Linear infrastructure in the tropical rainforests of far north Queensland: mitigating impacts on fauna of roads and powerline clearings

Transcription

1 Linear infrastructure in the tropical rainforests of far north Queensland: mitigating impacts on fauna of roads and powerline clearings Miriam Goosem Cooperative Research Centre for Tropical Rainforest Ecology and Management, School of Tropical Environment Studies and Geography, James Cook University, Cairns, Queensland, Australia ABSTRACT The impacts on rainforest fauna of internal fragmentation caused by clearings for linear infrastructure such as roads, highways and powerlines have been examined in the tropical rainforests of the Wet Tropics in far north Queensland. The impacts include habitat loss through clearing and edge effects, road mortality, disturbance from traffic movement, noise, headlights and pollutants and invasion along the clearings by weeds and fauna alien to the rainforest (including feral species). Barrier effects result from a combination of these factors. Rainforest fauna is often highly susceptible to these impacts due to specialised habitat requirements. Data collected over the past 15 years relating to vertebrate road mortality and trapping and tracking of small mammals, possums, herpetofauna and invertebrates confirm that many rainforest species suffer edge, linear barrier and disturbance impacts. Measures to mitigate these impacts on fauna have been or are currently being trialed and have proved successful for many species of wildlife. Mitigation may be achieved by maintenance of canopy connectivity or restoration of rainforest connectivity via corridors, faunal underpasses under roads and overpasses above them in the form of rope bridges. Preferable mitigatory strategies depend on circumstances and species. Where highways carry large numbers of fast-moving vehicles, funnelling animals under many high bridges with fencing to keep them away from the road will allow wildlife to move in the landscape while reducing road mortality. Alternatively, where roads carry few vehicles, keeping canopy above the road is a cost-effective means of maintaining connectivity for wildlife and aesthetics for tourists without a high road toll, while also reducing road erosion, edge effects, and weed and feral animal invasions. Key words: roads, powerline clearings, tropical rainforest, barrier effects, edge effects, impact mitigation Introduction Queensland s wet tropical rainforests The rainforests and associated natural ecosystems of far north Queensland s Wet Tropics (Figure 1) were inscribed on the World Heritage List in 1988 in recognition of their outstanding natural values with respect to ongoing evolutionary, ecological and biological processes. Listing was also in recognition of the area s exceptional natural beauty and its significance in conserving biological diversity including the habitats of large numbers of rare or threatened species (WTMA 2002a). For many groups of fauna and flora, these forests have the highest diversity of any terrestrial habitat in Australia. Although covering only about 0.25% of the total area of Australia, the Wet Tropics bioregion contains 26% of the total Australian species of vascular plants (2800 species in 1037 genera and 221 families) with representation in some plant groups being even greater (ferns 65%, conifers 37%, orchids 30%) (WTMA 2002b). In Australian terms, the region also has great faunal diversity, with 663 vertebrate species or 32% of Australia s terrestrial vertebrate fauna. Representation of Australian terrestrial fauna includes mammals 35%, birds 40%, amphibians 29%, reptiles 20%, freshwater fish 42% and butterflies 58%. Endemicity of Wet Tropics rainforest fauna and flora is also extremely high, with many species within the region being rare and restricted to small areas. Several of these areas of high local endemism probably represent refuges from environmental changes on evolutionary time scales. Unfortunately, of the vertebrates, 37 species fall into threatened conservation categories, having suffered substantial population declines due to a variety of threatening processes (WTMA 2002a). Therefore, besides the joy and privilege of working in an environment of great beauty (Figure 2), ecologists in the Wet Tropics also aid in the understanding and conservation of some of the highest levels of terrestrial biodiversity in Australia. Internal fragmentation Forest fragmentation caused by clearing for rural and urban purposes is recognised globally as one of the major threats to tropical rainforest fauna (Laurance et al. 1998; Laurance 1999). However, internal fragmentation of large tracts of rainforest is often ignored. Internal fragmentation occurs when natural habitat is fragmented and wildlife populations subdivided by linear clearings (Goosem 1997). Linear clearings associated with infrastructure for transport, energy and water supply take the form of roads, highways, railways, powerline clearings and pipelines. As human populations continue to expand, greater demands will be placed on these services. Routes chosen for linear clearings often traverse areas of otherwise relatively undisturbed natural habitat, subdividing faunal habitat into much smaller areas. Pp in the Conservation of Australia s Forest Fauna (second edition) 2004, edited by Daniel Lunney. Royal Zoological Society of New South Wales, Mosman, NSW, Australia.

2 Linear infrastructure in the tropical rainforests Figure 1. Location of the linear strip of mainly rainforest in north Queensland that comprises the Wet Tropics of Queensland World Heritage Area. Rainforests in the Wet Tropics occur mainly in a long, linear strip encompassing the rugged mountain ranges of the region (Figure 1), while remaining coastal lowland and tableland forests exist in a matrix of intensive agricultural and pastoral industries. Linear clearings dissect the forest strip on the ranges, because they connect the rapidly expanding urban areas on the coast with the rural areas of the tablelands. Currently, there are 320 km of powerline clearings and 1,213 km of roads and highways forming a network of linear clearings through the tropical rainforests of the Wet Tropics of Queensland World Heritage Area (WTQWHA). Roads through this region have long, winding climbs up the mountains. Steep gullies are crossed that require many large water culverts or bridges to cater for fast-flowing streams associated with heavy rainfall from monsoonal lows and tropical cyclones in the wet season. As populations increase, more traffic and demand for faster travel mean that several winding mountain and coastal highways will need to be straightened for safety and widened to provide extra traffic lanes (Maunsell McIntyre and Environment North 2000). Power suppliers are also under pressure to increase capacity and provide back-up distribution lines in case of failure (Powerlink 2002). Rainforest fauna are specialists reluctant to venture beyond the shelter of the closed canopy (Winter 1991). The rainforest habitat is characterised by great structural complexity with a cool, moist microclimate quite different to that of adjacent clearings or open natural habitats. Thus there is an intuitive expectation that rainforest wildlife should be more strongly influenced by linear clearings than species of open habitats (Goosem 1997). In a previous edition of this book, Winter (1991) highlighted conservation issues in the tropical rainforests of Queensland. He suggested that the relatively minor area of clearings for linear and other infrastructure must be considered a cumulative impact that extends beyond the clearing. He posed several relevant questions: To what extent are roads barriers to the movements of species? Is there a critical road width beyond which a significant reduction occurs in crossing movements of animals? Does it matter that a road is a partial barrier, provided the occasional individual manages to cross to ensure genetic mixing of populations on each side of the road? Conser ving Australia s Forest Fauna 419

3 Goosem Figure 2. An environment of great beauty Mt Pieter Botte, about 100 km north of Cairns. Photo from WTMA. Do underpasses and overpasses make a difference to the ability of animals to cross a road? Is the grassy swathe under a power line, with its community of grassland animals, a greater barrier to rainforest animal movement than a bare road because rainforest animals need to contend with the grassland community rather than just the vacant road? What is the actual intrusive effect of a road or powerline on a core area of rainforest? Do these clearings allow exotic species such as toads and pasture weeds, or grassland species such as finches and grassland rats to penetrate deep into the rainforest? How far from the cleared interface does the intrusive effect penetrate into the rainforest? Are there species in Australia that require deep rainforest unaffected by edge effect disturbances? The objective of this chapter is to review the progress made in answering these and related questions, mainly through recent research in the tropical rainforests of the WTQWHA. Ecological impacts of roads and other linear clearings have been identified in habitats ranging from temperate forests through grassland to desert (Trombulak and Frissell 2000; Spellerberg 2002; Forman et al. 2003). Here, their applicability to tropical rainforests is discussed and the potential of several measures designed to mitigate the identified impacts are considered. Impacts of linear clearings in tropical rainforests Habitat loss and alteration Terrestrial Although Winter (1991) pointed out that clearings for linear infrastructure are relatively minor in the context of clear-felling for forestry or rural or urban usage, often the extent of the linear clearing network results in relatively large areas of habitat being lost within otherwise natural areas. For example, in the WTQWHA, approximately 770 ha of habitat has been alienated in 320 km of powerline clearings, while 1610 ha is occupied by maintained roads and highways, and their verges, constituting 0.27% of total habitat in the World Heritage Area (WTMA 2002a). Even within conservation areas, verges and clearings are often maintained as grasslands or low, weedy, shrubby swathes by mowing, grading, burning or spraying with herbicides, resulting in a plant community that is floristically and structurally different from the surrounding forest. However, the area of forest habitat affected by linear clearings (ecological footprint) may be much larger than the actual clearing footprint due to edge effects and other disturbance impacts that penetrate the forest to varying distances. Aquatic Altered habitat can extend well beyond the cleared areas if roads are constructed near streams. Road changes to stream ecology can include erosion, sedimentation, altered flow patterns and channelisation, with consequent upstream and downstream impacts on aquatic and stream bank life (Eaglin and Hubert 1993; Brown 1994; Trombulak and Frissel 2000). Little is understood about this type of impact in tropical forests, although the alteration to stream flows and stream habitats resulting from the larger impact of water impoundments have been examined in Wet Tropics rainforests (Arthington et al. 1994; Bunn et al. 1996). During past logging activities in Wet Tropics rainforests, Gilmour (1971, 1977a,b,c) found that the major proportion of sediment load came from either undrained roads and logging tracks or earth and log-filled stream crossings. Drainage along roads and tracks was also considered vital to reduce high water velocities generated by overland flows during heavy rainfall (Bonell et al. 1981). In a recent study of erosion on unsealed roads in the Wet Tropics, Bacon (1998) found that less erosion and road damage occurred where canopy cover was maintained above the road surface. Erosion was probably reduced because rainfall was intercepted by the multilayered canopy and funnelled away from the road along branches and trunks (Goosem and Turton 1999). Streams may also be polluted by road and highway runoff that contains contaminants from vehicle emissions, lubricants, tyre and brake wear and transport spillages or from herbicides used to maintain low vegetation in clearings. The degree to which highway contaminants enter streams is currently being examined along Wet Tropics rainforest highways (Lottermoser 2002). 420 Conser ving Australia s Forest Fauna

4 Linear infrastructure in the tropical rainforests Disturbance effects the emission of matter and energy The emission of gases, liquids and solids may lead to pollution of air, soil and water adjacent to roads. Emissions of energy in the form of noise, headlights, vibration, movement and electromagnetic radiation may disturb animals, causing them to avoid the vicinity of a road or powerline clearing when selecting feeding or nesting sites. Noise, headlights and movement Many animals of temperate environments avoid areas near roads for a variety of reasons. Avoidance of hunters or tourist traffic associated with roads is common in large mammals (Pienaar 1968; Cole et al. 1997). Regardless of stimulus, such avoidance results in loss of valuable habitat and alteration of daily routines. Likewise, many birds avoid road edges for distances in excess of 200 m (Reijnen et al. 1996), depending on traffic volume and speed, suggesting that traffic noise and/or vibration may be the cause. Traffic noise may cause stress, hearing damage and altered behaviour particularly when communication during the breeding season is masked (Forman and Alexander 1998). Fauna known to be affected by such stimuli also include tortoises (Ruby et al. 1994) and amphibians (Barass 1985). Studies of disturbance from noise or other sources are few in rainforests, although Wilson (2000) found that ringtail possums of the Wet Tropics reacted to vehicular noise and noise from tourists spotlighting along walking tracks. Frequent human foot traffic reduced the intensity of trail use by mammals in Sumatran tropical rainforest and induced nocturnal rather than more usual diurnal behaviour in native pigs (Griffiths and van Schaik 1993). In the Wet Tropics, vehicular noise can penetrate to distances greater than 100 m on flat terrain and even further in gullies and creeklines (Marks and Turton 2000). Noise penetration was affected by type of vehicle, speed and road topography, penetrating further on steep slopes and from noisier sources such as semi-trailers with air brakes. It is likely that such noise could induce avoidance in certain forest fauna, particularly birds and amphibians, that use auditory communication. However, this remains to be investigated. Pollutants The impact on rainforest fauna from pollutants washed from the road surface during rainfall is unknown. In temperate areas, lead, cadmium and zinc accumulated in roadside vegetation and was concentrated through the invertebrate food chain by insects and earthworms to levels that may be toxic to vertebrates (Gish and Christiansen 1973). Small mammals near busy United States highways accumulate significant amounts of lead (Quarles et al. 1974; Getz et al. 1977), placing higher trophic level predators and scavengers at risk of heavy metal toxicity. Pesticides and herbicides also may be cumulative poisons. In the Wet Tropics, heavy metals accumulate in the soils and sediments immediately adjacent to highways, occurring in greatest concentrations at road curves where vehicles accelerate or brake (Diprose et al. 2000). This effect reduces significantly beyond the rainforest edge where dense edge vegetation acts as a screen for particulates (Diprose et al. 2000). It is possible that rainforest invertebrates, such as earthworms occurring near the road edge, could accumulate heavy metals, introducing these toxins into the food chain. Bioaccumulation in invertebrates and the distance of penetration of inorganic and organic pollutants form the first stage of continuing studies in the Wet Tropics examining this impact (Lottermoser 2002). Edge effects Edge effects comprise a diverse array of biophysical changes that occur at and near abrupt, artificial margins between natural habitat and clearings. Most studies of edge effects have concentrated on remnant vegetation and studies in tropical rainforest habitat have been no different. Elevated wind speed and turbulence and increased light penetration at edges result in greater air and soil temperature fluctuations than within the forest interior (Turton and Freiburger 1997). Consequently, evaporation increases while relative humidity and soil moisture decrease (Murcia 1995). Higher light levels promote the growth of disturbance-adapted plants such as weeds, pioneer species and woody vines (Fox et al. 1997; Laurance 1991; Laurance et al. 2001). Greater wind speed and turbulence plus a greater vine load near the rainforest edge causes more damage to canopy trees and even tree death (Laurance et al. 1998). Such alterations in microclimate, vegetation structure and floristics produce changes in habitats for fauna. Avoidance of edges by rainforest specialist fauna is observed with edges allowing greater access to gap and edge specialist birds (Lovejoy et al. 1986; Bierregaard and Stouffer 1997), and generalist insects (Didham 1997), frogs, reptiles (Schlaepfer and Gavin 2001) and small mammals (Laurance 1994; Malcolm 1994; Stevens and Husband 1998). The type of surrounding habitat modulates these changes in faunal community composition regenerating forest next to the edge provides greater potential for generalist species to colonise and may also reduce edge avoidance by specialists. Changes in species composition due to these habitat alterations may also have consequences for ecological processes such as pollination, dispersal, competition or predation (Murcia 1995). Edge effects along powerline clearings and roads through Australian eucalypt forest have been demonstrated for birds (Baker et al. 1998), although not necessarily for small mammals (Goldingay and Whelan 1997). However, studies of edge effects along rainforest linear clearings are relatively few. Certain species of forest interior birds avoid Amazonian road edges (Mason 1996; Laurance 2001). Predation on nests generally increases along the edges of temperate forests, and this effect has been seen in Belize (Burkey 1993) but not in Singapore (Wong et al. 1998). In Central Africa, small mammal abundance and diversity increased along logging roads (Malcolm and Ray 2000). The amount of alteration in both vegetation characteristics and small mammal communities was positively related to understorey density and to the amount of canopy damage. The greatest changes were seen along wide, graded roads (Malcolm and Ray 2000). Conser ving Australia s Forest Fauna 421

5 Goosem Figure 4. A narrow, unsealed road that carries little traffic. Closed rainforest canopy is maintained above the road surface, resulting in narrow verges with no weeds. Photo: M. Goosem. Figure 3. The swathe of grass and woody weeds in the 60 metre wide Palmerston powerline clearing. Photo: M. Goosem. In the Wet Tropics a suite of potential edge effects has been examined in recent years along roads and powerline clearings through rainforest. Edge changes in microclimate have been shown to penetrate to distances of 25 metres or more from a 60 metre-wide powerline clearing (Figure 3) for factors such as air and soil temperatures and vapour pressure deficit, whereas increased light levels were detected to distances of 7-11 m (Siegenthaler and Turton 2000; Maver 2002). Microclimatic edge effects were considerably less intense where linear clearings were narrower. The least microclimatic alteration occurred where the rainforest canopy was completely closed above a narrow road (Figure 4), in which case most changes were measurable only to a distance of 3-7 metres (Siegenthaler and Turton 2000). Microclimatic alterations are seasonal in nature, with dry season gradients into the forest often greater than those found in the wet season (Siegenthaler and Turton 2000). Where restoration of rainforest has been undertaken across a wide powerline clearing (Figure 5), trees only 4 years old and less than half the height of the canopy have significantly decreased the extent and magnitude of microclimatic changes impacting on the rainforest edge and into the forest interior (Maver 2002). Edge changes in rainforest vegetation structure and floristics have also been demonstrated. Canopies were more disturbed closer to linear clearing edges leading to increases in species linked with disturbance, including weeds, vines and rainforest trees characteristic of early Figure 5. Early stages of growth in rainforest restoration plots planted as trial corridors across the Palmerston powerline clearing. Photo: M. Goosem. stages of succession (Siegenthaler et al. 2000; Maver 2002). Again changes are less intense where canopy is retained above road surfaces (Siegenthaler et al. 2000), in comparison with edges adjacent to road verges without closed canopy or to the wide weedy swathes of powerline clearings. Recently, edge-induced changes in floristic composition along a powerline corridor cleared 50 years ago have also been demonstrated (Goosem 2002b). As expected, weeds and early successional species were more common near the edge to a distance of 3-7 metres. However, more insidious changes in floristic composition occurred with a species composition intermediate between edge and interior extending to metres although this zone has never been cleared (Goosem 2002b). Edge studies along linear clearings in the Wet Tropics have also compared faunal composition at the rainforest edge and interior. Changes in small mammal community composition at the edge have been demonstrated for a powerline corridor and for forest roads with wide grassy verges and narrow forest roads with canopy closure (Goosem and Marsh 1997; Goosem 2000a,b). The intensity of these changes varied with road width and degree of canopy closure. At the edge of a 60 metre-wide powerline clearing and at roads with wide grassy verges 422 Conser ving Australia s Forest Fauna

6 Linear infrastructure in the tropical rainforests Figure 6. Narrow, unsealed management road that carries very little traffic but has wide, grassy verges and no canopy closure. Photo: M. Goosem. (Figure 6), generalist species such as the Fawn-footed Melomys Melomys cervinipes increased with a concomitant reduction in interior species such as the Bush Rat Rattus fuscipes and Cape York Rat R. leucopus. Where canopy closure was maintained, many interior species did not avoid the rainforest edge but more specialised rainforest species, such as the Brown Antechinus Antechinus stuartii and Musky Rat-kangaroo Hypsiprymnodon moschatus were still restricted to the forest interior (Goosem 2000a,c; 2002a). However, in the breeding season, edge effects similar to those seen at wider clearings occurred even where canopy closure existed (Goosem 2000a). This seasonal effect was related to increases in juvenile Fawnfooted Melomys that colonise habitat close to the edge. Traffic volumes of up to 480 vehicles per day on narrow forest roads did not appear to increase edge avoidance in any small mammal species (Goosem 2002a). Unfortunately no data exist on road avoidance at highway traffic volumes (5000 cars/day), or where night-time traffic forms a major component of traffic volume, a topic that still needs to be examined. At least one rainforest skink, the Prickly Forest Skink Gnypetoscincus queenslandiae appears to be an edge avoider, whereas another, the Red-throated Rainforest skink Carlia rubrigularis, appears to be attracted by the altered microclimate of the edge (Larsson 2003). Although these varied edge effects penetrate the rainforest to different distances, the result is a much larger area of disturbed forest than is described by the linear clearing footprint. For example, if floristic changes that penetrate metres occur along all powerline clearings, the total altered habitat would be 1,600-3,200 ha on top of the 770 ha of clearing footprint, or % of the World Heritage Area altered from powerline clearings alone. Including roads and highways in the calculation would increase this area substantially. Faunal species likely to suffer greater edge effects include rainforest specialists and those that use large home ranges. In reality, the degree of canopy closure, varying widths of roads and the presence of areas where powerlines swing from the crest of one hill to the next (Figure 7) must be considered in calculating area of edge-affected forest. However, core habitat (otherwise unaffected by disturbance) in the Wet Tropics has been severely impacted by the intrusion of these linear clearings. Figure 7. Diagram of a powerline being swung from hillcrest to hillcrest. Gully rainforest connectivity is maintained. Diagram by Powerlink. Spread of weeds, feral animals and fauna from other habitats Powerline clearings and road verges through forests that are maintained as grasslands or low, shrubby swathes allow the penetration of weeds, pests and fauna alien to the surrounding forest habitat. Anthropogenic disturbance facilitates the weed invasion process by eliminating or reducing the cover and/or vigour of native competitors (Werren 2001). Therefore, most exotic plant species tend to remain associated with areas of gross human disturbance (Maillet and Lopez-Garcia 2000), whereas few establish in stable native vegetation. Prime habitat for weed colonisation is provided during road verge and powerline clearing maintenance that uses herbicide spraying, burning, mowing, grading or the removal of overhanging branches. Recent weed surveys along powerline clearings and road verges within the WTQWHA have recorded major weed infestations (Goosem 2002c), particularly of exotic grasses such as Guinea grass Panicum maximum and molasses grass Melinis minutiflora and shrubs including lantana Lantana camara and wild raspberry Rubus alceifolius (Figure 3). These weeds greatly impair ecosystem function and are considered to be transformer species that change the character, condition and nature of natural ecosystems over a large area (Richardson et al. 2000). The grasses form self-perpetuating swathes, promoting highly modified fire regimes, while the woody shrubs form dense, often monospecific stands that also tend to exclude recruitment of native trees and shrubs. Where restoration works were undertaken across a wide powerline clearing by weed removal followed by herbicide control (Dellow 2000), plots of rainforest trees established canopies quickly (Figure 5). Within four years, light penetration was reduced by 85%, weed cover reduced Conser ving Australia s Forest Fauna 423

7 Goosem by 70%, and grasses almost eliminated (Maver 2002). Similarly, where powerlines swung above the canopy from hillcrest to hillcrest (Figure 7), eliminating the need for ongoing maintenance of the linear swathe clearing, weeds were almost completely excluded over time, although woody weeds often fringed narrower connections along watercourses (Goosem 2002c). To monitor weed infestations in linear clearings, the potential of remote sensing imagery derived from satellite and airborne sources has been evaluated using GIS to relate imagery to a library of weed spectral signatures (Harriss and Gillieson 2002). Such techniques show potential for the future monitoring of weeds in rainforest clearings once remotely-sensed imagery becomes available that provides adequate spectral and spatial resolution and when variability in weed spectra due to phenology can be overcome by large spectral libraries (Harriss and Gillieson 2002). Habitat changes in linear clearings, including the establishment of weedy swathes and reduction in canopy due to maintenance practices, usually result in alterations to faunal composition along the clearing and possibly the ingress of species alien to the natural forest habitat. Feral predators, such as cats Felis catus and dogs Canis familiaris and the cane toad Bufo marinus often use roads with little traffic as movement pathways and bases for unimpeded hunting (May and Norton 1996; Seabrook and Dettman 1996). Toads have been trapped 100 m inside the rainforest edge and are found in very low numbers throughout the forest, but at present their relative numbers with respect to linear clearing edges have not been established. A recent study found that they prefer sites with little understorey, such as revegetated plots in the early stages of regeneration (Larsson 2003). Use of rainforest roads in the Wet Tropics by feral predators has been confirmed using sand-trapping and digital infrared-triggered cameras. Dogs, Dingoes Canis lupus dingo and feral cats were recorded using narrow rainforest roads (Byrnes 2002). Feral Pigs Sus scrofa also use rainforest roads and powerline clearings in the Wet Tropics as travel corridors as well as a base for foraging (Mitchell and Mayer 1997; Byrnes 2002). More pig activity was recorded close to roads and powerline clearings than further into the forest (Byrnes 2002). In a grassy Wet Tropics powerline clearing, the small mammal community was completely different to that of the adjacent rainforest, with grassland species dominating and rainforest species only penetrating the clearing where woody weeds grew near the rainforest edge (Goosem and Marsh 1997). Four years after revegetation was undertaken across the powerline clearing, rainforest small mammals and reptiles dominated in these narrow corridors, while grassland species still dominated the grassy sections of the clearing (Larsson 2003). Similar results have been observed along grassy rainforest road verges. Clearings comprising alien grassland habitat allowed the intrusion of grassland small mammals and even feral mice Mus musculus, whereas where canopy closure was retained above narrow roads, the extent of this invasion was reduced or eliminated (Goosem 2000a,c; 2002a). Where a tourist road traversed a strip of weed-infested, abandoned pasture less than 500 m wide occurring between two blocks of upland rainforest, grassland and feral species dominated the pasture and rainforest species were only able to exist in large areas of the woody weed, Lantana (Goosem et al. 2001). Therefore, invasions by feral predators and other feral species as well as native fauna alien to the rainforest environment have been shown to be a consequence of the presence of both roads and powerline clearings in the tropical rainforest of the Wet Tropics. Road mortality Mortality of wildlife is one of the most obvious impacts of roads on fauna. However, the magnitude of rainforest road mortality is poorly understood because relatively few of the animals living in Australian tropical rainforests are large enough for roadkill to be observed from a moving vehicle. On a 2 km stretch of rainforest highway near Cairns (Figure 8) that carried cars/day, more than 4000 road-killed vertebrates were recorded in three years of weekly walking surveys (Goosem 1997, 2000b), yet few of these would have been seen from a speeding vehicle. The majority (3000) of these animals were amphibians, with about 500 mammals, 500 reptiles and 100 birds. Longevity trials examined how long each group of roadkill would remain on the highway in an identifiable state under common weather conditions and average traffic volumes. This varied from approximately 1 day for small lizards to more than a week for large mammals, reptiles and thick-skinned cane toads. Using these results, the roadkill data were extrapolated to indicate between 3,400 and 10,700 vertebrate animals/km/yr being killed on that highway. Recently, similar weekly walking surveys over 12 months on 1 km of a rainforest highway on the Atherton Tablelands, with a traffic volume of cars/day, found 230 vertebrates that had been killed. The majority (140) again were amphibians but substantial numbers of mammals (20) and reptiles (30) and a higher proportion of birds (40) were found than near Cairns (Goosem 2003). In this case fewer vehicles using the road resulted in less roadkill. When numbers are extrapolated to a similar traffic volume as the first highway, mortality rate was even greater than near Cairns (1150 vs 630 vertebrates/km/yr on the Tablelands and Cairns highways respectively). However, longevity of carcasses was not measured and Figure 8. Kennedy Highway near Cairns where rainforest road mortality was studied. Photo: M. Goosem. 424 Conser ving Australia s Forest Fauna

8 Linear infrastructure in the tropical rainforests taken into account in the latter calculation and is likely to be substantially longer because of the fewer vehicles. In reality, mortality rates may be relatively similar. As yet, the effect on tropical rainforest faunal populations of this high annual road toll has not been quantified. Impacts of road mortality can be substantial. For example, following the upgrade of a tourist road in Tasmania, a population of Eastern Quolls Dasyurus viverrinus became locally extinct (Jones 2000). In the Wet Tropics, threatened species are also affected. A population of endangered Southern Cassowaries Casuarius casuarius johnsonii at Mission Beach suffered road casualties to 14% of known adult individuals over a three year period (Bentrupperbaumer 1988). Similarly, in a recent community survey of rare Lumholtz s Tree-kangaroos Dendrolagus lumholtzi on the Atherthon Tablelands, more than 10% of all observed animals were roadkills (Kanowski et al. 2001). In extreme cases, road mortality can act as a population sink, when mortality becomes greater than recruitment (Rosen and Lowe 1994; Fahrig et al. 1995; Jones 2000). Certain behavioural traits, such as mass migration of amphibians to breeding ponds (Reh and Seitz 1990; Vos and Chardon 1998) or seasonal large mammal migrations (Putman 1997), predispose wildlife to road mortality. These phenomena are not generally a feature of rainforest wildlife movements. However, several frog species are attracted to roadside drains to breed and are strongly represented in mortality statistics, while small mammals such as antechinus and rodents, have seasonal breeding or dispersal behaviour that is evident in roadkill patterns (Goosem 2000b). Dispersing young males form the majority of road-killed tree-kangaroos on the Atherton Tablelands (Kanowski et al. 2001). Seasonal factors can therefore be a major influence on roadkill abundance, particularly for groups such as the amphibians and reptiles whose activity depends on rainfall and temperature. Other factors influencing mortality of rainforest small mammals and amphibians include width of road clearing, presence of gullies, road embankments and cuttings and traffic speed (Goosem 2000b). In general, as road clearing widths increase, road mortality decreases. For small mammals, faster traffic speeds cause an even greater reduction in mortality at wide clearing widths, whereas at narrow clearings, faster traffic results in greater mortality. The reduction in mortality at wide clearings is likely to be due to a lack of animals attempting to cross, this avoidance of the road being exacerbated by the presence of fast-moving vehicles. More small mammals and amphibians are killed in the vicinity of creeks and gullies, while they tend to avoid the greater physical barrier posed by embankments and cuttings. Where gullies or creeks occur, many animals may be killed even at wide clearing widths (Goosem 2000b). In many rural areas, remnant habitat only exists in the vicinity of creeks and gullies, turning the riparian forest into natural movement corridors across the landscape. Where roads intersect with these movement pathways animals may be funnelled toward the road surface. On the Atherton Tablelands, Lumholtz s Tree-kangaroos are easily observed in these riparian corridors and are also commonly killed by traffic where high bridges do not provide a passage for the animals under the road (Izumi 2001). Linear barrier effects In combination, all of these road impacts mean that linear clearings create substantial barriers to many rainforest wildlife species. Physical barriers such as fences and concrete highway dividers also prevent animals crossing linear clearings and may increase this barrier effect. If such barriers result in complete subdivision of animal populations, demographic and genetic problems for the species may ensue. If individuals are unable to cross the barrier, populations on either side are smaller, less viable and therefore more likely to become locally extinct from reproductive failure, predation, disease or an unexpected catastrophe (Shaffer 1981). There is no opportunity for individuals from a population on the other side of the linear clearing to recolonise the area after a population decline or extinction (Bennett 1999). Long-term isolation can also result in the negative effects of inbreeding (Lande 1988). Overall, linear clearings reduce landscape connectivity through restricting movements of many animal species (Forman et al. 2003). The width of a clearing through rainforest has a large impact on the degree of barrier effects. Wide clearing widths restrict crossings of barriers far more than narrower clearings. Common rainforest small mammals have proven a useful indicator group for barrier impacts of linear clearings due to their ease of capture in large numbers. Smaller species were inhibited from crossing relatively narrow roads (8 metres wide, Figure 4) with canopy above (Goosem 2000c, 2002a). Crossings were only about 20% of normal movements within the forest. Wider clearings (20 metres, Figure 6) lacking canopy cover and with grassy, weedy verges, restricted crossing movements more (<3.5% of normal movements), particularly during breeding seasons when smaller animals did not cross (Goosem 2001). Minimal mortality of small mammals was recorded on a highway with a m clearing (Figure 8), suggesting that very few animals were attempting to cross the wide, busy road (Goosem 2000b). Finally, no crossings of a much wider powerline clearing with grassland habitat (60m wide, Figure 3) were made in any season by any small mammals, even the large species (Goosem and Marsh 1997). Therefore this clearing was a complete barrier to small mammals. However, where rainforest connections occurred along gullies (Figure 9), small mammals were able to cross this wide clearing. Large numbers of vehicles travelling at high speeds on highways probably increase the barrier effect of roads (Goosem 2000b). However, the addition of low levels of slow-moving traffic (up to 480 vehicles/day) on a narrow, unsealed road made little difference to small mammal crossing rate (Goosem 2002a), but these relatively low levels of traffic only occurred during daylight hours and not during the rodents nocturnal activity period. On narrower sections of a busy highway (3-4,000 vehicles/ day), small mammals attempting to cross suffered high mortality, particularly where traffic speeds were fastest (Goosem 2000b). Crossing success could be impacted by such high mortality, meaning the narrower highway may be a barrier in the form of a population sink with the potential to remove more individuals than can be replaced by normal population recruitment. Conser ving Australia s Forest Fauna 425

9 Goosem Figure 9. Corridors of remnant rainforest along gullies that cross the powerline clearing. Photo: M. Goosem. Other species suffering barrier effects in the Wet Tropics include rare rainforest ringtail possums (Wilson 2000). The Lemuroid Ringtail Hemibelideus lemuroides will not come down to the ground and is therefore incapable of crossing linear clearings without canopy connections. The Herbert River Ringtail Pseudocheirus herbertensis is similarly but less completely affected (Wilson 2000). Several other wildlife species were not recorded in highway mortality statistics or were under-represented. This suggests that they may not be attempting to cross. No evidence was found that any of these animals were avoiding road death through crossing quickly or when there was little traffic, or by using alternative crossing routes such as canopy connections or road culverts (Goosem 2000b). Such groups included microhylid frogs, several ground-dwelling birds and the Musky Rat-kangaroo, a species that will be intensively studied in the next few years with regards to road impacts. All these species may also suffer from population subdivision caused by barrier effects. A recent study of use of forest interior, rainforest edges and a grassland or revegetated powerline clearing has increased this list of species likely to be suffering barrier effects to include White-kneed crickets Penalva spp. and rainforest skinks including Gnypetoscincus queenslandiae, Eulamprus tigrinus, Saproscincus basiliscus and S. tetradactyla (Larsson 2003). Mitigation and management of linear clearing impacts for rainforest fauna Although many conclusions can now be drawn about the impacts of linear clearings in tropical rainforest from research over the past 15 years, research in the Wet Tropics concerning mitigation attempts is a much newer enterprise. This section will summarise results from the past 5-6 years of this research, much of which is ongoing. Reducing habitat loss - rainforest avoidance and rehabilitation of unused clearings The first principle to mitigate impacts of linear clearings through rainforest would be to avoid this sensitive habitat when planning new infrastructure or when upgrading existing routes. Ideally, some roads could be re-routed or not built. Unfortunately, in many cases this solution is impractical. Road, powerline and pipeline upgrades often cannot avoid sensitive habitat where existing infrastructure already traverses rainforest. For example, in the Wet Tropics bioregion the linear rainforest strip lying between the coast and the tablelands means that all linear infrastructure connecting the two areas passes through the forest. Continuing expansion of human populations and increasing requirements for transport, energy and water infrastructure are the driving forces behind infrastructure upgrades and are only likely to escalate. In an example near Cairns, the Queensland Department of Main Roads proposes to upgrade a major highway through the rainforest from two to four lanes to accommodate increases in both tourist and commuter traffic to the major population centre. A second recent proposal from power distributors is to upgrade a major powerline, following a new route that would avoid clearing of rainforest and allow the current clearing (Figure 3) to be rehabilitated. However, the new route has political opponents - landholders - above whose sugarcane properties the new powerline would be suspended. In both of these upgrade proposals, environmental protection in terms of protection of rainforest habitat and landscape connectivity has emerged as one of the major design principles that engineers must address. Rehabilitation of old, unused roads and clearings by revegetation would also reduce the intrusion of disturbances into core rainforest habitat by increasing habitat quality, reducing edge effects, restricting invasions of weeds and feral animals and thereby reducing linear barrier impacts. Unfortunately, rehabilitation is a costly exercise that can only be undertaken given political will and dollars. Current management strategies for unused roads in the Wet Tropics World Heritage Area are constrained by lack of funds. Passive rehabilitation by ecological succession is hoped to gradually allow roads to revert to rainforest. However, grass and woody weeds along roads and other clearings may either slow passive rehabilitation rates or self-perpetuate by fire and/or inhibiting the growth of rainforest species to almost completely arrest succession (Reynolds 1994; Maver 2002). Additionally, many rainforest species find difficulty in recolonising the compacted road surfaces. Maintaining canopy connectivity Various studies have shown that there is a common denominator that could be used to ameliorate many of the impacts caused by linear clearings through tropical rainforest. The retention of tree canopy over a clearing (Figure 4) would reduce edge changes in microclimate, vegetation structure and composition, and faunal composition, while also reducing linear barrier effects for many faunal inhabitants of the canopy, understorey and ground layer. Concomitantly, erosion on unsealed roads would be reduced as would invasions by weeds and subsequently by feral fauna and/or fauna from nonrainforest habitats. Together these improvements would minimise loss and alteration of both terrestrial and aquatic habitats. There would also be an economic gain due to the reduction in costs of road and verge maintenance such as mowing, grading or herbicide application. The latter also reduces potential pollutants entering the environment. 426 Conser ving Australia s Forest Fauna

10 Linear infrastructure in the tropical rainforests The success of the closed-canopy option is demonstrated by the reduction in linear barrier (Goosem 2000c, 2001) and edge effects (Goosem 2000a,c) observed for small mammals and by improvements in edge effects on microclimate and vegetation (Siegenthaler and Turton 2000; Siegenthaler et al. 2000). Weed invasions are greatly reduced where canopy closure is maintained (Siegenthaler et al. 2000; Goosem, 2002c; Maver 2002), as are some faunal invasions (Goosem 2000a,c, 2002a) and erosion on unsealed roads (Bacon 1998; Goosem and Turton 1999). This excellent strategy is applicable to unsealed roads in protected areas that are used as tourist access to the forest and also to many larger sealed tourist roads that carry low traffic volumes. The lack of weeds along the verge and ability to see into and enjoy the forest environment provide a pleasant experience for tourists and other road users. This could include the majority of unsealed roads still in use in the Wet Tropics as well as a number of tourist roads that traverse rainforest on the Atherton Tablelands and on the coastal strip. Unfortunately again, safety issues dictate that maintenance of canopy closure is not an option on busy highways and main roads where fast speeds are the norm and fallen branches could cause accidents. It is also not feasible above powerlines that require reliability of supply. Minimising clearing width or mortality? Another factor influencing impacts of linear clearings through rainforest is their width (Goosem 2000b). Restricting clearing to the minimum width required, or rehabilitating areas already cleared, retains greater areas of habitat and allows the maintenance of more canopy cover, simultaneously reducing edge and linear barrier effects, and biological invasions. However, restricting the width of roads and highways may increase the likelihood of wildlife mortality from traffic. This trade-off between mortality and barrier effects presents a conundrum. Narrow clearings with canopy cover provide greater connectivity in terms of faunal movements, dispersal and genetic and demographic interchange but greater mortality and consequent loss of genetic diversity. For the majority of narrow roads with low use for tourist and management purposes, minimising clearing widths would represent a net improvement in adverse impacts over a wider clearing with very little connectivity or mortality. Mortality occurring on a narrow road carrying few vehicles at relatively low speeds would not be expected to be a problem for common species. There are three cases where the increases in mortality expected on narrow roads could be considered a serious problem. The first occurs on high traffic/high speed roads, where the possibility exists that all attempts to cross result in death. In such areas, the road could act as a population sink, reducing wildlife populations to a greater degree than recruitment of new animals into the population, and thereby increasing the likelihood of population extinction. The second case relates to habitats of threatened or rare species, where the loss of one or several individuals could increase the likelihood of population extinction. Such is the case for the Southern Cassowary in the Wet Tropics on the tourist and local access roads at Mission Beach and in the Daintree lowlands (Bentrupperbaumer 1988; Moore and Moore 1999). A similar scenario exists in the third case where a species or one particular phase in its life cycle is particularly susceptible to road mortality due to road attraction (e.g. for warmth, foraging or breeding) or during migratory or dispersal movements. Lumholtz s Tree-kangaroo, several frogs, reptiles and scavengers suffer this type of impact in the Wet Tropics (Goosem 2000b; Kanowski et al. 2001). Fauna-sensitive road design engineering options Bridges Maintenance of rainforest connectivity under wider roads and highways can be aided by fauna-sensitive road design. Bridges over creeks and gullies at heights that allow canopy to remain under the bridge will allow movements of canopy, understorey and ground-dwelling fauna. Where riparian connections under the road are absent, creek and gully crossings often become areas of concentrated roadkill (Goosem 2000b; Izumi 2001). This is presumably due to preferences of some species for riparian habitats as well as the likelihood of wildlife aligning their home ranges and moving along these natural features. Riparian corridors also form the only possibility for movement of many rainforest animals in a cleared landscape (Izumi 2001). Therefore, incorporating bridges that span the canopy could ameliorate both linear barrier and road mortality impacts. High mortality rates that have been recorded in these areas can be useful in encouraging road engineers to include high bridges in their designs. For example, many bridges providing canopy or at least ground-level connectivity are proposed for the upgrade to four lanes of the rainforest highway near Cairns. Sites for many of these bridges were chosen in areas of known high road mortality (Goosem 2000b). However, noise and other disturbance impacts on movements of fauna are not yet completely understood and it is possible that disturbance-prone species may fail to use the bridge connections provided. Underpasses and fencing If such preferable but large-scale constructions are not economically feasible, large faunal underpasses, based on culvert designs form an alternative option (Figure 10). These have been trialed in a recent road upgrade on the Figure 10. A purpose-built faunal underpass constructed by Queensland Department of Main Roads in the East Evelyn upgrade. Plants that are unattractive as food to target rare species were planted near the road surface, while food species were planted at underpass entrances. Photo: Jonathon Munro. Conser ving Australia s Forest Fauna 427

11 Goosem Figure 11. Underpass interiors are equipped with refuge from predators for arboreal species (tall branches erected) and terrestrial species (rocks, logs). Photo: M. Goosem. Atherton Tablelands (Goosem et al. 2001; Goosem 2003), where the existing road and a strip of weedy pasture caused barrier effects for many rainforest species. The new fauna corridors consist of the underpasses together with rainforest revegetation of the weedy pasture using wildlife attracting plants. Corridors stretch from the underpass entrances through remnant rainforest to the edge of major rainforest blocks to the north and south. The interior of the underpasses are equipped with refuges from predators for arboreal species in the form of ropes swung from the roof and tall branches erected from the underpass floor, while rocks and logs provide protective cover for ground-dwelling species (Goosem et al. 2001, Figure 11). Monitoring since underpass installation has demonstrated movements under the road by many common rainforest species as well as an individual of one of the target species, Lumholtz s Tree-kangaroo (Goosem 2003). Movements are expected to increase as the revegetation matures and connectivity of the corridor becomes more structured. The previous low rate of road mortality of rainforest fauna in the vicinity of the Atherton Tablelands underpasses has been maintained. However, growth of vegetation on embankments adjacent to the underpasses, although not of plants designed to attract wildlife, may eventually necessitate fencing that prevents animals crossing the road surface and encourages them towards the underpasses. Design of such fences needs to consider the species present in surrounding habitats. For example, Southern Cassowaries react badly to wire mesh fences as they can see the habitat on the other side and will continually attempt to push through the fence, damaging both themselves and the fence. To prevent small animals such as amphibians, skinks and small mammals reaching the road surface, a solid barrier at the base of the fence may be necessary. Fences may need to continue underground to prevent animals digging underneath. Climbing species such as Tree-kangaroos may require the inclusion of floppy tops to fences to prevent them climbing over. Underpasses in the form of smaller culverts designed to carry intermittent streamflow under a busy highway have also been shown to allow movements of small ground-dwelling rainforest fauna (Goosem 2000b). Even culverts that are perennially wet may allow movements of larger species such as the Southern Cassowary (Moore and Moore 1999). Existing culverts have the potential to be fitted with ledges above the normal stream level to allow dry passage of smaller species (QDMR 2000). For highways, a decision must be taken on whether to use exclusion or guide fencing that prevents access to the road surface by fauna and subsequent road mortality. This fencing may also direct fauna towards bridges or underpasses. Knowledge of the species present in the adjacent forest and their vulnerability to road mortality as well as expected traffic volumes and speeds of the road will aid in planning and design. The final decision inevitably will be a compromise between increasing barrier effects for common fauna and protecting rare species from road mortality. In the case of the highway upgrade near Cairns, the design includes a fence and funnel strategy along the entire road. This is necessary as lanes travelling up and down the mountain range will be divided by concrete barriers for reasons of driver safety. Any animal reaching the road surface would therefore be trapped between concrete barrier and embankment. Fences will keep fauna off the road and will encourage them to pass under it via the many bridges. Overpasses Although canopy connections are preferred by rainforest arboreal species such as the rainforest ringtail possums of the Wet Tropics (Weston 2003), rope tunnel overpasses (Figure 12) have been shown to also facilitate movements across narrow roads where no canopy connections exist (Weston 2000). Some of these animals are strictly arboreal, will not venture to the ground and therefore suffer extreme barrier effects from linear clearings (Wilson 2000). Canopy bridges have also proved to be successful for arboreal fauna in Europe, Africa and North and South America (Goosem and Weston 2002). A simple rope ladder across a fifteen metre-wide gap caused by a tourist road through rainforest has also proved to be successful in ameliorating linear barrier effects, with almost all targeted arboreal species observed to use it regularly (Weston 2003, Figure 13). However, the effectiveness of rope bridges over the twenty or thirty metre wide gaps resulting from larger 2-lane highways or even wider 4-lane highways has not been trialed. It is hoped that overpasses over wider highways in the Wet Tropics will be erected and monitored in the near future. Figure 12. Rope tunnel overpass for arboreal species. Photo: Nigel Weston. 428 Conser ving Australia s Forest Fauna

12 Linear infrastructure in the tropical rainforests Figure 13. Simple rope bridge overpass being used by Herbert River Ringtail Possums. Photo: Nigel Weston Reducing Traffic Speed As traffic speed has been shown to affect mortality of fauna, particularly on narrow roads (Goosem 2000b), traffic calming to reduce speeds could reduce mortality. Voluntary speed reduction using warning signage has not been demonstrated to be effective in the Wet Tropics, although no scientific study confirms the anecdotal data. Elsewhere in Australia, Coulson (1982), Gardyne (1995) and Dique et al. (2003) have reported that warning signs do not reduce kangaroo and koala roadkills, whereas reduction in speed limits has reduced koala roadkill in south-east Queensland (Gardyne 1995). Efforts have been made to reduce traffic speeds along roads near Mission Beach in the Wet Tropics to prevent Cassowary roadkill. These measures were not completely successful in reducing speeds. Although tourists did slow down, the initial response of local drivers to perceptual measures diminished over time and speeds gradually increased. Measures trialed included transverse lines with narrowing spacing, guideposts and revegetation close to the road, bicoloured tarmac to give the psychological impression of a narrowed road, rumble strips which create a noise when driving and innovative signage (Quadrio 2002; Yates 2002). However, reducing the official speed limit from 100 km/hr to 80 km/hr did cause a general reduction in speed. Monitoring of traffic speeds will continue together with investigations of wildlife mortality rates and Cassowary crossings (Rainforest CRC 2003). Powerline Clearings The most effective way of mitigating the impacts of powerline clearings through rainforest is to build high towers by clearing only the tower footprint and then revegetating underneath the tower with low-growing rainforest species. The powerline can then be swung above the canopy, the resultant construction causing almost no impacts on fauna, beyond the occasional Figure 14. This powerline was upgraded to swing above the rainforest canopy and to be maintained by helicopter. Photo: Powerlink bird or bat strike. A recent powerline upgrade within the Wet Tropics World Heritage Area took this very successful approach (Goosem 2000b, Figure 14). Towers and line are now maintained using helicopter landing pads on the towers. This avoids the necessity for swathe clearing and thus the habitat loss, linear barrier, edge and invasion impacts caused by the presence of habitat alien to rainforest fauna. Where swathes have already been cleared, riparian vegetation in gullies (Figure 9) allow unimpeded crossing by small mammals (Goosem and Marsh 1997). Four-year-old revegetation plots forming narrow rainforest corridors across weedy or grassy swathes have a similar effect (Figure 5), reducing microclimatic edge effects and weed invasion (Maver 2002), restricting populations of some faunal invaders and allowing generalist rainforest mammals, reptiles and invertebrates to move into the revegetated plots (Larsson 2003). However, rainforest specialists such as the Prickly Forest Skink and Eulamprus tigrinus did not use the four-yearold revegetated plots and probably require longer-term restoration works before a corridor becomes a functional conduit for movement (Larsson 2003). Conclusions From the studies conducted over the last decade it is now possible to answer many of the questions posed by Winter (1991), with respect to certain rainforest wildlife species. Roads as barriers to movements?: Roads do form a barrier to movements of rainforest wildlife. In the case of small mammals, narrow road clearings inhibit crossings but do not completely restrict movements. Wider road clearings may form a seasonal barrier during the breeding season. Potentially, highways with wide clearings could be a complete barrier to movements. Rainforest ringtail possums and other species are also affected. Conser ving Australia s Forest Fauna 429