Proposed Fragmentation Indicator for Principle ER-4 (Wildlife Habitat Protection)

for the Larimer County Master Plan, Monitoring and Evaluation Report

 

David M. Theobald

Natural Resource Ecology Lab/Colorado State University

 

Principle ER-4. Larimer County shall endeavor to protect all areas identified as highest priority on the Important Wildlife Habitat Map, which is adopted by reference as part of the Master Plan.

 

Currently, the sole draft indicator is the number of acres of protected wildlife habitat in new developments. This is a useful indicator, but a second type of habitat impact, fragmentation, is widely recognized by biologists to be important as well.  That is, the location of disturbance in relation to the configuration of habitat matters.  Here I describe a couple of ideas for an additional indicator that can quantify habitat fragmentation.

 

Qualities of a useful indicator

  1. consistent over time
  2. intuitive and interpretable
  3. easy to calculate

 

Types of fragmentation impacts

  1. intrusion (I) – development occurs at the edge of a habitat patch
  2. perforation (II) – development occurs in the middle of a habitat patch
  3. fragmentation (III) – development breaks a patch into two or more patches

 

 

Candidate A. Number of polygons

1.      Count the number of patches of habitat, where larger number indicates increased fragmentation

  1. Simple, easy to calculate, must clip habitat with development
  2. Does not distinguish types I and II
  3. Number of patches can also decrease, as when a small patch is eliminated by development.
  4. Very sensitive to quality of spatial data.

Text Box: # polys

 

Text Box: area

 

 

 

 

 

Candidate B. Total length of edge

1.      More complex shaped patches have longer edge, larger number indicates increased fragmentation.

2.      Straightforward to calculate, must clip habitat with development

3.      Distinguishes types I, II, and III

4.      Again, length can decrease as patch size decreases. Can standardize by computing perimeter/area ratio, but still can decline if a patch is eliminated.

 

Candidate C. Average distance from development to habitat

1.      As development increases, average distance to habitat will decline.

  1. Straightforward to calculate, robust measure, clip distance bands to habitat map
  2. Distinguishes types I, II, and III
  3. Average value will always decline, never increase

                           

Text Box: I

Comments

1.      Can standardize each proposed indicators by using value calculated during baseline year (1998).

2.      Need to use the same IWH map to be able to compare change over time (or at least adjust the indicators when IWH map is modified).

  1. Need to define “developed” vs. “protected”, especially for calculating number of acres indicator
    1. developed: parcels (up to 35-40 acres) where a housing unit can be built (~80% of parcel is within 200 m of house in 40 acre parcel)
    2. protected: only if no buildings can be constructed, no significant change in native vegetation, domestic pets restrained, hiking prohibited at critical times of year (needs to be defined during site-review?)