crime, socioeconomics and vegetation after establishing controls for factors such as poverty, population density, and educational attainment. The vegetation data were based on satellite imagery.
"In previous research I’ve seen a strong relationship between socioeconomic status and vegetation, but I wasn’t really prepared to find such a strong association between vegetation and criminal behaviour such as assaults and robberies," Professor Mennis said of their findings. "In the existing literature, however, there is actually a lot of research supporting the idea that vegetation is connected to many mental health and behavioural issues."
Professor Mennis noted that the findings turned out not to be as surprising as he perhaps thought they might be, once he began to examine the literature related to vegetations effect on shopping behaviour, job satisfaction, and athletic performance to name but a few.
"I found articles where people had made connections between all of those things and the presence of natural settings or vegetation, so I guess it’s not surprising that vegetation and natural settings do influence our emotional states, how we feel, and – ultimately – our behaviour," he said.
The researchers are considering two main theories about the causal mechanism behind the influence the study documents. The first idea is that vegetated landscapes – particularly well-maintained vegetated landscapes – promote public participation in the community by having people come outside into public spaces and interact with each other. This is a longstanding idea, as Professor Mennis pointed out; the classic reference is to Jane Jacobs’ 1961 book, The Death and Life of Great American Cities
, in which she talks about ‘eyes on the street’.
"This is a pretty intuitive idea – that when you have neighbours outside in parks and similar settings, trust is built up between them and they will work together cooperatively to address issues in their community and keep an eye on those public spaces and each other," commented Professor Mennis. "When you build that sort of neighbourly interaction you will get a suppressive effect on criminal behaviour, because there are people who are not criminals outside, interacting with each other."
It may also be the case that a landscape with more vegetation provides some restorative or mentally calming effect which is able to reduce the psychological precursors to violent behaviour, the authors surmise. The extent to which green spaces can act as a deterrent to certain criminal activities may be dependent on the level of maintenance afforded these areas, but this is not a variable that the current research was able to account for.
"The measurement of vegetation that we used, which is derived from remotely sensed imagery, essentially conveys the magnitude of vegetation concentration, but doesn’t distinguish the degree of maintenance or care for that vegetation," explained Professor Mennis. "There are
studies that have concentrated on that, and common sense also tells us of course that an area of vegetation that is overgrown – weeds and shrubs on a vacant lot – will send a very different message than a well-manicured lawn or well-maintained park.
"Wild vegetation in a vacant lot suggests there is very little social control over an area, and that may encourage criminal activity. It is suggestive of a place where people aren’t paying attention; where neighbours are not coming together or looking out for their environment or each other. The direct opposite is conveyed when there is a very well-maintained vegetated landscape."
That maintenance of reputation is a key moderating factor in the relationship, believes Professor Mennis; an effect that other studies have highlighted. It is an important caveat that a certain degree of care for the surroundings is necessary for this relationship between vegetation and crime to persist. It will also be of interest to consider the role that urban greenery plays over the long term.
"This study was cross-sectional in nature and we found evidence that there’s some kind of causal mechanism related to certain types of criminal behaviour and vegetation. It will be more statistically sophisticated, but the next step is to do a longitudinal analysis, looking at changes in vegetation over time and changes in criminal behaviour over time to try to establish a chronological effect."
Obtaining data for such a study, in terms of vegetation rather than crime, will be much more challenging, however. The main point that Professor Mennis wished to emphasise was that initiatives to create healthier places, to reduce crime rates, and to produce better aesthetic landscapes are far from being in opposition to each other.
"Efforts to reduce storm water run off and improve air quality, for example, go hand-in-hand with increasing well-maintained vegetation in cities," he stressed. "The idea of a relationship between reduced crime and vegetation dovetails nicely with several of the other initiatives that many cities are undertaking right now with regard to environmental sustainability."