MEASURING THE ENVIRONMENTAL IMPACT OF TRAILS

Note: This post was written prior to the creation of this blog and as part of an earlier stage in the ongoing public debate over Boulder Open Space and Mountain Parks policies. While somewhat dated, the post still raises important issues which are still relevant.

An evolving quantitative approach by Suzzane Webel

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DEFINITIONS AND ASSUMPTIONS

DATA from Miller & Knight’s “Recreational Trails and Bird Communities” (COBOS, 1995)
can be extrapolated from birds to other species

BEHAVIORAL EFFECT = AVERAGE AMOUNT THAT VARIOUS SPECIES IN A STUDY AREA "CARE" ABOUT TRAILS (%) - (includes population counts, nest success, predation, flushing distance, change in heartrate, etc.).

• Ex: “Trails cause 2%/day decrease in nest success among some bird species”
  Assume 30 day incubation period, same risk every day; Nest failure =.02/day=(.98) to the 30 power = 0.545 success = 0.445 decrease


• Ex: “The probability of a nest occurring is 50% greater along trails than transects”

SPECIES EFFECT = NUMBER OF SPECIES IN A STUDY AREA THAT "CARE" ABOUT
TRAILS (%)

• Ex: “15% of grassland species were found in greater abundance farther from trails”


• Ex: “17% of forest species are found in greater abundance farther from trails”

TRAIL DENSITY = TRAIL MILES / SQUARE MILES Useful for comparing jurisdictions, amount of effective habitat, etc

• Ex: 70 miles of trail / 50,000 ac ( / 640 ac/sq.mi.) = 0.896

WIDTH OF IMPACT = (ASSUME ANY REASONABLE WIDTH (FT))

• Typically ranges from 0-600 ft from trail -- most measurable effects diminish beyond 100'. Assume ave. corridor width is 100’.


• The width conversion factor is 100 / 5280 = 0.019

ENVIRONMENTAL SACRIFICE AREA = TRAIL DENSITY x WIDTH FACTOR

Hypothetical area of 100% devastation along trail inside of which no bird dares fly, no mouse dares cross, no grass dares grow -- every day, year round.

• Ex: 0.896 x .019 = .017 ( = 1.7% of the entire jurisdiction is "devastated" by trails) CHANGE IN OVERALL HABITAT INTEGRITY = (E.S.A.) x (B.E. or S.E.) = Habitat Fragmentation Potential = The “bottom line”: What is the real impact of trails?


• Ex: 0.017 x (-0.15) = -0.00255 (= 0.3% decrease in grassland birds)


• Ex: 0.017 x (-0.17) = -0.00289 (= 0.3% decrease in forest birds)


• Ex: 0.017 x (-0.445) = -0.00757 (= 0.8% decrease in nest survival)
• Ex: 0.017 x (+0.50) = +0.00850 (= 0.9% increase in nest placement)

CONCLUSION: FOR CITY OF BOULDER OPEN SPACE AND MOUNTAIN PARKS, TRAIL USERS HAVE NO STATISTICALLY MEASURABLE NET EFFECT ON HABITAT INTEGRITY.

THE QUESTION: HOW MUCH RECREATIONAL ACCESS SHOULD BE GIVEN UP (CLOSED) TO ALLAY FEARS OF POSSIBLE REDUCTIONS IN HABITAT WHEN NONE CAN BE MEASURED???