Road hazards for wildlife more than just a collision risk

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When you think about roads and wildlife, the first thought that springs to mind is a collision with fatal consequences. But roads can impact wildlife in a whole range of ways and more evidence is needed on the most effective ways to mitigate those impacts.

Close up of long-tailed bat on tree trunk
Female long-tailed bat. Image credit: Colin O’Donnell, DOC

Lights at night, noise as a barrier to foraging, loss of habitat and habitat fragmentation – the consequences might not be immediately fatal, but they can still be damaging to the long-term survival of vulnerable populations. Knowing how best to avoid, remedy or mitigate those adverse consequences can be a challenge to even the most environmentally aware developers and land transport contractors and to those who make decisions on resource consents.

In a recent article in the New Zealand Journal of Ecology, Chris Jones, Kerry Borkin and Des Smith from Landcare Research and Wildland Consultants, review what’s known about roads and wildlife from research in New Zealand and overseas. They discuss why evidence-based decisions are needed when assessing roading impacts on wildlife and use the example of long-tailed bats as their case study here in New Zealand.

A bat wing outstretched
The NZ long-tailed bat has a wingspan of 25-28cm and weighs 8-11g. Image credit: Colin O’Donnell, DOC

‘Road ecology’ as it’s called, is a scientific discipline that hasn’t received a lot of attention in the scheme of things.

“Until relatively recently the discipline of road ecology was a minor, applied aspect of ecological and transportation research. Early studies focused on wildlife−vehicle collision rates, but as road networks and traffic volumes have increased there has been an increasing awareness of their wider environmental effects and impacts on wildlife populations and ecosystems.”

So just how much traffic do we have on New Zealand roads? Quite a lot as it turns out. The researchers supply some interesting transport statistics.

“New Zealand has a fleet of almost 5.1 million registered vehicles (February 2018 figures; MOT 2018) and is ranked in the top 10 nations globally for per capita vehicle use. With 11,000 km of State highways and 83,000 km of local roads, New Zealand has one of the highest available lengths of road per head of population in the world. Approximately 70% of all freight in New Zealand is transported by road.”

“National legislation, particularly the Resource Management Act 1991, requires that developers ‘avoid, remedy or mitigate’ the adverse environmental effects of their activities. How these effects are identified and managed in New Zealand varies because regulators and land transport contractors deal with these issues on a case-by-case basis.”

The researchers chose long-tailed bats for their case study as the bats have been a topical issue in recent roading developments.

“The effect of road projects on endemic long-tailed bats (Chalinolobus tuberculatus) has become topical, due to road projects (e.g. sections of the Waikato Expressway in the central North Island) in areas that still retain remnant bat populations in fragmented habitats. A lack of empirical evidence of the impacts of roads on New Zealand bat populations can lead to strongly conflicting opinions, leading in turn to a prolonged consenting process.”

“This absence of clear, robust evidence has resulted in a range of management conditions being imposed on road developments, with little certainty around the outcomes for bat populations. In this review, we summarise evidence on likely road infrastructure impacts on bat populations and the efficacy of mitigation approaches, used both internationally and in New Zealand.”

Because little is known of ‘road ecology’ in relation to New Zealand bats, the researchers first reviewed global knowledge of the impacts of roads on other bat species. They then assess the measures used to mitigate those impacts, both overseas and in New Zealand.

“Although we focus on mitigation, we acknowledge that avoidance of any impact is likely to lead to the best outcome for at-risk populations. Similarly, we acknowledge the emergence of ecological compensation as a management option, but there is significant doubt that this approach is practical for a number of reasons, particularly the time-lag between impact and compensatory ecological response, or whether compensation for ecological loss, e.g. through methods such as pest control, can be maintained in perpetuity.”

One example of the time-lag between impact and compensatory response is when new roads are cut through old forest. Bats tend to favour old trees for their roosts. Planting new trees to replace the old ones may not benefit the bats for another 60 to 80 years – the time it takes the newly planted trees to mature into favoured roost sites. In the meantime, the bat population is roosting in sub-optimal habitat. The loss of maternal roost sites, in particular, can have a wide-ranging impact.

“During spring and summer female bats gather into maternity roosts for a period of at least 12 weeks to give birth and rear their young. Usually only one pup is born annually and remains under close care for 4–6 weeks until it is old enough to fly and hunt independently. During breeding, the females need to hunt intensively and generally return to the roost several times a night to feed their young.”

“Once the pups are independent, the communal group may break up and the bats move to other roosts. Bats may gather from a wide area to form maternity roosts, so any disturbance or destruction of these roosts can affect bats over a large spatial scale. The same maternity roosts may be used every summer, as bats have a strong tradition of returning to the same site year after year.”

A road dividing habitat and foraging areas can also mean bats use up more energy finding food if they avoid crossing roads because of noise or bright lights for example.

“Individual bats use the same routes regularly for commuting between roosts and foraging areas. Alteration of routes can cause increased energy costs to bats in finding, possibly longer, alternative routes to feeding areas and therefore reduced foraging time.”

Bats have delayed maturity, high annual adult survival rate and slow reproductive rates. These are key factors in determining what management interventions are most effective or most essential for longterm population survival.

“For bats, management interventions aimed at maintaining high survival rates of adult female bats will have the greatest effect on maintaining population growth.”

The researchers identify the ways in which road development is most likely to effect bats in general and breeding females in particular.

Habitat loss

“At some point, a threshold of habitat loss will be unsustainable for individuals, leading to reduced survival rates and reproductive outputs. Loss of roosts is likely to be critical to bat populations given the important roles they play in breeding. In particular, it will be critical when roosts are occupied by females and dependent young, but loss of hibernation roosts will also be highly detrimental because of long-term fidelity to roost sites. Impacts of roost loss will become more pronounced when roosts are restricted to certain rare habitat types.”

“For many bat species, older trees have much greater value as roost sites, so their loss can have a disproportionately greater impact than that of younger trees. Direct mortality from the felling of roost trees is also a significant risk when forest areas are cleared for development, meaning that torpid bats and pregnant or nursing females and their dependent young are likely to be at the greatest risk because of their limited ability to escape.”

Habitat modified by noise

“Remaining habitat adjacent to roads may be changed by the presence of the road, its associated infrastructure and the vehicles using it. Bats depend heavily on hearing to navigate and detect prey and several studies have identified the negative effects of noise on bats.”

Habitat modified by light

“Increased levels of illumination from artificial light sources on and around roads have also been identified as likely to affect bats’ nocturnal behaviour patterns.”

Habitat change through creation of edges

“Road construction creates new edges in what was previously contiguous habitat. The associated reduction in cover, shade and humidity and increased light, wind and exposure, can have mixed effects depending on individual species’ requirements.”

Mortality through vehicle collisions

“In New Zealand, there have been reports of long-tailed bat collisions with vehicles but data are sparse. Both New Zealand bat species fly at heights that mean they could be within the path of vehicles, particularly large trucks. Long-tailed bats have been noted flying at 3−60 m above the ground.”

A caution kererū sign
These signs are on 12 streets in Otatara. Image credit: Supplied

“International studies have also recorded higher mortality rates of female bats in early summer, corresponding to the energy demanding late pregnancy and lactation periods of the life cycle, when females must forage more frequently, but are likely to fly more slowly and with reduced manoeuvrability.”

New Zealand studies to date match with what has been found internationally.

“Evidence is emerging of long-tailed bat activity rates being lower along busy roads than on other edges and where light levels are highest. Long-tailed bats are less likely to be present close to roads and where population density is high. This suggests that long-tailed bats are unlikely to be resilient to road projects.”

So what is or could be done to mitigate the effects of road development?

“The use of mitigation approaches in managing threats to New Zealand bats from infrastructure projects has been limited until recent years. Predator control aimed at improving breeding success and survival of bats, planting of indigenous vegetation to improve foraging habitats, and the provision of interim artificial roost boxes to provide roosts are methods that may improve the likelihood of persistence of long-tailed bats.”

“Bat management plans, which may be required by regional councils for resource consents or as part of designation conditions imposed by district councils, have since focused on these as potential mitigation methods. However, the lack of research, and therefore lack of evidence of their effectiveness for New Zealand species, has resulted in sign-off on bat management plans being protracted (and therefore expensive), and remaining contentious even after completion. Protracted consent processes may result in political pressure to make changes to legislation that may not be conservation friendly.”

It’s not just about the bats, however. ‘Road ecology’ is relevant to other endemic species too.

“Although we focus on the mitigation of road impacts on indigenous bat populations, the core principles outlined in this review apply to populations of any terrestrial vertebrate species facing similar issues. Our findings indicate that most mitigation methods have little, if any, scientific evidence of their effectiveness. We recommend that such evidence is essential to guide investment in mitigating road effects on bats in New Zealand.”

So how and where can such evidence of effectiveness be found? What research needs to be done? Who should be responsible and how can that research be funded?

“Given that such evidence is rare, future investment should be guided by an adaptive management framework that is justified by strong, inferential, evidence-based logic, and accompanied by robust, appropriately designed monitoring planned, in advance, to allow an objective assessment of a method’s effectiveness in mitigating an impact. Because such monitoring may be beyond what a single development project can realistically achieve, we suggest the development of a collaborative funding model for supporting the testing and development of mitigation methods. This work is likely to have a significant influence on the future planning and design of road infrastructure projects to minimise the impacts on bats and, more generally, on any native wildlife populations under threat from infrastructure development.”

“Although the primary focus of this review has been on the effects of roads, and their mitigation, on New Zealand bat populations the general principles described for assessing impacts from a population ecology perspective and thereby prioritising mitigation strategies, can be applied to bats facing the impacts of other types of development (e.g. wind farms, transmission lines or power schemes) and also to any indigenous vertebrate population. An approach that links the mechanism of impacts to effects at the population level by considering effects on vital demographic rates can help guide managers towards solutions that will sustain a whole population.”

The full research review is published in the New Zealand Journal of Ecology and is freely available online:

Roads and wildlife: the need for evidence-based decisions; New Zealand bats as a case study (2019)