Wildlife passages how to evaluate their effectiveness
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Wildlife passages – How to evaluate their effectiveness?. Edgar van der Grift Edgar.vandergrift@wur.nl. Worldwide many wildlife passages in all forms/sizes, but…. Dassentunnel. …do they work?. Much baseline information about passage performance available!. Knowledge gab 1.

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Wildlife passages how to evaluate their effectiveness

Wildlife passages – How to evaluate their effectiveness?

Edgar van der Grift


Knowledge gab 1
Knowledge gab 1 available!

Many studies about passage use…

But little knowledge about passage effectiveness!

Knowledge gab 2
Knowledge gab 2 available!

Some studies address effectiveness…

But in most of them the effectiveness of wildlife passages on population persistence remains unclear!

Wildlife passage use
Wildlife passage use available!

Studying use is not useless!


  • Use ≠ Effectiveness

  • Use does not provide evidence for effects of wildlife passages on population persistence !

Use effectiveness
Use ≠ Effectiveness available!

3 x 1 badger?


3 crossings of badger per time period T

But: what does it mean?

1 x 3 badgers?

Use effectiveness1
Use ≠ Effectiveness available!

1 x 3 badgers

After road construction:

Before road construction:

3 x 3 badgers

Usually no monitoring “before” !

Wildlife passage effectiveness
Wildlife passage effectiveness available!


The extent to which the objectives (= desired effect) for a wildlife passage are reached

no objective = no effectiveness

(And: no problem = no objective)

Setting smart objectives
Setting “SMART” objectives available!

S = Specific What are we going to do?

M = Measurable How much?

A = Acceptable Is there enough support?

R = Realistic Is it possible what we want to do?

T = Time frame When are we finished?

Objective for each project must be specific to the location, species of concern and nature of the problem

Working out a monitoring approach
Working out a monitoring approach available!

  • What are suitable research species?

  • What are suitable research sites?

  • What is the best study design (options)?

  • Which research methods (surveys/analyses) are most promising?

  • What measurement schemes should be used?

  • What are the estimated costs?

Choose appropriate research species
Choose appropriate research species available!

  • Species is (highly) vulnerable for road impacts

  • Proven use of wildlife passages

  • Road impact is measurable

  • Species allows for study design with high inferential strength

Choose appropriate research sites
Choose appropriate research sites available!

Species X: not viable viable highly viable

Select locations where wildlife passages are expected to make a large difference (e.g. by modeling population viability)

with infra

without infra

Choose appropriate study design b baci ba or ci
Choose appropriate study design: (B)BACI, BA or CI available!

(Roedenbeck et al., 2007)

Before road Before After

construction mitigation mitigation


Control 1

Control 2

Study design in literature review n 123 studies
Study design in literature review (n=123 studies): available!

  • Before-After (BA) comparison approach: n=15

  • Use of controls in <10 studies

  • Most studies retrospective: only survey of crossing structures after construction

    In practice: trade-offs between perfect study design and reality!

  • “Before” situation already in the past

  • No controls, no replication

  • Randomisation not feasible

  • Limited budget

  • ….

Do not only include the wildlife passage in your study design
Do not only include the wildlife passage in your study design!

  • <50% studies in literature review incorporated assessment of presence/abundance target species around wildlife crossing

  • Not measuring the population adjacent to road may result in wrong conclusions about wildlife passage performance


  • Reduction in roadkill does not necessarily mean the crossing structure is effective

Importance of population surveys
Importance of population surveys design!


Example 1:

Roadkill 10 5 50% ↓

Population size 100 20 15% ↑

Example 2:

Roadkill 10 10 0%

Population size 100 200 50% ↓

Monitoring! design!

Photo: Bart Siebelink

Photo: Bart Siebelink

Review: design!

  • Tracking pads (n=74)

  • Video / infra-red still cameras (n=36)

  • Collection/identification scats (n=16)

  • Direct observations (n=13)

  • Trapping (n=12)

  • Collection/identification hair (n=8)

  • Radio-tracking (n=7)

  • Trail monitors/wildlife counters (n=6)

  • Other (including dusting with fluorescent pigment)

Select appropriate research methods1
Select appropriate research methods design!

Go (sometimes) off the beaten track!

Large species over-represented in existing studies, partly because of chosen research methods

New techniques in development: e.g. genetics, PIT tags

Dry zone design!

Wetland zone

Distribution amphibians across overpass

Wildlife overpass design!

Distribution amphibians across overpass

Common toad

Common frog

Smooth newt

Crested newt

Green frog

Number of observations 2006-2007

adults/juveniles (n=968)

Significant more amphibians in wetland zone
Significant more amphibians in wetland zone design!

Number of observations

Efficiency method artificial refugia
Efficiency method “artificial refugia” design!

Number of observations

Crested newts: Belly patterns design!

Identification of individuals and individual movements across the overpass

Crested newts: Belly patterns design!

Identification of individuals and individual movements across the overpass

Appropriate measurement scheme
Appropriate measurement scheme design!

  • Appropriate monitoring period

    (review: 4 nights – 8 yrs)

  • Appropriate frequency of measurements within period

    (review: 1/day – 1/week)

  • Appropriate number of impact and control sites

Recommendation:Assessment of measurement scheme on the basis of dynamic population model simulations of monitoring

Appropriate measurement scheme1
Appropriate measurement scheme design!

Monitoring changes in populations

1 study site

10 study sites











Survey: 26 years

Survey: 12 years

Maximize inferential strength validity
Maximize inferential strength (validity) design!

Inferential strength

= high probability and low uncertainty that research result is true

Dependent on:

  • Number of competing hypotheses tested

  • Extent of extrapolation

  • Study design

  • Measurement scheme

Implications design!

  • Without studies with high IS it will be difficult to prove the effectiveness of defragmentation programs and to justify investments

  • Correlation between increasing IS and increasing costs

    However: investment in a few good but costly experiments should be prefered above numerous poor experiments at low costs

  • Experiments with high IS carried out over the relevant temporal and spatial time scales may not be feasible in isolated studies, but may be conducted in collaborative(international) networks of researchers

Complex problem time for collaboration
Complex problem: time for collaboration...! design!

This badger gate is for 94.68% effective!

Use effectiveness2
Use ≠ Effectiveness design!

Questions and discussion