Modelling Ecological Connectivity within the City of Calgary
The primary objective of this study was to assess structural landscape habitat connectivity within the City of Calgary for a broad range of wildlife species, with a focus on terrestrial habitat connectivity, using a circuit-theory model. The foundation of any habitat connectivity modeling in a GIS environment is a resistance raster, which is created to approximate the ease with which a species moves through the landscape. The resolution of the resistance raster is based upon the quality of the data available and the species under consideration, and for this project, we created a 5m x 5m resistance raster that covered the City of Calgary, plus a 20% buffer.
The resistance raster was created using a number of existing and derived spatial data layers, including land cover, slope, canopy cover, and traffic volumes. The creation and combination of these input datasets to produce a resistance raster required advanced remote sensing and GIS analysis and processing, as follows:
- The land cover layer was created using more than 15 different spatial datasets, which were provided for use by the City of Calgary. These layers included building footprints, roads, rail networks, golf courses, manicured areas, park boundaries, vegetation polygons, and hydrology. Compiling these existing data into a unified land cover layer required extensive data management, digitization, editing, and QA/QC checking, including boundary accuracy checks, topology correction, re-projection. Once a resistance value was assigned to each land cover class, the land cover layer was converted to a raster surface. If multiple land cover types were contained within a pixel, the value of the majority land cover type present was assigned to the pixel.
- Slope within the City was calculated using a 2 m LiDAR derived Digital Elevation Model (DEM) provided by the City of Calgary, which was then resampled to 5 m to be consistent with the spatial grain of the resistance raster. Slope outside the City, within the buffer, was calculated using a freely available 25 m provincial DEM, which was also resampled to 5 m. A neighbourhood level slope metric was calculated across four circular neighbourhoods (5, 15, 45, and 135 m) and the average neighbourhood slope value was then averaged with the original 5 m pixel value to derive a final slope
- Tree canopy cover in the City was calculated using a tree canopy model derived using LiDAR point cloud data that was processed using ENVI LiDAR. Within each 5 m pixel, the proportion of the pixel covered by tree canopy was calculated, and using focal statistics, we calculated the mean canopy closure for all pixels across four scales using circular neighbourhoods of 5, 15, 45, and 135 m. The neighbourhood level canopy metric was then multiplied by the original canopy closure value to calculate the cross-scale tree canopy cover value for each 5 m pixel.
- To account for the direct and indirect effects of road traffic volume on landscape connectivity, we used a quadratic kernel density calculation in ArcGIS 10.3 to convert average daily road traffic volume data provided by the City of Calgary into a smoothed traffic volume raster that extended 350 m from the road.
All of the input layers were combined together to create a final human modification score for each pixel within the resistance raster. We identified potential habitat connectivity linkages in the City of Calgary using this raster by modeling current flow using the software Circuitscape. The habitat connectivity model was then validated using two independent empirical data sets, as assess how well the model performed when considering the movement requirements of multiple species.
City of Calgary Parks Department
June 2015 to September 2016