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Final Report

REVIEW OF MITIGATION MEASURES USED

TO DEAL WITH THE ISSUES OF HABITAT

FRAGMENTATION

December 2008

Authors & Title: van der Ree, R., Clarkson, D.T., Holland, K., Gulle, N., Budden M., 2008. Review of Mitigation Measures used to deal with the Issue of Habitat Fragmentation by Major Linear Infrastructure, Report for Department of Environment, Water, Heritage and the Arts (DEWHA), Contract No. 025/2006, Published by DEWHA.

Acknowledgements: The authors would like to acknowledge the following for their advice and comments in the preparation of this report: Kelly Benson, Sylvana Maas, Vanessa Place, Chris Murphy, Joel Benjamin, Dianne Cameron, Miriam Goosem, Rhidian Harrington, Scott Watson, Leanne Maas, James Leslie, Kevin Roberts, Brendan Taylor, Tony Mitchell, Ian Harris, Mark Fitzgerald, Jay Quadrio and Trevor Ferris.

Photo acknowledgements: Australian Research Centre for Urban Ecology (van der Ree, R.), SMEC (Ferris, T.), Thiess Pty Ltd (Bax, D).

Copyright: The report was prepared by SMEC (Australia) and the Australian Research Centre for Urban Ecology for Department of Environment and Water Resources. The concepts and information contained in this document are the property of Australian Government Department of Environment, Water, Heritage and the Arts.

Disclaimer: The views and opinions expressed in this report do not necessarily reflect those of the Commonwealth Government. While reasonable efforts have been made to ensure that the contents of this publication are factually correct the Commonwealth Government does not accept responsibility for the accuracy or completeness of the contents, and shall not be liable for any loss or damage that may be occasioned directly or indirectly through the use of, or reliance on, the report. Readers should exercise their own skill and care with respect to their use of the material published in this report and that users carefully evaluate the accuracy, currency, completeness and relevance of the material for their purposes.

Table of Contents

1 Executive Summary 1

2 Terms of Reference 3

3 Effects of Linear Infrastructure on Wildlife 4

3.1 Introduction 4

3.2 Direct and indirect loss of habitat 5

3.3 Effects on wildlife 5

4 Effectiveness of Measures to Mitigate Habitat Fragmentation – A Literature Review 8

4.1 Introduction 8

4.2 Methods 8

4.3 Results 13

4.4 Discussion 31

5 Cost – Benefit Analysis of Habitat Fragmentation Mitigation Measures 38

5.1 Paucity of hard data for effective financial cost and ecological benefit 38

5.2 Variables in the cost – benefit analysis 38

5.3 Accounting for the differences in cost 39

6 General Principles 42

6.1 The Principles 42

7 Conclusions and Recommendations 45

7.1 Effects from major linear infrastructure 45

7.2 The need for crossing structures 45

7.3 The use of wildlife crossing structures 45

7.4 The efficacy of wildlife crossing structures 45

7.5 Pre-construction studies 46

7.6 Monitoring and evaluation 46

7.7 Information dissemination 47

7.8 Cost and benefit 47

7.9 General principles and national guidelines 47

7.10 Standard definition of terms 48

7.11 Future research 48

8 Summaries of Relevant Literature 50

8.1 Australian refereed journal and book articles 50

8.2 Australian reports, theses and conference proceedings 59

8.3 Australian ‘best practice’ manuals and guidelines 77

8.4 International refereed journal publications 84

8.5 International reports, theses and conference proceedings 103

8.6 International reviews and syntheses 106

8.7 International ‘best practice’ manuals and guidelines 108

9 References 109

TABLES

Table 1: Definition of engineering options to mitigate the fragmentation effects of linear infrastructure 9

Table 2: Location and type of structure across roads designed to mitigate fragmentation effects in Australia 14

Table 3: List of Australian species (or species group) observed using culverts, tunnels, underpasses, land bridges and canopy bridges 17

Table 4: Reference details for Table 3 and most commonly detected species using crossing structures 26

Table 5: Approximate cost of various structures 40

Table 6: Approximate Highway Construction Costs in 2006 40

FIGURES

Figure 1: An example of a land bridge, or “eco duct” with a front elevation nearing completion at Yelgun on the Pacific Highway in north eastern New South Wales. 10

Figure 2 and Figure 3: Examples of two operational land bridges in the Netherlands 11

Figure 2 and Figure 3: Examples of two operational land bridges in the Netherlands 11

Figure 4 and Figure 5: Culverts facilitating wildlife movement under the East Evelyn Road in the Atherton Tablelands, Queensland. 11

Figure 4 and Figure 5: Culverts facilitating wildlife movement under the East Evelyn Road in the Atherton Tablelands, Queensland. 11

Figure 6: A typical box-cell culvert under a railway line. 11

Figure 7: A bridge underpass, at Slaty Creek, Calder Freeway in Central Victoria 11

Figure 8, 9 and 10: Canopy bridges for arboreal marsupials across Wakehurst Parkway near Sydney, NSW (top left), Atherton Tablelands, QLD (bottom left) and across the Pacific Highway, NSW (right). 12

Figure 8, 9 and 10: Canopy bridges for arboreal marsupials across Wakehurst Parkway near Sydney, NSW (top left), Atherton Tablelands, QLD (bottom left) and across the Pacific Highway, NSW (right). 12

Figure 8, 9 and 10: Canopy bridges for arboreal marsupials across Wakehurst Parkway near Sydney, NSW (top left), Atherton Tablelands, QLD (bottom left) and across the Pacific Highway, NSW (right). 12

Figure 11: The first experimental glider pole, located on the Princes Highway in SE NSW (left).
Figure 12 and 13: An aerial fauna crossing during the day, and in use on the Karuah Bypass on the Pacific Highway (right) (Photos courtesy of David Bax, [Thiess Pty Ltd] 2006). 13

Figure 11: The first experimental glider pole, located on the Princes Highway in SE NSW (left).
Figure 12 and 13: An aerial fauna crossing during the day, and in use on the Karuah Bypass on the Pacific Highway (right) (Photos courtesy of David Bax, [Thiess Pty Ltd] 2006). 13

Figure 11: The first experimental glider pole, located on the Princes Highway in SE NSW (left).
Figure 12 and 13: An aerial fauna crossing during the day, and in use on the Karuah Bypass on the Pacific Highway (right) (Photos courtesy of David Bax, [Thiess Pty Ltd] 2006). 13

Figure 14: An example of local area traffic management. Traffic calming devices include signage (pictured), speed humps and crosswalks. 13

Figure 15: Typical culverts for drainage on the left, and a modified box cell culvert for the movement of Brush-tailed Phascogales on the right. 32

  1. Executive Summary

Ecological crossing structures on major linear infrastructure do work to mitigate habitat fragmentation. The crucial questions are how well certain measures work, in what circumstances, which structures work best, for which species, and how performance can be improved.

This report identifies and reviews an extensive body of literature on the effectiveness of measures that aim to mitigate the fragmentation effects of linear infrastructure, such as roads, railway lines, and utility easements (e.g. powerlines, pipelines). A broad purpose of this report is to provide information on effectiveness to assist the Department of Environment, Water, Heritage and the Arts evaluate proposals submitted to it under the Environment Protection and Biodiversity Act (1999).

It was a review of all key documents in Australia, including information published within scientific refereed journals, conference proceedings and consultants reports that provide data on the use of mitigation measures. Documents from overseas were included only if published in a refereed scientific journal or where it demonstrated a high standard of monitoring and of significance to Australia.

The general effects of linear infrastructure and traffic on the environment are numerous and varied, hence these overall effects are only mentioned briefly in the introduction section of this report. This report aims to identify the types of mitigation measures that have been installed, the species that use them and to evaluate the extent to which they restore connectivity.

Over 50 scientific articles and consultant’s reports were identified as possibly providing sufficient information on the type of mitigation measure, and the type and rate of use by different species or faunal groups. Twenty-nine publications described efforts in Australia to facilitate the safe crossing of linear infrastructure by wildlife.

Fragmentation of habitat has been mitigated by the installation of wildlife-specific structures (e.g. culverts, underpasses, overpasses, glider poles and canopy bridges), the use of existing drainage structures (culverts, pipes and bridges) and the design of linear infrastructure to maintain connectivity (e.g. maintaining canopy connection above the road or in gullies below powerlines, or elevating the linear infrastructure above the vegetation).

There is a great deal of ambiguity in the literature on the definition of structure type. The greatest confusion appears to be in the case of culverts, underpasses, tunnels and bridges. In these four situations, animals go under the linear infrastructure; henceforth, we propose that the structure be named by its design (e.g. 3m x 3m box-cell culvert), rather than its function (e.g. underpass).

Most mitigation measures facilitated the crossing of the linear infrastructure by a variety of species of wildlife. Crossing structures and other mitigation measures were successful at reducing the fragmentation effects of linear infrastructure for a wide range of species, particularly mammals. Different monitoring methods were used to document the use of structures by wildlife. Complete traverses were documented in a limited number of cases, while most inferred complete crossings by indirect evidence (e.g. footprints).

The stated or implied goal of most mitigation projects was to ‘‘restore connectivity’’. However it is difficult to assess effectiveness without a specific and measurable goal. Often the goal is not stated, is ambiguous or is imprecise. Thus, for example, it is not clear whether the goal of some of the mitigation measures were to restore connectivity to a ‘preroad’ condition, a ‘pre-road upgrade’ condition or to some other predetermined level that will maintain population processes.

There is sufficient evidence to demonstrate that many species of terrestrial vertebrates will use a range of crossing structures. The literature to date suggests that mammals are more likely to cross than other species. This is most likely an artefact of sampling methods, as they are targeted towards medium/large mammals. There is very little information on the use of mitigation structures by invertebrates, many of which are likely to be greatly affected by fragmentation effects of linear infrastructures.

In summary, many species of wildlife will use underpasses and/or overpasses to cross linear infrastructure, including devices not specifically designed for the passage of wildlife. The distribution and abundance of wildlife, as well as a range of landscape, habitat and structural factors, have been shown to influence the rate of use.

Many important and fundamental research questions remain unanswered, both within Australia and overseas. In the light of this we would advocate focusing on fewer, higher order research questions.

After conducting this review we identified areas needing attention and make some recommendations which are contained in Section 7 of this report. In summary some of these are that:

  • Preliminary ecological studies of the potential impact of possible new major linear infrastructure should be conducted at the earliest possible moment to give the best opportunity to design and conduct an adequate and useful base line study. These preliminary ecological studies should be properly designed, and adequately funded.

  • Greater use be made of the SMART approach to evaluate mitigation effectiveness (this requires that each goal is specific, measurable, achievable, realistic and timeframed).

  • Monitoring be an integral part of the construction and management process for all major infrastructure projects.

  • General principles be sent to all parties involved in the funding, commissioning, reviewing, specifying, designing and constructing of major linear infrastructure. These general principles are:

  • fragmentation is only one of the effects of linear infrastructure

  • avoid environmentally sensitive areas

  • identify the nature of the issues

  • better to connect than fragment

  • identify the goals for mitigation (use the SMART technique)

  • design mitigation structures for faunal groups, communities and ecosystem processes

  • mitigation structures should be for a wide range of species

  • understand conditions and populations adjacent to structures

  • conduct and support targeted research, and

  • monitoring should be an integral part of the construction and management process.

  • The Department of Environment, Water, Heritage and the Arts take a lead in developing national guidelines based on the general principles.

  1. Terms of Reference

The objectives (as set by Department of Environment, Water, Heritage and the Arts) for this comprehensive literature review were to assess the:

  • effectiveness of mitigation measures employed to ameliorate the habitat fragmentation impacts of major infrastructure

  • past and present monitoring programs of the mitigation effects of major infrastructure, including their scientific merit

  • cost-benefits of mitigation measures, in relation to overall infrastructure project costs.

  1. Effects of Linear Infrastructure on Wildlife

    1. Introduction

Roads, train lines, powerlines and other linear infrastructure are pervasive components of landscapes throughout the world. There is a growing recognition of their deleterious impacts on the natural environment and the need to quantify and mitigate these impacts (Andrews 1990; Spellerberg 1998; Forman et al. 2002; Donaldson and Bennett 2004; Goosem 2004; Davenport and Davenport 2006; Roedenbeck et al. 2007). The effects of linear infrastructure, and the traffic along them, are diverse and include many direct and indirect effects, such as:

  • loss, fragmentation and degradation of habitat

  • incursion of weeds, disease, dust, pollution and feral animals

  • direct mortality of wildlife, due to collision with vehicles

  • disruption of movements due to the creation of barriers

  • altered microclimatic conditions

  • disturbance due to vehicle movement, vehicle noise, headlights, and other light sources.

The type and severity of each effect, and the distance it extends into adjacent land varies according to the attributes of the road and surrounding landscape.

The ‘road-effect zone’ is defined as the area over which significant ecological effects of a road and its traffic extend into the adjacent landscape (Forman and Deblinger, 2000). Depending on the width of the road, traffic volume, vehicle speed and characteristics of the adjacent landscape (e.g. slope, vegetation type, direction and speed of prevailing winds), these effects can be observed for distances that may exceed 1000 metres for arterial roads with large volumes of traffic (see Forman et al. 2002 for summary of road effects and distances).

Much of the research quantifying the effects of roads and traffic on the environment has been conducted in North America and Europe, and along arterial roads that carry large volumes of traffic. There is limited direct information on these effects on Australian species and landscapes, or on narrow roads that have relatively low traffic volumes.

The extent of information on the effects of powerlines and other linear infrastructure on the natural environment is small compared to the study of road effects, but there is a growing body of knowledge in Australia (e.g. Goldingay and Whelan 1997; Goosem 1997; Goosem and Marsh 1997; Goosem 2004; Clarke et al. 2006) However, it is believed the general principles learned from the work in North America and Europe can be used to make inferences on the possible effects on Australian species, communities and landscapes. In addition, there are now an increasing number of studies on the specific effects of roads and other linear infrastructure on the Australian environment. Hence, what follows is a brief summary of some of the major effects of linear infrastructure and traffic on the natural environment. Readers interested in a more detailed summary and syntheses of the effects of linear infrastructure on the environment are referred to the publications given at the start of Section 4.1.

    1. Direct and indirect loss of habitat

      1. Effects on vegetation

Much natural vegetation along linear infrastructure corridors is lost during construction and in post-construction maintenance activities. To prevent and ameliorate this loss and degradation of vegetation, linear infrastructure should be:

  • aligned to minimise the direct loss of habitat

  • avoid rare or threatened species or communities, and

  • protect valuable habitat components, such as large hollow-bearing trees.

In locations where the alignment cannot be altered, options for ‘offsetting’ or compensating for the clearing of vegetation at the road construction sites should be taken. Currently there is considerable discussion about the merit and efficacy of the different compensation arrangements (Cuperus et al. 1999; Gibbons and Lindenmayer 2007).

There may be ongoing indirect loss of habitat adjacent to the linear infrastructure that occurs as a result of incremental removal or degradation of habitat. Altered hydrological, nutrient and microclimatic conditions along a linear clearing (e.g. Pohlman et al. 2007) may result in the growth of plant species that prefer more sunlight and water (e.g. Lee 2006), increasing growth rates and fecundity (e.g. Lamont et al. 1994a; Lamont et al. 1994b). These novel conditions are made available through increased water run-off from the road surface, opening up of the canopy and regular disturbance regimes (Spooner, 2005). Increased productivity of plants and especially of weedy species adjacent to the linear clearing is a commonly reported effect when the clearing traverses relatively natural areas. A study of the effects of roads and traffic on the plant species composition of heath-land vegetation in Hampshire UK, found that there was enhanced growth of heather (Calluna vulgaris) and grass species (Mollinia caerulea) near the road than further away (Angold 1997). This effect extended for up to 200 metres adjacent to a dual carriageway highway carrying approximately 35 000 vehicles during a 12-hour period. Data from nine sites adjacent to narrower roads with traffic volume ranging from 800 – 10 000 vehicles per 12 hrs indicates the edge effect is predicted to be 15 metres for 100 vehicles per 12 hrs, and 25 metres for 300 – 350 vehicles per 12 hours (Angold 1997). Vegetation adjacent to roads may also experience higher rates of defoliation due to an increase in the density of herbivorous insects compared to further away. Increased levels of foliar nitrogen through pollution adjacent to the road could also be a primary mechanism supporting higher insect densities and higher rates of herbivory activity (Port and Thompson 1980; Angold 1997). These factors are also assumed to be occurring in Australia.

    1. Effects on wildlife

      1. Loss of habitat

Direct and indirect loss of habitat caused by linear infrastructure will also affect fauna. Loss of habitat will decrease the viability of populations by reducing the size of the population that can be supported in that area. Quantifying the amount of habitat lost directly is relatively self-evident, while indirect losses are more difficult to quantify. A study of the Horned Lark in Illinois, USA demonstrated lower population densities in farm paddocks within 200 metres of country roads with 300 – 3000 vehicles per day (vpd) compared with further away (Clark and Karr 1979). A similar effect was evident near Boston, Massachusetts USA, where the presence and breeding of grassland birds adjacent to a two-lane highway (15 000 – 30 000 vpd) was reduced for up to 700 metres, and up to 400 metres when 8000 – 15 000 vpd was present (Forman et al. 2002a). The same study found that Traffic Flows Of 3000 – 8000 vpd Had No Obvious Effect On The presence or breeding of grassland birds (Forman et al. 2002a). In contrast, the abundance and richness of forest birds was reduced by 20 per cent adjacent to a powerline easement in southern New South Wales (Baker et al. 1998). Significant reductions in both abundance and richness of this bird community were evident up to 125 metres from edge of the easement (Baker et al. 1998). Similarly, Spotted Owls nesting within 400 metres of a logging road (with an estimated 5 – 50 vpd) had higher levels of stress hormones than owls further away (Wasser et al. 1997).

      1. Habitat fragmentation

The internal fragmentation of natural areas is recognised as a significant threat in parks and nature reserves. Some park agencies in Australia have programs to close and rehabilitate roads that are no longer required (Donaldson and Bennett 2004).

The division of habitat into smaller fragments results in lower population sizes. When roads act as a complete barrier or selective filter to movement, as is the case for many wildlife species (Forman et al. 2002b), these smaller populations may not be connected to other populations, and hence they are at a higher risk of extinction. This is because new individuals or plant propagules are unable to supplement a declining population or to re-establish a locally extinct population. The main factors influencing the barrier effect of a road relate to:

  • road width

  • traffic volume, and

  • behaviour of the species.

Species of animals most at risk of population fragmentation due to roads and traffic include species that are unwilling to travel across cleared areas (Forman et al. 2002b). Birds that favour large blocks of habitat, such as ‘forest-interior’ species, are more likely to be affected than ‘edge species’ that can feed in open areas but also occupy forest. Studies of the movements of understorey birds in the Amazonian rainforest found that roads with a 10 – 30 metres wide canopy opening typically formed a territorial boundary while birds more frequently crossed a road where the canopy remained intact (Develey and Stouffer 2001).

Many species of invertebrates and amphibians may also be at risk of population fragmentation because of low levels of mobility, avoidance of the unsuitable surface of roads and the relatively high potential for collision with vehicles (Mader 1984; Baur and Baur 1990; Reh and Seiz 1990; Gibbs 1998). Some species of reptile are at risk because they avoid roads, as was demonstrated for Blue-tongued Lizards in the suburbs of Sydney, where home range boundaries were aligned with roads, and they actively avoided crossing the roads (Koenig et al. 2001).

Even narrow roads (e.g. 3 metres in width) appeared to inhibit the movement of small mammals (Barnett et al. 1978; Swihart and Slade 1984), but did not completely eliminate road crossing. The response of species is often specific and even species within the same guild (e.g. small terrestrial mammals) have been shown to display different roadcrossing abilities (Goosem 2001).

      1. Mortality due to collision with vehicles

Mortality of wildlife due to collision with vehicles is related to the behavioural characteristics of the species, traffic volume and vehicle speed (Dhindsa et al. 1988). Numerous studies have clearly demonstrated that large numbers of creatures in a wide array of species are regularly and frequently killed along roads (Vestjens 1973; Davies et al. 1987; Coulson 1989; Rosen and Lowe 1994; Goosem 1997; Statham and Statham 1997; Scott et al. 1999; Haxton 2000; Hels and Buchwald 2001; Shuttleworth 2001; Taylor and Goldingay 2003). Road mortality has been identified as a major factor contributing to the decline of many species (Fahrig et al. 1995; Vos and Chardon 1998; Jones 2000; Hels and Buchwald 2001). Road mortality may even have significant impacts on population viability for relatively common species, such as the Swamp Wallaby Wallabia bicolor in peri-urban areas (Ramp and Ben-Ami 2006). It is important to note that in many situations, road mortality is not the only factor contributing to population decline. Other factors may include the loss, fragmentation and degradation of habitat, predation pressure, and competition. The rate of mortality is also related to the position of the road within the landscape. Brown et al. (1986) found a higher incidence of bird-mortality where roads and watercourses intersect.

The potential long-term and population-level effects of increased rates of wildlife mortality on local populations are unknown and probably complex. If mortality is a simple linear function of road length and traffic volume then doubling the number of vehicles and the amount of road surface could simplistically lead to a four-fold increase in the number of individuals killed (e.g. from 10 per year to 40 per year). However, these relationships are unlikely to be linear because each road death will reduce overall population density, potentially reducing the likelihood of other individuals of the same species being killed. The number of species that potentially scavenge on road-killed carcasses can be expected to be drawn to the greater feeding opportunities, thus increasing the probability that scavengers also being killed. Similarly, species that graze on mown vegetation on the road verge are also at higher risk of mortality. A high rate of mortality of Swamp Wallabies on North Stradbroke Island in Queensland occurred because they were attracted to feed on the road verges that were regularly mown (Osawa 1989).

      1. Incursion of feral species

Linear infrastructure can have a detrimental impact on wildlife through increased access and rates of predation by feral predators (May and Norton 1996) and Cane Toads (Seabrook and Dettmann 1996; Brown et al. 2006), as well as predation on bird eggs and young still in the nest (Donaldson and Bennett 2004). Increasing the width of the clearings associated with road and powerlines may potentially attract nest predators and lead to a decline in the abundance of some species of bird.

Roads and traffic may also facilitate the invasion of weeds and exotic plants as seeds attached to undercarriages in mud and dirt (Amor and Stevens 1976; Lonsdale and Lane 1994). Vehicles may bring seeds from a large potential catchment and move them across the landscape rapidly. Roadside verges (during and post-construction) are often modified and become more suitable for seed germination.

  1. Effectiveness of Measures to Mitigate Habitat Fragmentation – A Literature Review

    1. Introduction

The objective of this section of the report is to review the effectiveness of measures used to mitigate the fragmentation of habitat by major linear infrastructure.

The fragmentation effect of linear infrastructure occurs when the rate at which animals are able to traverse the developed area is reduced. If all movement is halted, the infrastructure becomes a complete barrier for the species. If a proportion of individuals are capable of crossing it, then it can be termed a semi-permeable barrier or filter. Filter effects may be species, sex or age-specific, or it may affect individuals within the population at random (e.g. van der Ree 2006). Barrier effects may occur due to avoidance of the novel linear infrastructure or due to increased rates of mortality as crossing is attempted. Avoidance occurs when animals avoid the linear infrastructure due to the establishment of unsuitable habitat or because of the road effect zone (see section 3.1). The avoidance zone extends for a variable distance on either side of the infrastructure and is caused by changes in the habitat, noise or other disturbance. Increased rates of mortality may occur due to direct collision with vehicles (in the case of roads and railway lines) or increased predation as a result of decreased shelter or protection or increased densities of predators.

    1. Methods

      1. Definitions

According to the Oxford English Reference Dictionary, the definition of mitigation is ‘to make milder or less intense or severe’. Our review focuses in part on the extent to which the barrier or filter effect has been made less severe.

Linear infrastructure is loosely defined as any linear landscape element that has been constructed or modified by humans to allow the passage of people or resources. Linear infrastructure includes:

  • roads

  • railway lines

  • pipelines, and

  • powerlines and other utility easements.

The most common linear infrastructure worldwide are roads, and most of the literature we encountered focuses on the effects of roads and traffic and the use of mitigation structures across them. Therefore, ‘road’ and ‘linear infrastructure’ are often used interchangeably throughout this report. Where specific to other types of linear infrastructure (e.g. powerlines or railways) they are mentioned specifically.

A wildlife crossing structure is defined in this report as ‘a physical structure that increases the permeability of the road or other linear infrastructure by facilitating the safe passage of animals over or under it and in the case of roads and railways, preventing collision with vehicles’. Wildlife crossing structures may be purpose built for wildlife or may primarily serve other functions (e.g. water drainage or access by humans). Other methods to mitigate the barrier effect of linear infrastructure include systems that are not strictly ‘wildlife crossing structures’ (Goosem 2004). These include maintaining canopy connectivity above the road or below a bridge, installing powerlines above the canopy of the forest with clearing of vegetation just for pylons, or establishing transverse strips of vegetation at low points (e.g. gullies and drainage lines) across powerline clearings (Goosem 2004).

In the literature there is considerable confusion and interchangeable use of terms when describing mitigation structures. We have developed the following terms and definitions (Table 1) to reduce confusion and provide consistency when describing the mitigation structures. In essence, we propose that a mitigation measure be described according to its specific structure, rather than its intended use. We propose to use the terms ‘underpass’ and ‘overpass’ as general terms that describe a collection of structures.

In this report we have classified and reviewed all structures in the studies according to their dimension and form, irrespective of the names given them by the authors. When we were unable to classify mitigation structures into one of our specific categories, they are referred to by the general term of ‘underpass’ or ‘overpass’.

Table 1: Definition of engineering options to mitigate the fragmentation effects of linear infrastructure

Title

Description

Overpass*

Allows passage of animals above the road

Land bridge

Also known as eco-duct or wildlife bridge. This is a (typically) wide (30 – 70 metres) bridge that extends over the road. The bridge has soil on it, and is planted with vegetation and enhanced with other habitat features (e.g. logs, rocks, water-body etc) (Figs. 1 – 3)

Overpass (small roads)

This bridge above the major linear infrastructure is typically to allow human access across the road. This overpass is typically narrow and not hourglass shaped. The road on the overpass is typically a minor road – it may be unsealed, single lane etc

Canopy bridge

This is a rope or pole suspended above the traffic, either from vertical poles or from trees. Typically installed for arboreal and scansorial species (Figs. 8 – 10)

Glider pole

These are vertical poles placed in the centre median or on the road verge, and provide species that glide intermediate landing and launch opportunities (Fig. 11)

Local traffic management

Devices to reduce the speed or volume of traffic – e.g. road closures, chicanes, crosswalks, lighting, signage (Fig 14)

Underpass*

Allows the passage of animals below the major linear infrastructure

Culvert

Culverts are typically square, rectangular or half-circle in shape and may be purpose built for fauna passage or water drainage, or a combination of both. They are typically precast concrete cells or arches made of steel (Figs 4 – 6). By definition, culverts were originally used to carry water. However, engineers and road designers are familiar with the size and shape of culverts, and hence we suggest the continued use of the term culvert to describe this type of underpass.

Tunnel

Tunnels are typically round pipes of relatively small diameter (e.g. < 1.5 metres diameter). May also be termed ‘eco-pipe’.

Bridge

A bridge is a structure that maintains the grade of the road or elevates the traffic above the surrounding land, allowing animals the opportunity to pass under the road. When used to mitigate the barrier effect of linear infrastructure, the primary function is often to facilitate water drainage or the movement of local human traffic, and secondarily to facilitate the passage of wildlife (Fig. 7)

Non-Structural Mitigation

This type of mitigation allows for sensitive road designs that facilitate ‘natural’ permeability

Canopy connectivity

The width of the linear clearing is kept sufficiently small to allow the tree canopy to remain continuous above the clearing, or where not continuous, sufficiently small to allow gliders (and other volant species) to safely traverse the clearing

At-grade crossings

Vegetation or other habitat features (e.g. rocks, fallen timber) are strategically planted or allowed to regrow such that animals are directed to preferred crossing locations where they are required to cross the linear infrastructure without the aid of any structures (i.e. similar to a pedestrian crossing)

Elevating the linear infrastructure

The road or powerline is elevated above the vegetation to minimise clearing (clearing only required for bridge piers or pylons) and allow natural vegetation to grow under the infrastructure

Corridor plantings

Are strips of vegetation, similar to that on either side of the linear clearing that traverse the clearing and provide corridors for animal movement.

* There was considerable overlap in use of terms to describe crossing structures, particularly for underpasses. The definitions in this table are an attempt to reflect their design and method of construction, rather than their potential use.



Figure 1: An example of a land bridge, or “eco duct” with a front elevation nearing completion at Yelgun on the Pacific Highway in north eastern New South Wales.



Figure 2 and Figure 3: Examples of two operational land bridges in the Netherlands



Figure 4 and Figure 5: Culverts facilitating wildlife movement under the East Evelyn Road in the Atherton Tablelands, Queensland.



Figure 6: A typical box-cell culvert under a railway line.

Figure 7: A bridge underpass, at Slaty Creek, Calder Freeway in Central Victoria


Figure 8, 9 and 10: Canopy bridges for arboreal marsupials across Wakehurst Parkway near Sydney, NSW (top left), Atherton Tablelands, QLD (bottom left) and across the Pacific Highway, NSW (right).


Figure 11: The first experimental glider pole, located on the Princes Highway in SE NSW (left).
Figure 12 and 13: An aerial fauna crossing during the day, and in use on the Karuah Bypass on the Pacific Highway (right) (Photos courtesy of David Bax, [Thiess Pty Ltd] 2006).

Figure 14: An example of local area traffic management. Traffic calming devices include signage (pictured), speed humps and crosswalks.

Mitigation structures are designed with the needs of the infrastructure and topography very much in mind. Therefore the particular designs of mitigation structures vary. The variation between similar functional structures between different projects can vary considerably. Just a small sample of some engineering drawings is included by way of example at Appendix 2.



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