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Spatial Graphs for Assessing Woodland Caribou Habitat Connectivity

Spatial Graphs for Assessing Woodland Caribou Habitat Connectivity. Dan O’Brien, MSc, RPBio (Cortex Consultants Inc.) Micheline Manseau, PhD (Parks Canada) Andrew Fall, PhD (Gowlland Technologies Ltd.) Marie-Josée Fortin, PhD (Dept. Zoology, University of Toronto)

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Spatial Graphs for Assessing Woodland Caribou Habitat Connectivity

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  1. Spatial Graphs for Assessing Woodland Caribou Habitat Connectivity Dan O’Brien, MSc, RPBio (Cortex Consultants Inc.) Micheline Manseau, PhD (Parks Canada) Andrew Fall, PhD (Gowlland Technologies Ltd.) Marie-Josée Fortin, PhD (Dept. Zoology, University of Toronto) Chase Habitat Supply Modelling Workshop November 23 – 25, 2004

  2. Presentation Outline • Using spatial graphs to measure structural connectivity of a landscape • Application • An assessment of Woodland caribou (Rangifer tarandus caribou) habitat connectivity in Manitoba • Methods for detecting an association between distribution of caribou and the structural connectivity of their range

  3. Habitat Connectivity • Connectivity is the degree to which landscape facilitates or impedes dispersal among resource patches (Taylor et al. 1993) • Structural connectivity • linkage of resource patches by physical adjacency • Functional connectivity • linkage of resource patches by processes that depend on dispersal and movement behaviour of the species

  4. Habitat Connectivity • Approaches to analyzing connectivity fall on a continuum STRUCTURAL FUNCTIONAL INDIVIDUAL BASED DISPERSAL MODELS SPATIAL GRAPHS INTERPATCH METRICS e.g., nearest neighbour distance INTRA-PATCH METRICS e.g., patch cohesion

  5. Spatial Graphs for measuring landscape connectivity • Nearest Neighbor links • Shortest links connecting patches, such that each patch forms at least one link

  6. Spatial Graphs for measuring landscape connectivity • Minimum Spanning Tree • The minimum set of links such that each patch is connected into a single component • There can be no other link with greater length linked to a patch • “Backbone” of connectivity

  7. Spatial Graphs for measuring landscape connectivity • Minimum Planar Graph (MPG) • At least one link between each pair of patches, • No link can cross any another link • All links are shortest path between pairs of patches

  8. Spatial Graphs for measuring landscape connectivity • The MPG is a Triangulation of Patches • Linked patches form a Delauney Triangulation • The dual is the Voronoi surface • Voronoi boundaries are equidistant from all patches • Each point in Voronoi polygon is closest to the interior patch than any other

  9. Forming links along a cost surface • Patches are linked along the least-cost (accumulated cost) paths • Links represents biological characteristics such as dispersal ability within and between patches within the matrix

  10. Application: Woodland Caribou • Boreal ecotype is threatened in Canada (COSEWIC 2002) • In southern Manitoba woodland caribou are generally sedentary • Habitat selection is strongest during winter months • Late seral Jack Pine stands and sparsely treed rock outcrops • High abundance of terrestrial lichens and low snow cover Photo: Jared Hobbs (www.hobbsphotos.com)

  11. The Owl Lake Woodland CaribouWINTER HOME RANGE GPS telemetry location data from Owl Lake herd (southeast MB) Popn Size: 65 – 75 11 collared adults (9 male, 2 female) Focused on winter points (Nov 1 – March 15)

  12. Habitat Map and Cost Surface

  13. HIGH QUALITY PATCHES LEAST COST LINKS Graph Extraction

  14. Graph Thresholding

  15. Graph Thresholding

  16. Graph Thresholding

  17. Graph Thresholding • At each threshold compute a landscape level metric, Expected Cluster Size (ECS) • ExpectedCluster Size: mean size of a cluster for randomly selected habitat cells (area weighted mean cluster size at threshold distance, d)

  18. Graph Thresholding

  19. Are caribou responding to structural connectivity? • Point Expected Cluster Size • Telemetry points associated with the closest patch • At each threshold scale points are assigned the area of the cluster containing the associated patch • ECS is then computed from cluster sizes measured for each location point • This represents the expected size of a cluster of habitat associated for a randomly selected location point.

  20. Are caribou responding to structural connectivity? Randomization Test • Point ECS computed for 100 sets of random points • distributed randomly in each habitat type in proportion to selection by caribou • Observed Point ECS compared to mean of random point sets • Distance thresholds where caribou points were greater than 95% CI, indicate scales at which caribou are more closely associated with highly connected clusters of high quality habitat than if randomly distributed within the home range

  21. Above 1000 cost units, PECS for late winter points is 7,500ha and similar to maximum cluster size • Differences greatest at scales between 500 – 1900 cost units Are caribou responding to structural connectivity?

  22. Current winter home range Historical home range Independent ValidationKississing Herd PATCHES FILTERED TO 25 ha • NE of The Pas, MB

  23. Between 500 and 4500, PECS for late winter points is ~7,500 ha, but less than Max Cluster Size • Greatest differences at scales between 500 – 4500 cost units Are caribou responding to structural connectivity?

  24. Summary • Strong association between distribution of caribou and connected clusters of habitat • Affinity for clusters ~7500ha, and link thresholds 500 – 2000 cost units. • In the Owl Lake herd, caribou associated with clusters near the maximum cluster sizes at these thresholds. • Kississing also show strong affinity to larger clusters, but associated with clusters below the max available. • In the Kississing range availability of clusters is greater in both sizes and numbers; hence, greater potential for range expansion.

  25. Conclusions • Spatial graphs are a useful method for assessing the connectivity of woodland caribou habitat • For identifying and mapping core areas of well connected habitat and can quantify how these areas contribute to overall landscape connectivity • For identifying scales where associations between caribou and connected habitat are strongest • Allows the patch definition to be scaled-up from inventory polygons to connected clusters which incorporate differential matrix quality

  26. Conclusions • Increasing matrix quality may improve utilization of existing high quality habitat • Facilitate movement between patches (optimal foraging) • Maintaining low densities (spacing out as an anti-predator strategy) • Supports the idea that definition of critical habitat should include not only abundance but also spatial arrangement of high quality habitat, in addition to the relative quality of the intervening matrix habitat

  27. Acknowledgements • Funding from Parks Canada Species at Risk Recovery Action and Education Fund • Field support was provided by Manitoba Conservation and Manitoba Hydro • Owl Lake GPS telemetry data collected as part of MB Hydro's Research and Development of Animal Borne Technology on Woodland Caribou Project Participating partners: • Manitoba Hydro • Manitoba Model Forest Ltd. • Manitoba Natural Resources • Natural Resources Institute of the University of Manitoba • TAEM Consultants.

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