spatial graphs for assessing woodland caribou habitat connectivity dan o’brien, msc, rpbio (cortex...
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Spatial Graphs for Assessing Woodland
Caribou Habitat ConnectivityDan 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
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
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
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
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
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
Habitat Connectivity
Approaches to analyzing connectivity fall on a continuum
STRUCTURAL FUNCTIONAL
SPATIAL GRAPHS INDIVIDUAL BASED DISPERSAL MODELS
INTRA-PATCH METRICS
e.g., patch cohesion
INTERPATCH METRICS
e.g., nearest neighbour distance
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
Spatial Graphs for measuring landscape connectivity
Nearest Neighbor links
Shortest links connecting patches, such that each patch forms at least one link
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
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
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
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
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
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
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
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
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
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)
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
The Owl Lake Woodland Caribou
WINTER 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)
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
Habitat Map and Cost Surface
Habitat Class
Cost
(OR-1)
JPD/STR 1
TMG/MCU 1.0077
MCL/IMU 1.7339
YNG/WL 3.5396
BURN 3.3029
WATER 3.5952
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
Graph Extraction
HIGH QUALITY PATCHES
LEAST COST LINKS
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
Graph Thresholding
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
Graph Thresholding
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
Graph Thresholding
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
Graph Thresholding
At each threshold compute a landscape level metric, Expected Cluster Size (ECS)
Expected Cluster Size: mean size of a cluster for randomly selected habitat cells
(area weighted mean cluster size at threshold distance, d)
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
Graph Thresholding
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
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.
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
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
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
Are caribou responding to structural connectivity?
Differences greatest at scales between 500 – 1900 cost units
Above 1000 cost units, PECS for late winter points is 7,500ha and similar to maximum cluster size
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
Independent ValidationKississing Herd
NE of The Pas, MB
PATCHES FILTERED TO 25 ha
Historical home range
Current winter home range
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
Are caribou responding to structural connectivity?
Greatest differences at scales between 500 – 4500 cost units
Between 500 and 4500, PECS for late winter points is ~7,500 ha, but less than Max Cluster Size
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
Summary
1. Strong association between distribution of caribou and connected clusters of habitat
2. Affinity for clusters ~7500ha, and link thresholds 500 – 2000 cost units.
3. In the Owl Lake herd, caribou associated with clusters near the maximum cluster sizes at these thresholds.
4. Kississing also show strong affinity to larger clusters, but associated with clusters below the max available.
5. In the Kississing range availability of clusters is greater in both sizes and numbers; hence, greater potential for range expansion.
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
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
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
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
Chase Habitat Modelling Workshop Nov 2004 Dan O’Brien
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.