Assessing the capacity of different urban forms to preserve the connectivity of ecological habitats

Abstract

This paper addresses the relationship between anthropogenic forest habitat fragmentation and the form of urban patterns. Using a two-step methodology we first generate 40 theoretical residential development scenarios following a repeatable procedure; the simulated urban forms are either moderately compact or fractal. Then, we compare the scenarios according to the functional connectivity of the remaining forest habitat using a graph-based approach. The methodology is applied to the urban region of Besançon (France), where forest surfaces are considered as a generic habitat for several animal species. Results obtained show that fractal scenarios of residential development are almost equivalent to moderately compact scenarios regarding the connectivity of forest habitat when the residential development is weak. In the case of a more intense residential development, fractal scenarios are superior to nonfractal scenarios when low dispersal distances of animals are concerned.

Introduction

Managing urban sprawl is a major concern in urban planning since it has marked negative environmental (air pollution, noise, and destruction of natural resources) and socio-economic (higher housing and commuting costs leading to social segregation and social inequity) effects. Yet urban development is a real necessity in many countries owing to the growing numbers of inhabitants and households. Consequently, land consumption is a major planning issue. Given that new residential buildings often require only moderate amounts of land compared to the associated road infrastructures (Camagni, Gibelli, & Rigamonti, 2002), the recurring question is where might urban expansion be located without worsening the effects of urban sprawl?

One major impact of urban sprawl on natural ecosystems is the fragmentation of wildlife habitats (Forman, 1995). The spread of artificial surfaces reduces available habitats through the loss of favorable areas and the break-up of the remaining habitat areas into separate patches. The viability of a species in a fragmented habitat depends on the ability of individuals to reach one patch from another by crossing unsuitable habitat. Consequently, landscape connectivity, combined with the size and the quality of habitat patches, proves to be a key notion for the conservation of animal species. Alongside structural connectivity, functional connectivity is recognized as being ecologically relevant (Taylor, Fahrig, & With, 2006). Functional connectivity may be defined as the interaction between a given species and the elements of a landscape. Methods for evaluating functional connectivity often use landscape metrics (Magle, Theobald, & Crooks, 2009) or spatial simulation models (Tischendorf & Fahrig, 2000). In order to set up tools that are easy for planners and landscape managers to use, we turn to graph theory, which provides a happy compromise between the need for intensive measurement as part of a biological approach and the constraints associated with data acquisition (Calabrese and Fagan, 2004, Fall et al., 2007). Moreover, graph theory is a preferable alternative to spatially explicit population models for species conservation in heterogeneous landscapes (Minor & Urban, 2007).

While numerous studies have analyzed responses of animals to anthropogenic habitat fragmentation, little research has addressed the relationship between habitat fragmentation and form of urban patterns (Bierwagen, 2005, Bierwagen, 2007). The limited transfer of knowledge between the eco-physical and spatial planning domains, underlined by Termorshuizen, Opdam, and van den Brink (2007), may partly explain the lack of knowledge about the relationship between urban forms and ecological systems. As Alberti (2005) points out, we do not know how clustered versus dispersed and monocentric versus polycentric urban structures differently affect environmental conditions, nor how urban development patterns influence ecological systems along the gradient of decreasing density from urban center to its periphery. Alberti (2005) also remarks that ecological studies dealing with urbanization simplify the consideration of urban structures to such an extent that the results are no longer useful to urban planners and managers. For example, Tratalos, Fuller, Warren, Davies, and Gaston (2007) compare and contrast several urban density measures with a series of measures of environmental quality and biodiversity potential. Their study yields no conclusive results and shows that similar urban forms may induce a varying environmental quality. Conversely, Bierwagen (2005) shows that urban areas that differ visually may nonetheless have similar ecological connectivity scores. Her study also shows that ecological connectivity declines with the increasing size of the urban area. However, it cannot be inferred from such a statistical relationship that there is any functional relationship between certain characteristics of urban forms and the ecological system. Urban forms are highly complex, which may account for the difficulty in identifying key variables for use as a lever for wildlife conservation. To overcome this difficulty, it may be helpful to work on simulated rather than real-world urban forms, since their morphological characteristics can be controlled. For instance, Geurs and van Wee (2006) use a system called Environment Explorer in which land-use and transport modules are dynamically interconnected to simulate scenarios of urban development. The 500 m resolution of the land-use cells, however, was too coarse for precise measurements of environmental impacts at local level.

In this paper, we aim to better understand how different patterns of residential development may impact the shape of animal habitats, and therefore affect their connectivity. Two categories of built patterns are considered: compact built patterns characterized by high built densities, uniformity, and sharp (i.e. non-sprawling) boundaries (Geurs & van Wee, 2006); fractal built patterns that are intrinsically nonuniform across scales, and exhibit longer and more sinuous boundaries (Frankhauser, 2004). In urban planning, the compact city model is the common answer to the problem of urban sprawl. But the model's limitations have been pointed out, especially the congested roads, reduced access to green and natural areas, higher housing prices, and reduced living space (Breheny, 1992, Burton, 2000). Accordingly an alternative urban model has come to the fore combining reasonable densification, as in the “wisely compact city” (Camagni et al., 2002), and a polycentric urban organization (Davoudi, 2003). The fractal city model, in keeping with this tendency, appears promising since several commentators have suggested that the fractal city could satisfy people who consume various urban and rural amenities by improving access to both built and nonbuilt spaces (Cavailhès et al., 2004, Frankhauser, 2004).

In this paper, we adopt a two-step method. First, we generate 40 theoretical scenarios of residential development using a repeatable procedure that explicitly takes into account fractal or nonfractal urban development models. Then we compare the scenarios in terms of the functional connectivity of the remaining habitat using a graph-based approach. Our aim is to identify the urban form that best preserves habitat connectivity.