Urban connectivity
Sonja Braaker1, Martin K. Obrist1, Fabio Bontadina2, Marco Moretti2
1 Swiss Federal Research Institute WSL, Zürcherstrasse 111, 8903 Birmensdorf
2 Istituto federale di ricerca WSL, Via Belsoggiorno 22; 6500 Bellinzona-Ravecchia
Summary
Despite the fact that urban environments belong to the most fragmented landscapes, the ecological relevance of habitat connectivity in cities remains largely unknown. In our study we investigated the importance of habitat connectivity in an urban environment for species groups with contrasting dispersal abilities and locomotion modes. The model species groups are hedgehogs (Erinaceus europaeus), ground dwelling mammals with home ranges of 10 to 40 ha, ground beetles (Carabidae) ground dwelling insects with dispersal abilities of a few hundred meters, spiders (Araneae) with the ability of many species to disperse by ballooning, weevils (Curculionidae) small sized beetles with limited flight abilities and bees (Apidae) with high dispersal ability in all three dimensions. The study was conducted in the city of Zurich.
To assess habitat connectivity for hedgehogs we tagged 40 hedgehogs with GPS loggers. The high tem- poral and spatial resolution GPS locations were used to investigate habitat preference of urban hedge- hogs. Based on these we employed novel electrical circuit theory algorithms (Circuitscape) to model connectivity maps for the hedgehog. We developed a stepwise analytical framework to assess habitat connectivity in an urban environment using a data-based approach. Additionally, behavioral response of hedgehogs to common stressors of present urban infrastructure, light and noise where investigated in a masters study. The major results show that:
i) Habitat connectivity maps on the one hand highlight connectivity pinch points critical to the movement of hedgehogs and on the other hand can be used by city planners to identify habitat corridors and derive management and conservation actions.
ii) Habitat connectivity, in addition to habitat quality, plays a substantial role in the prediction of animal movement of urban hedgehogs.
iii) At the individual’s behavioural scale, animals showed disorientation in response to traffic noise and pronounced startle effects to car headlights. Both behaviours potentially decrease survival rate during their trying to cross connectivity pinch points or corridors on roads.
To study the role of habitat connectivity for urban arthropods we examined arthropod species composition on 40 ruderal ground sites and 40 extensive green roofs. We were especially interested to investigate whether and to what degree green roofs enhance urban habitat connectivity for arthropods of different abilities to move, both in terms of distance and dimension (vertical). We measured local, regional and connectivity variables at all sites and compared their relative influence on species composition of the four arthropod groups using variation partitioning. The results reveal that:
iv) Green roofs and ruderal sites support a highly variable abundance and diversity of arthropod species.
Parts of the communities seem highly adapted to the respective environment, as many species were caught exclusively on one habitat type (roofs or ground sites).
v) Highly mobile species communities at ground level, as those of bees and weevils seem mainly shaped by local variables.
vi) Regional and connectivity variables have an more pronounced effect in shaping roof communities.
vii) For species of lower degree of mobility, such as carabids and spiders, local variables have generally a greater influence on ground and roof communities.
vii) Overall, habitat connectivity has an impact on urban arthropod community composition, and green roofs have a great potential as both habitat and stepping stones for urban biodiversity.
State of the art (pre-ENHANCE)
High anthropogenic pressure on the environment in the last decades has led to widespread and funda- mental changes in landscape composition and configuration causing habitat degradation, fragmentation, and isolation for many species living in the wild. At the same time, it has been demonstrated that within a city the mosaic pattern of habitat distribution within urban environments can continue to support consider- able numbers of animal and plant species. Urbanization processes and particularly habitat fragmentation may act as an ecological filter at both taxonomic and functional levels. Species of different degrees of mobility, thus abilities to disperse and to move between populations should be differentially affected by fragmentation. Weakly mobile species likely are hindered by increasing fragmentation of suitable habitats, while more mobile species which are able to cross large impervious areas should be able to survive by maintaining vital metapopulations in the urban environment.
However, the exact effect of habitat fragmentation on animal species and whole communities in the urban environment is still unclear, and especially the vertical dimension of urban habitas, such as green climb- ers on buildings, flowered balconies, and green roofs, has so far been conspicuously neglected in con- nectivity studies. The highly patchy character of urban areas poses major challengees to the assessment of habitat connectivity. This might be one of the reasons why the role of connectedness of particular habi- tats, such as urban forests, ruderal environments, humid areas/sites, has rarely been studied explicitly in urban environments.
Most studies who assessed habitat connectivity using a spatially explicit approach base their assumptions on literature reviews or expert opinion. Additionally, spatially explicit modelling has never been applied to such highly fragmented fine-grained habitats as urban environments. Spatially explicit connectivity mod- els have a great potential to assess current connectivity pinch points as well as to model effects of future landscape changes. As such, they could be of great value for planners to identify habitat corridors and derive management and conservation actions.
Concerning the urban invertebrate fauna, green roofs and vertical green could have a great potential to enhance urban biodiversity. To the present date most researches were limiting their research on the positive effects of green roofs on the urban climate (storm water retention, urban heat island mitigation, energy use reduction) and as valuable habitat for some invertebrates and plants. The functional aspect of green roofs as stepping stones for dispersion of urban species across impervious urban environments has never been investigated, Factors shaping arthropod community composition and distribution on green roofs are in fact still unknown, since no study has so far investigated arthropod communities on green roofs with a larger sampling scheme, including comparable ground sites, which allows to draw robust statistical conclusions.
Motivation and research questions
Currently, the worldwide fast development of cities, either by rapid expansion of urban and peri-urban ar- eas, the trend to urban densification, or the movement of human residences from urban centres towards rural areas, leads to an increased fragmentation of natural or semi-natural habitats. A small scale patch- work of suitable and hostile habitats is the result of these processes. Despite these massive landscape changes the importance of habitat fragmentation and habitat connectivity in urban environments is largely unknown. To mitigate future landscape changes and develop sustainable urban planning strategies the understanding of the importance and the role of habitat connectivity and fragmentation in urban environ- ment is essential.
The main goal of our study was to investigate which habitat and landscape structure and connectivity metrics optimally describe and predict the movement and the distribution of species of different mobility in an urban environment.
Specific questions are:
1. Which spatially explicit connectivity pathways for urban environments explain empirical movement data of ground-dwelling vertebrates?
2. What is the role of habitat connectivity in comparison to habitat quality for urban ground dwelling ani- mals in cities?
3. How can behavioural response to omnipresent disturbances modulate overall movement patterns?
4. Which is the relative influence of local, regional and connectivity variables on the community composi- tion of urban arthropods with different dispersal abilities?
5. What is the functional role of green roofs as stepping stones for urban arthropods?
To answer these questions we selected four model species groups characterized by different mobility abilities and life-histories, i. e. hedgehogs (Erinaceus europaeus), ground dwelling mammals with home ranges of 10 to 40 ha, ground beetles (Carabidae), ground dwelling insects with dispersal abilities of a few hundred meters, spiders (Araneae) with the ability of many species to disperse by ballooning, weevils (Curculionidae) small sized beetles with limited flight abilities and bees (Apidae) with high dispersal abili- ties in all three dimensions.
The study was conducted in the city of Zurich, the largest city of Switzerland, located in the Swiss plains.
Technical issues: material, methods, sampling, etc.
Habitat connectivity for urban hedgehogs
As ground-dwelling, insectivorous mammals, hedgehogs are directly confronted with the urban patch- work of habitats in their nocturnal roaming and foraging. Forty hedgehogs were captured at 11 sites and equipped with a custom-built GPS logger and radio transmitter. The hedgehogs position was recorded every 10 seconds during 1–5 consecutive nights. In total 727 hours of activity were recorded comprising 304 to 7837 location fixes per animal. From these data, we identified habitat selection of urban hedge- hogs using a multivariate analysis. We modelled habitat connectivity with an electric circuit theory method (Circuitscape). Based on a movement resistance landscape Circuitscape models identified areas of differ- ent degrees of connectivity in the urban matrix. Based on the results from the habitat selection analysis, we evaluated several alternative resistance surfaces in the electric circuit modelling to achieve an optimal connectivity model. Finally, in order to understand the very local disturbance effects of urban traffic on the movement behaviour of the hedgehogs, we additionally set up behavioural experiments under semi-natu- ral conditions. In these controlled observation experiments, 45 hedgehogs were tested for their locomotor response to car head-lights and noise.
Role of habitat connectivity for urban arthropods
Ground beetles, spiders, weevils and bees have contrasting life histories, and their average movement abilities range from a few hundred meters in the horizontal dimension to several kilometres including the vertical dimension. To study the role of habitat connectivity for urban arthropods we examined the arthropod species composition on 40 ruderal ground sites and 40 green roofs with extensively managed vegetation.
We installed a flight interception trap and six pitfall traps at each ruderal ground site and on each green roof. Sampling was carried out for 15 weeks, trap stations were emptied weekly. Ten week samplings out of 15 were retained for the analyses. The resulting samples were sorted to higher taxa and consecu- tively determined to species level by specialists. We measured local environmental variables, such as
plant species composition, proportion of forbs and bare soil, flower abundance and the age and area of the site. Regional environmental variables were assessed in detailed GIS maps on four different radii (100–400 m; 100 m steps) around the sampling sites. Measures comprised the following five proportional variables: buildings, roads, meadows, urban green with structures, and forests within the respective radii.
Connectivity was quantified in the four investigated radii using various measures based on spatial pat- terns as mean nearest neighbour, mean proximity index, contrast weighted edge density and Shannon diversity index.
We tested the relative influence of environmental and connectivity variables on species composition at local (i.e. habitat conditions at the trap site) and regional scales (i.e. landscape composition and con- figuration in the four different radii) with the use of variation partitioning based on canonical redundancy analysis. For each of the four selected arthropod groups we performed separate analyses of communities occurring on roofs, on ground sites, and those occurring on both roofs and ground sites.
Innovation, gains, new insights and main results thanks to ENHANCE
Habitat connectivity for urban hedgehogs
Overall our results showed that habitat connectivity, in addition to habitat quality, is an important aspect which should not be neglected in urban planning strategies.
For the first time high resolution GPS tracking has been used to investigate the movement behaviour of small mammals in cities. Furthermore, it is the first time that detailed GPS movement data is used as basis to model connectivity with circuit theory. Our study provided a novel analytical framework which is able to identify spatially explicit connectivity pathways for urban environments (Fig. 1).
high low Current flow
200m
Fig. 1. Optimal connectivity map model for the hedgehog in a neighbourhood of the city of Zurich calculated with Cir- cuitscape. The GPS-radiotracked hedgehog is in bright blue dots. The coloured ground corresponds to different current flows (connectivity) ranging from yellow (high habitat connectivity) to dark blue (low habitat connectivity). Bright yellow areas highlight pinch points and indicate essential connectivity areas. Infinite resistance values are coloured black and core habitat patches in green.
The analytical framework is very flexible and can be applied to different habitat use data depending on the focal species. It further allows to model and evaluate the effects of future landscape changes on habitat connectivity by highlighting connectivity pinch points, thereby representing an important tool for city plan- ners to identify habitat corridors and derive management and conservation actions.
Our controlled experiment on the effect of traffic light and noise on the movement behaviour of hedge- hogs demonstrated that light exposure startles the animals, thereby reducing their ability to escape and prolonging their presence on the roads. Hedgehogs reacted with an avoidance response to traffic noise, turning away from engine noises, which might help to avoid them crossing busy roads. A similar but delayed avoidance response was eventually visible after the visual startle, but with obvious detrimental timing.
Role of habitat connectivity for urban arthropods
Our comprehensive sampling allowed us to demonstrate, that green roofs and ruderal sites support a highly variable number of individuals and species. Overall we captured 48 086 individuals of 480 spe- cies. Green roofs and ruderal sites support a highly variable number of individuals (mean ± sd: 531 ± 361, 706 ± 451 respectively) and species (mean ± sd: 59 ± 11, 79 ± 19 respectively). Many of the species were caught exclusively on one habitat type (roofs: 68, 14 %; ground: 159, 33 %).
Our results show that a) habitat connectivity has an impact on urban arthropod community composition and b) that green roofs have a great potential as habitat for urban biodiversity and c) that they might act as stepping stones enhancing the permeability of the urban landscape.
The combination of extensive arthropod sampling at ruderal ground sites and green roofs enabled us to identify the variables shaping the local arthropod community. The importance of local, regional and con- nectivity variables vary depending on the arthropod group studied and on the community (roof or ground) investigated.
Preliminary results show that for high mobile functional groups such as bees and weevils regional and connectivity variables have an increased importance for roof communities (Fig. 2). In contrast, for func- tional groups with lower mobility, as carabids and spiders, local variables have generally a greater influ- ence on ground and roof communities.
a b
Fig. 2: Variation partitioning among local (green), regional (blue), and connectivity variables (purple) of a) roof visiting bee communities (high mobile group) and b) roof visiting ground beetle communities (low mobile group). For roof visit- ing high mobile group (a), regional variables play a major role (5.36+5.73= 11.09 % expl.var.) while both regional and local variable are structured by connectivity (5.73 % expl.var.). In the contrary, for roof visiting low mobile group (b), local variables play a major role (7.86+2.12=9.98 % expl.var.) compared to regional (1.55+2.12=3.67 %) and connectivity (2.12+2.12+0.01+0.01=4.26 %). Local and regional variables are only partially structured by connectivity (2.12 % expl.var.).
In addition, increasing the proportion of forbs on roofs increases the number of species. Given the sig- nificant higher number of species on ruderal sites compared to green roofs and in the light of previous studies on arthropod diversity on urban lawns and meadows, green roofs should not be promoted as substitute for ruderal ground habitats but rather as an added value for urban species diversity and the general possibility to increase the area of urban green in cities.
Thanks to this ENHANCE project the importance of habitat connectivity for a wide range of different spe- cies groups could be demonstrated for the first time in a highly fragmented urban environment. This study brought us a big step forward to understanding the role of fragmentation and connectivity in urban land- scapes. Furthermore, the interdisciplinary framework of ENHANCE allowed many enriching discussions with researchers of other modules (especially with module 1 and module 3) and with researchers of other tasks within the same module but working at other institutes.
References
List of publications
Braaker S. et al. in prep.: Assessing habitat connectivity for ground-dwelling animals in the urban environment. Aimed journal: J. Applied Ecology.
Jafar-Zadeh S. 2009: Urban ecology of hedgehogs Erinaceus europaeus: behavioural response to light and noise. Master thesis ETHZ.
List of presentations
Braaker S. et al.: Are green roofs stepping stones for arthropods in an urban landscape? ECCB, Glasgow, September 2012 – Talk.
Braaker S. et al.: Habitat connectivity for hedgehogs in cities – PACE conference, Zurich, 02–04.2.2011 – Talk.
Braaker S. et al.: Study design of PhD project: Habitat connectivity in an urban environment. IFLA World Congress, Zurich, 27–29.06.2011– Poster.
Braaker S. et al.: Habitat connectivity in an urban environment. IALE, Laufen, Germany, 20–22.09.2011– Talk.
Braaker S. et al.: Artenvielfalt und Lebensraumvernetzung in der Stadt – Modul Grünflächenmanagement ZHAW Wädenswil, 14.4.2011 – Talk.
Braaker S. et al.: Habitat connectivity for hedgehogs in urban environment – DEG Seminar, WSL Birmensdorf/ZH, Septem- ber 2011 – Talk.
Braaker S. et al.: Habitat connectivity in an urban environment – Lecture Series in Community Ecology University Bern, 24.10.2011 – Talk.
Jafar-Zadeh S., Obrist M.K., Braaker S., Bontadina F., Ghazoul J.: Behavioural response of hedgehogs Erinaceus euro- paeus to light and noise in urban habitats – Oxford & Cambridge Student Conferences, London 24.03.10 – Poster.
